Recapping the 535 HV (was Re: 535A HV issue)


Dave Wise
 

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@yahoogroups.com [mailto:TekScopes@yahoogroups.com] On Behalf Of Mike
Sent: Sunday, November 07, 2010 4:30 PM
To: TekScopes@yahoogroups.com
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.


samd65
 

Thanks for the information. I actually already replaced all of the caps in the trippler circuit with ceramics. It seems to be working fine right now. It's still not 100% (2nd time base doesn't work due to lack of tubes.), but right now I'm working on my 556 and Hickok tube tester.
Still, that's a lot of useful info.

--- In TekScopes@yahoogroups.com, David Wise <david_wise@...> wrote:

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@yahoogroups.com [mailto:TekScopes@yahoogroups.com] On Behalf Of Mike
Sent: Sunday, November 07, 2010 4:30 PM
To: TekScopes@yahoogroups.com
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.


Dave Wise
 

UPDATE 20210324: Murata DHR series is EOL, Vishay 615R150GATD10 is ~ $5, and the Russian surplus has dried up.

SUGGESTED PART NUMBERS
Nominal specs from 545
In stock at Mouser

There is about 22mm vertical space between the baseplate and the tubes.
Box film caps fit just fine provided the three lower-voltage
parts are skinny so you can stack them.

Grid/Feedback/Cathode: 6.8nF 3kV, use 10nF 3kV box film, Wima MKP1W021005B00JSSD $0.89 6x15x27mm
If you allow 2kV, you can use Kemet F463DB103K2K0Z, R76UN2100SE30J or R76UI182050H4J,
or Epcos/TDK B32672L8682 or B32672L8822, or Nichicon PPB2201680KGL.

Tripler 1: 6.8nF 5kV, use 10nF 6kV box film, Wima FKP1Y021006F00KYSD $1.83 15x26x32mm
Or 10nF 6kV Z5U ceramic, Vishay 564R60GAS10, $5.86 ouch

Tripler 2/3: 470pF 10kV, use 1nF 15kV Z5U ceramic, Vishay/Roederstein HVCC153Y6P102MEAX $2.44 8x15mm


PLACEMENT/ASSEMBLY
Between the two rows of ceramic strips except as noted.

On the left, place Grid flat with leads facing left.
On top of Grid, place Feedback with leads facing right.
Place the feedback resistor string to the right of Feedback.
On top of Feedback, place Cathode flat with leads facing right.
At the lower right, place T1 flat with leads facing left.
At the upper right, hang T3 vertically against the strips.
Below the lower right strips, hang T2 at an angle, head down/leads up,
avoiding the ground lug and straddling the filament wires.

Dave Wise
Not a Component Engineer but I like playing one


Dave Wise
 

ADDENDUM

Grid/Feedback/Cathode caps that are short enough to lie on their
backs can be installed that way. The baseplate is 70mm wide.
T1 is 26mm + 3mm for wire bends. The feedback resistor
string is 5mm wide. That leaves 12mm per cap, which opens
up more part numbers, especially if you include 2kV.
We're still single-sourced on the tripler.

Grid/Feedback/Cathode:
3kV Wima MKP1W021005B00JSSD $.89 6x15x27mm
Panasonic ECW-HC3F822
ECW-HC3F103 $1 8x16x27mm
2.5kV Kemet F464BP103J2K5A $1 9x15x18mm

2kV Wima MKP1U021004D00KSSD $.70 7x14x18mm
MKS4U021004D00KSSD $.68 7x14x18mm
FKP1U021006B00MSC9 $1.45 11x21x32mm
Epcos/TDK B32672L8682J $.67 6x11x18mm
B32672L8822K000 $.70 6x12x18mm
B32613A2103J008 $.84 11x17x27mm
B32652A2682K000 $.94 11x19x18mm
B32653A2682K000 $1.30 9x17x27mm
Kemet R76UI210050H7J $.76 12x13x18mm
R76UI182050H4J $.67 6x12x18mm
R76UN2100SE30J $1 6x15x27
PHE450SB5100JR06 $1 7x13x18mm
F463DB103K2K0Z $1 6x15x26mm
Nichicon PPB2201680KGL $1.30 6x15x27mm

On the left, place Grid and Feedback on their backs.
Place the feedback resistor string to the right of Feedback.
To the right of that, place Cathode on its back.
Near the upper right, place T1 flat with leads facing left.
Above the upper right strips, hang T3 vertically.
Below the lower right strips, hang T2 at an angle, head down/leads up,
avoiding the ground lug and straddling the filament wires.