PARASOUND HCA-800II amplifier with protection circuit issue

[jpcar]

Hi,

I'm having trouble with the protection circuit of this amplifier.
The amplifier seems to be working fine. I'm feeding in a test tone and seeing it present at the relay switch (RY1 - circled in red to the right of the sch). Both channels are working, no DC offset. But the relay switch is not closing and signal is not continuing the the speaker terminals. And there's a red standby light.

I removed the relay switch to test it out of circuit and it's working fine (I tested it with a 12vdc power supply). And I'm reading around 120 ohms across the relay coil. With the relay switch removed, the amp has 13vdc across the diode D006 (which is where the relay switch coil was). And the standby light turns green. But when the relay switch is in circuit there is no voltage across the relay coil.

Any thoughts on what I could do next?

Thanks.

[Enzo]

I wish the voltage readings had come first, we never would have removed that relay. BAd relay is way down the list.

The diode is reverse biased, so without the relay coil, it blocks DC So I suspect you get 12v on one end and zero on the other.

Put the relay back in, Now you get no voltage drop across the coil, may I assume that means +12 at both ends, rather than zero at both? SO we need to get the relay energized. Follow the path. I will assume for now you have 12v on the coil. Got 12v on the 10 ohm R011? Got 12v on the collectors of Q003, Q004? How about their bases? Got some positive volts or is it zero?

[jpcar]

Thanks!

I had removed the relay to sort out one of the switches which wasn't making contact. I do prefer to not remove parts if possible!

Tracing through now. So, yes the relay coil has no voltage drop and +12v is on both sides. There is +12 at R011 and Q003, 004 collectors. There is +.5v at Q004 base and +.7v at Q003 base. Then at the other side of that zender (D005) there +2.3v.

Should I suspect Q003, Q004? They are testing ok with diode test in circuit.

I didn't like the look of cap C007. It measured out of tolerance so I replaced it.

Just noticed R012 connected to Q003, 004 collectors. And Q005, 006. I need to check voltages around there.

[Jon Snell]

Start from the other direction, you will confuse yourself otherwise! Always assume the protection circuit is working and it is being told to power off until the detection and drive circuit has been tested thouroughly.
Firstly, check for 0volts on L8 and L9. Any DC voltage on those points will stop the relay from pulling in.
If there is no DC voltage on these points, follow the DC voltage to through the circuit up to the driver transistors, if needed check for +ve voltage on the driver pair base. It must be at least 5volts to allow the relay to pull in.
Look at what may pull it down; 1n4148 diodes are unlikely but could be etc etc etc.

[jpcar]

Yes, good idea. I had originally tested with a signal passing through and didn't see any DC but I should look a little more closely.

0v at L8 and L9.

follow the DC voltage to through the circuit up to the driver transistors, if needed check for +ve voltage on the driver pair base. It must be at least 5volts to allow the relay to pull in.
Look at what may pull it down; 1n4148 diodes are unlikely but could be etc etc etc.

What is the driver pair base? Is that Q809 and Q810 or Q807 and Q808?

The base of Q809 is +0.5v and Q810 is -0.5v

And base of Q807 is +1v and Q808 is -1v

I'm seeing around +2v at D803 which is the 1n4148. Should I be seeing above 5v?

[Jon Snell]

0v at L8 and L9, that confirmed, one must find the leak to ground that stops the voltage from rising ... the supply on R010 is a good place to start.
What voltage is on R010, because if there is no supply, the switch voltage will be held low, stopping the relay from switching.
C007 leaky or R008 open circuit will stop the 12volts from rising.

[jpcar]

Ok found the problem.

C005 - (which is at the bridge rectifier D002) was bad. It's a 10uF and was measuring around 1 or 2uF - which must not have been enough. Replaced it and Hooray! Relay switch clicks on! So, it wasn't fully shorting but leaking some voltage I guess. I should check the ESR.

Red voltages show with the problem, and Green voltages are the circuit working properly.

The working version: has 3.6v at the D005 and D004 intersection. Then 1.5v on the base of Q005. And 0v at Q003, 004 collectors.

I don't understand D003 which seems to tap off the AC side of bridge rectifier (D002) and bring positive voltage up to R010 where it is blocked by D004.

There's a lot I don't understand. But I'm happy this is working!
Thanks for the help!!

[g1]

I don't understand D003 which seems to tap off the AC side of bridge rectifier (D002) and bring positive voltage up to R010 where it is blocked by D004.

I think that's just a turn on delay (mute) at power up, relay is disabled til C005 charges up.