Electric & music

Big Fuzzy Distortion pedal

admin — Mon, 17 Mar 2025 01:39:15 +0000

Big Fuzzy Distortion pedal

I've been noodling with audio circuits since finishing my amp project last year. I mostly played with some different kinds of fuzz on a breadboard, then built a small fuzz box with mostly parts from Ax-Man.

There's no foot switch, just an on/off toggle and controls for the output volume and bias. The bias control changes the voltage on the transistor and hence the tone it creates, from a strong fuzz to glitchy and broken up.

For a first proper pedal box build, i thought i'd make another version of the bass distortion that i built from a kit last year. I like that pedal, but the blend function doesn't work smoothly for some reason that i haven't been able to figure out yet. So until i have the skills to dig into that further, i thought i'd try something new.

The main circuit is really just an Electra distortion with an added LPB-1 boost added after.

I wanted to build a distortion circuit that would work directly with the Guitar PCB Buff 'n Blend Board to mix the clean and distorted signals together to give a deeper sound on bass. The blending needs to have both signals in phase to work properly. Since a single transistor circuit inverts the signal, I added the LPB-1 boost circuit to hopefully get the signal back in phase (inverted twice).

The transistors and diodes are from Ax-Man, my local surplus store, and i basically just tried a few different options for both (with variants in bias) until i got something that sounded good to me. Ax-Man has a suprisingly great array of electronic parts, but the more popular parts are long gone, so you end up trying a few alternates and seeing what happens.

With the circuit working well on the breadboard, i started planning the case. I had a small box that should just barely fit it all, but it was hard to tell with an undrilled box. I created a design and layout with small crosses where i'd drill holes in the top.

While researching designs I saw some really cool etching and thought i'd give that a try. I used the blue press 'n peel film and my laser printer, using an old travel iron to transfer the toner. It went well aside from the border, but i knew i'd want a sort of broken, decayed look, so i left that as is.

This is the box after drilling. I'm really pleased with how the etching came out. It was in the acid bath a couple of minutes too long, the toner was coming off and the lettering for the top 2 labels is not very legible, but the effect is just what i wanted.

This is the inside circuits, wired up. The diagonal board at the bottom is the Buff 'n Blend, it works as an extended breakout board for the foot switch. It's off-kilter to give better access for the in/out jacks. I also had to bend the pots down, pulling the board higher to make room for everything. It's tight, but it fits with no interference or shorts.

Before assembly i gave the top a light coat of satin black paint and after it was dry I polished off the top with some fine sandpaper.

I did have it apart a few times after initial assembly because i thought it had a bad ground, it would hum every time i turned down the volume on the guitar or bass. Turns out that was actually some problem with the cable i built, not sure exactly why, but the pedal works perfectly with commercial cables, so i'm going to call it done.

This was a fun build, i learned a lot and already have a couple more in the works.

admin Sun, 03/16/2025 - 20:39

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Electric & music

lightweight bass tuners

admin — Sun, 09 Mar 2025 16:11:09 +0000

lightweight bass tuners

I've been playing the new kit bass as my main bass for most of a year now, and while it's been staying in tune really well all winter, it has a couple of issues. The pressed-in front hub piece on a couple of the tuners had worked loose, so they were pressed back with a bit of glue to hold them in. The action on the neck was a bit higher as well, so it needed a truss rod adjustment to bring it back. And, i wanted to correct the bit of neck dive the bass has - not terrible, but enough to get annoying.

I found a reasonable deal on some Gotoh Res-o-Lite tuners, and weight-wise they're a big improvement. Original Fender-style tuner on top, replacement on bottom. The full set is almost 8 ounces lighter than the original tuners.

The mounting is different too, instead of 4 screws on the back of the headstock, there's a single screw to locate the back. On the front instead of the pressed-in flange there's a nut that more positively clamps the spindle through the headstock.

Everything about the new tuner is smaller, most noticeably the handle and mounting plate, but also the spindle that mounts through the headstock. This isn't an issue for winding the string, but it's not a great fit in the original holes.

