Tektronix 524AD oscilloscope

[ianbatty311]

Hi, folks,

Thanks to Martin (and others), I've finished my "adventure" with AVO valve testers - you can pick up the three Youtubes (intro, and 2 x questions) at our website:

https://hrsa.org.au/

and open up the Youtube Channel, then find my AVO talks.

For my next challenge...

I have a Tektronix 524AD that was used as the lineup monitor at Channel 9 here in Melbourne from the early days probably until the late 70s.

I have the main HT supplies working, but the EHT (two at –1500 V and one at +2500 V) are low: the –1500s are only about –500 V, and the +2500 is only about +1300.

The fault symptom is a low-brightness spot that will not focus and that I cannot remove astigmatism from.

The EHT supply is derived from a 6AQ5 free-running oscillator (the notes do not describe whether it's a sinewave circuit or a blocking oscillator circuit, but I can check the waveform and find out).

The 6AQ5 tests OK, and it has supply voltage. There is a 12AU7 that controls the oscillator's operation, but pulling that valve out has no effect.

There are three subminiature diode rectifiers (one each for the two minus supplies, one for the positive), so I desoldered the two minus supply diodes in case there was loading/leakage at their outputs. I got no measurable change.

Has anyone worked on these 'scopes, and:

1. Have I missed anything that I should have tested before going to the EHT supplies?

2. What should I be looking for in the EHT supply circuit itself?

Among other places, you can get the manual at:
https://www.manualslib.com/products/...s-9014188.html

...the EHT circuit is towards the back of the handbook (3rd last page), but I've attached it for convenience.

Thanks for any and all help.

Cheers,
Ian.

[Alphonse01]

Hi,

looking at the circuit diag. If you pulled the 12AU7 then the 6AQ5 should be Oscillating with a capital O and all voltages should be over spec.

Check R801 for going high >82k. Check C801 for being leaky.

What voltage have you got on pin 6 of the 6AQ5? The 82k resistor should be pulling it up toward the 450V.

Cheers

Alphonse01

[factory]

If this hasn't been used for decades & being a 1950s design, it probably has numerous paper capacitors, I note the two next to the 82k resistor have been replaced in one of the pictures from Tek wiki here; https://w140.com/tekwiki/wiki/524#/m...tom_front2.jpg

You should be able to test the capacitor for leakage, or try a replacement for these, also check the voltages as mentioned in the post above.
Don't forgot to use solder with a small amount of silver content, to avoid damage to the plating of the ceramic tag strips.

David

[6AL5W-Martin]

a very old scope, amazing.
looking from this situation:

1.) all PSU Voltages are correct
2.) Tubes 6AQ5 and 12AU7 are checked OK
3.) The Isolation from the filament of the CRT to ground is checked at 5kV to be OK,

swap the .001 1kV C what is parallel to the primary coil of the HV transformer and test please again. This peace is known a typical problem: Identifacte by IR picture: it goes warm = replace. (not with the finger please, HV )
Tek use there always a foil C, never a ceramic.

greetings
Martin

[ianbatty311]

Hello, Martin

Thank you for your helpful reply.

I tried (i) testing the 6AQ5, and it is OK, (ii) removing the 12AU7, and this made no difference, (iii) testing the 12AU7 anyway, and it is OK.

I will try your suggestions and see if I get any better output.

Thank you,

Ian.

[ianbatty311]

Hello, David,

Thank you for the suggestion - I'll take it, and those from Martin, on board.

Time is getting a bit short now (Auction will be on Sunday), but I will have a quick look. It would be good to sell it as "works pretty well", so I'll give it a go.

Cheers,

Ian.

[Diabolical Artificer]

Try giving the "EHT adjust" pot, R807? a wiggle. With these EHT supplies the insulation in the EHT tfmr can go... I usually change the glass diodes for SS ones, a few 1200v rated ones in series.

Although big & heavy and power hungry these scopes are well worth keeping if you work on valved gear.

Andy.

[IanG3XYV]

https://www.vintage-radio.net/forum/...d.php?t=171117

Ian

You may care to take a look at this long thread that Andy and I took part in a while ago.

It’s really about the eht supplies for TEK 500 series scopes which need many kVs but your pdf has much in common. Lots of suggestions for you there but I fear it may be your transformer breaking down. TEK went from encapsulation in beeswax to epoxy and the epoxy in time lacks sufficient protection to keep out moisture and thus breakdown. I note your 524 does not need such high eht but…

Watched your YouTube videos on the AVO valve testers and found your explanation of the back off principle the best I’ve seen. Thanks for that and good luck with the 524.

Ian

[IanG3XYV]

https://www.vintage-radio.net/forum/...d.php?t=171117

Ian

You may care to take a look at this long thread that Andy and I took part in a while ago.

It’s really about the eht supplies for TEK 500 series scopes which need many kVs but your pdf has much in common. Lots of suggestions for you there but I fear it may be your transformer breaking down. TEK went from encapsulation in beeswax to epoxy and the epoxy in time lacks sufficient protection to keep out moisture and thus breakdown. I note your 524 does not need such high eht but…

Watched your YouTube videos on the AVO valve testers and found your explanation of the back off principle the best I’ve seen. Thanks for that and good luck with the 524.

Ian

[John_BS]

I assume that you can't adjust the -1500V rail using R807?
You could try disconnecting R814 from the anode of V805 and seeing if the -1500V comes good. If so, there's obvioulsy an issue with the diode or (more likely) C808.
(More tricky to perform the same trick with the other supplies!)
One possiblity would be to first eliminate C808 as being faulty, and then swap it for C810 (keeping R814 disconnected) and then see if the -1500 returns.
Problem is perhaps more readily tackled if you can get hold of some 3kV capacitors to temporarily sub for C807, C702 and 703 - values don't have to be precise.
John