Randall RG80 popping noise

[pontiacpete]

This amp has a popping like noise with or without an instrument plugged in. Going out the send jack to another amp shows that the pre amp is good, no popping.
The sound makes me think that it is an electrolytic on the power amp board so I change all of them, with no change.

How do I attach a small recording of the noise? I like it to be heard.

So where do I go from here? Semi conductors, diodes or transistors. Any suggestions welcomed, please.

Also, I can't even upload a zip file of the service manual.
Does anyone have the schematic for the RG80. The service manual looks different than this amp perhaps the wrong schematic after all.

edit: here's a youtube clip
https://www.youtube.com/watch?v=CLSVelUYkvI

[Jazz P Bass]

Have we eliminated the obvious?
Like: Jumper the send & return jacks.

Randall-RG80_100 guit amp.zip

[pontiacpete]

Yes, I jumpered them and clean the jacks with burnishing tool

[pontiacpete]

The voltages on the 2 main filter caps are +/- 36vdc.
When the popping occurs the voltage fluctuates between 33v and 36v
Checking the power transformer at the secondary, theres' a steady 55vac across the winding
Did the short test on the winding with the neon bulb and across the winding it looks good, bulb flashes.
There's no flashing between either end and the center tap. I'm not sure if that's normal or not.

I don't know what I have to do to get interest in this thread, but suggestions would be appreciated. Seems like have a problem getting interest in my threads,
although I'm not getting a complex

[Jazz P Bass]

I for one don't know what to add.

It does not sound like a generic problem.

[nevetslab]

Usually when I run into the symptoms you describe, I find it in the preamp section, but you've already verified to your satisfaction it's not occurring when you route the preamp output to another amp. Did you dead-patch the Return jack during that test? Just curious, since the Effects Send jacks rarely interrupt the signal to the power amp. (Block diagram of this amp and the schematic not withstanding).

It's interesting you're seeing a slight dip in the DC supplies when the popping occurs. Do you have the means to monitor the AC mains voltage/current? I assume this issue happens wherever you plug into the mains. I was wondering if there's anything occuring on the AC mains that is passing thru to effect this. Doubtful, or all of us would be going nuts with all the interference running on the mains coming thru our amps. I'm guessing the drop in DC supply voltage feeding the power amp is the result of current usage feeding a speaker, corresponding to the popping transient.

I'm chasing a similar issue on a Fender Hot Rod Deville amp at the moment, and while I hadn't made the crackling noise occur on the clean channel yesterday, I did pull the main PCB up and out for close inspection with my headlight and surgical loupes. Found a good collection of solder fractures that needed mending. I'd start there...on the solder side of the power amp board....hoping it's not a nightmare to access. While I've never made a habit of using Freeze mist to try and stimulate a problem like this, many do. I do make use of a chopstick to go probing all over the PCB, seeing if I can find any particular component that seems to be the source of it. How often does this popping transient occur. Do you have any similar sized power supply filters to substitute for the two supply filters (6000uF/50V).

Assuming that too doesn't yield results...what to use to stimulate the occurrence?

I just noticed the odd feedback shunt networks. R44 & C33 are typical, but then there are two more...C36 and R52 and C34, sitting between R45 and R46. You've changed all these, including the bootstrap cap C35. I'd be inclined to lift out C36/R52, and C34. Why IS C34 there?

[pontiacpete]

Thanks JP and Nevetslab!
To measure the mains current I usually pull the fuse and put ammeter in its place, but this amp has one of those IEC fuse combos. So I used the Kill-A-Watt device and it is showing a jump in current from .15A to about .35A after the pops.
The popping does start immediately after start up so I never thought of it as heat related but perhaps I will squirt some freeze spray on some components and see if it reacts.
But first I will just solder the whole board. I had already soldered a lot of it so, good idea, just do the whole thing. Banging it doesn't appear to affect the popping.
I have to look through this board and see if the schematic lines up with what i have.
Thanks again

[nevetslab]

Wow...that's a significant current increase....150mA to 350mA AC mains current. I assume it drops back down after it settles. How often is the occurrence (rep rate, if somewhat continuous)?

Since this begins immediately after start-up, you're right...not heat related. It does seem current related. That's what made me wonder about the supply caps. Any idea how old the amp is? Often, if the supply caps and other parts are US made, there will be a 4-digit date code on them, such as 8237 = 1982, 37th week. If you find several date codes thruout the amp...if the power transistors are metal TO-3 or even TO-66, there normally will be date codes on them. Usually the product will have been built within a year of the latest date codes (assuming not replacement parts).

