Replacement caps for Tektronix 434 Power supply board.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Yes Q995 ok so is Cr924/5, Q924/5 and C926.

[Agent24]

Re: Replacement caps for Tektronix 434 Power supply board.

Is the new Q995 still OK?

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Q995 arrived fitted it and put all plugs back in their positions.
Turned on scope after a few seconds R926 started smoking again.
When I turned off the scope there was a flash of light on the phosphor.
Removed P126 no smoke.
I say again what is causing R926 to smoke???
I don't know what else to do.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Redwire I tested P128 plug
Pins 1/2 = 85kohm
Pins 1/3 = O/L
Pins 2/3 = O/L
Longbow thinks there is a short.
I am waiting for transistors to be delivered and thought I would double check CRT connections but when I removed cover plate the plastic plug was very stiff and didn't want to cause any damage to pins so thought I would ask.
Would Q995 cause R926 to smoke?

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

How did we end up worrying about the CRT?
What did you want to test on the CRT. The CRT cannot damage Q995.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Sent for 2N3439 should arrive Thursday.
What is the easiest method for testing CRT?
How do I remove the plastic cover at the back of CRT.?
Best way to remove charge?

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Redwire - Thanks for cross reference. Sorry I haven't been around to help out, things got a little hectic around the house......

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

Q995 in HV shunt regulator

Tektronix P/N 151-0279-00 NPN HV transistor 300V hFE 40 TO-39 package
Original part SE7056 or SS2821

For substitutes I suggest 2N3439 or 2N5058. If you add a small heatsink a MJE340 would do.

Here is catalog of Tektronix part number to vendor part number cross reference 27MB .zip from Dave Miller AA4DF.

[Agent24]

Re: Replacement caps for Tektronix 434 Power supply board.

http://w140.com/tekwiki/wiki/SE7056 might help you find a substitute

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Correction its Q995 shorted.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Redwire found Q955 shorted B to E.
Can you suggest replacement part as I can' t find SE7056 in UK.
Many thx, John.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

VR944 (51V zener) gets shorted from 'Intensity Limit' switch pg 167. This seems to boost CRT brightness at higher sweep speeds. If you measured a short on VR944 it might have been the sweep-speed switch position.

What I would do is put the multimeter on diode test and roam around looking for shorted semi's. Going across C-E etc. so find anything obvious.
The components left that I see could load down the +115VDC rail are Q940/Q950/Q955. Q924 and Q925 I would also in-circuit test.
R926 burning up is still weird.

Just to review,
On diode test, a multimeter outputs ~1mA at up to a few volts - this is above a diode/transistor drop so any semiconductors are forward biased and start conducting so you can measure their forward voltage.
An in-circuit low V measurement usually means a short but it can be wrong if there are low value resistors or inductors across a semi junction.

On Ohms, a multimeter outputs under 0.7V - below a diode/transistor drop so any semiconductors stay off and you get an ohms reading of any resistors or caps charging in-circuit.

The big difference is a shorted semiconductor will be show up on the low ohms scale.

Some multimeters have a continuity (beeper) for the <200ohms range and usually it's for wires and connections.
I've seen some beep under 10 ohms, others under 100 ohms, others under 200 ohms. You have to know your meter.
Some multimeters have a continuity (beeper) for the diode test too.

Unless it's wires/connectors, I ignore the beep and look at the values I'm getting.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Redwire I think you are on the right track.
Tested ohms to gnd P126
Pin1 = 9.93kohms
Pin2 = 17.58Mohms dropping slowly.

Left P126 unplugged switched scope on
Voltages present
Pin1 = 117.8vdc
Pin2 = 117.7vdc
Pin3 = 2.945vdc

Pins 1/2 are across vr944 is this correct???
Cathode side should be 51vdc.
This is a new Zener.
Question at what low ohms does continuity not buzz?

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

It's not making sense to me, which is a good adventure.

So we are seeing an overload on the PSU +115VDC rail and R926 smoke, and unplugging P126 stops that. Is that right?

With P126 unplugged and the scope off, I would measure ohms to GND on P126-1,2 and see if there is low ohms on the 115V rail to track down.

R926 appears to be part of a snubber for Q924/Q925 voltage multiplier that generates the CRT grid bias voltages.
The only way for R926 to cook is if CR924/CR925/C926 have shorted - and you say they were replaced and test OK. Hmmm.
Can you check for solder bridges or carbon tracking etc. There's no components left to fail here, it could only be isolation, as I see it. Something overlooked, just look things over a bit.

There may be another path for R926 to smoke, for current to backflow through R926. But that would need an open circuit+short circuit.
It's grasping at straws and really convoluted. Quick diode test VR944.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

My apologies Redwire, one on the HV caps had a broken solder joint.
I tested the components around the HV cap some were suspect so decided to replace them.
Status of scope at the moment replaced plug P126.
Turned on scope R926 smoked again C926 is new cap. Cr924/5 test good.
What is causing R926 to smoke???
Voltage at node R926/C926 1.839v
Hope this makes sense.

Voltage at cathode VR990 = 290vdc not 617vdc.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

John, I can't follow what has been fixed and what hasn't.
The power supply was dead, then something fixed it.
Then R926 was smoking and you had a fat trace if P126 on the Z-axis PCB was unplugged.
Now you have no trace?

Pulling P126 disconnects +115VDC from the CRT circuit but there's still +617VDC, +250VDC, +/-15VDC there.

What was arcing, and was fixed?
When repairing things, I find it best to make ONE change and see what results.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Replaced:
VR935, VR937, CR935, C926, R926.
Turned on scope no panel lights or trace.

Removed P126. 3 pin plug.

Panel lights came on front of scope.
Primary voltages where:
-15volts spot on.
+15volts was within spec.
+115volts was +116.7volts.
+250volts was +255.4volts.
-75volts was -73volts but dropping by 1 millivolt slowly still dropping at -70volts.
Still no trace.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

I tested -15v supply to ground on secondary power supply board connection P115 page 170 on PDF. and had continuity is this correct?

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Q924, Q925, R927, C926 Cr924, Cr925 Test good.
Will replace C926 with new Cap tomorrow.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Did you try removing C926 from the circuit and testing it? I agree with redwire about testing the transistors. I think you should also test R927 .