Randall RH200 - Unknown Resistor

[EstynEstyn]

I just got a Randall RH200 off of a friend in pretty bad condition. The power amp PCB has a hole blown in it from a minor explosion and R47 no longer exists in the physical realm. Unfortunately the schematic lists the value of it as '*' and Randall tech support aren't much help. Does anyone know what the value is, or can recommend around what it should be. Many thanks.

http://www.pdf-archive.com/2011/03/25/rh200-schematics/

[teemuk]

Something around few hundred ohms, but the exact value is experimental. Some Randall amps following the same architecture use a 180-ohm resistor.

You want to use resistance that biases the output stage correctly, so with proper resiatance value the DC voltage measured in between driver's bases (that is, across the biasing circuit D2, R47 and collector of Q6) should approximately equal the number of following base-emitter voltage drops. In total that's four (two per each side NPN and PNP, parallel's count as one), so you should measure something ranging from about 2V to 2.4V. Depending on design you may wish to bias colder or hotter, but this would be in the ballpark at least. Note: Let the amp run at idle for few minutes so temperatures can stabilize.

[Jazz P Bass]

Yeah . the asterisk means "fit at assembly".
It is a tailored component.
I guess it saved them the cost of a pot.

[EstynEstyn]

Thanks for the responses guys.

I replaced it with a 180r but it burnt up again. Can anyone give me advice on tracing the root of the problem? I'm only used to dealing with valve amps, so this solidstate power output is all a bit new to me.

[Jazz P Bass]

SS amps do not draw a whole lot of mains current at idle.
Basically the whole output section is off awaiting a signal.
If R47 went open, something is pulling way too much current through it.
I would start by checking every transistor with a DVM set on diode check.
I am leaning towards a shorted Q7 or Q9 with Q11 in second place.
R47 is part of the bias circuit.
The bias circuit sets the "slightly on" condition of the output stage, which is a requirement to eliminate crossover distortion.
Obviously something is more than slightly on.

[EstynEstyn]

Looks like Q9's had it. Ordered a new one, thanks for the help! Just had a msg from Randall Tech Support saying R47 should be 15k! Could this be right?

[teemuk]

No.

See above comments about biasing the stage. Whatever you do, you do not want to introduce too high resistance to the bias string since this will lift the operation mode to class-A, the idling current will be enormous and the output stage will literally toast itself to death.

A shorted R47 is actually much more optimal starting point since it will mean colder bias. The worst that can happen in that mode is slight increase in crossover distortion - slight, because there already are two other biasing elements in the string already. Many Randall amps actually include only those. Anyway, you want to experimentally increase resistance from the short circuit state until proper bias point is reached. Most likely you'll end up in a resistance value close to few hundred ohms. Kilo-ohms are simply too much.

Naturally, all this assumes you fix whatever else problems the power amp circuit is having first.

[Sussmayr]

Looks like Q9's had it. Ordered a new one, thanks for the help! Just had a msg from Randall Tech Support saying R47 should be 15k! Could this be right?

Installed in the RH200 I am working on was a 100ohm 5% 1/4 watt at location R47