You can see the sloppy fit. The tuner would mount, but it wouldn't be very stable under tension. It's a large enough space that it needs more than just a couple of turns of tape. Luckily i have a metal lathe and aluminum bar, so in an hour or so i had 4 spacers made to fit the existing hole and spindle for the new tuner.

The inside is a slip fit over the tuner spindle nut and the outside a light press fit into the wood headstock. The length is about a mm short of the thickness of the wood, so the clamping action will be on the wood and not the spacer.

The tuner is a much better fit. The new tuner uses a beveled washer under the nut flats, so the top of the hole will never be seen after installation.

I used a smear of light oil inside the spacer and on the nut threads to make sure the nut would turn easily and hopefully avoid corrosion for any future maintenance.

The back of the tuners has a convenient flat that lets you easily align the tuners with a straightedge. I'm not doing anything about the unused holes for now.

I was a little worried that the new tuner handles would look too small, given how large the originals are, but i think it looks fine. They are smaller, but i think not too small, just not traditional. The new tuners are very smooth, easy to turn, and the overall balance is much improved. It feels balanced, neither top nor bottom heavy, just neutral and comfortable. The lighter weight overall will be nice too.

The completed bass, ready for more practice.

admin Sun, 03/09/2025 - 11:11

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Electric & music

bass overdrive pedal

admin — Sat, 01 Jun 2024 02:10:42 +0000

bass overdrive pedal

I've never used an effect pedal with a bass, i was always pretty happy with a good amp and never really needed to change the tone beyond what the amp could do. But my older kid had a cheap programmable pedal that broke, and i thought it would be a fun project to make a pedal kit together as a replacement.

One of the more generally useful effects is adding some dirt to the sound, and when i looked at Build Your Own Clone, the overdrive kit for bass was also one of the simpler kits to make. Of course i got 2 kits so we could each have a pedal when we were done.

The kits come complete with all parts and unpainted cases, all we had to do is finish the case and (This photo is from the BYOC kit page)

We painted the cases first so they'd have time to dry, then got started on the circuit boards. The instructions are well done: granular enough to keep track of the 45 electronic components, taking each group separately. The board itself is also nicely laid out, with clear space for each part and a neat layout when you're done.

With the board populated, you build the rest of the components in place in the box, which gets increasingly fiddly as you have more parts to work around. The trickest step is the first one going into the box, because you have all of the pots and the LED loosely installed and have to line them all up. In the end it's not hard to break that down into smaller steps that keep the alignment and required spacing below the board.

The cases just needed a little deburring around the holes and degreasing. We ended up painting them, picking different but equally punny theme. Elton found a cute photo of a baby plover bird, so named their pedal the Ploverdrive. I got some water decal paper to create labels and an outline, then painted a rendering of Super Grover to make mine the Groverdrive.

Mine worked on first try. Elton's worked when on but the bypass didn't work, turns out they had two of the switch wires swapped, an easy fix.

I've used mine at a couple of band practices so far, and i'm still learning how to dial it in. There's a lot of range in the drive adjustment, and it's easy to dial in a balanced level between on and bypassed volume. I like it, and i'm finding some space to use it on a couple of songs.

admin Fri, 05/31/2024 - 21:10

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Electric & music

Bass guitar kit build

admin — Thu, 09 May 2024 20:41:42 +0000

Bass guitar kit build

I recently began playing bass again in a band with friends, and it's been super fun. It seems so easy, i think partly because we've all been playing our instrument (in my case with a large gap) for 30 years or so, but also because our two singers already have many high-quality songs in the hopper. As our guitarist said, "joining this band is like being born on third base".

I still have my '70s Guild bass and it works great, but it could frankly use a bit of repair. I like the idea of having a spare bass for a backup and something that has a different sound for different uses. I also like the idea of not spending a pile of money on a guitar, no matter how many cool options there are that i'd like to own. Also i also like to make things, so started to look at kits as a cheapish way to get a guitar without having quite so much woodworking.

