Randall RH100 repairing distorting amplifier

[Leandro Dias]

Hi again.

Well, bad news, it started to failure again after of almost a week of working well as it should.

The pre-amplifier aparently is still ok, sending good sound through the "send" output.

The headphone session (that i didnt tested that well before i think) has the same bad sound as the outputs. It comes out of the power board not the preamp one as I said before.

The values I get now are as following:

Q14 (three different measures)
B = 54v
C = 0v
E = 54v

Q15 (three different measures)
B = -54v
C = 0v
E = -54v

I get no voltage in R24 and R25, how much should i get?

I could not measure the C7, C8 and C9 because my multimeter cant reach it (goes only to nF), but apparently they are working. I can't replace them right now.

 

[Leandro Dias]

Update: changed c7 and c8 and C5 and it got back to work for like 2 hours with great sound, just to start failing again. I think i'm fighting the symptons not the cause. Tomorrow I will check the preamp section.

 

[spec]

Disconnect the pre amp from the power amp

Short pins 2 & 3 and see if the power amp survives.

See if you can put an audio signal between pins 1 & (pins 2&3).

spec

(it sounds very much like your power amplifier is oscillating)

 

[Leandro Dias]

"see if the power amp survives" lol.

It does survive, constant low hiss coming out of speakers when i short it.

You mean to feed the power amp with another audio source?

I'll try that

 

[spec]

"see if the power amp survives" lol.

You mean to feed the power amp with another audio source? Remove the pre connection (pins 1-2-3) and feed it another source?

Yes, but you must connect pins 2 and 3 together on the power amplifier board.
Without pins 2 & 3 connected together the power amplifier will almost certainly oscillate (because there will be 100% feedback). I have mislead you by using the word survive. There is no danger in connecting pins 2&3. That must happen somewhere in the pre amp and it may be that connection that is going wrong.

(1) remove pre amp conections
(2) connect pins 2 & 3
(3) feed another audio signal between pin 1 [input] and (2&3) [input 0V]

You need to do it in that order.

spec

 

[Leandro Dias]

I fed another source and the problem persisted. But in very low volumes the sound seems ok. When i fed more volume it does get the same ugly distortion again, thin and .

 

[spec]

Did you definately short pins 2&3

 

[spec]

Hi again.

Well, bad news, it started to failure again after of almost a week of working well as it should.

The pre-amplifier aparently is still ok, sending good sound through the "send" output.

The headphone session (that i didnt tested that well before i think) has the same bad sound as the outputs. It comes out of the power board not the preamp one as I said before.

The values I get now are as following:

Q14 (three different measures)
B = 54v
C = 0v
E = 54v

Q15 (three different measures)
B = -54v
C = 0v
E = -54v

I get no voltage in R24 and R25, how much should i get?

I could not measure the C7, C8 and C9 because my multimeter cant reach it (goes only to nF), but apparently they are working. I can't replace them right now.

All voltages correct.

The current through R24 and R25 should be around 20mA, so the voltage across each resistor would olny be around 6 mV.

spec

 

[spec]

It is very difficult to fault find audio power amplifiers remotely ( shame I cant get my hands on it), but my theory is that:

(1) something is intermittent: solder joint, crack in PCB..
(2) something is failing slightly as it heats up or with voltage across it: capacitor would be favorite for this
(3) some part of the amplifier is oscillating, not necessarily all the time, but only under certain conditions. The circuit made by Q8, Q12, and Q13 are always suspect in that type of output arrangement.

spec

 

[Leandro Dias]

Did you definately short pins 2&3

Yes, I solder them using a spare cable.

Did some joint/cracks observations and there was a possible minor suspect, q7. Since i had spares I changed it (A970) and again it went back to normal. I've left it on for like 1h and no problem so far. I'll leave it for my clients to test.

My suspects are the caps too. But I've changed most of the suspects.

 

[spec]

Audio power amplifiers can be a real pig- I speak from years and years of experience and dead transistors (and speakers).

Anyway keep at it and you will find the root cause.

spec