Replacement caps for Tektronix 434 Power supply board.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

I used 2x 200meg in series then 100k fitted inside Dmm plastic probe. Soldered 200meg to probe and gnd and neutral to end of 100k. Then red wire to junction of 2nd 200meg and 100k. Ptfe around all solder joints then shrink wrap then put all inside plastic tube then all inside another plastic tube about 10" long.
Resistors rated for 10kv measuring -2000v.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Red wire what is aquadag my multimeter is 8.4meg.
Measured old 9v battery
Dmm = 6.78v
HV probe setup = 1.6mV
Ratio = 4237.5:1
Double checked same result.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Interesting way of figuring out the division ratio. I calculate 4000:1, so on this tube measurement, 4KV would read 1 volt. Be sure the ground is connected solidly to the chassis and to your meter (-) terminal.

Aquadag is a term used for the grey conductive coating near the front inside end of the tube. Essentially that coating is the "plate" element. TV sets put the +30KV on the aquadag w/r to ground. More coating was sprayed on the outside of the tube to produce a swell HV filter capacitor. This scope has the aquadag at ground, and the cathode at -4KV, so you don't have to go probing around in the usual place. There should be a HV test point near the tube socket. Naturally you will want to try out your probe AFTER replacing the faulty diodes.

p.s. I'd like to see your finished probe!

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Just a quick comment. If you're looking to build your own HV probe look for an old resistor from Beman. There nicely built, my Polaris 651 probe has an 800Mohm in it. Found this really quick using google.

http://www.electronicsurplus.com/bem...ic-1090meg-ohm

P.S. Great thread, it's reminding me of everything I've forgotten.....

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Srhofmann glad your going down memory lane as you get older you realise you forgot more than you know. I have to try to learn just some of what you forgot ha ha.
Longbow will post photo of finished result.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Tried HV probe today on my Hameg which has a fault doh. Anyway -1900v read -1892
-1877 read -1868 and -1962 read -1960 seems ok.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

On a non-storage tube, 2KV is typical. Now quit playing with that probe and fix your 434.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

I am still waiting for the components ESR said there was a delay with pay pal but my account shows payment sent last Saturday very annoying phoned today again told in post said the same on Wednesday when I phoned.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Hi Gents and Mr Longbow, parts arrived today fitted them tonight.
See photo's. Zoom in on photo's.
The trace does not cover the 10x graticules almost 1/2 a graticule short.
What is the bright green dot at the start of the trace never noticed it before.
The signal shown is with a 1x probe its the scope calibration signal 0.6v @ 1Khz.
See settings on scope.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

I see insufficient horizontal sweep in those pics. It looks like it should be 11 graticules wide and you are getting 10.
The bright spot at the beginning of the trace is normal per service manual 3.L,M.

Can you comment on the action of the horizontal position control <->
I notice it's a bit shifted. Does it move the trace further to each side?
It is probably the H-sweep a bit out of cal but it could be the H-oscillator, at this point.
Can you check the timebase, if it is accurate or compressed 10/11's.

The H-amplifier uses +250VDC and the +/-15VDC rails (pg. 199)
I don't know the history of your adjustments but I would make sure the rails are correct and then consider doing a CAL on the H-sweep.
Adjusting the +15V rail affects the entire scope calibration! It has to be bang on per A.1 and changing it "the oscilloscope will require complete calibration". I'd leave it unless you affected that...

If there is no problem seen, I'd try calibrate the H-sweep.
Center the H-pos control and adjust R703 until things are in the middle, per D.1.f
If it is low sweep amplitude, adjust R817 until you get 11 graticules width per D.3.f

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Red wire are you looking at the correct serial number BELOW 500000.
I don't see any voltages on p199 but I will check them from my PDF.
Many thanks.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Redwire is right it does look like horizontal sweep is compressed. Did you make sure the the var knob on the time base selector is locked? It looks like the signal is only 840-850 uSec. Should be a lot closer to 1,000 uS, 5 full divisions wide. The built in calibrators are usually pretty damn close 1Khz+-33. Might check it with a frequency counter if you've got one or use a known external source. Step down transformer with a diode/bridge to measure the ripple frequency.

If you've got a "find the trigger" button you might give it a couple of pushes, the one on my Tek scope occasionally acts up.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Redwire see photo's after alterations you recommended.
Longbow see photo diy HV probe (worked out ok)
Many thanks to everyone any other advice is welcome.
Special thanks to Longbow for staying the distance and helping me your a gem mate cheers.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Can someone check frequency from these photo's
10x probe see settings on scope.
0.6v 1khz calibration signal.

Many thanks.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

It looks close to 1 KHz. Take a look at something that you're pretty sure is pretty accurate. Got an old analog TV take a look at the colorburst crystal. It should be around 3.58 MHz for NTSC. Got an old router pop the top and look at the clock in it.

Don't forget 90+ percent of the time you're just looking for a signal with a scope. Is it there? Does it look like it should? Is it about the right amplitude and frequency?

Good job getting the scope working again.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Srhofmann thanks for replying I have a HP 3325B injected 1khz signal very accurate so scope should show 1khz but it looks wrong to me I thought there should be ten signals for each gradient or am I wrong.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Srhofmann thanks for replying I have a HP 3325B injected 1khz signal very accurate so scope should show 1khz but it looks wrong to me I thought there should be ten signals for each gradient or am I wrong.

Good catch, just proves I shouldn't do basic math before I've had an entire pot of coffee. Approximately 2 divisions for a 1Khz signal at .5 milliseconds per division. At 50uS or .05 milliseconds per division you shouldn't see a pulse train only a transition edge. It's like you're off by a factor of 10.

1. Does the output look the same if you use the internal calibrator?
2. The time/div knob can be pushed in or out and it controls a zoom feature. I don't remember exactly how this works you might check this feature out.

Here's a picture of the internal calibrator of my scope at .5 mSec per division

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Yes it's the same with the internal calibrator.
I think you are right about the factor of 10 but what would cause this is it possible for the Timebase dial to slip out of cal by a factor of 10 if yes how do you correct this???

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

If you've got the Timebase switch in the normal operating mode my guess is that something mechanical has failed in the rotary switch itself. Look in the last few pages of the schematic and you can see that the timebase switch actually has around 20 cams that open and close individual switches to change the timebase by switching different components in and out of the circuit. I've never opened one up and never had to troubleshoot one so I have no idea where to even begin.

Have you tried looking at different frequencies from the generator. Maybe it's just a failure in this particular mode. Just occurred to me, is it possible that the knob isn't aligned correctly and it's rotated from where it really should be? Or could you just rotate the knob so it's correct for what's on the screen?

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Same on other frequencies knob hasn't moved may try opening up.