Looking for a good ESR meter

[Marcel Birgelen]

Don't know if this is the right forum for the topic, but I'm trying to revive a Dolby CP750. This thing isn't really used for production anymore, but instead of letting it catch dust on the shelf, it would be nice to do something with it. But in the currently broken form, the thing is just a heavy paperweight.

The PSU seems fine, but it refuses to properly boot. My first suspect is a bad cap on the mainboard, so I did an in-circuit capacitance check with my trusty Fluke multimeter, but I couldn't really find a bad cap this way.

Since measuring capacitance in-circuit is kind-of flaky and capacitance alone isn't everything, I want to test the Equivalent Series Resistance or ESR of each of them. I've had an old-school ESR meter for years, but I loaned it out and it never returned... So, I'm looking for a new one and I was wondering if someone around here has any advice on which one to get.

[Marcel Birgelen]

For those who are interested in how the story continued.

I ended up buying this ESR meter. I measured all of the caps on the motherboard which took a few hours. Since I did this at home at night and the ESR meter makes an intriguing beeping sound, I woke up the wife, who thought someone left the refrigerator open...

Although none of the caps had an ESR that was entirely out of spec, I fired up my soldering skills and ended up replacing those 6 caps with the worst ESR. I ordered a bunch of fresh capacitors as close as possible to the original ones, as I didn't really trust those lying around here.

I somehow didn't really expect the thing to actually work, but to my surprise, the unit booted up again... Although I'm not able to pinpoint the problem to a single component, I'm pretty proud at my achievement anyway.

[Steve Guttag]

Some components don't show their failure until they are powered up and under load. I've had a few diodes do that to me too. Check them out of circuit...find...in circuit...once they heat up..eh...maybe more semi than conductor. Transistors are notorious for changing characteristics when heated up...probably not as much today as in yesteryear when most were discrete components. All good benches would keep a can of freeze mist around.

There is definitely a satisfaction in taking something from non-working to working. Congrats!

[Sean McKinnon]

Awesome work! Welcome to the official "Dolby Power Supply Repair Club!"

[Marcel Birgelen]

Thanks for letting me into this excclusive club.

The first potential use may be a pop-up Drive-In cinema, once the organizers get it worked out with the local authorities.

Some components don't show their failure until they are powered up and under load. I've had a few diodes do that to me too. Check them out of circuit...find...in circuit...once they heat up..eh...maybe more semi than conductor. Transistors are notorious for changing characteristics when heated up...probably not as much today as in yesteryear when most were discrete components. All good benches would keep a can of freeze mist around.

Yeah, load could definitely have been a factor. Although the thing refused to boot entirely, so you could assume most components weren't really warmed up already. I suspect that at least 5 of the 6 capacitors I replaced were just decoupling capacitors though.

[Marco Giustini]

Thanks Marcel for sharing. I've seen that ESR meter on several repair videos on youtube and it seems quite good.

Indeed the feeling of fixing something dead is very refreshing! Particularly if you spent hours on it!