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Solid State Amplifiers => Amplifier Discussion => Topic started by: DJ5D on February 11, 2021, 05:06:20 PM

Title: 1983 Fender Showman Channel Switching/Noise Issue
Post by: DJ5D on February 11, 2021, 05:06:20 PM
Hi,

I have a 1983 Fender Showman, a solid-state amp from the Rivera era, and it has an interesting problem.

(https://i.imgur.com/16Fs6XI.jpg)


On the front panel, Channel 1 works great. However, I cannot switch to channel 2.

So I tried plugging in the footswitch to see if it was a bad switch on the front panel, and this is where it gets interesting. When I plug in the footswitch and hold down the Reverb button, I CAN switch channels, and play thru it with all channel 2 controls working, but only as long as I hold the Reverb footswitch down. As soon as I let it go, nothing.

So the switches themselves appear to work but somewhere there is a problem. The only thing i have done so far is to replace the input jack with a generic alternate- I'm not sure if that would affect channel switching or not.

On the schematic you can see where the front panel switches and the footswitch paths merge, so i have to assume that the issue is happening on the preamp board and not the logic board (but honestly i have no idea)

I'm attaching link to a PDF of the schematic- if anyone can help me trace the signal path and recommend some possible solutions that would be greatly appreciated!

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

So far I have tested IC's 1 (the schematic seems to indicate this is involved with channel switching) with a multimeter in diode mode. I don't know if this is useful info or not, i dont have an o-scope and a multimeter is all i have.

Here is IC 1 and the values I got:

(https://i.imgur.com/idCmBBD.png)


Hopefully someone can give me an inspiration or two on this. Thanks in advance!
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: Loudthud on February 11, 2021, 05:39:09 PM
Google is telling me the schematic file you linked does not exist.

Found this thread: https://www.ssguitar.com/index.php?topic=3605.msg27527#msg27527
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DrGonz78 on February 12, 2021, 02:35:21 PM
The first thing to do is to make sure all those grey colored ribbon cables are making connections from each end to the other end. I would literally check for continuity between each wire on all of those ribbon connectors. They tend to bend and wear out badly. What happens is that the connections become unreliable and introduce many problems between each board's connections. Only after making sure that these ribbon cables are making good connections on each wire can you expect to know they are not the main problem at the start. I attached a few pics of the ribbon cables that I am referring to on that amp.

Update: Make a note that Pic 2 and Pic 3 show a different ribbon cable. I took another cable I had and soldered the each leg to the corresponding pin. I left the ribbon connector on the logic board side and soldered the wires directly to the preamp board. This was the most important factor when fixing this amp in my opinion. There was no way to depend on that original ribbon cable after I ohm out each connection on each wire only to find the connections were failing. Also, note that the original ribbon cable in that spot was actually white.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on February 12, 2021, 06:43:28 PM
I updated the link to the PDF.

To the best of my knowledge I have confirmed that all the connector cables are working (they have been replaced) but i have not confirmed that the pins they are attaching to are actually making a connection to the board. Some may have corrosion or wear.

That will be my next step!
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on February 12, 2021, 06:50:06 PM
@DrGonz78

I see that you soldered an orange wire directly to IC2.

Can you explain this?

EDIT: nevermind, I saw your other post, I wonder what they heck they were trying to do?
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DrGonz78 on February 13, 2021, 02:23:08 PM
I think that whoever had opened up the amp before me was trying to figure out if the TL604 or TL607 chips were a problem. I got the amp that way and made for a confusing moment indeed.

Side Note: If you are looking for replacement TL604 or TL607 chips I found a good reputable source. I bought a few to have on hand after this repair. I made a test setup to make sure the analog switching was working and that they were actual genuine chips. The chips passed the test with flying colors.

https://www.utsource.net/itm/p/147198.html

https://www.utsource.net/itm/p/713686.html
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: Enzo on February 15, 2021, 10:41:12 PM
And another alternative:   Peavey used a TON of TL604 in the 1980s, and when TI stopped making the chip, PV designed a small drop-in board to replace it.  They sell them through their parts department.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on February 16, 2021, 02:10:16 PM
Thanks, this info about the replacement TL604s & 607s- immensely helpful.

