As you might remember from the Test Equipment-Subforum, I bought a PM3320A about 2 weeks ago.
When I turned it on, it worked and showed no sign of the defects the seller mentioned.
I thought: Well, maybe nothing wrong with it afterall/transport-knocks have solved the problem.
As we say in Germany: "Pustekuchen!"
Today I finished the first part of a self-made "Almost All-in-One Bench-Tester" (Will contain Signal-Gen, LCR-Meter, Multimeter, 10W Amp to simulate Generators, etc.) that grew out of the desire to build something to sort of calibrate my PM3350, and wanted to test it with the PM3320A.
On a hunch I checked the Battery Compartment and yep: The batteries (Ni-Cd Accumulators) were done for! 300mV on the first one, 600 on the second one. And the contacts were corroded. Some time in the past, batteries must have leaked in there.
After cleaning the contacts and replacing the batteries, I turned the scope on and thats when the trouble started: I had previously observed some pretty rough calibration-waveforms when I first turned the scope on, but now that the RAM was wiped, the scope didn't start with the stored parameters, but instead started a "Auto Set"-Routine set everything up. And it hangs up during that routine.
So I opened the case and started measurements with the 5V-Rail.
The result was kinda expected: Highly contaminated with random noise: In analog-mode the trace is just a 2 divisions wide blurred green band. Digital Mode shows it jumping all over the place.
(Yes, the new LCD for the PM3350 arrived. Thx again to JohnnyBerg. I just didn't have the time to install it
)
The upper trace is Channel A, which measures 5V on the Display Memory-Board, the lower one is Channel B, which measures 5V on the Processor-Board.
Next I measured the Calibration-Output: The 1Vpp 2Khz Square-Wave was horribly garbled and also modulating around something else. I suspect Mains-Frequency, but didn't measure it. Maybe I do tomorrow.
You can see how both look like in this video:
https://youtu.be/oz7jPVhGgyUAt the time I have three culprits:
- Reference-Voltage (5V-Rail is regulated by a LM358 that uses the Reference to compare the output-voltage):
- 5V Regulator (Something might be wrong here, hopefully not the LM358, because its not socketed and I hate desoldering DIP-Packages^^):
- Capacitors inside the Power Supply:
As you can see, one of the Caps has what appears to be a scorch-mark. I don't have a IR-Thermometer and I won't go anywhere near that PSU when the scope is powered up with my fingers or a thermocouple, but I guess that cap deserves a treatment at the recycling plant.
I think, I'm going to replace all the Electrolytic capacitors with "Old New Stock" my dad got from Philips when he worked there 20 years ago. They are the same style and should fit without problems. He even has 2 of these big fat "Mother-of-Caps" beasts with several hundred µF and rated for over 200V
Do you guys have any more ideas where to look for sources for that kind of problem?