Fender Performer 1000

[Langdon]

Hi from England, looking for advice on diode replacement...

Symptoms - big nasty hum at switch on, no response to input.

I've examined the board and all I can see is brown, heat damaged area around CR56, a 1N5353B diode, 16v 5W.

What I don't know at my level of knowledge of electronics (basic) is did it just flame out, or is it probable that another fault caused it to overheat. Do diodes just do that with age? Is it likely that replacing it will solve the problem completely or can I expect the replacement to get over-stressed and burn out sometime soon if that's all I do?

I have the schematic - thanks to Ed Treat at Fender, a multimeter and that's about it.

Thanks in advance, I've already learned a lot just browsing the forum and checking out all the other Performer posts.

[J M Fahey]

Hi langdon.
*Visually* examining a failing amp provides little or no clues, since electricity is invisible (to us).
That's what test instruments are made for.

Symptoms - big nasty hum at switch on, no response to input.

points to burnt/shorted power transistors.
Post your schematic, so we all talk about the same, disconnect those speakers *now*, and measure the DC voltage at the terminals which were connected to them, I bet you have a lot of DC there.
Good luck.

[Langdon]

Thanks for reply...

I've tested at the ends of the internal speaker output leads, speaker disconnected, volume pots at 0, and get 45v DC. So does that confirm a dead power transistor? And if so, how to id which one?

Schematic attached.

Thanks again for your reply.

[pdf64]

It would be prudent to replace every device in the power amp (inc the ic). They're all dc coupled, so there's no telling where the problem started or where the damage stopped.

[J M Fahey]

Good advice, parts cost is nil compared to time lost.
Also check R112 to R115, one or more may be open.
If you want to save some time, diode check (search the forum for instructions) the output transistors, specially those TIP142.
Build a bulb lamp limiter (search too) to be able to test the amp safely .
There is nothing more frustrating than spending time and $ replacing 9 parts, turning it on, and have a 10th one, previously undetected, burn the new ones.
Play it safe.
Only when you have no DC voltage in the output can you re-connect the speakers.
Good luck.

[Langdon]

Thank you pdf64 and JM, plenty to bite down on there, I think. And there's me thinking it might be one little diode....Thanks for your help, guys, much appreciated as I don't have any friends or contacts who know about this kind of thing.

[J M Fahey]

Well, it *might* even have been caused by a little diode, or any other humble part.
Or worse, a bad contact or a tiny crack in a track.
Point is, most failures leave no visible clues.
A few do.

[Langdon]

Understood. Okay, so the way to proceed is to replace all the devices in the power amp circuit. Does this mean just the transistors and the IC or should I replace everything? It looks like I can get all the components, the only one that is proving difficult is the IC - MC1436. Is a substitute possible?

[J M Fahey]

Go step by step: diode test all 4 transistors : your multimeter in diode test setting, each transistor has 3 pins, making 3 pairs possible, check each pair both ways, you should read either around 500/600 or over 1000 (including open or over range, typically by blanking some digits).
Very low values (or straight 000) show shorts.
In that case pull the suspect transistor and re-measure it on its own, out of the circuit.
Substitute it with a known good one and see what happens.
It's considered good practice, anyway, to replace the "still working" ones too, becuse after the overload they may " work but ....... "
*If* it works, probably you can trust the IC, which is not a very common one.
It's *similar* to a regular TL072 but it's *guaranteed* to support +/-22 Volts, or a little more, which regular Op Amps can't do.
Worst case, you can order it through an authorized Fender service center or some shop.

[Langdon]

Hi, JM, just tested the transistors in place in the circuit. Q11 and Q12, both TIP142 transistors check out as okay. Both Q13 and Q14, both TIP147, gave readings of 1781 or so between base and emitter, and between base and collector, but near zero between collector and emitter. So I guess it's replace those two and then see what happens.

I found a supply of the IC, MC1436 - on ebay, either from the US or Hong Kong. I should have guessed I might find them on ebay.

Thanks for pointing out that test to me.

[J M Fahey]

OK, start by changing those two. Be careful with mica, grease, desolder/clean well the pads so you don't tear them, pamper the board.
Probably your amp will come back to life.
Remember to use a series lamp at the beginning, until everything tests OK.
I trust more an original 1436 than a Hong Kong one. We have been reading horror stories lately.

[Langdon]

Don't you hate those threads where someone pops up, wants help, gets help and then disappears and nobody ever knows what happened, how it turned out? Nobody gets to learn if the fault diagnosis and the proposed solution was correct.

Well, life happens, other things take precedence, but I finally, 3 years later, this week got 2 new power transistors for my amp. I fitted them today and the amp works! I have never done a repair like that before and wonder what it was that caused the original failure so am a bit wide-eyed that it is working again. How long for? Who knows?

I'm on a such a tight budget for things musical that repairing the amp myself with some help was the only way it was ever going to get done so my heartfelt thanks to all those who helped me to the solution, especially J M Fahey, sir - thank you very much. I raise a glass and toast you all.

[Tom Phillips]

Thank you for the report and congratulations on fixing your amp.
Would you please re-post the schematic. The old file was lost due to a problem with the forum server and it would be nice to have it here in the thread again.
Cheers,
Tom

[Enzo]

Let me try, I am on my other browser now, and I need to know I can upload.

[Tom Phillips]

Thanks Enzo. It's nice to have the full service manual posts.
Regards,
Tom