开云体育

I've done some research, and I think I've found the cheapest parts source
(other than Stan Griffiths) for recapping the HV section. This applies to Types
531, 535, 531A, 535A, 541, 545, 541A, 545A, and any other scope that used paper
caps here during any part of its production.

Don't ask me why I did this, it just seems to be an itch I have to scratch.

The 535 et al HV uses eight HV paper caps for which replacements are hard to find cheap.

Six caps on the HV board:
1. Cathode input, grid input, feedback: 6.8nF 3kV,
2. Tripler first stage: 6.8nF 5kV,
3. Tripler second and third stages: 470pF 10kV.

Four caps in the HV aux compartment:
Cathode output, external Z-axis, grid mid, and grid output: 15nF 3kV.

Replace all the 6.8nF and 15nF with Russian military surplus 10nF 4kV film caps.
These keep showing up on eBay at about $15 for 32 caps or enough to do three scopes.
On the grid output, use two caps in parallel. (There is precedent for this -
Tek switched to all 10nF in late 535A production and did the same thing.)

Tek switched to ceramic, but for these positions don't use ceramic unless
you have genuine Tektronix NOS. The manufacturer's parts were carefully
evaluated for voltage coefficient of capacitance. Modern Z5U's in this
voltage range are trash (more than 50% capacitance loss at operating voltage),
and X7R's are over $5 each. I tested a bunch of parts and have the data to
back this up. The TDK 10nF's may be only $0.25 each, but you have to use
two to get 10nF in the scope. This is awkward on the HV board. The
tripler first stage is the worst; there simply isn't anything new for
less than $5. Those surplus film caps are truly the only good deal.

Although higher-voltage ceramics also lose 50%*, 1nF 10kV or 7.5kV units can be gotten
cheap and do a fine job. They're also quite small, making it easy to find room for
the film caps. Replace the 470pF 10kV caps with Murata DHR4E4A102K2BB (1nF 10kV),
available at Mouser for $0.75 each. The next-cheapest parts, DHRB34A102M2BB
and Vishay/Ceramite 564R75GAD10, run about $2 each and perform the same.

* At rated voltage. In the scope, they only see 3kV, more than 50% derating even
for the 7.5kV parts. Why did Tek use 10kV? Because even new, those paper caps
were marginal.

If you do three scopes, this works out to about $9 per scope. You can't beat that
unless someone gives you parts for free.

Dave Wise

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Mike
Sent: Sunday, November 07, 2010 4:30 PM
To: TekScopes@...
Subject: [TekScopes] Re: 535A HV issue


Great! Downhill from here.

Besides the HV filter & multiplier caps there's a lower-voltage BB across the HV transformer, C808 I believe, that causes havoc in a different way (part of the HV oscillator tank). This one needs a stable high-Q replacement, either poly tubular or mica.