- 2025
- Oct
- 13
A Viz WP-705 Power Supply part 2: Musings on faults
When I opened this Viz WP-705, it seemed to be a fairly clear cut case of the A/D failing. But, after some more tracing circuits, there are a couple of other things here.
Each side of this device’s displays are driven by their own regulator, so each side has a LM309H LDO 5V device in a TO-5 package. So, that could also be a problem. I decided to wait until I received the manual I ordered for it.
The manual arrived eventually after being scanned delivered on one day and showing up the next. Unfortunately, it wasn’t as useful as I’d hoped, and I’m not sure why it is like it is. The manual copy is dated 1978, and my power supply is dated 1980. Since RCA was obviously no longer slapping their name on the units, you’d think that, eventually, all of the RCA parts would be removed in favor of other commercial equivalents. The manual does suggest this, as it’s using different A/D / Driver pairs than what my unit has.
My unit still has RCA parts. I would think a later model would have the non-RCA parts, but I guess not. Perhaps they made these with multiple variants depending on what chips were available? Regardless, other than the power supply portion itself which seems to follow the manual, the actual portion I’m interested in isn’t laid out in the manual I have. Oh well.
Anyway…
I decided to check the 5V lines for both sides, since I now know there are two individual rails. The working side is ~5V. The non working side is down near 2.6VDC, and the input is about 4.6VDC. That’s certainly wrong.
Time to pull it out.
There’s some crusty flux under the device, I’ll clean that off with some alcohol. Now the input reads about 12VDC, which is probably fine as it’s input is unregulated. I decided to pull the small capacitor beside the regulator, it wasn’t shorted but it had an ESR of 28Ω so it’s dry. I’m sure the other one is too. I have a couple of 4.7μF parts laying around, I’ll replace both.
Something is drawing the voltage down, but is it the regulator? I’m not sure - I didn’t feel anything getting hot during testing.
I’m going to go ahead and check all those transistors for shorts, and try to do some comparative measurements with the other side. I also have some LM309H parts laying around somewhere, it’s just a matter of figuring out which bin I put them in. Stay tuned for part 3 and hopfully a full diagnosis.
Next part of this series: Coming soon.
Previous part of this series: https://wereboar.com … ply-part-1-analysis/