I was planning on replacing all capacitors as well, I was asking someone at amprepairparts.com for a kit, and the told me that the 11000uF/60V capacitor was no longer being manufactured. I'll likely need to find that one as well if I want the job to be a complete restoration.

Hopefully within the week i'll be able to post a follow up with how things are going.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: g1 on February 18, 2021, 01:28:34 PM
You can spend a fortune trying to match the original caps, especially if they are screw terminal type. 
There is no point in wasting that kind of money.  The modern common equivalent would be a 10,000uF 80V snap-in type.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on February 19, 2021, 01:32:40 PM
Success! Well mostly. Channel switching now works.

Thanks for the recommendations on replacement parts!

When you replace cabling, you really have to replace ALL of it. The wiring, the connectors, the sockets, every bit of it. Also IC7 and IC8 had to be replaced. (CD4013BE).

So it's in a working state now, as far as channel switching goes, but there is one last issue...

There is a light distortion tail when playing clean, accompanied by intermittent crackling and pops. This happens on both channels- I'm hoping the last bit of cable/socket replacement will take care of it but I fear its in the caps.

I'm going to replace them all regardless, but does what i'm describing sound like a capacitor problem?



Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on February 19, 2021, 03:55:10 PM
Here is an audio clip of the noise, if this helps anyone to ID an obvious problem.

The noise is happening regardless of whether anything is plugged in, knob position, i can even have the volume down all the way and it is still audible.

https://drive.google.com/file/d/1ZGVHAwuoPCNL8kdBITldN0QJwp078zU7/view?usp=sharing


Anyone familiar with this and its cause?
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: phatt on February 20, 2021, 07:34:49 AM
Try plugging your guitar into the power amp input on the back,, that will (normally) disconnect the preamp circuit from the signal path.
So if that stops the noise then the main power supply and power amp power amp is likely ok and that tells you the issue is in the preamp.

Let us know what you find.

Could be a dying cap,, most likely in the preamp.
Phil.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on February 20, 2021, 08:19:41 PM
Thanks Phil.

Yep. Plugging into the power amp input, signal clean as a whistle. Nice way to quickly eliminate suspect components.

Next step replacing all electrolytic caps on the preamp board, and the logic board as well, for good measure.

16 in total:

1 16V 100uF
2 16V 10uF
7 25V 10uF
6 50V 1uF

Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on February 25, 2021, 05:34:07 PM
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.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: phatt on February 26, 2021, 08:12:00 AM
No never remove Elcaps if you don't know the polarity. :duh
If they are part of supply they will certainly blow.

Just replacing parts without First establishing what has failed and WHY is just going to lead to tears.
You would need a signal injector to find what part of the circuit has failed but as the switching circuit is on the blink as well then it's likely that won't help much.
All I can say is
This is a rather complex circuit and maybe you should just take it to a repair service.
Phil.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DrGonz78 on February 26, 2021, 01:59:40 PM
Post pictures of the board and all the caps that you replaced. Which exact caps were the one's you were unsure during installation?
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on February 28, 2021, 01:46:29 PM
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.

(https://i.imgur.com/mCesZkX.png)

Updated Schematic with Caps/Critical Components Highlighted:

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



Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on March 02, 2021, 07:11:21 PM
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?
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: phatt on March 02, 2021, 10:10:19 PM
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.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on March 02, 2021, 11:43:32 PM
@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?
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: g1 on March 03, 2021, 03:07:05 PM
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.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on March 04, 2021, 02:16:09 PM
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?

Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on March 05, 2021, 01:37:23 AM
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.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: g1 on March 07, 2021, 10:55:00 PM
Double check whether that slight distortion is there when you go straight into the power amp.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on March 11, 2021, 10:04:38 PM
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.



Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on March 11, 2021, 10:27:17 PM
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
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: phatt on March 12, 2021, 08:06:12 AM

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.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on March 12, 2021, 02:33:38 PM
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?
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: phatt on March 12, 2021, 10:40:41 PM
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,,, xP
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. :tu:

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. 8|

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. :-X

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.


Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: Loudthud on March 13, 2021, 12:00:23 AM
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.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on March 13, 2021, 12:21:42 PM
@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. :-X"


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!





Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: phatt on March 14, 2021, 07:58:18 AM
Hi DJ,
Ok thanks, now I see where you are coming from an that's cool.
Just be aware this stuff can become very addictive, my wife thinks I'm quite disturbed,  :lmao:.

Most young guitar players will tend to fall for the hype that if you have the latest model Amp/Gear/Guitar you will be in mojo heaven and will play better and you will be BROKE $$$.
Reality is with some basic gear and some understanding of electronics and Sound you can escape the hype bubble easy.

Read Bobs comment for just how cheap you can run a big rig setup.
https://www.ssguitar.com/index.php?topic=5003.msg39230;topicseen#new
Note the Amp used is one of the cheapest model amps you can buy.
Very Clever for a chap who has little electronics knowledge  8|

Re modding amps;
Yes you can alter some PCB's but depends on how much work is involved. Some circuits leave no room for much alteration and it's just not worth the trouble. (your amp is likely one I would not bother to try and alter).

For me, Having tried all sorts of ideas I've found from experience that a lot of Dedicated guitar circuits are way to extreme and many owners of multi channel amps tell me they like one channel and end up back at relying on there pedals for all the other sounds.

So as you wish to learn my advise is buy a Bread board and start building circuits to see what suits your playing style. You are welcome to my circuits if you so wish ,, most of them are on this site anyway but there have been a few alterations in the last couple of years.
-----------
To understand whatzhappining; :duh
 Bandwidth.
By shear design the power stage of old Valve amps limited the bandwidth of the signal.
WHY?
Most valve amp circuits have a lot of Decoupling between each stage including the power amp. They also have a Transformer coupled output to speaker.
Most guitar OTx's are often low speced and can't transfer really hi frequencies so straight up limited bandwidth by design. Add things like miller effect and you have an amp that sounds sweet even when distorting.

NOW Enter SS where most power stage circuits are DC (Direct Coupled) so no caps to wipe off a little bass,,, and no OTx to wipe off excess hi freq content. The result is often HARSH TONE. :'(

But don't panic,, there are many ways to recreate the sweet stuff most of it can be done in the preamp stages.
It's not about the Mil Spec transistors or the Gold plated print on Valves it's about understanding what is happening inside a circuit.

It's the RC coupling and boring maths stuff that makes the mojo not the cost of fancy mil spec Chips/Valves actives.
-------------

90% of the freq of guitar is under 1khZ,, add for some harmonics gets up to around 3khZ. Past 5khZ is just adding crap that will destroy the CLARITY and Definition of the notes you play.
Of course we all tend to fall for the MORE is always better than LESS,, humm.
Not so once you understand what is really going on. To cut through you only need to focus gain at say 1khZ to maybe 3khz.
This is where the whole system has to come together to create that TONE FOCUS. So when you turn up the treble knob you don't want to turn up frequencies way past what is needed,, otherwise your tone will be way too harsh/brittle. Add a lot of distortion and now you just annoy people.
My cab sim which is permanently on for that very reason to help tame that top end fizz.

Regards to OD/Distortion.
I have 3 OD pedals,  ALL 3 the gain is never above 2. I've found trying to get the OD out of one stage or circuit never works.
The idea is you build that OD each pedal adding a little more Grit/Edge.
With all 3 on, I'm into ZZ Top kinda distortion.
No reason you could not build some of the these into a rack case but you will still need a control board.
My pedal board can go direct into FX return of the Laney but I do like my Spring reverb and that extra Volume control on the Amp Cab is handy. :tu:
I've added a pic of my home made pedal board. 4 are obviously brandname the others are my own designs.
The ugly black box has a Compressor and Cabsim.
The white GeOdrive is my design and my favorite the PhAbbtone.
Enough for now I have work to do. bummer.
regards,, Phil
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on March 15, 2021, 05:24:40 PM
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?
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: phatt on March 16, 2021, 12:32:33 AM
Re distortion on clean,
                               As mentioned a lot of circuits will have some clipping once the signal level gets high enough.
Maybe try and turn off the reverb and GEQ and just use the 3 pots of the clean ch.
If the clipping is still evident then maybe the gain is a bit high at some place in the preamp circuit or there is an issue inside all that switching crap.

