Speaker protection relay issue

[mckaymckaymckay]

Hello fellow DIYers, im wondering if someone could shed some light on a speaker protection circuit that is not engaging.

All output transistors have 8mv on the emitters so i have no shorts and it works if i mimic the final transistor to shunt half of the supply to the relay to ground.

The relay has +44v on each side, so im guessing once the final transistor has +0.6v on the base it will shunt one side to ground and enable the relay. However this transistor has 0v on the base, and im at a loss to what could be causing it.

Ive replaced all 4 diodes shown with new exact matches and also replaced the final 4 transistors (first 3 have 0V on the base, so these are not engaged). Voltages shown in the schematic are when the relay unlocks (half of what they are).

Any ideas?

An externally hosted image should be here but it was not working when we last tested it.

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[Ian Finch]

As you only show a part of the schematic, can you quote the make/model or post a link to the full manual/schematic?

 

[mckaymckaymckay]

Apologies, its the Toshiba SA7100 Receiver.

 

[mckaymckaymckay]

Toshiba SA-7100 Manual - AM/FM Stereo Receiver - HiFi Engine

Service manual is there, however it has a few errors, namely Q805 is a 2SA841.

 

[el156]

Strange you say that you have +44 v in the relay,where you should have +26 v shown in the schematic (emitter of Q804).

 

[mckaymckaymckay]

If i short Q807 collector and emitter the relay will engage and that voltage drops to 26v

 

[Mooly]

Is the negative voltage on C806 OK ?

 

[mckaymckaymckay]

Edit, sorry:

That cap is reading as 0.6v

 

[Mooly]

There should be a negative voltage on the cap derived from D802. If that is missing check the cap, the diode and the feed resistor R819.

 

[mckaymckaymckay]

Replaced cap and diode already, will check R819.

 

[Mooly]

It could well OC. Q806 base/emitter junction puts a clamp on the voltage at the top of R815 which means that come what may that missing negative voltage can't be 'pulled positive' by anything going on above... assuming Q806 etc is OK.

 

[mckaymckaymckay]

Actually its ok, discovered another fault while ive been changing parts...

One of the STV-4H diodes has disconnected from the cabling, and its not left me with any leads to solder onto

 

[Mooly]

If that is one of the bias setting diodes then you may well find damage to the output stage. Based on that I suspect your relay fault will be down to the emitter resistors being open and the 'current limiting detect' part of the relay driver coming into play thinking there is a massive overload.