Mystery Randall

[Valvehead]

I have a 1990 randall here..i believe the model is " Hardrock" or " Hardrock Stack" but its not labeled. it came as a mini stack with 2 , 1x12 cabs . Its a mini size head with 2 output transistors, says 50w on the back. The tag inside says model RG 150 Es . but I cant find a schematic for that anywhere online and Randall will not respond .

It works but has a very burnt resistor . cant see the bands. The resistor is right before the speaker jacks .

Can anyone tell me whats needed here by looking at the circuit ? I know its a long shot ....but Im at a dead end .

I drew up the circuit by tracing the pcb when i could get to the bottom

[dmeek]

I'm guessing it's part of the Zobel network. Probably 10 ohms. Can you post photos of the whole circuit board, both sides?

[Valvehead]

. ......

[Helmholtz]

Make sure the 0.1µ series cap is not shorted.

[Valvehead]

Make sure the 0.1µ series cap is not shorted.

not shorted .

[Helmholtz]

not shorted .

Then either the power rating of the resistor wasn't sufficient or the amp has a stability/oscillation issue.

[Valvehead]

Not sure if this will help but here is a RG 100...similar amp. But mine just has the 2 output transistors and only 1 of the .27/7w

randall_rg100_sch.pdf

[The Dude]

I can't find anything literature wise, but I think dmeek's stab at 10 ohms is probably correct since the first band is brown and normally that part of the Zobel network is a low value resistor.

[TomCarlos]

VH: Different site but could be useful reading to you. Read Teemuk's entries in this posting - Randall RG140 - need information about a burnt resistor - or schematics

[dmeek]

The model appears to be, or similar to RG50-112. That model has the pull switch. I can't find a schematic though.

[g1]

Did you measure it? Is it close to 10 ohms?

[dmeek]

Found the schematic

RG50 - Schematic.pdf

[Valvehead]

Found the schematic

[ATTACH]n985293[/ATTACH]

That IS pretty damn close !!!!!!!!!

im going with 10 ohm / 1 w !

THANK YOU !!!!!!!!!

[Helmholtz]

If you just replace with the same power rating, chances are the resistor will fail again.

[g1]

If you just replace with the same power rating, chances are the resistor will fail again.

Still no report (post #11) as to whether the original part has actually failed or not.
It does look like it has been excessively overheated though.