1983 Fender Showman Channel Switching/Noise Issue

[DrGonz78]

Post pictures of the board and all the caps that you replaced. Which exact caps were the one's you were unsure during installation?

[DJ5D]

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

[DJ5D]

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?

[phatt]

Q, Is it more like a Hiss?  Hiss is often normal, especially with high gain circuits.
A lot of amplifiers do have a certain amount of circuit noise (google resistor noise for info)
When in a quite bedroom circuit noise can be quite annoying but at gig level you never hear it.
Phil.

[DJ5D]

@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?

[g1]

If it's a hiss type of noise, and more than normal amount, then I think it's more likely a semiconductor than a cap. 
So you plugged into the 'power amp in' on the back and it's quiet?  Then the noise is coming from the preamp.
How about the graphic EQ sliders, do they affect the noise?  If so, then the noise is from before the graphic eq section.
If you can isolate the area the noise is coming from, you will have a much better chance of solving it.

[DJ5D]

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?

[DJ5D]

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.

[g1]

Double check whether that slight distortion is there when you go straight into the power amp.

[DJ5D]

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.

[DJ5D]

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

[phatt]

Q Does it get worse when up very loud?
My ears tell me its a fairly large signal and although a lot of clean amps might sound clean there is often a bit of distortion if you listen hard but at a gig you would not notice it. To me that is normal.
If you have another Amp try sending a signal to it from the Preamp output of the showman, see if that makes a difference.

One possible reason for a tiny fizz is speaker Voice coil starting to rub the magnet, often worse at certain frequencies.
To check for that,
You can often tell a speaker is starting to rub by gently pressing the cone. You should hear nothing but if it rubs you will hear a scratching sound through the cone. Apply gentle pressure to several points around the cone. You may need to remove the speaker if you can't get access to the cone easy.
Phil.

[DJ5D]

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?

[phatt]

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

That tells you the preamp circuit is Noise Prone,, not uncommon even with big name brands.

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

Most likely because something has changed (different circuits and diff speakers)
Your Fender has Excess hi frequency again a common issue with clean channels,, not enough thought in design stage.
They build these things and unlikely they do any R&D

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

The signal in this case might not be passing through the rather complex switching and that might be where the signal is clipping,, way over my head to guess why.
I steer clear of amps like this that try to do all this fancy switching,,,
IME I often find the Audio is fine it's the switching crap that fails.

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 noticeable noise.

The sim is likely limiting the bandwidth and hence a much sweeter result because you have bypassed the crappy preamp
The Fender preamp has likely way too much bandwidth and that is evidenced by the harsh brittle sound in your recording.
without a scope it's a guess as to what part of the preamp is causing the problem.

So the speaker definitely is not the problem.

Agreed.

Does this experiment rule out the filter caps?

You have already proved the rectifier and main filter caps are working as they should,,, don't try and fix what ain't broke.

Understand that amps like this are very complex are often a nightmare to resolve.
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.

Only other option might be to try the stomp box and Sim into the front end and see if that improves the outcome.
My hunch is the switching chips maybe at the root of this,, along with bandwidth issues.
Others here might have better options for you to try as some here have way more experience that myself.
Phil.

[Loudthud]

I think it was mentioned in this thread that many of the connections on the PCB were re-heated and/or re-soldered. Was any flux from re-soldering removed ? I would use a chemical intended for flux removal or isopropyl alcohol, a tooth brush and paper towels. Pay attention to the areas around channel switch IC's.