Salas DCG3 preamp (line & headphone)

[Salas]

When turning on and after waiting few secs for the delay circuit, then a reassuring click emanates from the output relay if all is well. Qa is the relay coil's driver transistor. Qa BC550C should show healthy Vbe, Za Zener should show 9.1V across its legs, Dd flyback diode should show the coil's working voltage across its legs. Somewhere around the nominal 12V. With enough +/-V margin, no problem.

 

[Russellc]

Im speculating but if that relay controls output from the board, might that explain the no sound, and thus I hope, no D/C ofsett?

 

[Salas]

If the relay doesn't work for some reason it surely cuts the signal output route when at rest. Your DMM could be just measuring noise in this case (?)

 

[Russellc]

No sound of any click at all. It is very hard to get to (and I don't want to short against the heatsink) but I am not raising any voltage across the diode behind the relay

 

[Russellc]

If the relay doesn't work for some reason it surely cuts the signal output route when at rest. Your DMM could be just measuring noise in this case (?)

That's what I am hoping! Now, is the relay defective, or is it part of something that senses something is wrong and wont allow it to turn on?

 

[Salas]

see #6,221 for some indicative DC voltage measurements you can make around the relay's driving circuit.

 

[Russellc]

Or is diode dead...better check orientation. I might be remembering the I-select relays, but it seems it did click before. I really cant say for sure. I've had lots of stuff with turn on relay noises over the years.

 

[Salas]

Also check that between IN and J1's gate pin there is 3.3K R1 bridging value to be measured and not some broken line or pad interruption. Ohms you measure with power off only.

 

[Salas]

Or is diode dead...better check orientation.

Dd diode is for fly-back voltage control on the relay's coil. If all is well it should be showing 10V or more across its legs. Its the same as on the coil because in parallel. Red probe on its cathode for correct polarity

 

[Russellc]

I though no voltage across diode behind relay, but getting on it better, I get the same "hunting" like on Offset. The tiny Za zener is 4.7 mV.
Now for Qa, when you say "across the legs" you mean the 2 outside legs?

 

[Salas]

Vbe as in the other BCxxx transistors

 

[Salas]

Do you get good dc voltage across the selected input's relay diode on I-select?

 

[Salas]

If the three Leds don't shine, one or more could be wrongly oriented or broken. Cutting off the power line to the relay.

 

[Salas]

The three Leds normally shine after the turn on delay right at the moment when the relay should also click.

 

[Russellc]

If the three Leds don't shine, one or more could be wrongly oriented or broken. Cutting off the power line to the relay circuit.

The three leds are lit up fine. I have better measurements now, dont know if they are good, but here they are:

diode behind relay...................................................1.3 volts a/c
Qa....................................................................................3 mV
Za....................................................................................4.3

 

[Russellc]

The three Leds normally shine after the turn on delay right at the moment when the relay should also click.

Then maybe it does work? I dont hear it, but everything lights up that is supposed to light up

 

[Russellc]

1.3 volts A/C above ( post #6235) doesnt look right. I dont know about the Qa measurement of 3 mV, or Za meaurement of 4.3?

 

[Russellc]

Do you get good dc voltage across the selected input's relay diode on I-select?

when not selected, I measure 0 across diodes. As they are switched on, 12 volts, and it moves as I switch. It must be selected to showthe12 volts.

 

[Russellc]

When turning on and after waiting few secs for the delay circuit, then a reassuring click emanates from the output relay if all is well. Qa is the relay coil's driver transistor. Qa BC550C should show healthy Vbe, Za Zener should show 9.1V across its legs, Dd flyback diode should show the coil's working voltage across its legs. Somewhere around the nominal 12V. With enough +/-V margin, no problem.

no flyback diode only measures 1.3 volts. The diodes on the I-select measure 12 volts, once they are switched to, otherwise they measure zero.

 

[Russellc]

Dd diode is for fly-back voltage control on the relay's coil. If all is well it should be showing 10V or more across its legs. Its the same as on the coil because in parallel. Red probe on its cathode for correct polarit

Also check that between IN and J1's gate pin there is 3.3K R1 bridging value to be measured and not some broken line or pad interruption. Ohms you measure with power off only.

not sure what you want me to measure here? There is a 3.3k resistor there. It indeed measures correctly.