Power supply question... Rackmount delay unit repair

[Mackin]

Edit: If there are no numbers, you could measure the voltage drop across the working zener in the +15V supply and replace the shorted one with a zener of that voltage. It ought to be close enough. Just be sure to observe polarity.

The voltage drop across the other one was 15V.

I was wondering about reverse current and power dissipation. I don't want to drop a huge one in there just because I don't know

For a small audio circuit like this one that has, say, 6 or 8 op-amps and a handful of transistors, would 1W be all right ? I'm just guessing, you can probably tell I'm pretty clueless at this point, haha !

Thanks again guys

[The Dude]

A 1W 15V zener should work just fine. The main current path is C-E of the transistor (most likely- without seeing a schematic). The zener just regulates/clamps the output. There shouldn't be that much current passing through it, that is unless voltage gets too high on the device...... for instance if the transistor shorts.

[Mackin]

A 1W 15V zener should work just fine.

Can't wait to order some from Mouser and get this thing working !

I'll post a video.

[The Dude]

Not sure where you live and/or what stores are available, but it might be cheaper to just pick one up locally than pay shipping for a single small part.

[mozwell]

the TO-220 on the right is marked I - O so that is most likely a 7805 regulator for the 5V supply, especially as it looks to not have an insulating pad behind it.
The tab on 7805 is 0V, so the heatsink would also be 0V

2SA794 is a 100V 0.5A PNP transistor hfe 100 or so
BD140, MJE350, would both seem to be good replacements
If the NTE replacement transistor has been "overly hot" during your testing, i would suggest replacing it when you replace the zener as it may have been stressed & may not be reliable, if you are ordering parts, get a few transistors & zeners

[Enzo]

You really need to determine where that diode is wired. if it is a 15v zener, and it likely is, then it should have 15v across it. If however it is just a protection diode across the 15v output, it will also have 15v across it. In other words just finding 15v across it does not tell us what it is.

1N4744 is the common 1w 15v zener. ANy electronics shop should have them. Even non-audio shops. Mouser sells them for around 25 cents each, or 10 for $1.60. But shipping socks it to you, so even if someone local charges $5, it is probably cheaper and faster. Someone walks into my shop, I'd charge them a dollar if I was not in a good mood. If they seemed interested, I'd just give them one.

[Mick Bailey]

I'm pretty sure the filter caps on the +15V & -15V section are good as they both measure the same value when using the capacitance function on my DMM.

It's worthwhile knowing that a DMM won't tell you if a cap is good or not. It just reads the value at a low voltage. Under operating conditions a cap can go open or short, or have excessive internal resistance (ESR). So a totally usueless cap can measure 'good' on a DMM.

The value of a cap is rarely an issue, particularly in a power supply. It will fail for some other reason rather than a value change.

[Mackin]

If the NTE replacement transistor has been "overly hot" during your testing, i would suggest replacing it when you replace the zener as it may have been stressed & may not be reliable

Yup, I already have a spare for that in case I mess up the other one.

[Mackin]

It just reads the value at a low voltage.

True, I hadn't considered that.

I was just messing around making resistance measurements ...hoping to find a short somewhere.

But then again, like you said, a short might not have shown up given the component was not under operating conditions - in this case - a larger voltage than what the DMM actually puts out. Correct ?

[Mick Bailey]

That's right. But also a cap is imperfect - it has an inherent series resistance (incidentally also inductance, but in this case it's unimportant). The resistance can rise to an extent that the cap shows a fault in circuit, but a DMM doesn't 'see' the resistance and only measures the capacitance.

I've had electrolytic coupling caps that won't pass an audio signal that still read their correct value on a DMM.

The same thing goes with the diode check on a DMM. It will tell you a diode or semiconductor junction is bad at a test voltage of 2v, but one that reads good can still go open at operating voltage.

[Mackin]

I've had electrolytic coupling caps that won't pass an audio signal that still read their correct value on a DMM.

Wicked !

Thanks for sharing

[Mackin]

All right !! It's alive

Thanks to all for your help !

Here's a short clip of me playing guitar through it.

https://youtu.be/RqNCRfQUoz0

[mozwell]

That's great, job well done
For the record, and to help others in the future who may have the same problem, what did you finally do to repair it.

[Mackin]

[...] what did you finally do to repair it.

I replaced a shorted 15V zener diode and the burnt transistor it was connected to in the power supply section.

That's pretty much what it boils down to :P

Thanks again to all who jumped in to help, I truly appreciate it !!