Ok try these tests,

Short Q1 base to ground, what happens to the led?

Short Q2 base to ground, what happens to the led?

Short R28 to ground, what happens?

 

Grounding the Q1 base will cause Q2 to switch on, in turn pulling down the base of Q3, causing it to turn on and pull up the gates of the two FETs. What is not clear to me is if 0.6 volts is enough to turn them, Q4 and Q5, on. It looks like they are a pulse stretcher, so that when IC5 conducts and charges C24, Q1 is biased on until C24 discharges. But if C24 has developed leakage then it will not work that way.
Connecting the junction of R27 and R28 to ground should switch off the LED and the two lights.

 

From post #20' the TS has removed the opto, so Q1 should be on, and the fets should be Off.. so something is wrong with the transistors...

 

Hello,

There is an inconsistency between the schematic and the PCB.
In the schematic the optocoupler is called IC5 and on the PCB IC4.
So remove IC4 from the PCB.
Use pin 5 of the optocoupler to connect to one of the speaker outputs.

Bertus

 

Hello,

There is an inconsistency between the schematic and the PCB.
In the schematic the optocoupler is called IC5 and on the PCB IC4.
So remove IC4 from the PCB.
Use pin 5 of the optocoupler to connect to one of the speaker outputs.

Bertus

How about instead just ignoring the error in the labeling.

 

Short Q1 base to GND - LED on - no effect

Short Q2 base to GND - LED on - no effect

Short R28 base to GND - LED on - no effect

 

OK then. If removing the gate drive to the pair of FET transistors does not switch the LE#D off, then it appears that at least one of those has drain to source leakage, or a short circuit. So the next step is to remove the two power devices, shown as Q4 and Q5 in the drawing, since it appears that one or both are defective. But first try using an ohm meter to check the source to drain resistance. If it is a low value in both directions then at least one has failed. Also do the same check after removing them, in case the problem is some strange connection issue. But removing those two should cause the LED to not light.

 

Yep....sounds like Q4, Q5 are faulty D/S shorted...

 

Ive decided to call it a day on this, some of the tracks have lifted off and its now getting more trouble than its worth.

only way forward I guess would be to try and get the circuit board reprinted and transpose the parts but some of these are obsolete so it may just be time to let this one slip.

Appreciate all the help guys.

 

Maybe your multimeter cannot accurately measure the gate voltage of the Mosfets because its frequency response is not made for audio (it accurately measures 50Hz and 60Hz).

The datasheet for the IRF540 Mosfet shows that the most sensitive ones barely conduct only 0.25mA when the gate-source voltage is 2V.

 

OK, do you remember the bit about not being an electronics guru? There's been a monumental cock up here. The unit was sent away and someone looked at it a few months back, it was returned then mothballed until I decided to dig it out again.

The Dome light output is on a 2 pin push in plug and the 12v power input is utilising 2 pins of a 4 pin socket so there was no chance of a crossover. Seems whoever looked at the unit last time removed both and put them back in the wrong order.

I've therefore been putting 12v through the Dome light output which will explain why the LED was constantly on. The funny thing was though that the voice part of the modulator still worked in this configuration, which is why it took me a while to figure it out. I had no reason to assume that the power input and dome output had been switched. On the plus side, the power on off switch works now which is about it!

I've now swapped the leads over to the correct positions but now I still get the LED and Dome lights on all the time and absolutely NO sound at all or even a crackle when I connect the speakers or the mic input. "Diddly squat". The TDA2003s have been changed, so has the Optocoupler just in case. A few broken tracks that Ive had to bypass with link wiring and I've rechecked all solder joints and they're fine.

I'm open to suggestions, I can either bin the damn thing, get a track repair kit and repair the PCB or just opt for a newer voice modulator and call it a day with this one.

Appreciate everyone's help anyway.

 

OK, there is life in the old girl yet. I got the voice modulation working again but the LED is still perm on and this time there are no shorts. We have a permanent 12v op to the dome lights irrespective of volume or VR position.

 

Hello,

Did you connect the sound to light in the origenal way?
(so with the optocoupler).

Bertus

 

Everything is back as per the circuit diagram now, so back to original

 

Hello,

Did you check the mosfets as it has been asked before?
If you replace them, use IRL540's.
It is sensitiver as the IRF540.

Bertus

 

Yes, removed the MOSFETS and tested them but they were open. Tried shorting oit the pins to see if they would trigger and they wouldnt so could be that my multi-meter on DIODE check, didnt give it enough V to close the gate. Ordered a few new ones anyway.