Test Equipment Anonymous

Damn - that was a sketchy repair! Nothing like leaving the bad cap in circuit and installing an underrated replacement. Yeek!

-Pat

More PSU re-cap.

Replaced C644 in the +300V circuit and C642 in the +100V circuit. Both were 2 x 125uf/350V in parallel. I wanted to use a single 270uf/350V for each but they are hard to find and expensive. So used 2 x 150uf/350V in parallel for each. Was able to fit them nicely under the chassis and left the original cans in place. Disconnected of course.

More re-cap activity.

C666A and C666B. Original 20uf/450V each. Replaced with 22uf/450V. C627. Original 2 x 20uf/450V in parallel. I should have replaced them with a single 47uf but I screwed up on my order. So 2 x 22uf/450V in parallel. I was able to mount all the capacitors under the chassis as shown and leave the original cans in place. This completes the re-cap of the PSU itself but there are 4 more capacitors to change out in decoupled (isolated) circuits.





Initial check of the PSU voltages after setting the +150V reference. +100V reads +99.1V. +300V reads +302.8V.

Next is replacement of C801A and C801B. Both were 20uf/450V. Replaced with 22uf/450V. These capacitors are in a decoupling circuit for the CRT and the HV Oscillator. The can was installed outside the HV enclosure suspended upside down. There was not enough room in the HV enclosure to install the replacement caps so the can was removed and the replacements installed on a terminal strip under the HV enclosure.






That large brown disc cap is not original and must be the HV cap that the owner said he had to replace. It looks kosher so it will remain as is. The HV will be checked and adjusted as needed once I start the calibration. There is one more dual can capacitor to replace and will most likely have to be pulled in order to install the replacements.



I am considering removing the 5642 HV rectifiers and installing HV silicon rectifiers but if the HV falls into spec I will leave them alone.

One consistent issue I noticed is that there was no ability to achieve a DC Balance on Channel A. Channel B was fine. I would have bet the farm that the issue was an out of tolerance resistor in the DC Balance circuit. Nope. One of the GE branded 6DJ8's, V423A/B, is weak. Replaced it with a National branded 6DJ8 and got good DC Balance.

Bet the farm? Never knew you had a farm to bet with.

The last capacitor to change out is C488A and B. Original were each 40uf/450V. Replaced with 47uf/450V. The can had to be removed and the replacements installed on a terminal strip. These capacitors are in the Delay Line circuit and I'm not sure what their function is. The circuit description in the manual doesn't give any info. They are essentially across the +300V and +100V. Additional filtering perhaps?





After a half hour warm up checked the HV. -1676V vs -1675V. No adjustment required. So now that the re-cap is done all the switches/controls will get a deoxit and then calibration.

Started calibration. Vertical section first.

The vertical compensation for both channels was spot on. No adjustments required. Channel A needed a slight gain adjustment due to changing out V423A/B. The manual goes into great detail on adjustments of the delay line and specific warnings that if not done properly will result in gross high frequency issues. Displayed is a 1MHz square and for a 15MHz bypass scope looks damn good. So I am leaving the delay line alone. And the scope triggers correctly too which is an indication that the delay line is OK.



This is the delay line. Each one of those circular holes is a trimmer cap adjustment.



Next will be the trigger circuits and sweep. I know the sweep is slightly off and will need an adjustment.

Continuing with the calibration.

The Trigger circuits required no adjustment. The sweep timing was definitely off. I was able to do a credible job without the aid of a Marker Generator. I'd like to have one but Ebay prices for a functional unit are stupid and basket cases not worth the effort. 100kHz displayed and aligned.



This completes this project. Put the scope on burn-in for at least 6 hours.

MED6753 wrote: ↑Tue Nov 21, 2023 2:59 pm The sweep timing was definitely off. I was able to do a credible job without the aid of a Marker Generator. I'd like to have one but Ebay prices for a functional unit are stupid and basket cases not worth the effort. 100kHz displayed and aligned.

I too yearned for one in the early days and luckily a mate had a Phillips one that I borrowed on occasion until I later got an AWG with Pulse capabilities. < that's all you need these days.

tautech wrote: ↑Tue Nov 21, 2023 7:48 pm I too yearned for one in the early days and luckily a mate had a Phillips one that I borrowed on occasion until I later got an AWG with Pulse capabilities. < that's all you need these days.

I've got a Waugh scope calibrator. It does a few neat things such as producing a sub ns square wave and square wave with an amplitude of 100V.

To set up the timebase of a scope accurately I use an AWG. Even a cheap one with a plain crystal (not a TCXO or OCXO) will be good within 50ppm, which far exceeds the claimed accuracy of a scope timebase. Usually there are presets for the timebase on one of the low settings and one of the high settings and you rely on the precision and stability of the components for the other ranges to be right. I've found if I follow the maker's instructions for adjusting the timebase, sometimes some of the other ranges can be silightly out, but usually within spec, which is something like < 3%.