various similar faults on psu 12v lines - zener or capacitor or what?

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mutetourettes

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Hello all, I've had two very different (but both late 80s early 90s tape) machines recently whose 12v psu lines have given up, and in both cases it looks like a very similar fault. I just want to run this by your good selves to discuss and see:
a) have I understood this right
b) have you all found this to be common too

One circuit is a series-pass transistor, one is a darlington, but they're both taking a larger DC input into a resistor/13vzener voltage divider to put 13v into a transistor (for the 12v line after the transistor's drop). They also have a capacitor parallel to the zener.

The issue is that the transistor is getting 0v, rather than the 13v the resistor/zener should control.

I'm thinking four possible cases for both machines
a) the resistor has failed open
b) the zener has failed short circuit
c) the capacitor has failed short circuit
d) some strange fault with the transistor where the base is going to ground

I have not unsoldered anything yet.

I suspect the capacitors (for no good reason), though they do not look bad. are they common to fail short-circuit? What is the function of the capacitor (smoothing power supply ripple?) if I take the capacitor out and test the circuit briefly would that commonly work ok enough to tell me "yes it was the capacitor"?

Attached to this post the Rotel RD865 12v line schematic

 

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abbey road d enfer

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mutetourettes said:
Attached to this post the Rotel RD865 12v line schematic
A very common failure with this arrangement happens when there is a direct short to ground. The capacitor then discharges into the b-e junction, which destroys it.
 

mutetourettes

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Thanks - Do you mean the short to ground being somewhere Downstream on the 12v rail - so the capacitor (and zener) is effectively across the b-e junction?

So would your instinct here be that the transistor is the first suspect?
 

JohnRoberts

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It may just be coincidence, so troubleshoot each individually looking for actual faults.

For simple pass regulators check device junctions for reasonable voltage drops, failures from overheating often indicate as short circuits.

JR 
 

mutetourettes

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Thanks John, do you mean remove the transistor and put it in a test rig with expected voltages? Do you also intuitively suspect the transistor first? Hi
 

mutetourettes

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I should add that the transistor in the TOA has been replaced (not by me) with no succesful improvement. That’s i think why i’m thinking why is that base sitting at zero volts in both designs.

The replacement of the transistor lifted traces a little.. it looks precarious now. Hence i’m trying to understand the likely issues and hoping to lift as few more component legs as possible..
 

mutetourettes

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Thanks. Yes with the toa it’s easy enough to work on just the psu as it’s a separate board, that’s what i took the readings from (no load). The rotel is all-in-one, so i’ll have to see what needs to be disconnected to isolate it.

These 12v supplies tend to run the reel and  capstan Motors. Not sure if that means likelihood of nasty negative voltages etc
 

JohnRoberts

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with a VOM you can generally probe for shorts while still in circuit... removing component just to test them is tempting fate that you won't cause some other unintended bad connection.

JR
 

mutetourettes

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Hi John - thanks for your comments - do you mean probing while the power is on? I've tried to analyze the TOA board as much as possible with the power off, confirmed that the transistors are ok (BE and BC both give good results in diode test), the two closest resistors are ok
R670 does indeed measure 10ohms,
R671 does eventually settle on 4.7kish (presumably having charged up all sorts of capacitors around the circuit),
I can't really work out how to better test between C628 and D620
They're in parallel and both settle on half an ohm in either direction,and diode test gives a quick drop to nothing both ways, nothing visually bad
could you give me an example of what else would you probe there?
 

JohnRoberts

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mutetourettes said:
Hi John - thanks for your comments - do you mean probing while the power is on? I've tried to analyze the TOA board as much as possible with the power off, confirmed that the transistors are ok (BE and BC both give good results in diode test), the two closest resistors are ok
R670 does indeed measure 10ohms,
R671 does eventually settle on 4.7kish (presumably having charged up all sorts of capacitors around the circuit),
I can't really work out how to better test between C628 and D620
They're in parallel and both settle on half an ohm in either direction,and diode test gives a quick drop to nothing both ways, nothing visually bad
could you give me an example of what else would you probe there?
Test for shorts or device junction integrity with power off... If nothing obvious, and not catching on fire you can start troubleshooting with power applied.

#1 first schematic... marked up voltages shows 16v drop across 220 ohm resistor. Ohms law instructs us that would draw 72ma x 16v is over 1 watt... Unless that is a power resistor sized to dissipate watts that will get hot. Either the capacitor or zener are suspected short circuits.

#2 second schematic ... less current through 4.7K (only 4 mA) but again the zener and capacitors are suspect short circuits.

Try unsoldering one lead of the cap(s) to see if voltage rises, measure resistance across cap/zener with VOM while power is off...

JR
 

mutetourettes

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Thanks John

on the TOA (#2 schematic - easier to work on as it's a separate psu board) Thanks to you i got this far and could then lift a single leg of the capacitor with some hope that was an intelligent thing to do rather than 'shotgun'... Amazingly the solder came off easily and the leg didn't stick too awkwardly in the board...

yup that capacitor is a dead short both ways. diode (in circuit) now gives diode type readings.

Reconnect the mains transformer (but no downstream load circuitry) and voila, we have 12v again... all circuit voltages where they should be (with no capacitor in place).

I'm hoping  that it wasn't a downstream fault that did this. In this design of circuit that capacitor looks fairly well 'protected' from downstream nastiness being the cause of its failure, would you agree? It's at least 32 years old.. I hope it's not too daft to just put in a new capacitor (when I can find one) and reconnect the rest of the machine?

I think I should probably order a full set of caps for this PSU, but I want it to be working first, so I can replace them in small sections, checking "do no harm" as I go..
 

JohnRoberts

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congrats those are good simple circuits to learn troubleshooting from.

Ohms law is your friend.

That capacitor where it was located is unlikely to be damaged from the load side or from a PS spike... sometimes parts just fail. 

JR
 

Whoops

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mutetourettes said:
Hello all, I've had two very different (but both late 80s early 90s tape) machines recently whose 12v psu lines have given up

With PSU's from the 80s and 90s it will be good practice to replace all the Electrolytic capacitors.
Panasonic rated at 105 degrees are pretty good and reliable
 

mutetourettes

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Whoops said:
With PSU's from the 80s and 90s it will be good practice to replace all the Electrolytic capacitors.
Panasonic rated at 105 degrees are pretty good and reliable

Thankyou, You headed off my Obvious next “which to get?” Questions!! Bravo 👍
 

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