Randall RG75/G3

[bsco]

Good day to everybody...I have this one here and after going through the circuit, I have audio up to the relay.....the relay is not being activated so the audio is not being fed to the speaker....the relay is controlled by a power transistor which checks out fine.....thee is 14V on the tab so the transistor is not being turned on?? traced back to the other transistors and low value resistors and everything checks out fine....diode checking the transistor junctions also checked out fine....I did find a capacitor with a relatively high esr and replaced that but same thing.....powers up no sound....Line out/ Send works fine.....signal is available at the relay but won't switch.....I will post the schematics for viewing....
Cheers

[g1]

No DC along with that signal getting to the relay?

[bsco]

No DC along with that signal getting to the relay?

Hi g1. Thanks for the reply...the is DC to one side of the relay coil.....14V.......that is the same voltage as on the tab of the driver for the relay.....for some reason the transistor is not getting turned on....I back tracked through the circuit but everything checks out so I must be missing something...

[Enzo]

No, I think he is asking if that signal you find blocked at the relay also has DC sitting on it. Not the voltage on th relay coil.

You have a bunch of circuits there to protect the amp. But Q5 turns the relay on to enable the speakers. Briefly ground its tab with a clip wire. Does that pull in the relay? If not the basic relay coil circuit is open. But I bet that works, so then what voltage is on teh base of Q5?

Q2 kills the relay by grounding the base of Q5. Now look further left for AC SENSE. The AC supply is rectified to turn on the base of Q1, whwich then turns OFF Q2. THus if there is AC supply, Q2 is disabled, allowing Q5 to work. So what voltages are around Q1, Q2?

Q3, Q4 both work the same except for polarity. The two of them sense DC on the output, and if it is there, they turn off Q5.

SO if the basic relay works - grounding the tab - then sort out which of these protect circuits may be bad. And don't overlook that Q5 itself can be bad.

[bsco]

No, I think he is asking if that signal you find blocked at the relay also has DC sitting on it. Not the voltage on th relay coil.

You have a bunch of circuits there to protect the amp. But Q5 turns the relay on to enable the speakers. Briefly ground its tab with a clip wire. Does that pull in the relay? If not the basic relay coil circuit is open. But I bet that works, so then what voltage is on teh base of Q5?

Q2 kills the relay by grounding the base of Q5. Now look further left for AC SENSE. The AC supply is rectified to turn on the base of Q1, whwich then turns OFF Q2. THus if there is AC supply, Q2 is disabled, allowing Q5 to work. So what voltages are around Q1, Q2?

Q3, Q4 both work the same except for polarity. The two of them sense DC on the output, and if it is there, they turn off Q5.

SO if the basic relay works - grounding the tab - then sort out which of these protect circuits may be bad. And don't overlook that Q5 itself can be bad.

Hello Enzo. Thanks for the reply....I will double check for DC mixed with the audio signal......but I think not......the relay itself works as I removed it and bench tested it.....so the problem is in the circuitry somewhere...I have to go over to the shop later this morning.....I will go through the circuit again and see what is going on and will post my findings back here later today....
Cheers

[bsco]

Ok Enzo.. I was over to the shop this morning...the relays works....and I did test it with the jumper from the tab of Q5 to gnd.....so for voltages....
Q5.....6.8mV base...14V on the collector....Q1 base=.605V.....Collector =53mV..........Q3 base=approx 200mV........ Q2 base=83.9mV.......Collector=6.8mV.....
Q4 base=200mV and slightly fluxuating.....Collector=0V....no signs of DC on the audio.....there is 14V at the junctions of the two diodes for the AC sense line.....it goes to a 22k resistor and then on the other side is the .605V....I checked the resistors and diodes and everything checks out fine.....I also did diode tests on the transistors and they checked out as well....I tried putting pressure on the ckt board to flex it somewhat to see if there is a hairline crack somewhere but no difference.......anyway, let me know what you think.....I have a couple of pressing matters when I get back to the shop so I'll do some more testing later this evening......
Cheers

[bsco]

Finally got back at this one...The problem was C3....was shorted...once I replaced that, it was up and running...
Cheers.....

[Enzo]

Ah. C3 shorted. It is the power on timing cap. It can never charge, so the base of Q5 can never turn on.

Glad you found it.

[bsco]

Ah. C3 shorted. It is the power on timing cap. It can never charge, so the base of Q5 can never turn on.

Glad you found it.

I'm glad as well..The customer picked it up earlier this afternoon......and he was quite happy.....
Cheers