As the schematic is very hard to read I'm guessing that after the signal leaves the preamp it runs through IC1, IC2, IC3, IC4 & IC5.
If it's possible you could break into the preamp output at R18 then plug direct into power amp input.
 That would bypass all the switching circuit.
If you still have the distortion then you know it's in the clean preamp and not the switching circuit.
Phil.
Title: Re: 1983 Fender Showman Channel Switching Issue
Post by: DJ5D on March 16, 2021, 01:55:55 PM
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.

Title: Re: 1983 Fender Showman Channel Switching/Noise Issue
Post by: phatt on March 17, 2021, 07:51:19 AM
Yep not surprised at all as there is likely a fair bit of loss and noise in all that switching circuitry.
I doubt it is faulty it's likely working as designed but these things are called design over sights. :-X

I have no idea why the gain pot does not work?
Regards finding where the noise might be;
You could try and bypass IC4 (TL604) & IC5 (TL607).
To do that take the Graphic output at R19 (P1-1) then connect that wire to terminal 6 (P3) which is on the bottom page. (the preamp output/ poweramp input sockets)

You may have to disconnect pin7 of IC4 as well as pin4 of IC5 as signal and hiss might bleed through.
This is all guessing as that schematic is missing quite a few connections. so do check 8|

That leaves the Channel switch & Reverb still stock but graphic EQ is permanently engaged wit FX loop bypassed.

If hiss is still an issue try lifting one end of R2 (at pin4 of IC2) that is the return mix resistor for the reverb.
Some reverb circuits can introduce a lot of buzz/hiss.
Phil.
Title: Re: 1983 Fender Showman Channel Switching/Noise Issue
Post by: DJ5D on March 17, 2021, 11:44:26 AM
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.





Title: Re: 1983 Fender Showman Channel Switching/Noise Issue
Post by: DJ5D on March 19, 2021, 05:28:42 PM
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! :)



Title: Re: 1983 Fender Showman Channel Switching/Noise Issue
Post by: phatt on March 20, 2021, 12:42:19 AM
Good work and you learned from it all.  :dbtu:
Don't worry you will find a new project soon enough. ;)
Phil.
Title: Re: 1983 Fender Showman Channel Switching/Noise Issue
Post by: DJ5D on March 23, 2021, 07:17:34 PM
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...
Title: Re: 1983 Fender Showman Channel Switching/Noise Issue
Post by: phatt on March 24, 2021, 04:17:25 AM
Sorry but you are doing it the hard way.  xP
It's a common mistake to assume that the ACTIVE components make or break the mojo.
OK you had a win by replacing some opamps but mainly that improved noise issues.
But note;
wide bandwidth requires hi spec chips OK fine BUT if the bandwidth is limited then by design you remove most of the hizz fizz and buzz which means that lower spec chips work just fine. (That's a general comment and not always so but still worth noting)

Tone is all about the circuit design and that comes from the passives, not the quality of the passives but the Values of those define the final result.
Sure speakers and power stage can have effect on this as well as PU's in the guitar but a whole world of tone mojo comes from knowing how to tweak the circuit and most of that is PASSIVES. in guitar circuits it's often just understanding some R/C maths

Doing this by willynilly swapping out parts and hope for the best can end up destroying the PCB tracks so I'd suggest learning how to use simulations and a Bread board and recreate some of the Showman preamp to test out the many options before you open up the lid on the amp.

I would recreate the 2 preamps on a testboard then link into the power amp of the Showman to hear how tweaks alter the sonic result.
No need to do all the switching as that has little to do with tone shape.

The Dirt circuit is a little tricky but still doable.
It has the CD4007 which I'm assuming is just a bunch of mosfets setup to create a lot of Dirt but as there is little tone shaping after it then likely a lot of fizzle and hash is the result.

Note that a lot of amps are built to what the market wants and many amps are built to create a whole lot of hashy distortion which the kids with tin ears seem to like,, so if you want sweet OD then it's unlikely to work well for a sweet OD tone without a lot of tweaking.

You can always Bboard some cab sim circuits and temporally insert them in the FX loop or Preout /PwrAmp input to wipe of the excess bandwidth. That would be less work than testing out the whole preamp section.
I'd guess that the preamps will still be an issue.

If I had a clear schematic to work from I could simulate some of the preamp sections to get some idea where the tone is going wrong,, no chance as that Schematic is unclear.
Phil.