Re: Hello from newcomer Fabio Trevisan - My first Tek Scope 464 + DM44

 

On 16 Sep 2016 17:07:00 -0700, you wrote:

Hi David, Just to give a feedback:

It's a long message and OF COURSE don't feel obliged to either to read it or answer... telling it to someone is my way to organize the ideas and - maybe - the outcome can be disclosed later and help someone.
Parsing a problem so it can be described to someone else is a powerful
technique.

1. When the HV drops, cathode voltage drops to around 1000V, and then, in 20~30 seconds, it drops to 920V. when I shut it off.

2. Checked the Unregulated +15 V and it reads about 24V (just a tad higher than what shows in the schematics). The ripple is about 1Vpp while in normal operation and, after the HV drops, the ripple increased to almost 2Vpp until when I shut it off...
This supply is unregulated so the variation from nominal is expected.
A change from 1Vpp to 2Vpp indicates that the current draw is
doubling.

2.b. Still at the Unreg +15V, as the manual doesn't state how much is the acceptable ripple, I did the following... I added a brand new 6800uF cap in parallel (to the original which is nominal 5500uF +100% -10%)... and the ripple under normal condition dropped from 1Vpp to 0.5Vpp... I didn't care to look after the HV drops because it doesn't add anything to the conclusion.
So, IMHO, the original cap is performing roughly the same as the brand new 6800uF cap... In other words... C1487 is GOOD.
Yep, doubling the capacitance will halve the ripple. The rule of
thumb for the 120 Hz output from a full wave 60 Hz rectifier is that
8200uF will yield 1 Vpp of ripple at 1 amp.

3. I don't have a temperature probe, but using my finger (with the scope off, and after discharging!), and while the HV is still normal... both the driving transistor and the HV transformer are hot, but not too hot... I can keep my finger on them, so they're about or below 70 Deg Celsius.
But when HV drops and I leave it on for about 20~30 secs which is as far as I went, both the transistor and the HV transformer becomes VERY Hot, to the point that I cannot keep my finger on one of the pins from the HV transformer (I can keep the finger on its plastic body though)

4. I don't have a reference stated on the manual, but the Post Acceleration anode is - while in normal operation - at 7.22kV. I didn't measure it yet after HV is down. I suppose that it will be down by the same amount (33% below the 7.22kV)...
My notes say it is 8.5 kilovolts. That is enough difference that I
would suspect failure of the high voltage multiplier is causing the
problem.

4b. I measured the 5 resistors of 3M Ohms in the HV multiplier and they're all up by 10%, like 3.3M each.
It's not ideal, but I don't think it's the cause for the cathode HV drop, which is the main issue.
The resistors could not cause the problem anyway.

4c. I managed to find some ESJA53-12 diodes, which are 12KV, 5mA fast recovery diodes (actually better than the originals, at 100ns, while the originals are 250ns, only the forward voltage is worse, at 37V while the originals are about 25V and this doesn't seem to be a problem here).
The PDA voltage will decrease a little bit but the effect on
deflection calibration will be minor.

I already tested them reverse biased at the first stage of the HV multiplier (3.61kV) and they're quite promising at 1.1nA (an indirect measure... I actually measured how much voltage it allowed to leak to a 10Mohm voltmeter, and it read 11mV!!!)
I will try to replace - first only CR1503 at the cathode - and see if it helps.
Specially this one, which is mounted right under the transformer and soldered very closely to its pins, if it's leaking too much, as it gets heated by the transformer, it may be causing the whole mess.
Leakage through either of the diodes will cause problems. The
capacitors could also be causing the problem.

I have not ordered from them yet, but these guys look like a good
source for high voltage diodes and capacitors:

<http://hvstuff.com/>

If replacing the diode doesn't help, however, I'm afraid I will need to start looking for an HV transformer, as I`m not seeing much more that can be ruled out...
The HV caps seem to be in rather good condition, and they're HUGE (I imagine they're very robust)...

Rgrds,

Fabio
I would change the high voltage capacitors before messing with the
transformer. Make sure the board around the high voltage multiplier
is clean when you finish working on it.

The oscilloscope and CRT will still work with the PDA missing although
deflection will be lower and brightness and sharpness will be
affected. You could temporarily remove C1582 to verify if the high
voltage multiplier is the problem.

Then I would take a close look at the cathode supply rectifiers and
capacitors around CR1503. And then the +600 volt bias supply built
around CR1512.

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