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

Fabio Trevisan
 

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.


On the DM44 issues, I installed the missing R1142, R1143 and R1147 on the A&B Timing Switch board, and now it can measure time on all horizontal scales.


Oh... And there's a TEK trick there...


On the 464 service manual "A&B Timing Switch Diagram 7", it shows the 3 resistors above as 2K, 1K and 1K respectively... and I naively installed them exactly as such... just to find out that the measurements were wrong in scale. (but at least, this time it was measuring in all horizontal time-bases, and not just on the 2s multiples).
Went to the Parts list of the 464, and it stated: Check DM44 manual for Resistor Values...!!!
Then, went to DM44 manual and, next to the DM44's parts list, there's an additional section describing all the values of all components that need to be added, removed, or changed values, for each scope model that the DM44 fits in...
Guess what, the correct values are 500R, 250R and 250R..!!! And not 2k, 1k and 1k as it was drawn on 464's diagram.


DM 44 is still sometimes crazy (less now than it was at the beginning), but I assume it's some bad contact somewhere which I will eventually find out.


Now, regarding the HV supply... I did some progress in making various checks but still didn't find the root cause for the drop of HV after the scope heats-up...


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...
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.


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)...


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.


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).
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.


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

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