One of our guitarists has a Harley Benton guitar, a discount brand sold through a shop in Germany, and that was appealing but i wanted to find something that didn't involve $90 in shipping cost. The more i shopped the more i wanted to find something that wasn't a P- or J-bass or Hofner clone, which make up most of the cheap kits around.

I ended up with this P-J kit from Vibeworks Guitars, $207 after shipping.

The kit included all parts, even some basic strings and a cheap cable to connect with an amp. The only extra materials are some solder for the pickup connections and whatever finish you want. It has a jaguar-style body and jazz-like neck, with the combo of precision split pickups and a single coil jazz pickup at the bridge. This seemed like a good option for flexible tone, and i've generally liked the jazz basses i've played even though the neck is a bit narrower than i'm used to on the Guild.

The wood parts were completely unfinished, so everything would need some level of work before assembly. The neck is maple and fully built with frets, markers, and nut already installed and prepped. The body is alder with everything milled and the primary holes pre-drilled. The body and neck are numbered to match together.

As shown, the neck comes squared off, since the most common headstock styles are trademarked. I drew out and cut it to a general Fender-like shape and filed/sanded it smooth. The bass had some milling marks and needed a bit more sanding, but probably less than 2 hours total before starting the finish.

The finish is some Epifanes spar varnish that i had left over from the earlier dashboard project. It was brushed on and wiped off to keep the finish thin and matte. The neck got 4 coats and the body 3, and the finish feels like smooth wood; not glossy, just a slight texture to it.

While the neck stayed quite light the wood on the body darkened a couple of shades with the same finish. I was hoping to keep a natural finish rather than painting a color, so i was happy to see that the wood looked pretty good.

This isn't part of the instructions, but once the varnish was dry i used some aluminum tape to add shielding inside the body cavities to help keep the electronics quiet.

As much as i like the wood finish, i also wanted something different with it, something a little funky to keep it from being too serious. I enlisted one of my kids to paint a cute thing with too many of something, and got this amazing character:

The assembly process was very straightforward, it can really be done in a couple of hours if everything is ready to go. I took some time to lube the tuners and break them in a bit, but everything seemed to fit up fine until i got to the bridge.

The stock bridge in the kit is the same as an average Fender Squire bass, but i really like the heavier bridge on my Guild and that kind of sustain, so i swapped the kit bridge for a Gotoh with brass saddles. It fit perfectly, but i realized that something wasn't quite aligned right. When using the pre-drilled holes and the neck in place, the E string was right at the edge of the neck by the 12th string. Either the neck was a hair out of alignment or the bridge holes were in the wrong place. At this point it was way easier to move the bridge, so i drilled new holes 1/8" over and the alignment is much better.

In hindsight, it was a mistake to install the pickups first, and i had to move the p-pickups. With the fit of the pick guard and control panel, there's a very small window of fit locations, so the best approach is really to place the pickups there and let the pick guard determine the placement, since the pickup holes are also a close fit. As a result, there are 2 sets of many of the holes, but thankfully most aren't visible on the finished bass.

After assembly, the neck truss rod only needed a minor adjustment to give a slight relief for the strings. I spent some time getting the action dialed in as low as practical with the bridge, then the intonation and pickup height. It worked on first plugin, no issues with the wiring or controls.

The tone is very good, with a little more meatiness and weight than the Guild's single pickup. The bridge jazz-style pickup has a bit of noise as all single coils do, but the split precision pickups are dead quiet. I've used the bass in a couple of band practices already and i'm really happy with it.

admin Thu, 05/09/2024 - 15:41

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Electric & music

Champ clone guitar amp

admin — Thu, 02 May 2024 19:36:50 +0000

Champ clone guitar amp

As noted in the Fanfare guitar amp and speaker cabinet story a couple of years ago, that wasn't actually the first guitar amp i tried to build. I had bought a small box of parts along with a couple of chassis that had been pulled out of old Hammond organs that would have provided most of the pieces needed to make a working amp.

That project had multiple problems: reusing a partially-cleaned out chassis made working in there messy, i didn't really know what i had for transformers, and all that made it difficult to really trace what was going on to prevent it from passing a signal.

