Replacement caps for Tektronix 434 Power supply board.

[Agent24]

Re: Replacement caps for Tektronix 434 Power supply board.

If R1032 burnt up then it is passing too much current. Assuming it was not the wrong value or too low wattage, then:

Maybe Q1032 or CR1032 is bad, or something they are supplying current to is shorted, or

There is something wrong with Q1032 drive circuitry. Since your neons are not flashing correctly as per the manual, I would say that this is likely where to start looking.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

If R1032 burnt up then it is passing too much current.

Duh - sorry, couldn't help myself.

Maybe Q1032 or CR1032 is bad

I agree, it looks like Q1032 is shorted. Wasn't there a previous "accident" that caused this? Also, if Q1030 is shorted, Q1032 would always be turned on. Both are NPN transistors, both easy to test in-circuit. Just try E-C resistance.

Previously, R1032 was hot and had increased in value to 3,1K. That fact would point to a shorted Q1032 or Q1030 also. CR1032 "disconnects" Q1032 as soon as the supply has started, also turning off current through R1032.

This is a good time to check resistance between CR1032 cathode and primary ground (1). If C1034 is shorted, you will see more smoke. Good luck.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Latest.
Q1030, Q1032, Cr1032, all tested good out of circuit.
R1032 is correct value and I used a 3w so over spec.
The majority of the transistors are socketed so easy to test, used diode function on Dmm.
New C1034 10uF 160v fitted although original tested good esr.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

First point is that in a previous post, R1034 measured 1K, not 10K. I would say that's impossible, but in electronics... You see resistors increase in value or open but I don't recall seeing one decreasing in value or coming up s/c.

OK, reality check. CR1032 cathode should NOT be at ground potential. R1034 should be 10K, but other parts in the regulator may decrease that value ...a little. Let's say hypothetically that CR1032 cathode to ground resistance is 10K. Now we will assume that Q1032 AND CR1032 are both shorted. That would put R1034 and R1032 in series accross the HV which is about 325 volts.

My trusty calculator says that 28 mA would flow through R1032, creating 1.36 watts of dissipation. So a shorted Q1032 would not smoke R1032. (It might smoke R1034, but we are going to solve a few problems before plugging in the unit again.

We already know that Q1032 is supposed to pulse at the same rate as the neons. My guess is that Q1032 would turn on and off no more than 2 or 3 times before the supply starts up. Q1025 turns on and off at the same rate. Its collector goes from ground to 51 volts in time with DS1024. That action causes Q1032 emitter to pulse from ZERO to 50 volts in time with the neons, right at the first of startup. After a few cycles, that transistor should not be delivering current any more, thanks to CR1032.

Finally, a philosophical comment. Often we discover an obvious problem with our repairs whether it is lawn mowers, blenders or electronics. But assuming that the single problem is the only problem is mistaken logic. Also it is human nature to ignore problems until something actually catches on fire or just stops working. I'm afraid we are dealing with multiple problems in multiple parts of the scope. Go slowly and think!

[Agent24]

Re: Replacement caps for Tektronix 434 Power supply board.

Duh - sorry, couldn't help myself.

Haha! All good. I know that was obvious, but without a magic crystal ball I can only provide the obvious answer for as to why the resistor burnt.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Retested all transistors out of circuit on Primary Board

All OK.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Desoldered one leg of Cr1032.
Resistor ok.
Neon flashing as before.
94.5vdc across C1020.
0.12vdc across Vr1025.
Tested R1026 = 100ohms.
Q1030, Q1032 test ok out of circuit???

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Desoldered one leg of Cr1032.
Resistor ok.

I assume you mean resistors R1032 and R1034 are OK?

Neon flashing as before.

Good sign, but we don't know if the frequency is correct just yet.

94.5vdc across C1020.

This is correct, voltage is set by VR1020 100v. Zener diode.

0.12vdc across Vr1025.

Don't just stand there, Sargeant, a problem is staring at you! More measurements necessary. See below

Tested R1026 = 100ohms.

Fine

Q1030, Q1032 test ok out of circuit???

Fine, then you know that R1032 got hot because the neon circuit is turning Q1032 on much longer than it should be. Or also possible is something shorted to ground along the line connected to C1034 (+). As I said earlier, my guess is that Q1032 would operate 2 or 3 times before the supply starts up, then disconnect itself.

------ What about that .12 volts? We are talking about a 51 volt zener diode here. That means if current flows from ground, through VR1020 and then through R1028, you will see 51 volts on the cathode of the zener. Since Q1025 is in parallel with VR1025, we have 4 parts that need to be tested. 1) VR1025 2)R1028 and 3)Q1025 4)R1025

Conditions that would lead to .12 volts at VR1025 cathode: Shorted Vr1025, Open R1028, and Shorted Q1025 (E-C short).

