Randall RG75 G3 Combo, hum then pop, then everything OK

[Emetal]

Hi everybody, got Randall RG75 G3 combo for repair, when turned ON it hums for about 5 seconds, then pops and from there on everything goes normal. Only happens when it was OFF for longer time, like few minutes. If turned OFF and ON within minute or so it starts normaly. First thing coming to my mind is some cap, or Relay? Anybody had similar problem? Any ideas?

[glebert]

There is a relay that cuts the signal to the speaker outs that is supposed to have a delayed turn-on. Do you hear the relay click to on, and does it happen immediately with power-on?

[Emetal]

I have just actually opened the amp, look like those schematics I uploaded are not correct, I do not se any tube or a relay as shown on these schematics, looks like just the last schematic that says RG75-A (preamp) is correct

[Emetal]

There is a relay that cuts the signal to the speaker outs that is supposed to have a delayed turn-on. Do you hear the relay click to on, and does it happen immediately with power-on?

There is no click there is no relay, only hum and pop from the speaker, looks like I have wrong schematics...

[Emetal]

Does anybody please have correct schematic ?

[glebert]

What power amp module do you have (including date code or anything else that might be a a clue)? In my experience with Randalls like that they mixed and matched preamp and power amp modules, and have had ones where the "right" schematic wasn't available but I could piece it together from two others that were similar.

[Emetal]

What power amp module do you have (including date code or anything else that might be a a clue)? In my experience with Randalls like that they mixed and matched preamp and power amp modules, and have had ones where the "right" schematic wasn't available but I could piece it together from two others that were similar.

Here are the photos of the power amp module:

[g1]

Does this one match?

[Emetal]

Does this one match?

Yes sir, seems to be that. Now do you have any idea what could be the problem, maybe one of those big 4700 uF /50v caps? Or something else?

[glebert]

Since you mentioned a hum and pop, I would look to see if there was DC coming out on the speaker outputs for that first few seconds. You could measure this with a DMM or even just look if the speaker is being pushed and held (in or out). If it is not DC then I would try to figure out if were 60 or 120Hz that comes from the AC line.

[Emetal]

Since you mentioned a hum and pop, I would look to see if there was DC coming out on the speaker outputs for that first few seconds. You could measure this with a DMM or even just look if the speaker is being pushed and held (in or out). If it is not DC then I would try to figure out if were 60 or 120Hz that comes from the AC line.

Exactly as you sad, speaker is PUSHED and HELD for few seconds. There is 25 VDC on the speaker while it hums. So what now, where the DC is coming from in those first few seconds? (I am sorry for asking everything I don't do repairs every day, just sometimes)

[glebert]

Exactly as you sad, speaker is PUSHED and HELD for few seconds. There is 25 VDC on the speaker while it hums. So what now, where the DC is coming from in those first few seconds? (I am sorry for asking everything I don't do repairs every day, just sometimes)

In that case I would not have the amp hooked up to a speaker as it is dumping lots of power throught he voice coil with the DC, even if only for a few seconds. I assume your main voltage rails are +/-25V? Seems like one polarity of the output is being turned fully on for some reason. Why? That is the tough part. I would work with the amp with no load and measure voltages, starting with the power transistors and working back upstream and look for voltages that start high/low and then drop to normal levels. Someone else may have a smarter approach though.

[g1]

I believe there is a turn on delay circuit around C13 and D14 area.
Like glebert said, do not connect the speakers til the amp is repaired. Check if DC volts at Q3 collector change during turn on til a few seconds after.

[Emetal]

I believe there is a turn on delay circuit around C13 and D14 area.
Like glebert said, do not connect the speakers til the amp is repaired. Check if DC volts at Q3 collector change during turn on til a few seconds after.

DC volts on Q3 between ground and collector DID change, went up for few seconds and then down, i checked it once, and when I wanted to check it again R15 and R16 smoked out and turned black...

[g1]

R15 and R16 smoked out and turned black...

The output transistors and everything else still ok?
Maybe you will get lucky and that string will have been what was causing the issue. Suggest replacing Q7,8,9 as well as those burnt resistors and D5. Check R17 as well.
Power up with no speakers or load, preferably on a lamp limiter. Check if the output still goes DC at turn on.