When i got interested in trying again this winter, i decided on largely re-starting. I kept the main board that i had wired, because i felt pretty confident in that part of the build. I set the rest of the parts aside and started with a clean schematic for the Champ, the board, and the power transformer. I was able to measure the output, so i at least knew what i had for the PT. I also had the output transformer, but was less confident in that since i didn't know anything about it.

I used Rob Robinette's site for schematic and layout ideas but wanted more of a head-style cab that i could use with different speakers so i made a more horizontal layout. I splurged and bought Harbor Freight's cheapest bending brake, and it was barely adequate for the ~16ga steel i had in my materials pile.

A little hammering to square up the corners and some paint to cover the dings, and i had a chassis. I spent a lot of time tracing and cross-checking the board to make sure everything was correct. And i corrected some things. The jacks and pots were all cleaned up and reused from the first attempt.

I didn't take many pictures along the way, so here's the mostly finished internals:

Feeling good about thing so far, i carefully followed Rob's Amp Startup procedure. That was going great until i put in the rectifier tube and got no DC voltage at all. That 5U4 tube was old and i don't know how to test it, so i decided to order a new one and get a correct output transformer while i was at it.

When the parts arrived i grabbed my meter and plugged in the tube to find no change. Retracing everything for the rectifier i realized i had one of the 5v heater wires on the wrong socket connection, so that's why the tube never even got warm. With that fixed, the old tube worked fine, so now i have a spare.

The rest of the start up went well, with no drama and good tone, so i moved on to making a small head cabinet for it.

I planned the chassis size to be a little smaller than the surface of the speaker cabinet i made earlier. The small vents on top are probably overkill, but i added a power resistor to drop the voltage a bit, and didn't want that to get hot against the top of the cabinet (it barely gets warm).

I covered it with the left over tolex from the speaker cab, along with some perforated metal from Ax-Man and a new handle.

The controls are very simple: a single input, a toggle switch to turn on the negative feedback circuit, and a combined on/volume switch, with an indicator lamp. I had originally thought about using the switch for high/low input so i could have 1 input instead of 2, but i got really hung up on how to wire that correctly and thought the NFB switch would be more useful anyway.

I was right about that, the overall volume is pretty low, so no real need for the dual inputs and the NFB switch does make a real difference in tone and how the amp distorts.

The back of the amp is pretty standard, but i decided to wire up both impedance options from the output transformer. To keep it simple there's just one speaker jack but a 4/8 ohm switch to match the connected speaker.

Here's the full stack:

It makes a cute combo, maybe 2.5' tall. I doubt that the 12" Peavey Black Widow is the ideal speaker for that amp, but it sounds ok. A little peaky with the treble, but i'm not sure if that's the speaker or the complete lack of tone stack. It's quiet enough for an apartment through about 1/3 volume, and get a really nice growl around 1/2-2/3 volume. Even fully cranked up it's not painful and has a nice grit to it. The distortion kicks in at a lower volume with the NFB turned off.

On bass the volume is lower overall but the tone is good, if still a bit peaky on high notes.

For future changes, i'd like to try sealing the back of the cabinet and cutting a vent slot at the bottom of the baffle. I think that speaker will sound good with bass with a more sealed up surround. I might try the small capacitor "ice pick highs" mod from Rob's site to see if that smooths out the treble. That's about it though, i don't want to use this amp as a platform for endless tweaking, i'm happy having it work as a decent bass practice amp.

For next amp projects, i'm thinking of building a good mid-size bass amp, something like the '60s Ampeg B-15 flip-top.

admin Thu, 05/02/2024 - 14:36

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Electric & music

a modern 78 portable

admin — Fri, 01 Mar 2024 01:00:02 +0000

a modern 78 portable

This is a weird niche project that i've been thinking about for a while. I have a portable record player that works great for multiple speeds, and i've used in in the yard several times to listen to records, but it's kind of a drag to also string out a power cord.

I also have a wind-up 78 player that works great, but it uses steel needles, which only works with acoustic 78s, those needles are too rough on the later electric recordings.