If all 4 parts test good, then you can assume that DS1024 neon is turning Q1025 on (collector voltage goes near ground) nearly all the time (confirm with scope). Well, is DS1024 on all or most of the time? Or is Q1025 shorted?

Although it is not likely, neon tubes can get flakey after many years of service. You can put a scope accross them and check that they both have nearly equal firing voltages - probably 70-100 volts. Check the parts list on those. After doing those tests, we will review the paragraph labeled "Startup" again.

Whilst playing with this circuit, you might put a 1 megohm resistor in series with CR1032 to prevent the risk of fire.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

The manual says the neons should cycle at a 5 second rate.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Sorry for delay been busy.
Found short in secondary.
My fault fitted new electrolytic wrong polarity do oohs!!!
Blew up fright of my life.
Anyway scope has a trace.
Tried calibration signal but wasn't triggering properly kept moving.
When turning time base control to right towards u/sec.
High pitch buzzing.
No control on intensity very bright trace.

Took voltage measurements.

-15v actual was -14.66vdc ??? Out of spec.
+15v actual was +14.75vdc??? Out of spec.
+115v actual was +113.6vdc ok.
+250v actual was +247.8vdc ok
-70v actual was -70vdc dropping slowly ??? Out of spec.

Getting there ha ha.

[Agent24]

Re: Replacement caps for Tektronix 434 Power supply board.

Buzz noise and very bright trace sounds like something is running at too high\or low voltage?

Maybe more old resistors that have gone out of spec in the high voltage circuit?

What was shorted? Maybe there is other damage in the same area.
Which capacitor did you replace backwards? In which circuit?

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

C1104 in secondary power supply.
Will do more testing tomorrow.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Latest on Tek 434.

I still have problems with bright trace, focus and triggering.
Power supply seems ok.
These are the latest voltages on photo.

Many thanks for all your help.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Intensity no control still too bright.
when turning knob anti clockwise stops at 8.0Vdc fully ccw 89Vdc. TP 924

Can anyone explain whats going on here simple sine wave from generator.
See photo.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

I still have problems with bright trace, focus and triggering.

Pick one. I recommend brightness.

Power supply seems ok.

Not! Recheck all low voltage supplies in this area. Intensity will be controlled by G1 control grid to cathode voltage on the tube. But, the cathode has -4KV on it w/respect to ground. Use the waveforms to the left of the schematic.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Voltages on P141.
- 69.93Vdc
- 14.60Vdc
+ 14.74Vdc
+ 114.1Vdc
+ 251.4Vdc

TP1236 = - 69.5Vdc

Vr935 continuity test across = shorted.
Cr931 continuity test across = shorted.

Can anyone suggest equivalent components for above?

Waveform 1 from page 173 crt circuit & calibrator.
Does this look right waveform going from right to left camera never caught complete signal but if you zoom in dim trace is there.
If this is ok I will take more pics.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

If you could indicate the manual page you are working on....please. In my opinion, we should be looking at sheet 11 showing Z-axis and HV control circuitry. Keep in mind that what is displayed on-screen will be affected by the setting of the trigger controls. The storage functions should be turned off. If the display is at max. brightness then you have a problem somewhere on sheet 11, z-axis blanking, HV control, grid bias control. And, I know you have already thought of the following. If troubleshooting seems at a dead end, begin with the alignment procedures for this section. At some point, you will not be able to complete a step, for obvious reasons.

There are lots of parts labeled with d.c. voltages and various waveforms shown at the left. (BTW, your photo does not show the trace.) Your assignment, should you choose to accept it, is to compare a number of your d.c. voltage readings with those given in the schematic. Ditto for the test point waveforms. I'm fairly sure you will come up with some discrepancies, which you should note in your post. Keep in mind that you will need a HV probe, or HV meter, to measure and set HV.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Alas Capt. I don't have any high voltage measuring equipment only my fluke 85 1000v.
I can measure other voltages on sheet 11.
Storage is off.
Will get to it Asaph. Ha ha.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Voltages on P141.

Where is this plug? I can't find it on sheet 11.

TP1236 = - 69.5Vdc

? Where is it? What does it measure?

Vr935 continuity test across = shorted. Cr931 continuity test across = shorted.

Maybe. You are in the right place - d.c. restoration and tube bias. I would remove the tube socket, after making sure you bleed off the HV. Make a suitable bleeder using a 2W 1 Meg resistor clamped to an insulating plastic rod, ground wire with alligator clip...you know the drill. Inspect the tube socket and rear tube pins for possible arcing or leakage tracks. Also check for shorts between cathode and G1. Then re-measure CR931, VR935 and VR937 again. You might have to desolder one lead. Those last 2 are 33 volt zeners. No mystery there.

I wouldn't start replacing parts until you know these parts are really shorted. Time to check ebay for a HV probe

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

P141 on sheet 12 storage.
Sec pic for more voltages.
Never touched high VOLTAGE before.