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I’m wondering what are the symptoms of a bad or failing HV transformers in these scopes.
I’m still awaiting for some more tunnel diodes to arrive (2.2mA and 4.7mA), but the trace is not very stable horizontally, and I can’t properly adjust the time base. The -1850V is very steady though.

This is a fairly recent acquisition, and I’ve never had one of these before.

PS: It has been entirely recapped., and all the voltages are good and strong.

Could someone tell me what are the signs to look for regarding the HV transformer to make sure it’s good or failing? I’d really like to save this scope and give it back it’s former glory.

Thanks in advance.

Hi Steven,

I do not have a 547 yet myself, but from what I have read is that one of the main signs was that the trace would dim out making you turn up the intensity until the trace gets really dim and blooms. This happens on 547's, and 545B's. I am sure that there are other scopes that used this potting compound as well. The transformer gets lossy when the potting absorbs moisture making the transformer heating up. The one way to see if the transformer is lossy is to check the grid voltage of the 12AU7 tube, (or is it 12AX7?) that's driving 6AU5 tube. The grid will go high trying to compensate for the lossy transformer. I am not an expert on the 547's and certainly not an expert myself. But if your high voltage is staying where it needs to be, in your case, -1850V, and stays at that voltage for at least half an hour, there is hope yet for that transformer.

I have several scopes myself, (503, 504, 515A, 533A, 535A, 2- 561A, 564 and 565) and I think most of them use the beeswax potting compound with the exception of the 503, 504, and the 565 as I can't see what potting they use. My 503 is starting to give me issues with having to crank the intensity up all the way and I still can barely see a trace. I don't have a high voltage probe so I really don't have a way to safely measure the HV.

Brenda

Search or "547 V800". Albert

On Wed, Apr 13, 2022 at 01:00 AM, Brenda wrote:

Hi Brenda,

Interleaved.

Hi Steven,

I do not have a 547 yet myself, but from what I have read is that one of the
main signs was that the trace would dim out making you turn up the intensity
until the trace gets really dim and blooms. This happens on 547's, and 545B's.

This is not the case with mine. Even after about 45min to one hour, the trace is still bright with intensity around 11 o’clock, and rather steady.
What I do notice, however, is that I cannot adjust the horizontal time base. Even with both x1 and x10 the pots all the way down, it’s still too wide.
I remember reading somewhere that horizontal stretching was a sign. But it might be caused by something else in my case. I don’t know.

I am sure that there are other scopes that used this potting compound as well.
The transformer gets lossy when the potting absorbs moisture making the
transformer heating up.

While researching this, I’ve read that as well.

The one way to see if the transformer is lossy is to

check the grid voltage of the 12AU7
tube, (or is it 12AX7?) that's driving 6AU5 tube.

12AU7

The grid will go high trying to compensate for the lossy
transformer.

Will do.

Brenda

Thank you Brenda

On Wed, Apr 13, 2022 at 10:17 AM, Albert Otten wrote:

Search or "547 V800". Albert

Hi Albert,

Will do.

Thank you

On Wed, Apr 13, 2022 at 10:29 AM, Stephen wrote:

What I do notice, however, is that I cannot adjust the horizontal time base.
Even with both x1 and x10 the pots all the way down, it’s still too wide.
I remember reading somewhere that horizontal stretching was a sign. But it
might be caused by something else in my case. I don’t know.

Hi Stephen,

Which pots do you mean? Sweep lenghts are determined by internal trimpots in the Miller run-up circuits, calibration steps 24 and 44.
Is time/div OK or also expanded?

Albert

On Wed, Apr 13, 2022 at 02:39 PM, Albert Otten wrote:

Which pots do you mean? Sweep lenghts are determined by internal trimpots in
the Miller run-up circuits, calibration steps 24 and 44.
Is time/div OK or also expanded?

Albert

I didn’t get to that point yet. I’m getting ahead of myself. I was just monkeying around with R544 and R566 to see if they had any effect.

Many things were bad in this scope.
I changed several broken or just bad tubes, some transistors in the Vertical amp, which I matched as best as I could. That seems to be ok. Had to change 4 bad tunnel diodes out of 5. Basically all of them but D85 which tested good. I used the method shown by Alan (w2aew) in one of his videos.

Now I still have to figure out why I don’t have any ALT trace neither in A/B ALT nor in B INTENS/A ALT. B INTENS is also just a vertical trace at the far left side of the graticule, and nothing else. The Delay-Time Multiplier has no effect whatsoever.

Then I’m gonna have to address the problem of Channel 1 not showing any trace, and the down arrow on the scope indicates that the trace is way down (or way up if I pull the invert button)

Thanks Albert

I see Stephen. It's good to set R544 and R566 first and do steps 24 and 44 after that. Steps 24 and 44 have no effect on sweep speed, only on sweep length.
Albert

Albert,

I think I fixed the x1 and x10 horizontal issue. I can now quite dial them in. The problem was that the contacts of the x2, x5, and x10 wafer-switch were very dirty. Even at zero, the trace was way too wide, and was at x2. Wafer cleaned. All is now good as far as that issue goes.

If anyone has had this problem before, and has any idea where I should start to investigate the reason why I have absolutely no ALT anywhere, I’d be grateful.

Thanks.