Classic Analog Board repair

[bibilit]

I still had a pair of non-working Analog boards (early ones) the design is slightly different from the late ones.

https://68kmla.org/forums/index.php?/topic/31279-dp4-on-classic-ii-analog-board/

https://68kmla.org/forums/index.php?/topic/31245-low-voltage-classic/

Both of them were not working at all (no outputs from the secondary side where new caps have been replaced )

These boards have a Zener Diode (18V) on location DP1 (just above the TDA 4605, location IP1 in the picture below)



In the first board, replacing the Zener solved the problem (i replaced also the diodes DP3, DP4 and both capacitors at the same time)

The second one didn't worked first time ( voltage at pin 6 of the TDA was a bit better with the new Zener, but still below the 12 volt range)

DP3 and DP4 were replaced also and the two caps, nothing else was apparently bad.

In the end i replaced the TDA 4605 also, and the Classic worked (the logic board is shot, only use for testing purposes)



So, all these parts can be responsible for Analog failures.

 

[JNizzle]

Awesome! This may come in very handy with my Classic, which I've hit a bit of a wall with. Thanks!

 

[bibilit]

Two more boards at home...

Those have the same issue, but new to me, Pin 6 of the TDA 4605 is reading 45 volts, instead of 12 expected.

The 22k 3 watt resistor is reading OK and usual suspects have been replaced, can the resistor be bad in board ?

 

[techknight]

zener diode? Since the absolute maximum rating of that pin/chip is 20V, id say you would need to change the TDA as well as finding out why your VCC is running high. Usual suspect is an open zener diode, the resistor is supposed to be the burn-off resistor when the diode clamps, forming a crude regulator. 

 

[bibilit]

Thanks TechKnight,

Can't see any Zener in the neighborhood, these two boards are the late design.

I removed the TDA on one of the boards, for some tests, and VCC was still the same.

Probably missing something...

 

[apm]

Perhaps a bad TDA chip? I don't see anything in the late-model schematic that would clamp the voltage so perhaps that happens in the TDA itself, as a result of its current draw through the 22k resistor.

Edit: also check bridge rectifier DP9.

 

[bibilit]

I did remove the TDA for testing purposes and voltage was still the same.

Voltage at the bridge is ok, and of course ok at the first leg of the resistor.

yes i agree with you, the design is straightforward, don't see anything either...

 

[apm]

Did you swap in a known good replacement for the TDA? As far as I can tell it's the TDA chip itself that would limit the voltage appearing at pin 6. If it fails open circuit, you wouldn't see any difference removing the defective part.

 

[bibilit]

That's a good point, not sure, i made a note for each board with replaced parts, will have a look.

Thanks.

 

[pep_one]

Two more boards at home...

Those have the same issue, but new to me, Pin 6 of the TDA 4605 is reading 45 volts, instead of 12 expected.

The 22k 3 watt resistor is reading OK and usual suspects have been replaced, can the resistor be bad in board ?

In my Mac Classic II I have similar problem. At the Pin 6 of the TDA I read 138V! (AC voltage) Is this possible? What can be the culprit?