HP 410C VTVM specialists around?

[inse]

I bought a HP 410C from the sixties in fairly good condition.
DC V and A and Ohms are very good but I am not able to calibrate the AC V ranges.
The supply voltages are all inside their specified range.
The first issue is that I am hardly able to do the AC zero alignment, the coarse adjustment pot A3R31is almost  at the cw limit, I had to bypass the A1R7 resistor to have any headroom for adjustment.
The 0.5V, 1.5V, 5V and 15+V trimmers are at their ccw limit to just or not quite get a correct reading.
Is anyone here familiar with this specific piece of equipment?
First I suspected the AC probe to have somehow deteriorated, but even when feeding DC voltage into the AC socket, I am not able to do an alignment.
Furthermore I noticed that the Orange Drop capacitor A3C1 is quite sensitive to temperature changes - do those fail this way?
Any help would be appreciated.

[factory]

These usually have a Polystyrene capacitor for A3C1, is the orange drop a replacement?
Polystyrene capacitors are usually quite reliable, but are easy to damage with too much heat when soldering, some cleaning chemicals will also destroy (dissolve) them. They are usually chosen for their temperature stability, i.e. they have very little change in value with temperature.

David

[inse]

It’s a Sprague 715p 100nF polypropylene capacitor,  polystyrene are not common in this capacity range, afaik

[RichardM]

I repaired one of these recently. There is a recent thread if you do a search.

My A3C1 was shorted and that was the problem. I just replaced it with a poly cap. The only remaining issue I have is in DCV mode below the 0.5V range I read high.

[inse]

Thanks, Richard - of course I have been following your thread, you had no reading at all iirc.
The funny thing in my case is that everything works fine except AC alignment.
I will give A3C1 a close inspection, though I wonder how it could fail.
It’s an epoxy coated polypropylene capacitor, rated 600V for whatever reason (if you look at the schematic).

[inse]

Just checked the capacitor, isolation resistance is beyond measurement, capacitance excellent…

[RichardM]

Hi anse

Yes, I have no idea what my A3C1 would fail. It was one of the last things I checked after a number of weeks I am happy to take some readings from my unit if it helps ? It's the least I can do after the support I have received from this forum from other members.

Regards

Richard

[inse]

Thanks, Richard, I might come back to your offer if necessary.