Randall KH-120

[carlosraj]

Hi

I am new to this forum and trying to fix this Randall KH-120 guitar amplifier. I have replaced all the 5 transistors on the heatsink (A1941, C4793, C1815, A1837, C5198). I have checked all the transistors, doides, resistors and caps, they are all OK. Now when I turn on the amp, it is distorting and the volume is very low even at maximum level. The power board is labelled RG75-B and I could not get the exact schematic. I downloaded the schematic for board RH2-100-B from the net and RG75B from this forum, but they are slightly different. The RG75B board is similar but this actual circuit has 2 additional transistors (Q16,Q17) and few resistors. The value of some resistors are different from each other. On one board , pin 3 is connected to ground, but on the other, pin 1 is connected to ground. Does anyone has the actual schematic for this amp. Please assist.

Thanks

 

[unclejed613]

does the amp have a headphone jack like the one on the -100? if so it could be bad, so you have little or no audio going to the speaker.... or the speaker could be bad too... since you replaced output transistors, one of the things that would fry the outputs is a bad speaker...

 

[Nigel Goodwin]

does the amp have a headphone jack like the one on the -100? if so it could be bad, so you have little or no audio going to the speaker.... or the speaker could be bad too... since you replaced output transistors, one of the things that would fry the outputs is a bad speaker...

Hopefully you meant 'speaker socket'

100W directly to a pair of headphones might perhaps damage your ears?

Checking voltages would probably be a good idea, it's possible that other parts have blown - and also that some of the new parts have, if all faults haven't been replaced before turning ON. A sinewave input and scope could help as well, as it often helps to see in what way it's distorted (lacking negative or positive peaks etc.) A common failure is the emitter resistors, if the outputs have gone short.

 

[carlosraj]

Hopefully you meant 'speaker socket'

100W directly to a pair of headphones might perhaps damage your ears?

Checking voltages would probably be a good idea, it's possible that other parts have blown - and also that some of the new parts have, if all faults haven't been replaced before turning ON. A sinewave input and scope could help as well, as it often helps to see in what way it's distorted (lacking negative or positive peaks etc.) A common failure is the emitter resistors, if the outputs have gone short.

I am sure that all the components are OK, because I have double checked them. This amp has been repaired by someone earlier. When it came in, the C5198, A1837 and emitter resistor on the right side was blown. It was fitted with 100 ohm emitter resistors. I replaced all the blown components and turned on the amp. When at low volume, it was distorting badly and volume very low. If I set the volume at maximum, it was sounding very much better. I then refferred to the RG75B circuit and it shows that the emitter resistors are 330 ohms. So I replaced with 330 ohms resistors. Now there is no sound at all, but when I set at maximum volume, there is slight distorted sound. I suspect the emitter resistors value are too high. When I reffer to the RH100 circuit, it shows 47 ohms emitter resistors, all other components are exactly the same. I am confused, which emitter resistor value is correct, 330 ohms, 100 ohms or 47 ohms.

Thanks

 

[Nigel Goodwin]

Distortion at low volume is a classic symptom of crossover distortion, the bias is set by Q8 and D5 and isn't adjustable. You measure it by the voltage drop across the output emitter resistors (0.15 ohm in the 100W schematic) and should usually be in the low 10's of mA (measured as 1.5mV per 10mA across 1 of the resistors, or 3.0mV per 10mA across both).

The emitter resistors of the driver and constant current source aren't critical, and won't stop it working, and probably wouldn't make any audible difference.

Static checks on components isn't a useful service procedure, and rarely used in servicing - fault finding to locate the problem is what's normally done.

 

[unclejed613]

When at low volume, it was distorting badly and volume very low. If I set the volume at maximum, it was sounding very much better. I then refferred to the RG75B circuit and it shows that the emitter resistors are 330 ohms. So I replaced with 330 ohms resistors. Now there is no sound at all, but when I set at maximum volume, there is slight distorted sound. I suspect the emitter resistors value are too high. When I reffer to the RH100 circuit, it shows 47 ohms emitter resistors, all other components are exactly the same. I am confused, which emitter resistor value is correct, 330 ohms, 100 ohms or 47 ohms.

yes sounds like no bias voltage on the output transistors (and most likely the drivers as well). check R17, Q8, and D5. check the voltage between the collector of Q8 and the collector of Q9. there should be about 1.2V to 1.8V between those two transistor collectors. if there isn't, then either Q8, or D5, are shorted, or the protection transistors (Q10 and Q11) are shorted, or turning on hard with the amp running. Q10 and Q11 are monitoring the voltage across the output device's emitter resistors. if there were a fault in the speaker wiring, or the speaker itself, Q10 and Q11 clamp the output stage bias so that the output devices will be starved for current.

 

[carlosraj]

Today, I replaced all the transistors on the board expert the 5 transistors attached to the heatsink. I turned on the amp, and wow its back to normal. It is working perfectly fine now. I guess one of the transistors was leaky.

Thanks to all who provided guidance and assistance.

Rgds