The solution: a hybrid, using a spring-powered motor to spin the record and a small electronic amp that can be powered by a battery. Not as convenient as a fully electric rig, but hopefully able to play music for a good amount of time on a battery charge, or until i'm tired of winding up the spring.

The starting point: a (probably 1930s) Phonola portable that i found at an estate sale for $20.

It's complete and the motor probably needs a good cleaning, but it works well enough for some testing and not too nice or rare to modify a bit. Most components are riveted on, which is a hassle, but the dremel can grind them a way pretty quickly.

Next i trolled ebay until i found a tonearm that would be easy to mount and has cartriges available. This one was taken off of a Technics turntable, it's not super fancy but has a nice gimbal mount and i found a cheap 78-specific cartridge that fit the P-mount. Also the straight tube was easy to remove and shorten to fit.

I removed 3", which was probably a little too much, but it works. I made a mount for it to get it in the ballpark of the right height with a record in place.

It would track better if the head weren't angled, but it's going to be wired for mono, so i'm not worried about perfect separation or fidelity. I'll make another mount that's nicer and moves it about 1/4" out so this could also work for 12" disks.

I'm using amp boards found on Amazon, the output wasn't rated but they take 5-24 volts, which is a handy range for battery power. I ended up needing two boards in series to get enough signal boost, the volume was too low with just one.

This is my testing setup:

Power is from a DC power supply, and output is going to a guitar cabinet. I also tested a small 4" speaker, which worked, but i probably want something with a wider range. This setup sounded really good, clean signal and good volume range. Everything from 6-24 volts seemed to work equally well as far as the sound.

Lots of work yet to do, but happy that the concept works. For next steps, I'm going to try another preamp board that provides some tone control to compensate for the pre-RIAA sound curve. I also want to see what speaker options i can fit in the case. It needs a proper tonearm mount and plate to mount the volume knob, battery and charging port.

admin Thu, 02/29/2024 - 19:00

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Electric & music

Columbia Viva-tonal portable

admin — Sun, 08 May 2022 13:53:46 +0000

Columbia Viva-tonal portable

I've been looking for one of these portable wind-up players for a while now. I have a couple of electric portables that work great and play a wider range of formats, but I also like the idea of being able to play records where we might not have electric power.

While trolling local estate sales, I saw this one pop up, and somehow it was still for sale when I arrived:

It seems like a pretty standard portable player from the 1920-30s, but I can't find an exact match for it online. Most similar Columbias have the brake and speed controls on the opposite sides, and the speaker sound hole exits at the rear of the unit. It's possible this was a European model or something, but I'm not really sure.

You can find similar units on eBay for $300-400, and (as far as I can tell) this isn't a particularly rare or fancy machine, but for $100 with all parts complete, I thought it was a bargain. On first test, the motor turned and the reproducer diaphragm looked intact, so I figured anything else could easily be replaced or fixed.

When I tested it at home it was obvious that the grease was pretty sludgy and it needed a good cleaning and lube, so I opened it up. Some of these players used pot metal for the tone arm and mount castings that would swell and crack over time, but this one seems to be a little better metal.

After removing the platter (slightly stuck on the taper fit) I got a look underneath and found this little brass number tag. I assume it's some kind of serial number, but apparently there's no surviving record of Columbia hardware from this era, so I can't do much with it. You can see the brake lever on the left, with the leather pad that presses against the inside edge of the platter to stop it from turning. The lever on the right is a speed control, which adjusts the speed of the governor in the motor.

With the 5 screws removed, the motor board lifted right out. Underneath the board you can see the formed cloth/fiberboard sound horn, which is a little soft on the bottom but fully intact, and the spring-powered motor. Look closely at the platter and motor and you might see a familiar brand.

The guts of the unit were made by Thorens, better known in modern times as a brand of high-quality turntables from the 1970s on. Apparently they made some of their own branded players in the early 1900s, but also supplied motors and assemblies to other makers as well.

