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Author Topic: 1983 Fender Showman Channel Switching/Noise Issue  (Read 4279 times)

DJ5D

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Re: 1983 Fender Showman Channel Switching Issue
« Reply #30 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!






phatt

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Re: 1983 Fender Showman Channel Switching Issue
« Reply #31 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

DJ5D

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Re: 1983 Fender Showman Channel Switching Issue
« Reply #32 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?

phatt

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Re: 1983 Fender Showman Channel Switching Issue
« Reply #33 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.

DJ5D

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Re: 1983 Fender Showman Channel Switching Issue
« Reply #34 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.

« Last Edit: March 16, 2021, 03:48:42 PM by DJ5D »

phatt

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Re: 1983 Fender Showman Channel Switching/Noise Issue
« Reply #35 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.

DJ5D

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Re: 1983 Fender Showman Channel Switching/Noise Issue
« Reply #36 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.






DJ5D

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Re: 1983 Fender Showman Channel Switching/Noise Issue
« Reply #37 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! :)




phatt

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Re: 1983 Fender Showman Channel Switching/Noise Issue
« Reply #38 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.

DJ5D

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Re: 1983 Fender Showman Channel Switching/Noise Issue
« Reply #39 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...

phatt

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Re: 1983 Fender Showman Channel Switching/Noise Issue
« Reply #40 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.