1971 Randall RG-90 head, smoking speaker!

[olddawg]

Dude.. you are scaring me.. with an amp drawing current like that, every time you replace a fuse and power it on you are probably damaging more components. At some point the power transformer may fail from abuse also. AT THE MINIMUM you need to learn how to static check transistors and diodes in circuit. You need to know how to check them out of circuit. You need to use a lightbulb limiter or a variac with a current meter when you power it on. If you do not know these things you need to start reading. The concepts are not that difficult but take a little time and concentration. I don’t want to sound harsh or demeaning.. but you will not be able to rock hammer this amp into working order.

[Randall]

"You mean the big metal RCA's or one of the small black ones?"

No offense, but if you don't know where and which are the power transistors, you have no business trying to fix this amp.

[Enzo]

We don't just throw parts at it, we need to take an ohm meter to the various transistors and find out if they are shorted. We need to check associated resistors for open condition. And yes, both big and small transistors can be involved.

DC on teh output is a very common failure mode, but of course a thousand things can cause that. Just like "my car won't start" is a very common car complaint, but could be caused by a thousand things.

[czech-one-2]

Well, if anybody knows a tech in central Europe that could fix this amp I would be extatic. Its a pretty early Randall and deserves to be restored I think.
Did I mention that I'm a guitar player not a tech, :-D
Sorry, my terminology is terrible but yes I know that the 4 large metal can transistors are power transistors. But I dont know what those large back mosfet looking transistors do, nore do I know the function of all the little black transistors on the pcb. I guess a schematic would help alot!
I dont know of anybody in Prague that works on old solid state amps, but I would be willing to send the head to Germany or Austria for repair if anybody has any recommendations. I should also mention that I bought the amp head to GIG with it, not turn around and sell it once its fixed. I've used transistor amps for 30 odd years as they are the only amps that have unlimited clean headroom at volume, which I need.
I guess I'd just like to try and narrow down probable causes so I can at least attempt to replace any parts that are faulty before it ends up in the dumpster with knobs and parts for sale on ebay...
The fact that the pcb traces are still intact [there was one trace in the suspect area that I had to jumper] and the PT is working shows me that nothing catastrophic has happened [yet] and there is hope for this old amp.
I did manage to get it from 'dead as a door nail' to ''lighting up and smoking speakers' with new filter caps. Wheres the 'pat on the back' emoji?

[czech-one-2]

That horizontal resistor that was smoking when a speaker is not connected was 220ohm, not 22ohm. This seems to be the most obvious problem area. It goes like this:
+ terminal of 1st 6000uf mallory filter cap > red wire to pcb > 220ohm resistor > F&T 470uf/63v + > F&T - terminal to ground.

And is this correct? : the + terminal of the second 6000uf filter cap goes directly [via black wire] to the negative terminal of the speaker jack? [photo] Seems like that would put dc on the jack, yes? Thats the orientation when I got it though so I assume its correct.


I ordered a set of four power transistors and will change the 220ohm resistor.
I would sure love to see a schematic for this amp if anybody has one!

[g1]

I think those larger plastic transistors you refer to are probably the drivers. And they get damaged by shorted output transistors.
I'll have a look for schem. but some of the older Randall are hard to find.

[Randall]

Not so hard, first google page. Here is one for sale:
https://www.musicparts.com/products.asp?Company=Randall

[g1]

This schematic should be the general idea though it will not be a complete match. You will have to check for differences.

[czech-one-2]

^ thanks G1! But the attachment isnt opening, just a blank page. Could very well be my laptop....thats the next thing that needs repair :-/

[czech-one-2]

Not so hard, first google page. Here is one for sale:
https://www.musicparts.com/products.asp?Company=Randall

Thanks, I saw that too. But thats half of what I paid for this 'project' !

[nosaj]

^ thanks G1! But the attachment isnt opening, just a blank page. Could very well be my laptop....thats the next thing that needs repair :-/

It's not you, he needs to zip it then upload it.

nosaj

[g1]

^ thanks G1! But the attachment isnt opening, just a blank page. Could very well be my laptop....thats the next thing that needs repair :-/

Should be fixed now, try again.

[nosaj]

Should be fixed now, try again.

Ba-Bam works here.

nosaj

[czech-one-2]

Works! ( the link...not my amp ;-)
I noticed a 4558 opamp in there. Mine pre-dates opamps I think! School me, when did opamps hit the scene?
I'm gonna replace the power transistors and the TIS-30 and TIS-29 FET transistors [mine are different than the schematic] when the replacements arrive. ( Sorry Enzo, I know I'm not supposed to throw parts at this thing but thats all I got :-(
So this thing is on hold for awhile. I spent some time checking all the resistors and they all seem ok.A couple more with hairline cracks were replaced. I dont want to power up again until the new transistors and FET's are in. I'm really not sure how to test the small black transitors, and thats gonna be a nightmare if theres a bad one....

[tdlunsfo]

Man if it was me, I’d also build me a dim bulb tester. This will help protect those new parts if you end up still having a short somewhere.