Tektronix 434 s/n below 500000 faulty ps.

[johnmcgivern]

Today I replaced the following components.

VR935, VR937, CR935, C926, R926.
Replaced P126 3pin plug.
Turned on scope no panel lights on front no trace.
Turned off scope removed P126 3pin plug.
Turned scope back on, panel lights came on front of scope.
Primary voltages where:
-15v spot on.
+15v was good.
+115v was +116.7v.
+250v was 255.4v.
-75v was -73v initially then dropped millivolt by millivolt down to -70v I stopped recording then.
C926 tested good out of circuit but replaced with new cap.
I turned off scope quickly when the front panel lights never came on as I didn't want to smoke R926 again.
What else could cause R926 to smoke.

[Craig]

It can only be overheating if it is passing too much current. Where do you think the current is going?

Hint: The only components R926 is connected to are CR924, CR925, C924, C925 and C926.

So measure the voltage between the junction of R926 and C926 and tell us what it is.

[johnmcgivern]

Plugged P126 back in turned on scope R926 started smoking.
Voltage at node R926/C926 was 1.839VDC
Turned scope off.
Tested Cr924/Cr925 in circuit test good.

[Craig]

113V across 100 ohms is not a recipe for a happy resistor.

Of course the 115V line will not be able to supply the current needed to sustain that voltage - it will be pulled down, and probably most of the other rails too, which explains why the scope is not happy.

Keep testing - there must be a hard short to ground somewhere.

[johnmcgivern]

Craig please excuse my inexperience I am truly sorry mate.
3pin plug P126 can you tell me what voltages to expect at each pin.
Pins 1 and 2 have continuity this is the plug pins not the z axis board pins.
Very easy to get mixed up.
To be honest I am struggling to follow the voltage rails due to secondary board having x amount of wires but connections on other boards just show for instance 115v in several places on schematic with no detail as to which pin it is connected to on the other boards. Hope it makes sense .
Sorry for waffling. All good learning experience starting to feel as though I am serving my apprentiship on the Tek434. Ha ha!

[Craig]

All the voltage connections are on there. You just need to know that a schematic is made up of bits from several boards. For instance P/O A2 (Part of A2). You just need to find the schematic with the particular part of A2 that has the power connections on it. Similarly with the other boards.

I'm currently fighting two plugins of mine. A 7L12 spectrum analyzer that has a 20dB suckout at about 1GHz, and a 7S14 sampler that is fighting - there is some evidence that someone was in there before me; never a good sign.

[johnmcgivern]

No problem Craig that is fluent Japanese to me (you know what I mean) best of luck I'll carry on may get lucky.
Kind regards, John.

[johnmcgivern]

Craig there is an ongoing debate on Tekscopes forum re: 7S14 sampler may be helpful to you. Above my station. Best of luck.

[Craig]

Hi John

Thanks - been following that, and made a post this morning about the (ongoing) repair. Basically replaced the totally dead and leaking mercury batteries - with silver oxide coin cells, and checked the sampling diodes (diode in one direction and 7 megohms in reverse - so perfectly OK), but neither trace responds to the offset control, and one is pegged off the bottom of the screen.

Onwards...

[Craig]

OK - got Channel 2 of the 7S14 resurrected. Dirty switches - Deoxit and working the switches got that channel working. Ch1 seems to be a problem with the 2-diode sampling bridge - mismatch of reverse leakage. Will replace with an HSMS-8202 and pigtailed leads.