Customer returned fixed Fender Princeton 112 reverb: Suddenly fuse keeps blowing

[kka]

Please ignore my previous post. My DMM acted up and I used a different one, could take V readings all over and here is what I got:

(measured in between diodes):

CR5 - CR6: 0,6V

CR6 - CR7: -0,6V

CR7 - CR8: -1V

CR9 - CR10: 1V

CR10 - CR11: 1V

CR11 - CR12: -0,8V


C38: 27,6V (el. cap 47nF 50V at +)


Transistors:

Q1 Base: 1,2V
Q2 Base: -1,2V

Q1 Collector: -42V
Q2 Collector: 42V

Q1 Emitter: 1V
Q2 Emitter: -1V


U5 (MC1436):

Pin1: -27.6V

Pin2: 1.1V

Pin3: 1.1V

Pin4: -27.6V

Pin5: -27.6V

Pin6: -130mV (DC taken with DMM, I need to measure the AF with scope later)

Pin7: 27.7V

Pin8: 0V


I hooked up my cell phone (music player) to input of amp and signal is very faint coming out of speaker even when volume turned up. Amp hums quite loud. I just did this for 5 seconds. Wonder what next steps I should take.

Thank you.

[DrGonz78]

What voltage did you get at the junction of R65/R66? As I noticed once you got your DMM working on the next posting there is nothing there showing readings for that part of the circuit. Not sure that matters much but it could. Just pointing it out in case you forgot to measure that location once you got your DMM working. Best regards.

[DrGonz78]

One other thing to point out here that just occurred to me... Did you measure the DC voltage output going to the speaker w/ the DMM? Also, I am not sure but would a cell phone be a good input source to test on a guitar amp? Have you tried using cell phone before and it works well on other amps? Just curious because I have never tried doing that and am interested in the answer, thanks.

Edit: Also is there 16v on the TL072 chips, for example, U3A?

[diydidi]

Dont have the schematic in front of me, but replace MC1436..

[kka]

Thanks for coming to my aid, Doc. Really need a doctor right now :-)

Junction R65/66 read 1V , forgot to include

Yes, I. Sometimes use my cell phone (Pandora radio) to feed a signal into an amp for the testing and burn in period after I fixed it. It's like an iPod

I will measure the output and the other chip later

I ordered the MC1436 just in case and will also use my scope on pin6 while running an AF signal through the amp

I will also check C40/41 which might cause the hum noise.

[kka]

Folks, just wanted to update that I managed to fix this amp, customer picked up about 2 weeks ago. Using a scope and Sig Gen I ended up tracing a few more broken components including one of the preamps. It was not easy, but I learned a lot in the process and the customer is very happy, that's what really counts.

Thanks again to everyone for your support !

[Chuck H]

That's great. I had an old Kustom to repair that ended up taking more research and learning than the ticket allowed. The experience was good overall. But I do need to disagree on what really counts. I think it's the bottom line. After all, unless your independantly wealthy there's no way to continue participating in life if satifying others is your primary goal. But we can't make future ventures more efficient if we don't learn. And some things are best learned by experience. If your doing what you want then it's time well spent. Experience is expensive... And priceless.

[DrGonz78]

When you say say "preamps" that is referring to an opamp chip?

[R.G.]

If you start shotgunning which many people on this forum seem to suggest, expect failure rates to zoom up and your reputation will drop. Avoid replacing any part unless you know what its contribution to the fault mechanism was, as cause or symptom. If any tech tells you on the forum to just replace a bunch of parts, such as replace all caps as a matter of course in a repair, you are advised to ignore everything they ever write again. They are incompetent as a diagnostician. Don't introduce more variables into the mix when diagnosing problems.

I agree with what I think you meant to say, but I think it needs some clarification.

There are really two objectives in any repair. One is to get the thing running again by fixing the (perhaps few) conditions that made it fail. The other is to make it reliable again. That first objective is much like solving one of A. Conan Doyle's Sherlock Holmes mysteries, finding the one hidden clue that makes the mystery all clear. The second is like an auto mechanic telling you "yeah, we fixed the bad spark plug that was making it miss, but you really need to replace all of them, not just the one that was failing. They're all old and will start failing soon."

If you have a true mystery on your hands, and are trying to find the one mystery component that's making things not work, you do just confuse things by subbing in parts, perhaps at random, with no particular clue if that was the problem. I have no respect for this approach. There are times when measuring, thinking, and reasoning until you can come up with the root cause of the problem is the only good way to proceed.

However, there are also times when replacing a number of parts makes sense; not perhaps as a diagnostics procedure, but as a maintenance process. I've dealt with a lot of old equipment, often 40-50 years old. Electrolytic capacitors do have a finite lifetime; the just age and die at some point. It's not possible to know what that point is. If you find the one mysterious part that failed was an electrolytic, do you then replace just that one and send the amp out to make its way in the world?

My opinion on this is that for my own gear, I would replace them all. The reasoning is that if (1) all of them are decades old and (2) one has failed and (3) there is a known wear-out mechanism, then it is logical to conclude that soon another will fail. If I were doing repairs for a customer, I would feel it was my obligation to the customer to advise them that they may have issues with others in the near future and offer them the option to do some preventative maintenance instead of repairs.

Another case is Fender plate resistors. Experience has shown that the (often) 100K plate resistors in old Fenders get crackly after several decades. Fixing one fixes the amp. Replacing them all fixes the next one before it gets bad.

The difference is whether replacements are used for diagnosing a problem or whether it's used for preventive maintenance. The first is a bad practice. The last is doing things in the best interest of future operation. They are different.

There is another case, the only one where I would accept parts replacement instead of thinking. Solving mysteries is sometimes a slow process. There are times when speed is more important than cost or the intellectual achievement of finding the subtle, hidden fault. In today's world of very cheap parts, it can **-=> sometimes <=-** be faster (and cheaper, in terms of total cost to the customer when bench labor is taken into account) to remanufacture one section of the amp. If you have the problem isolated to one amplification stage, and the customer is about to take off on tour or go on stage, replacing a tube socket, all of the seven? eight? resistors and three caps in one stage can be faster than the intellectual exercise of finding the hidden clue.

So I agree with what I think you meant to say. I disagree with how I think many less-experienced people will understand it.

[soundguruman]

Your not arguing with me, you're just contradicting me.

[R.G.]

Your not arguing with me, you're just contradicting me.

??

[J M Fahey]

Your not arguing with me, you're just contradicting me.

Hi soundguruman, you've posted nothing on this thread, who are you answering to?
Just curious.

[kka]

When you say say "preamps" that is referring to an opamp chip?

Yes, I replaced 1 faulty OP amongst a few other components that got fried.