cap checker repair

[pete g]

hi, yes, there's not much left. wish I could find a bad part. because of all the parts I changed, not one tested bad.

[Nusa]

You could try measuring contact resistance on various switches, if you haven't. If you get any measurable resistance on closed contacts, you have probably found a problem.

[Armadillo]

Won't it be easier to know the actual voltage/current readings?

1) Are you using the with milliampere/uampere technique or without?

2) Under Voltage Test  - Please provide all the 16 voltages as measured with the voltage meter at the Test Terminals.

with that at least we have a better foundation to start tracing.

[pete g]

I was scoping the line that feeds the grid of the bn8, ans saw a lot of noise with big spikes intermittently. trying to localize, I disconnected pin 6 and 10 of the B-D-L switch. need to find the source any ideas?

[pete g]

will do that and post back.

[pete g]

not using a milliameter. will check voltages.

[floobydust]

6BN8 grid is a high-impedance node in leakage test, it should be sensitive but quiet.
There is nothing really for noise filtering for the 6BN8 aside from R7 100R so I'm not sure how it does with modern RFI. Move your cellphone or Wi-Fi stuff.

I would suspect an open switch contact. You can measure ohms from 6BN8 grid through the rotary switches.
I use a small pencil eraser to clean oxide off the wiper and gently drag paper through the wafer contacts to clean them, or use contact cleaner spray.

[pete g]

I hung a 20ufd cap on the line that feds the grid of the bn8 and that seems to have solved the problem as far as setting the calibration of the min lytic position. the mica,etc. cal is still not working. sometimes I can get the eye to close and then it opens again?

[floobydust]

Putting a capacitor at the 6BN8 grid will filter hum and noise there for leakage-test mode.
But that part's leakage current alone will upset tests and it will kill the bridge mode for measuring capacitance because that uses AC. So I don't think it's a fix.
I would not scope the 6BN8 grid, but instead the plate if looking for hum or oscillating.
If it's a problem, the 6E5 you can pull and see if it's C4 or C11 are making it worse.

You are sure the new filter caps C1, C2 are working, at least 20uF 700V?

I would check the grounding, from the line cord to chassis and R41/R42 and the ground on the 200V side. It also goes to the bridge transformer. It's a few minutes for continuity tests.

The paper, mica leakage test, I still can't find the current-sense resistor. Thought it would be at pin 8 on the rotary switch.

Beyond this, need some pictures inside.

[pete g]

there is noise on the plate of bn8 sometimes reaching 1-2vpp. I did find a wiring error to the cal trimpots and corrected it. I would think the solution to this problem is being told by the voltage readings on the bn8. I think if they can be analyzed, the problem would be solved. what do you think? i'am able to calibrate the minllytic and mica,etc, but the adjustment won't hold the eye tube opens again or flutters.

[pete g]

hi guys, i'am going to shelve this project for now. wanted to take a minute, to thank those folks who contributed. much appreciated.

[pete g]

hi, the problem with this tester, turned out to be off the wall. after years of me dousing the phenolic switches with contact cleaner, they have become conductive and leak voltage erratically. never knew this could happen, I guess the moral of this drama is not to douse those switches.

[Nusa]

Glad you finally spotted something! How are you going to fix it? Slide switches are easy enough, but those wafer switches are likely unobtainium.

[pete g]

I think its curtains for this meter. but i'am going to use some of the parts to build another one.

[TERRA Operative]

Can you pull the leaky switches out, clean them thoroughly in isopropyl alcohol and bake them in an oven to completely dry them out?

Failing that, maybe modern replacements could be shoe-horned in?

[Armadillo]

The 16 readings would have exposed the leakage switches,  yes?

2) Under Voltage Test  - Please provide all the 16 voltages as measured with the voltage meter at the Test Terminals.

[pete g]

baking it might be worth a try. but, I wonder if it would get really brittle. I don't think alcohol is s good choice as a solvent.

[pete g]

the voltage readings were not affected, but the wafer did start to smoke at 600v.