Help me diagnose a faulty amplifier.

[Maker]

It's a Sony TA-F120, my dad bought it around 1990 so no wonder it need a bit of maintenance.

It's only very slightly faulty, works just fine as an amp but the indicator LEDs on the front panel have stopped working. It sat in the loft for years and when I plugged it in for the first time the LEDs initially lit but quickly faded out.

The LEDs are supplied 15v and connected to ground through an NPN transistor. I spent ages following the traces by hand to figure that out before I thought to look for a service manual.



There was no +15v supply to the front panel, turns out the joints on the regulator has cracked, I reflowed them and the LED's briefly lit and faded out again. Presumably then the transistor is busted (Though I'm not sure how to test it. I swapped it with the first NPN I could find, with the leads twisted to match the pinout. Again the LED's briefly lit and faded out, I assume that means there's a problem elsewhere that's cooking that transistor. There's no evidence of burnt resistors or leaky electrolytics, nothing I've probed has drifted far from it's original value.



The service manual shows this:

So do I measure it within a minute or after ten minutes? The second I turn it on the points measure about 3.8mV, once it's all warmed up they measure about 12mV.

 

[Seadog]

The bias current adjustment should have nothing to do with the LEDs. It's balancing the quiescent current in the output stage.

However, looking at the circuit, the LEDs are switched by a line from the protection circuit. If the bias levels are way off it may well stop the LEDs illuminating. Does the amplifier work?

It says that the levels are checked from cold, within one minute of switching on..

 

[Maker]

I didn't think so but it was the only fault (If it even is a fault) I could find.

 

[Seadog]

The voltages are close enough. The 120 is spec'ed at 4mV, so you're close enough at 3.8 for it to work.

 

[Seadog]

Are we talking about all of the LEDs? If there's a problem with the protection circuit or the regulator you'll have problems. Try getting some noise from it to see where you stand.

 

[Maker]

Yeah both sets of LEDs, the power and the source direct ones.

It was making noise before I took it apart but I will go and test it again to make sure I haven't broken it.

 

[Maker]

Got it back together and wired in, still working, just no lights.

 

[Seadog]

Well that's a good sign. It shouldn't be too difficult to fault find. At least it's all discrete components.

 

[voipio]

What is the "gunge" all over the main PCB? Is it the remnants of an old spill, leaking electrolytics, or ???

 

[Maker]

On the top side it's just dust and IPA. There's some sticky crap on the bottom of the board which I initially thought was capacitor gunk but nothing looks like it's leaked and there's no components on the bottom anyway so I think I must have spilled something on it ages ago.

 

[MattF]

Warm it with a a hairdrier, & see if it's a thermal fault. Easy starting point.

 

[Screwdriver]

Sounds like a failing power supply or voltage regulator. First obvious culprit will be the electrolytics. They do fail over time and a duff capacitor could give you the sort of fault you describe.

I'd swap them all out for new.

 

[Alan Reynolds]

Are we looking at the LED board? They are in series and if one breaks down, then they all go out if it goes O/C. LED’s can do funny things when they decide to go wrong.

 

[Maker]

Sorry, I forgot to say the first thing I did was test the LEDs, they all work fine.

 

[Seadog]

Is the +15V still present? after the LEDs extinguish.

 

[Maker]

Yes.

 

[Seadog]

And present at the anode of D903?

 

[Maker]

Yep.

 

[Seadog]

Is C705 OK? The drive for the LED switch is on the ac detect pin of the protection IC. If that's is going low just after you switch on then that is certainly suspect.

 

[Seadog]

However, if it was pulling that pin low then the protection circuit should shut down the amp

I assume that the collector of Q704 is sitting at +15V? Shorting collector to emitter should enable all of the LEDs.