Randall RH100 G2 amplifier repair

[Omnitek]

Hi,

I'm trying to repair a Randall RH100 G2 with a blown power amp.
So far, I've replaced the output and driver transistors and some burned resistors.

Now the amp is working fine but I only get half of the power (about 60 watts) at 4 ohms.
At 8 ohms I get the full power.
Does anybody knows what could be the culprit?

Thank you!

Nicolas

 

[MikeMl]

Power supply sag?

 

[Mr RB]

Did you replace the output transistors with parts that have lower beta? Or identical part numbers? Does the schematic have minimum beta specs for the parts in the power stage?

 

[Omnitek]

Hi,

Thank you for replying.

The power supply doesn't seems to sag very much. The supply voltage is at 53 volts and goes down to 47 volts at full power.

As for the power transistors, I replaced them with the exact same model numbers as the original ones. I didn't see any information about the beta on the schematics.

I was suspecting the limiter circuit made of q10 and q11 so I bypassed it and then I got the full power at 4 ohms.
Maybe it's just the way that amp has been designed and doesn't really deliver 120 watts at 4 ohms with that limiter circuit.
Any opinion on that?

Thanks!

Nicolas

 

[Nigel Goodwin]

Have you replaced the emitter resistors?, if you have did you use the correct value? - if you haven't, then the resistors may have gone high in value following the failure of the amp.

 

[Omnitek]

Hi,

Yes the emitter resistors are the correct value.

 

[Nigel Goodwin]

Hi,

Yes the emitter resistors are the correct value.

But were they blown?, and what values did you replace them with?.

 

[Omnitek]

Hi,

No, they were ok so I did not replace them.

Nicolas

 

[tunedwolf]

Nigel is right on the money with this. Whenever any output device is blown and then replaced, I automatically fit replacement emitter resistors, however they may be reading. Most of them have a smaller resistance than the average Multimeter is capable of properly reading due to lead/ connection/ switch resistance etc. After being super heated by the failure of the driver, either the value has changed, or it's now hanging on by the skin of it's teeth and will likely fail when the amp is driven hard, causing yet more damage to the amp, or popping a voice coil. I dunno which is worse, but replacing them is cheap insurance in my book

 

[MikeMl]

Checking a 0.33Ω or other low value resistor is quite easy if you have a constant-current adjustable lab power supply. Set the current limit to 1A, connect it to the resistor, and measure the resulting voltage across the resistor to 3 significant figures using a DMM in volts mode. R =E/1, you don't even need a calculator...

 

[Mr RB]

Does the amp label actually say the power output spec into 8 ohms and into 4 ohms?

Maybe it's just an 8 ohm amp, and the limiting is working correctly.

 

[Nigel Goodwin]

Hi,

No, they were ok so I did not replace them.

The reason I asked, is because (as others have pointed out) they are 'difficult' to test - and they quite commonly go high in value (but too little to measure with a multimeter). Having gone slightly high would cause exactly the problem you're having.

In the case where they had gone O/C, and you'd replaced them with the correct values, the resistors that monitor the voltage across them are quite often damaged as well, which could cause a similar problem.

 

[dobz3]

Sorry same problem different thread

 

[Nigel Goodwin]

Do you have a schematic of it you can post here?.

Sorry - I see it's in another old thread - check R21 to R23, they are potential dividers on the current limiting.