The spring is a sealed box, but seemed to work fine, so I didn't open that up. All exposed bearing points were cleaned and related with fresh grease (gears) or sewing machine oil (pivots).

Next I checked the reproducer:

This is the orthophonic style, with an aluminum diaphragm rather than the mica diaphragms often seen on earlier home players. I have a mica reproducer in an adapter for my Edison player, and have rebuilt that one with fresh rubber gaskets. I took the cover off of this one, and found the diaphragm intact, and no obvious issues with the rubber, so left it alone.

Here it is on the first outing, to our back yard, playing records at the picnic table. It's surprisingly loud for such a small machine, even using soft needles. There's no volume control at all, short of covering the sound hole, which doesn't do much.

I did a little cosmetic work on the case after this to clean up the mildew powder inside the case lid and reglue some loose covering material, but it seems like a really solid little unit.

admin Sun, 05/08/2022 - 08:53

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Electric & music

Fanfare guitar amp and speaker cabinet

admin — Thu, 28 Apr 2022 14:14:32 +0000

Fanfare guitar amp and speaker cabinet

Lacking another big diversion, I spent a good part of this past winter learning how tube amps work and trying to piece together a small guitar amp. I read all about the Fender Champ and even bought an amp chassis and parts from an old Hammond organ to try to make one.

That project got stuck on some still-undiagnosed problem, but keeping my eye on estate sales, I found one where a guy had been fixing and reselling old radios, with tons of related parts and tools. I got there way too late for the great stuff, but I picked up a working AM radio and this non-working tube-based intercom (for free):

The pen is there for size comparison. This is a Fanfare, made by Fanton, probably from the 1950s. I can't find a lot of info on the, but it appears that they worked wirelessly by transmitting a signal through the power line, and used the built-in speaker as the microphone (I couldn't find another mic in the box).

The lever at the top is a push to talk setup, and the knob is on/off volume. Here's the back:

And here is the chassis and guts:

It's similar to the AA4 radios of the time in that the tubes are all running directly off of the mains, rather than from a power transformer. This isn't the safest setup for electrics, but it was a simple and cheap way to build things. I don't have a shot of the underside, but there is a bus ground, and I was able to confirm that at least the case isn't electrically hot. I added a polarized plug as a basic precaution, and may add an isolating transformer at some point.

The tiny speaker was crumbling. The paper box at the top is just housing the push-to-talk lever and related switches. It's a relatively complex switch that I forgot to photograph separately. The 35W4 on the left is a rectifier tube to cover AC to DC, and the 50C5 tubes are both power outputs.

The attractive thing about this for me is the 12ax7 tube, which is the basis for many guitar preamps. After studying many different circuits, I figured I could strip away everything that wasn't part of the audio path and hopefully end up with a reasonable guitar amp.

I'll have to update this post with the actual schematic I ended up with, but it's very simple. The signal comes in through a 1/4" jack, through 1/2 of the 12AX7, out to the volume, back to the other 1/2 of the preamp, then out to just one of the 50C5 tubes. The 2nd tube seemed to be wired more for boosting the wireless signal, so I routed around it and added a resistor to the circuit to keep the electric load correct on the remaining tubes.

The plan worked, and after swapping out a few small components with values more in line with other amps I'd seen, it works! I did initial testing with the bare chassis, the photo above was the first test back in the case.

Now the tiny amp needed a speaker cab. I had planned for this to be a fairly small practice amp, but I found this 12" Peavey Black Widow speaker for only $25 at the local guitar store. Also, the amp is surprisingly loud, so it seemed like a decent-size speaker would be worth a try. It would make the speaker cab more versatile for other amps if I end up completing any of them.

I didn't get any pictures of the bare cabinet, but it's just 1x10 pine from the lumber yard, and uses box joints at the corners. This is how it looked with tolex (black, Vox weave pattern):

It's a bit tall so I can store the amp inside the cabinet and keep everything together. You can see the speaker offset in the baffle panel:

The baffle is plywood I had in my scrap pile. I got the vintage grille cloth from Ax-Man, who just happened to have a huge roll of it at the store.

