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Fender Showman 1983 Rivera Era Switching logic problem

Started by DrGonz78, November 02, 2014, 05:15:14 AM

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DrGonz78

Okay I have a Fender Showman solid state amp in for repair. The symptoms are Reverb and Equalization lights are stuck on, but cannot turn them off. The reverb was not passing signal but I am betting that was due to the faulty wiring in ribbon connectors between preamp and logic board. The EQ sliders are working fine, but cannot turn off the effect. This occurs whether you try the push buttons on the amp or the foot switch. Pushing the momentary switch on the amp and holding it down turns off the indicator light on the foot switch, not the light on the amp. I can only guess that the problem might be a TL607 or TL604 chip, which I know are rare birds. I am a bit confused on how to test the TL604 or TL607 chips to know if they are the culprits.

Also, to note is that the channel switching works fine. The effects loop switching was working, but has quit working now(indicator light stuck off). I had noticed continuity issues with both ribbon connectors that go between preamp board and logic board. I have straightened out that mess to make testing less of a guessing game and have good connections now.

Looking at the preamp schem page, look for P1 (Power IN) for the related voltages. (Strange that P1-1 & P1-3 should be +/-18vDC.)
P1-1 = 15.87vDC P1-3 = -16.65vDC
P1-4 = 14.75vDC P1-5 = -15.4vDC

Also look at P4 voltages. P4(on the preamp board) is what connects to the logic board. It connects to the P4 connection on the logic board. Notice on the preamp schematic on the switches it will have P4-6 for the reverb push switch for example. Here are some voltages with channel 1 selected on.
P4-1 = 15.77vDC (Which should be 18v)
P4-2 = 15.78vDC
P4-3 = 1.4vDC
P4-4 = 1.4vDC
P4-5 = 1.79vDC
P4-6 = 1.79vDC

So I need help understanding this logic board better. The switching logic on this amp is the most elaborate that I have worked on yet. Any help would be greatly appreciated, thanks.  :dbtu:

Edit: Just to note that all the TL607 and TL604 chips are getting -15.75vDC and 14.75vDC power on pins 5 and 8.
"A person who never made a mistake never tried anything new." -Albert Einstein

Roly

If your indicator lights are stuck on then this thankfully has nothing to do with the analogue switch IC's.


Circuit: Page 3 of 4, Logic and Switching

IC7 and IC8 are dual D-type flip-flops which store the current status of each of the four Channel Switching, Effects, EQ, and Reverb.

Initially they are all R)eset by the switch-on reset line from IC6 pin 8, and should disable all effects and clear all indicators.

After the switch-on reset has been released they are free to be toggled by an edge arriving via IC6.

P4-1 Vcc+ (logic high)

P4-2 n/a  (high)

P4-3 Channel Switching (low)

P4-4 Effects (low)

P4-5 EQ (low)

P4-6 Reverb (low)

I'd guess that the footpedal isn't providing the right stimulus at P4-n for some reason (open Vcc+?).




Aside...

Quote from: DrGonz78I am a bit confused on how to test the TL604 or TL607 chips

They are pretty much relays.  They have a couple of signal connections and an associated control pin.  The way i'd do it is have the chip data sheet, or at least pin cheat, and use a CRO to see what was passing signal and the logic state of its control pin, e.g. CD4016;


If you say theory and practice don't agree you haven't applied enough theory.

DrGonz78

#2
Great info Roly, my hat goes off to you as usual  :tu:.

So now I am first just observing voltages and how they relate to the CD4013 chips (IC7 & IC8). I now understand that pins 1 & 2 (IC7) toggle the ch. switch and I have observed how there is 15vDC toggling between pins. So low and high or on/off flip flopping is working on the channel switch.

Looking at reverb switch pins 12 & 13 (IC7), pin 12 toggles 15v to 0v(24mv low) but pin 13 stays high at 15v no toggle. EQ switch pin 1 (IC8) stays at 15v no toggle and pin 2 stays low at 0v no toggle. Effects switch pin 12(IC8) stays low at -222mv no toggle and pin 13 stays high at 15v no toggle. Also note that the effects loop channel led is off. At one point while taking voltages the effects led came on and stay on. Turn off amp and it is now off again.

Also IC7 is hot as hell at 45C degrees. All the other IC's are only around 29-32C and IC7 heats up right away. So definitely makes me think that IC7 is shot. However, it is strange that IC7 is connected to the channel switch part of circuit and it is working fine.

Well I have yet to invest the time of scoping anything at this point, but this does help so far. Thanks again for the post Roly!
"A person who never made a mistake never tried anything new." -Albert Einstein

Roly

A hot IC is certainly a sign of something.   8|

Yeah, seeing that it's a common, available, and cheap IC I'd take a punt and just replace it, however you need to keep in mind that it may be getting hot because it is driving (or sinking) and excessive load/current.  If, say, one of the transistor buffers driving a LED packed it in it could produce a similar symptom.

