Dirty A-720 with no lights/sound, seems to get power though

[zaibatsu]

A gentleman dropped this off to me today to see if I could fix it (sentimental value), there are no lights when switched on, but there does seem to be power going through the unit so I don't think it's the transformer.

It's a 240V unit, on DBT (77W), I'm reading ~202V at the switch while it's off, drops to 156V when on.

There's idle voltages at the emitter resistor measurement points on both channels, which seem to change OK with the adjusters too (~10mV).

No relay click. I'm not seeing any burnt flameproof resistors on quick glance, and the outputs don't seem to be shorted either.

It's very dirty and there's a lot of corroded jumpers etc, could it just be possible that the LED drive circuit is messed up (along with whatever else)? Is it worth removing the obvious corrosion and DSBG before attempting to move forward with it?

Appreciate suggestions

 

[zaibatsu]

Just quickly removed the DSBG, buffed the corrosion off the bus bar, and replaced 2x out of spec 1000uF Muse with fresh KZs.
The Blackgates measured in-spec so I just cleaned up the glue and popped them back in for now.

No change, but figured a cleaner starting point would be welcome.

 

[avionic]

No LED or lamps illuminated. No relay click. Start at the regulated DC power supplys. Verify all DC supplies are working and getting to there proper destinations.

 

[zaibatsu]

I'm getting +42V and -42V DC at the +B / -B points on both channels.

There's also a -B / E (-43V / 0V) pair and a +B / B (42V / 0.08V) pair which run to the center of the board.

Bulb starts bright and dims normally straight after as it would if there was no short.

Let me know if there's anywhere else I should test voltages.

 

[avionic]

No LED's illuminated ? Ever?

 

[zaibatsu]

The story was - the guy bought it new back in the 80s, it was working fine when put away for ~3 years, then when he tried to use it again there was no action. Fuse was never blown (still good).

I haven't seen the lights turn on even once.
Something tells me whatever is supposed to drive those LEDs might be problematic, since the rest seems to check out?
And maybe a separate problem holding it in protection?

Edit - I'm getting -38V and -42V on the red and white cable going to the LED board (wires labeled LEDL and LEDH).. if it means anything.

 

[avionic]

If not. Start at the LED and work your way back to the power source for them. Find where and why the voltage is missing. The HFE schematic suxs ...

 

[avionic]

Most likely the same voltage source as the protect circuit .

 

[zaibatsu]

Thanks, that would make sense.

I'll have a look.

 

[avionic]

Nice clean-up job so far. Happy hunting..

 

[avionic]

Look at voltages around TR208. . Make sure you have around 50 volts on both ends of R254 (47Ω fusible most likely).

 

[willyrover]

Look over that board very carefully. I had a half-dead Yamaha M-85 tuner (same era as the A-720) with that brown glue.

I looked over the boards several times before finding a diode with a leg eaten away.

 

[avionic]

I had a half-dead Yamaha M-85 tuner

T-85 tuner and M-85 power amplifier.

 

[zaibatsu]

Funnily four diodes had cracked/crumbling clear bodies: D211, D212, D213, D214
All the same type (1SS82 > BAV21 sub). Three measured fine, one had flaky numbers, replaced all.
No change.

Look at voltages around TR208. . Make sure you have around 50 volts on both ends of R254 (47Ω fusible most likely).

TR208
B: -40.85 V
C: -40.87 V
E: -41.50 V

R254
-41.5 V
-41.8 V

 

[avionic]

What voltage are you getting at the cathode of zener D219 ?

TR208
B: -40.85 V
C: -40.87 V
E: -41.50 V

R254
-41.5 V
-41.8 V

Close enough for now.

 

[zaibatsu]

What voltage are you getting at the cathode of zener D219 ?

18.10 V at cathode of D219 zener (12.03 at anode)

 

[avionic]

18.10 V at cathode of D219 zener (12.03 at anode)

Perfect.

 

[avionic]

What voltage do you have at the base of TR235 ?

 

[avionic]

What voltages are you seeing at LED High and LED Low jumper pins.

 

[avionic]

Have you measured the DC offset before the protect relay ?