1971 Randall RG-90 head, smoking speaker!

[DrGonz78]

Also, it has a plastic base, my new BR is metal. Does it need to be isolated from the chassis in SS amps?

None of the legs of the bridge rectifier should have a connection to the metal base. Get out your meter and check to see. Just different heat sink potential.

[czech-one-2]

^ Thats what I thought, but you know with these ss amps, one short and there go all the traces.
what about the continuity between all the legs on the bridge rectifier?

[g1]

Continuity is a vague term. Please give us readings in ohms.
If you have very low resistance between all legs, it could be the bridge, or the circuit it is connected to. Are you measuring with all legs disconnected from the circuit?

[czech-one-2]

No, measured with everything hooked up. Just surprised that there is continuity between all the legs.

[czech-one-2]

double post...

[czech-one-2]

"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.

double post...

[czech-one-2]

Well.....6 months later and ITS ALIVE! :-)
Its really nothing short of a miracle as this amp has had ALOT of techs and non techs working on it over the years, including sharpie marks by all components.
I replaced the TIP29C and TIP30Cs as well as the four 2N6254 power transistors. [these were most likely the problem].
The tremolo works and the amp is LOUD, CLEAN and very quiet.
The only thing not working now is the spring reverb. If I gently strum the springs I here it faintly through the speakers, so the rca cables are ok and I have a few tanks so the tank is ok. These amps have a great spring reverb sound so I really want to fix it so that its working 100%. The reverb trim pot seems to be working.
If you guys can help me trouble shoot just the reverb circuit I would be grateful! :-)
Still interested in finding a schematic for this 1971 Randall RG90 head.

[czech-one-2]

Unfortunately, solid state amp repair can be a real 'bear'.

Tube amps are relatively easy as the high voltage stages are coupled to the next stage by Vdc blocking capacitors.
Which makes things unlikely to blow up.

SS amps on the other hand are 'direct coupled'. No caps to stop the voltage.
So when things go bad , they go bad.

A tried and true tool is a 'lamp limiter'.
If the amp is in a failed state and tries to pull huge amounts of current, the filament in the lamp will help absorb it.
Along with a visual interpretation of what is going on. (bright is bad)

Another useful tool is an ammeter on the mains.
I use a device called 'Kill A Watt' along with an ammeter.

If you want to learn I would suggest fellow member Teemuck's online book.
http://www.thatraymond.com/downloads...ttala_v1.0.pdf
(Download Caution: it is 419 pages)

And thanks for that, I now have a lamp limiter! :-)

[g1]

All I can say without a schematic is that if you can't get a real loud response from the springs with the reverb and volume turned up, then I think the problem is on the recovery side of the reverb circuit.
Have you looked at the RG80 schematic? Is it similar? You may have to draw out what you have there.
If it's like the RG80, the coupling cap at the reverb output, or the recovery IC, would be suspects.

[czech-one-2]

Actually, if I shake the tank it rattles loudly through the speaker, just nothing through the guitar input.
I gotta fix it now cause everything else is working great, and these amps have one of the best spring reverb tones of any solid state amp ever made.
Unfortunately, I'm at a loss. Also, there are no IC's in this amp, it is a 1971 model.

Here's the latest pics with the amp working. Maybe someone could point out the reverb coupling cap? I would really like to know what the original transistors were, as all the small black transistors were replaced at one time and most have been installed with different orientations from the original 'triangle' transistor holes. In other words, some have different C B E orientation than the originals. Hopefully the tech that did this knew what he was doing!

[g1]

Do either of these schematics match up?

[czech-one-2]

Thanks for alot for those, I'll get back to you on that. Maybe those TIS58 transistors are the original parts.

[J M Fahey]

This is the Tina Turner / The Jackson era Randall.
Far from Metal, it was a "Twin type" amp, at least as far as features are concerned.
They were used by many a Funky player, even Country guys
2 channels, Tremolo, Reverb, etc.

Practically same power amp, but very different Reverb: driven by a single transistor, no transformer, capacitor coupled, requires a high impedance tank.

[g1]

If yours has a coupling cap into the tank, like C43, check that signal is getting through it.

[The Dude]

Also, check the resistance of the tank input. You may have a broken transducer wire or open transducer. FWIW, I check them at the cable end that plugs into the amp first. That checks both cable and transducer. If you get an infinite reading there, go directly to the tank input and see if it's the cable or the tank.