{Paradoxically, in a board full of IC's it is often the cold one that has failed dead short.}
If you say theory and practice don't agree you haven't applied enough theory.

DrGonz78

#4
Okay I found a couple NOS ECG4013B chips today. As far as I can tell ECG4013B and CD4013B are identical. Here is a pic of the chip where SYLECG4013B is printed on the top and PHIL on the bottom. I am guessing the SYL stands for Sylvania and the PHIL for Phillips...? ECG4013B I believe is the part number.

Just wanted to confirm that I correct in my thinking here, thanks.
"A person who never made a mistake never tried anything new." -Albert Einstein

Roly

Generally speaking it's only the number bit, 4013, that matters, 'tho the following B)uffered or UnB)uffered can matter in analogue applications.  Here you just need a dual D-type flip flop and I don't think it matters a hoot who makes it.
If you say theory and practice don't agree you haven't applied enough theory.

DrGonz78

Thanks Roly! Yeah just right before reading your last post I was reading up on ECG, SK, and NTE Components. I know about NTE of course but SK & ECG not as familiar. I know better about this whole thing now. I tend to stray away from these types but it is what was easily found.

Anyhow, a quick update. I replaced both 4013's IC7 & IC8. I now have the EQ part of the foot switch working. So, channel and Eq switches work! Now I have reverb light stuck on and effects loop stuck off.

IC7 is not getting hot anymore.  :tu: Back to the drawing board :trouble.
"A person who never made a mistake never tried anything new." -Albert Einstein

Roly

{I'd actually read the "PHIL" on the bottom as "Philippines", the country of manufacture.}


Quote from: DrGonz78I replaced both 4013's IC7 & IC8. I now have the EQ part of the foot switch working. So, channel and Eq switches work! Now I have reverb light stuck on and effects loop stuck off.

Humm ... and reverb is in the one that was getting hot, and now is not.

But the EQ and Effects are in different IC's and not working.

Top and bottom too, couldn't be less related really.

Have you confirmed that you can see all the pedal closures at the P4 connector?  I.e. is the problem now before or after P4?

I'd be inclined to start at the pedal and confirm that it is generating/sending a signal for the non-working functions.  If not, why not?  And if so, where is it getting lost?

If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

Aaaaannnnddddd nnnooooowwwww ffoooooorrrrrr seeeeexxxteeeee thooooousssaaaannnndddd doooollllaaaarrrrssss!
If Phil comes from the Philippines ...... where does Roly come from?
You have sixty seconds !!!!!!

Roly

If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

Didn't know about your French roots  :o

I always thought that "Roper" meant in Ye Olde English : "the man with the rope" meaning :



but I stand corrected :)

Roly

Yeah, very distant French roots ... via a long line of Irish publicans.


Quoteroper
1   n    a craftsman who makes ropes
2   n    a cowboy who uses a lasso to rope cattle or horses
3   n    a decoy who lures customers into a gambling establishment (especially one with a fixed game)
If you say theory and practice don't agree you haven't applied enough theory.

DrGonz78

#12
(Yeah my first gut feeling was Philippines, but then I thought of the connection between Phillips/Sylvania to go for the long shot!)

Well it is fixed!!  :dbtu: First off I went and started figuring out how the circuit works. I started checking voltages on Q5 and Q6 which help turn on/off the effects light channel. I stopped on Q6 measuring a short between emitter-base. I was about to replace it when I realized there was just a tiny drop of solder bridging the two together. The bridge was very tiny and it was later in down the trace, not right between the legs. So that fixed the problem I created with the effects channel after it stopped working.

At that point I got to thinking about pins 12 & 13 on IC8 and how pin 13 always stayed on, where pin 12 toggled correctly. So, I measure continuity between the two legs of pin 14 & 13. I removed the solder to confirm it was not the legs of the IC and it was at the traces/pads pin 13 & 14. I knew right away that Vcc was bridged somewhere else on the board, so I went about tracing about with no luck. The whole time the problem is staring me right in the face!! A little red wire connected from Q12's collector to pin 3 on IC2. I remember the first time looking at the circuit board thinking what was Rivera thinking putting that thing in there? Anyway I then made the connection looking at the schematic that it was some hack job by a horrible tech. Simply removed the wire and reverb light switching is working now!

The next thing is to bust out scope and check the signal routing on those TL604's and T607's, especially IC2. Thanks Roly for the help steering me into a solution. Also, those ribbon connectors are horrible and are very prone to breakage. P4's ribbon connectors had the wires break on both ends. I had thought one side was good and broke off the connector to hard wire that side to the preamp board. I was having problems with the switches on the amp not working. That fixed it for good too. Those ribbon cables are total crap  :trouble.

Well thanks again!!  :dbtu: 

http://youtu.be/d1nNTmAXa-o
"A person who never made a mistake never tried anything new." -Albert Einstein

Roly

Well there are a few lessons in that one. 

Yeah, those various multi-way's range from excellent to the single core grey belts that bust so easily.

Well done.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey