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:Parsing a problem so it can be described to someone else is a powerful
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.This supply is unregulated so the variation from nominal is expected.
A change from 1Vpp to 2Vpp indicates that the current draw is
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.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.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
4b. I measured the 5 resistors of 3M Ohms in the HV multiplier and they're all up by 10%, like 3.3M each.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!!!)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:
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...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