Here's the back of the assembled cab with amp on top:

The speaker is upside-down to keep the wiring more compact, with storage space at the bottom. The cabinet itself isn't terribly heavy, but that speaker weighs a ton. It's luggable, but not light.

The front:

I kept the baffle simple and screwed it on. The cloth isn't as straight as I'd like, but not terrible, and fairly tight.

The overall package sounds pretty good with my old Ibanez guitar and my very mediocre playing skills. It's surprisingly loud, and gets some nice grit in the sound at anything over 1/3 volume.

But at least it plays, and it also looks pretty cool at night (the light only comes on after the tubes are warmed up):

There's a persistent hum that I still need to chase down, most likely a grounding problem somewhere. I might also add a tone control and some kind of attenuation circuit to keep the volume down a bit. Fun project though, and fun to play through.

admin Thu, 04/28/2022 - 09:14

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Electric & music

1960s record changer rebuild

admin — Mon, 07 Feb 2022 14:06:33 +0000

1960s record changer rebuild

This is a 1964 RCA VGA11T record changer that i found at our local surplus store:

As advertised, it powered up and turned the platter, but no audio signal. I was looking for a project and had $20, so it came home with me. Having recently tuned up an old Dual automatic turntable for sale i was happy to not have to get into the mechanicals of a record changer.

In some initial testing with a meter, the cartridge didn't seem to be putting out any signal, which should be around 1 volt for this Astat 346 cartridge (from online research), so it should show something on meter. I found a good deal on an NOS unit online with a LP/78 flip needle. Unfortunately, installing the cartridge did nothing for the overall audio output, so I dug in further.

I'm no electronics expert, but tracing the wiring I realized there didn't seem to be any power going to the amp circuit. With no schematic to work from I decided it would be easier to try to replace the amp entirely than try to fix it with my limited knowledge and test equipment.

At least I was able to trace the signal path and tap into the input line. The 2 very thin wires here are coming in from the cartridge, the signal goes through the tone and volume pots, then out to the heavier red wire and back to the amp. By tapping in here I can use the original knobs & position and have volume control. I clipped that and added some connectors for easier testing.

I did a fair amount of research about the various small amp circuits available, and I'm still certainly no expert. I ended up with a small amp kit based on a LM1875T chip that I had to assembly myself, but only cost $6. It's rated for 20W, which is probably way more than needed for a single 4" speaker, but I figured the extra overhead might help, not having a separate pre-amp.

I clipped the new circuit together for testing to see if the new amp would provide enough power and a decent sound. It worked great! I'm testing it here with a 78 disc. The amp board is rated up to 25 volts, so I tried it at different levels to see how different voltages worked. There wasn't any real difference in performance from 6-18 volts, so I dug through my pile of wall warts to see what might be a good fit.

Happily, the old style crystal cartridge puts out a strong signal with great tone balance, so the sound and volume was spot on, and didn't overload the speaker. Again, I'm not expert, but it looks like newer MM or MC cartridge would require a preamp to get the right RIAA tone curve before the standard power amp. This set up has a very small hum at idle, but I'm just going to live with it.

I chose a fairly lightweight wall transformer putting out 12VDC and 1 amp, and tested it in place of the power supply to confirm before pulling it apart. With the back of the case off I could solder in the power leads (in place of the wall plug) and finalize the input & output wiring.

The power supply was glued in to the case bottom, so it hot glued that to the player case, and the amp board screwed in to the side with some cork spacers. There's easily room to fit both under the speaker board and nothing is visible once the cover it back on. I added a couple of staples to keep the wires from flopping around.

Here's the final assembly, everything buttoned up and working. I tapped into the main unit power after the on/off switch, which is triggered by the Manual/Automatic switch at the lower right corner. That way the amp is powered on at the same time as the turntable motor. All controls work just like the original did, just going through some different guts.

Here's a view all closed up and ready for travel. This is going to be really nice for back yard listening and maybe even some camping trips.

admin Mon, 02/07/2022 - 08:06

tag

Electric & music