Again hum and smoke... but from another component

[slidincharlie (Carlo P)]

I had hum and smoke in a Classic 30 recently, and it was due to a burnt diode: see my recent thread below ("Hum and smoke from Peavey Classic 30").
The burnt component was one of the two diodes tied to the power tubes' plates. I replaced the diode with two 1N4007 in series.
When I turned the amp on again to check, a resistor got burnt and went in smoke: it's R67 (see schematic attached).
I must say that I don't know if the resistor was burnt before replacing the diode, or if it got burnt after the diode replacement.
I apologize for the brutal question, but I do not know what to check to cure this amp: what can make this resistor burn?

[slidincharlie (Carlo P)]

Here's the schematic...

Sorry, I forgot the schem...

[Tom Phillips]

R67 is in the negative low voltage power supply. I'd say that the cause of it burning is unrelated to the previous diode problem. R67 is burning because there is excess current flowing through it. The -30V supply and the -15V supply are downline from R67. If R73 is not getting excessively hot then it indicates that something hooked to the -30V supply is drawing the excess current. You need to trace down that line and look for problems.

[Enzo]

Not much anything will be related to those flyback diodes failing. They fail from either random component failure or from loss of speaker load during amp operation.

If the -30V rail is loaded down, it is used to drive the reverb IC and the FX loop driver xstr. The xstr wouldn't burn up the resistor, so we are left with the IC, the little cap next to it C20, and of course the filter cap next to R67, C47.

If the -15 were grounded, I don't think enough currrent would be allowed through R73 for R67 to burn.

[slidincharlie (Carlo P)]

1) I pulled the reverb IC out, and the R67 resistor does not burn any longer.
Does it mean that the IC is bad, or that something else downline from the IC can be bad?

2) In the former case, is it enough to ask for a "4558" IC to the local dealer, or should I ask for something more specific? Is it a special Peavey part?

[Tom Phillips]

It's most likely that the 4548 itself is bad and you can just buy the standard replacement from your supplier. It's not a Peavey special part. I'd also replace R67 since it has been so stressed.
Here is a link to a discussion about the 4558: http://www.jt30.com/jt30page/micKeff...58-Op-Amp.html

[slidincharlie (Carlo P)]

One more dumb question:
does the small circle mark on one IC's corner indicate pin #1?
I was a fool to not take a note of its mounting position before pulling it out...

[Steve Conner]

Yes, the circle is a fairly standard way to mark an IC's pin 1 on PCB screenprints.

[slidincharlie (Carlo P)]

I can't find the 4558 IC locally, and don't know when I can find one.
What happens if I leave the IC slot empty? Do I simply lose the reverb? Or is the amp going to be harmed without that IC?

[Tom Phillips]

I don't see any problem with running the amp without the reverb IC installed. I would not worry about doing it if it were my amp.
Tom

[TD_Madden]

that reverb chip went south on a early 90's Classic 50 of mine many years ago (since sold the amp)...think I found a replacement at Radio Shack (and if THEY have 'em, the parts should be pretty common.)

Peavey was pretty good about sending the schematic as I recall, and even included information about "updates" to the circuit.

[Ptron]

Yeah, the 4558 is a very common dual op-amp chip. There are probably a ton of substitutes. Don't quote me on this, but I think most 8-pin dual op-amp chips have the same pinout and would work, although some are known to be not so hot for audio.

[Enzo]

4558, 4560, 5532, TL072 These are common dual op amps. This is not a critical application.

[slidincharlie (Carlo P)]

This amp is driving me crazy!

The local electronics dealer gave me a LM358N KE6530, saying that it is equivalent to the 4558. I replaced the bad 4558 and turned the amp on, and a very strange thing happened: the two central EL84 had red glowing plates, and some bad crackles and pops came out of the amp.
I turned it off, then on again: the power tubes were not glowing and hey, the preamp tubes did NOT lit! At the same time some loud intermittent noise came from the amp.

Considering that this PCB is definitely hard to handle, what should I check? The heaters? The small bent jumpers joining the three PCBs? I am not able to identify the heaters from the PT: are they the red and yellow wires?
What else can cause the preamp tubes stay off?

(crying bitter tears...)