1971 Randall RG-90 head, smoking speaker!

[czech-one-2]

Hello,greetings from Prague, CZ!
I hope Mr Fahey, Enzo or anybody can helpme repair this 1971 amplifier head, as you folks are real knowlegable about these old solid state amps!
I recently took a gamble and bought a 1971 RG90 head, sold as 'not powering up'. I figured if I'm lucky its just a fuse....no such luck :-(
First thing I did was change the 2 large filter caps..and PRESTO! We have power!
Now the problem seems to be when speaker is hooked up, the cone sucks 'in' and the speaker will start smoking in no time.
The good news is:I examined the board and all the traces look fine and show continuity and obviousy the PT is ok, so this amp is definately fixable! But I dont know where to begin.
If you have any time to reply I would be very gratefull for your time and advice.
I will post gut shots today!
Thanks in advance for an advice you guys an offer!
Bill

[czech-one-2]

I replaced few suspect resistors and caps in that area. Also the board mounted large caps with F&T caps. There were a couple cracked resistors that were replaced. I tried swaping the 4 RCA 2n6254's temporarily with 2n3055's and there was no change, so I put the RCA's back in. All wire connections are solid. I'm thinking its maybe a bad pcb mounted transistor? But as you can see, my diagnostic skills leave much to be desired, I am a guitar player first and foremost! ;-)

[czech-one-2]

This photo shows the area of the speaker connection:

[Helmholtz]

the cone sucks 'in' and the speaker will start smoking in no time.

Means that there is DC at the speaker output. Do not connect the speaker again! The DC probably has already caused some damage to the voice coil (smoke!).
Please measure DCV at the output without any load.

[czech-one-2]

Fortunately, I used an old no name speaker for the test. It still works though!
Ok, went to measure DC on the speaker jack and it sparked like crazy, so didn't get a reading. Also, without a speaker connected a resistor started smoking. its the red/red/blk? 22ohm horizontal resistor in this photo:

[Helmholtz]

Can you get a DCV reading if you connect the meter leads via clips before turning the amp on?

Might be DC+HF oscillation. Do you have a scope? Bulb limiter? Schematic?

[czech-one-2]

Can you get a DCV reading if you connect the meter leads via clips before turning the amp on?



Might be DC+HF oscillation. Do you have a scope? Bulb limiter? Schematic?

Answer A:
I read zero voltage at the jack with the amp off.

Answer B:
No / No / No :-( Anybody have a Randall RG-90 schematic?

If i can get this beautiful 1971 amplifier up and running, I promise to lay down the soldering iron and focus on other forms of entertainment!

[Helmholtz]

Answer A:
I read zero voltage at the jack with the amp off.

Sorry, I meant connecting the meter before turning on but read DC voltage after turning on.

[czech-one-2]

Sorry,
Wierd, but I dont measure any voltage on the speaker jack when the amp is turned on?

[xtian]

Your meter is probably fried now. Try testing a 9v battery for voltage as a test. Hopefully you just took out a fuse inside your meter.

[g1]

When you got sparks, did you have the meter set to current (A or mA) range?

[czech-one-2]

Well, the amp popped a fuse, so no voltage reading...
Anyway, new fuse in, voltage across speaker jack is 42v. That cant be a good thing....lol

[czech-one-2]

The first set of photos were before I changed the caps, heres after re-capping and changing a few cracked resistors, a few caps and adding 10w brick resistors.


Apparantly, this amp was worked on many times over the years. The red marker dots by all components are not my doing, and all the small black transistors have been replaced at one time and soldered on to cut off legs of the original transistors. All contacts seem well soldered and intact though.

[Randall]

One or more of your output transistors is likely shorted.

[czech-one-2]

You mean the big metal RCA's or one of the small black ones?
'' I tried swaping the 4 RCA 2n6254's temporarily with 2n3055's and there was no change''