well it looks like the story isnt over yet.

as ive been playing with this now functional amp, ive noticed that the mids and highs are quite harsh.

i have to work very hard to dial in a pleasing tone, and the range of tones that work are quite limited.

i think i'm going to pull it apart again and replace all the RC4558s (6 of them) on the preamp board, and double check the caps, make sure there arent any of those cheap JACKCONs in there.

i'll post the results soon...

UPDATE:

The amp is now 100% functional!

The gain and bass knobs had some bad traces, one was close enough to join with solder and the other i connected the circuit with a small wire.

Happy jamming with a classic fender tone shall now commence!

Thanks to all of you for your invaluable help! @phatt @loudthud and everyone else!

Major victory! the tide has turned in this battle!  :dbtu:

After reading up in the forums about IC's, particularly the RC4558, I decided to take a shot in the dark and and replace the one on the logic board.

Fortunately I found a local source, who had a whole batch of compatibles (part number MC4558CN) and the tech also inquired about the replacement caps I had already put on the board. Some of the caps were the JACKCONs (apparently a quite inferior brand judging by his reaction) so he suggested that I pull those off and replace with high-grade Nichicon brand.

and Voila! The white noise is completely gone. I don't know what made the biggest difference, the IC or the caps, but the noise is solved!

However, I'm not quite done. Both the Channel 2 Gain and Bass aren't working as they should.

Since this Fender has a Vol, Master, EQ and two mid controls on channel 2, I can dial in a nice dirty sound even without the Gain and Bass- but I really have to tweak it a LOT to overcome the harshness of the tone- and I get the feeling that this amp was meant to sound much better.

So, next step, replacing ALL the 4558s on the preamp board, as well as any other JACKCON caps I find.

GREAT suggestion, Phil!

I bypassed the logic board using Channel 1 preamp P3 pins 9 & 10 to poweramp P1 6 & 7 (from R18 as you suggested)

....and it gives me a full, clear and louder signal! (with the exception a small bit of ground/pickup noise) All snowy hiss is gone.

I was convinced that the logic board was not the source of the problem here, but this seems to prove otherwise.

With the bypass, on channel 1 I can control treble bass and volume as expected, but when I tried connecting channel 2 (P3 11 & 12) and all tone shaping knobs work except the gain, for some reason. (note that gain does work properly when logic board is all plugged in)

I'm guessing the gain circuit is somehow processed on the logic board...? Nonetheless it has no problem functioning as the "dirty" channel with just the vol and master volume.

So now I focus in on the logic board. All caps have been replaced, along with 2 IC's that fixed the original channel switching problem. So I'll have to test every single component on the board, looks like.

I spied a vintage Hitachi oscilloscope in the local classifieds for $70....the addiction is getting hungry LOL.

This inspires me to make something that looks like a fender amp on the surface (i like the vintage look) but inside uses an entirely different preamp.

But before that........I still want to crack this slight distortion problem.

I noticed something new, when mic'ing up the amp, i get a clean tone as long as the volume does not exceed 40%.

Does this tell you anything?

@phatt Thanks for the detailed answers, very much appreciated! Also @loudthud thanks for those tips, it could probably use a more thorough cleaning with the proper chemicals.

I tried feeding the amp sim into the front and it sounds exactly like it does with just the guitar.

@phatt you said:

"I've built several dedicated preamp systems for players and bypassed the whole front end of there crappy combo rigs.
The chance of finding let alone fixing the problem is low. All I can say is either sell it or bypass the preamp. "

The primary goal of this project is to learn how these electronics work, instead of just being able to do repairs using youtube tutorials. So even if its ultimately not 100% I've learned a LOT, which will help me on some plans I have in the future.

So Phil when you say you have built pre-amps for exactly this type of situation, this caught my attention. I may be interested in doing this myself.

Did you build them from scratch, or modify existing parts? I was thinking about converting it to a tube amp if some kind of kit or instructional exists out there for that kind of thing.

Thanks for all the help!

well that was an interesting experiment. the speaker is A-OK.

preamp out to another amp: white noise and some distortion- but of a different character.

preamp out to mixing board: same result, white noise and occasional static, but not the light distortion tail.

stomp box/amp sim into power amp in: near-perfect. If i just wanted a speaker only, all would be well. Going in this way it sounds great, aside from barely noticable noise.

So the speaker definitely is not the problem.

Does this experiment rule out the filter caps?

Here is what the noise sounds like.

You can hear the slight distortion as the note fades.

I hope someone can recognize this symptom instantly....

https://drive.google.com/file/d/1hOBJVYW_dBoDQ3LQefN9l19W4dl_97Nm/view?usp=sharing

nope its not there when straight in to the power amp.

i've reflowed or re-soldered everything cap wise, except for the filter caps and the low voltage power supply ones.

SUCCESS.

I went thru with a magnifying glass and did some cleaning up with the soldering/traces (it pays to take detailed photos of everything prior to work) and the vast majority of the noise is now gone. It sounds like fairly normal noise now, a soft snowy one that can be minimalized with the right EQ settings.

A little more work to do though- there is still a very slight (and unpleasant) distortion tail on the clean channel especially on the lower frequencies. So I'm going to trace the whole clean channel preamp circuit again and check for any leaks. But it looks like i'm in the endzone now.

yes the noise is pre-EQ, and the power amp line-in is entirely unaffected.

I'll start researching semiconductors immediately, but what about the big caps?

@phatt

Its a bit too loud to be normal amp noise. And its constant regardless of volume or anything plugged in.

I think you said that if you plug in to the back and there is no extra noise, then the power amp is likely OK. Does that mean that the low-voltage power supply is ok as well? There are 4 2200uf caps on that board. And of course there are the two BIG 60V 11000uf ones...i didnt replace those.....yet. (maybe this is it?)

However, I did discover 2 extra caps in the power amplifier section, and one was bulging a bit- i was able to replace it because I had an extra, but there is still one 50V 1uF Bi-polar left to replace there.

I've got my fingers crossed that it is the last link in the chain.

But if not, then its on to the low-voltage and the big caps, unless those are totally ruled out by the plugging into the back test.......are they both ruled out by that?

Well good news and bad news.

I replaced all the caps and everything is functional.

However I still get a static noise even with volume down. It's not quite the crackles and pops I used to get, its more a soft static, but still there.

All the caps are now replaced, so what else could be causing the static when the volume is down?

a DISCOVERY.

The reason why most of the caps were not marked with polarity is because they are NON-POLARIZED. On the 1983 Showman there are only 3 polarized caps total on logic/switching and preamp boards.

Being a novice at electronics I didn't know that these non-polarized capacitors existed, so this explains the partial functionality. it was right there on the schematic but I just didnt know what "VNP" meant.

Next step: Getting proper non polarized caps and installing them.



Updated Schematic with Caps/Critical Components Highlighted:

https://drive.google.com/file/d/1N5OFU8sjBvo5nWdV8ukadsTESLImRg8U/view?usp=sharing

well things did not go as planned.

When getting new caps i was given a different variety, they assured me that they would work fine despite having higher voltage ratings but the same uF.

So when putting them on, I had to guess as to the polarity of some because most of the original fender caps had no polarity markings on them. So i had to guess that the orientation was correct based on the few that were marked on the circuit board.

Nothing blew up, the channel switching and all buttons still work, i can hear the EQ change when i move the sliders, but alas NO SOUND from the input jack. The external input on the back still produces sound but channel 1 & 2 are not producing signal to the outs.

Any suggestions? How can I make sure all my cap polarities are correct?

EDIT: i have sound on channel 1 including EQ and Reverb. The switches are working but going to channel 2 no sound. right back where i started, almost.