[pontiacpete]

It actually goes up to 470mA to 520mA but yes, it always comes back down to the 150mA. The frequency is irregular. It will pop, then about 5 to 8 or 10 seconds later it will pop, but then several pops occur, one after the other.
This amp has been to another tech and the problem, obviously, wasn't solved. But the 2 main filter caps that were in place when I got it were 3300uf/50v. So, thinking this is probably a filter cap problem, I installed a couple of new 3300ufs. Then I looked at the schematic and I saw the 6000ufs, but i"m convinced that this schematic is for the RG100.
Could this be the problem? The lack of filtering? I've been wondering about that.
I can't see a pair of 6000ufs fitting in the allotted spot on the board, but who knows, maybe there are some out there.

Thanks for the information on dating the components. That's very useful. I will look at the numbers on the old caps and the transistors.

[nevetslab]

I see 1981 in the title block of the schematic, so this was built in the 80's, would be my guess.

Just a thought. See what the amp is biased at, by looking at the voltages across both R53 & R54 (100 ohm emitter resistors, which feed the output xstrs' bases. You could try setting the bias pot for 350mV-400mV....if all is good, you should see about the same voltage across R53/R54. Prior to that re-setting, take and exercise the bias pot....rack it back and forth a bunch. If it's an open frame pot, like you might see on Fender's Hot Rod series amps, you might add a touch of contact cleaner to the bias pot and work the pot to get any long-term oxide off of the track. Enzo's alternate method of watching the AC Mains current draw, and adjust the bias so you just begin seeing an increase in mains current, and hold it there is likewise a good method. Not sure off hand what that would read across R53/R54. In both cases, you're not turning on the output xstrs...just getting them closer to that point.

[pontiacpete]

I called Randall today and found out that the amp is from 2012 and therefore a different schematic.
Here's the schematic of the power amp board.
He recommended soldering the board over also, which I just did and still there's the popping. I will probably go over it again because there are some short leads that don't go through the board very much at all. I'll also get some of those voltages on the emitter resistors. I have a gig this afternoon but I hope get this solved soon. Thanks again for the help.

Oh well the site isn't letting me upload the schematic. Is that because there's a protection on it? Or is there another issue?

[pontiacpete]

Is anyone else having problems uploading files?
I've tried many times to do so and tried different browsers and nothing. It's just a one page pdf from Randall. I'm able to email to myself so perhaps there is not protection on it and can also save it to my desktop without problems.

[nevetslab]

Besides the thought about the bias control, once that has been reset, and assuming the control isn't involved in this continual popping, monitor the current thru the pre-driver stage (voltage across R53 & R54), and see if there's current increase in that stage that correlates to the popping. But, since there's feedback in the overall circuit, it's no doubt going to show that increase. If the pre-drivers Q11 & Q12 were removed, that effectively opens the feedback loop. You'd need to tie the inverting input Q9 to 0VDC, by grounding the junction of R45/R46. That should leave the front end stable, enough so you could monitor it and see if this transient is originating in that section. You might need to lift one lead of the emitter resistors of the drivers and outputs, just to be sure the output doesn't try to latch up.

Trying to find if it's a transistor that's beginning to fail, it's hard to do when it's within the overall feedback. If the front end circuit is stable, not being the source of the popping, you could then put Q11 & Q12 back in, but leave the emitter resistors of the drivers & outputs lifted, AND removed the ground from R45/R46, since now there's feedback to stabilize the loop. It still without the popping, re-connect the driver stage emitter resistors R55 & R56, and monitor them. If still noting, that points to the outputs.

The driver and output stage emitter resistors are wire wound ceramics, no doubt. I wonder if one of them is becoming intermittent?

[pontiacpete]

There is no trim pot on this version of the amp. And I still can't upload the schematic for the 2012 RG80. The components are not in the same location as the 1981 RG80.

[nevetslab]

Several of us reported the upload problem yesterday, so that usually is a temporary problem. OK, no bias adjustment, so it was fixed at the factory at a set value. We did the same at BGW Systems, fixing the bias level on our power amps. Still, that puts us at the next step as I had suggested in my previous post # 13 yesterday. While the schematics are different, you'll be looking for the same principal with the one that matches your amp, in looking for where the origin of the sudden increase in current that's creating the popping transient(s).