Tektronix 434 s/n below 500000 faulty ps.

[johnmcgivern]

The -69.7 is out of spec.
I took readings on storage board because P141 was easy to access all voltages.
Don't have high voltage probe or meter but may be able to borrow one from electrician I know.

[Craig]

What is the spec of the -75V? I can find no mention of the tolerance of that (or any) voltage rail other than +15V.

What leads you to think that a 7% error in the -75V is the source of your bright and uncontrollable intensity trace? And what makes you think that an error in the HT is the cause? If it is low in value, there is less acceleration voltage and the trace will be dim or unfocused.

[johnmcgivern]

Sorry Craig, I don't have the experience to analyse the circuits I was just going off Table 4.2 page 73, sheet (4-7)
Voltage tolerance and maximum ripple.

[Craig]

Well, every other voltage is slaved to the -15V line, which in an earlier mail you reported to be -14.66V (which is out of spec). If you adjust that to be -15V, all the other lines will come up much closer to the specified voltages. As a ratio the -69.7V will come up to about 71.5V, about 4.5% low. Which is now*slightly* out of tolerance.

And from what I can see it is only used in two locations on the storage board into high impedance points. Which will be supremely indifferent to the -75V being a tad out of spec.

Your problem is not with the power supply! It is a red herring. You need to dig elsewhere, almost certainly on the CRT circuit diagram 11, not the storage circuit diagram 12.

There might be a proviso to that statement about the supply though:

You are right to check the HT. -4.1kV comes onto that board from a voltage adder circuit *on the power supply* (Ha!). Which means that you need to be careful poking around, because getting your finger on several kV DC is likely to spoil your day. You will need a 1000x high impedance probe to test that. There some highly stressed capacitors in there which can break down with the passage of years under sustained high voltage - particularly C990 and C934, 10nF 5kV. These are high-K (Y5S), wide tolerance ceramic caps (+80%, -30%). Check with a magnifying glass for damage to those - areas of blackening, pieces of the encapsulation that have broken away, cracks etc.

And if you are testing around that circuit, use the high voltage engineer's trick - keep one hand in your pocket. A jolt from one hand to ground is less lethal than one that goes from one hand to the other via your heart.

Are we having fun yet?

[johnmcgivern]

Just had a look around eBay and Google for them high voltage caps.
Why is everything on Tek scopes so difficult to source.
Had to send to China for Q1080 waited nearly 3weeks.
Then you have the special Tek numbers with no data sheets or equivalent part number.
You have to be a designer to work out component values.
Anyway enough whinging I'll be back at it tomorrow. Ha ha.

[Craig]

Well, you don't know that there is anything the matter with those high voltage caps yet.

[johnmcgivern]

Qservice have them but don't have cr931 that may be shorted.

[johnmcgivern]

I tried to adjust -15v but it was maxed out.

[Craig]

OK - leave the power supply alone now, other than checking the HT voltage.

Did you do what I suggested and desolder one leg of CR931 and check it without the complication of any attached circuit?

[Craig]

And if it should be fried when you test it, eBay 262085963645 are NOS 1N486A from a UK seller.