Replacement caps for Tektronix 434 Power supply board.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

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

Yes, and don't start now. Use the proper probe. Luckily, a simple HV probe can be obtained on ebay for very little. As for the power supply voltages on P141..the first 3 voltages are low by more than I'm happy with, but not low enough to prevent operation.

TP1236 is simply measuring the same point as P141 pin6. This switch is part of the storage function which should be off until you get reasonable operation from the main scope portion. Storage=Off

Check +/- 15 volts. Are both these lines free of ripple? (25KHz is your switching frequency). Are those 3 diodes really shorted?

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

I had a look on ebay for HV probe £100+ cannot justify that price when hardly ever used.
I will attach pics of VDC and AC ripple of first three voltages just been doing that this morning.
Can I check across C1107 with scope for ripple?
These voltages taken at P141 with 10x probe.
See setting on scope.
jpg 497 = -75Vdc
jpg 498 = AC ripple of -75v

jpg499 = -15Vdc
jpg500 = AC ripple of -15v etc.etc.etc.

Hope this makes sense.
John.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

I don't see anything unusual there. You don't need a scope to confirm d.c. levels....The ripple on the 75 volt line seems Ok. You already checked accross C1107 in jpg500 and it looks ok - back to sheet 11 and the waveforms at the left side of the page.

Previous post #54 doesn't make sense. The wiper should swing from 0vdc to -15vdc. How could it NOT, given that your -15 supply is OK? Sheet 11 suggested dc operating voltages indicate -4.9v on pin 3 - that's the input from your brightness/intensity control. Your measurement says -4.7v. No problems there, and other voltages in post #53 check good. Please fill out the d.c. on the rest of the pins on U940 - they are part of the HV regulation. But we will disregard that section for now. These days, a HV probe isn't as popular as before, so they can be found for very low prices. Those sellers who hope to get a fortune for a new Fluke probe are kidding themselves.

The main issue at this point is the maximum brightness and lack of intensity control. From the tube's point of view, that means that the G1 to cathode voltage is wrong. G1 needs to be more negative, or the cathode needs to be more positive than it currently is. U940 is supposed to control this relationship, along with the d.c. restoration circuit. Please confirm waveform 3 at TP924 which should show a 40 volt negative going p-p pulse...a larger version of waveform 2.

You mentioned some shorted parts. That would be a good place to double check.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

U940 Vdc Voltages.

Pin 1 = +5.82
Pin 2 = -0.238
Pin 3 = -4.79
Pin 4 = -4.76
Pin 5 = -4.81
Pin 6 = -4.70
Pin 7 = -14.65
Pin 8 = +0.001
Pin 9 = +3.511
Pin 10 = +0.020
Pin 11 = +14.32
Pin 12 = +4.72
Pin 13 = +0.113
Pin 14 = +0.664
Pin 15 = -14.60
Pin 16 = +0.001
Pin 17 = +1.269
Pin 18 = +14.29
Pin 19 = +3.048
Pin 20 = -12.12

Tried waveforms on sheet 11 but cannot get them to stablise. see pics. 2 different t/div.
The settings on sheet 11 for waveforms are walking across x axis tried all settings these are the best.
John.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Q989, 990, 995 all collectors = 0volts.

R986 = 250 one side 1.3v other side.
R987 = 250 one side 0v the other. Wrt gnd.
Parallel resistance approx 3.66 megohms.

R990 resistance colours look like brown/green/blue, silver.
This is supposed to be 22megohm but it's obviously been there a long time.

Where does the collector voltage Q995 617volts come from.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Just cut legs on Cr931 andVr935 both shorted is there an equivalent for Cr931.

Cheers, John.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Can anyone recommend an equivalent for 1N486A.
Would 1N486B suffice?

Cheers.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Shorted diodes? I guess that could be a problem. Looks like a 1N486A diode is no longer made. There are some specs online and many current parts will work ok. One number I came up with a a BAV21 which is equal or superior in specs. I'm sure there are others. 1N486B does not seem like a good fit, especially when there are lots of common diodes that are easily available. Note that most of the diodes in this circuit are the same part number, so get a handful of whatver you decide upon.

I would definitely replace both zeners together, VR935, VR937. These are common 1/2 watt zeners.

The next step is to ask the question why are there shorted diodes? Did one fail and cause the other to smoke? Are there problems on another board? Is the tube cathode shorted to the G1 grid? I'd like to hear your theories. You can certainly replace the faulty parts. It might work fine, or it might smoke again.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Q989, 990, 995 all collectors = 0volts.

R986 = 250 one side 1.3v other side.
R987 = 250 one side 0v the other. Wrt gnd.
Parallel resistance approx 3.66 megohms.

R990 resistance colours look like brown/green/blue, silver.
This is supposed to be 22megohm but it's obviously been there a long time.

Yes, all those parts should be checked, and it may be necessary to lift one side of the large resistors to make sure. Also note R960=56 MegOhm might be a bit difficult depending upon your meter. (about 20 nS if you have that function). The circuit is confirming that HV regulation isn't working. That might be the main problem, or it might be a result of the faults you discovered in the d.c. restoration circuit. (shorted diodes).

Where does the collector voltage Q995 617volts come from.

The 3 transistors, Q989, 990 and 995 are in series with the "cold" end of the HV stack - sheet 10. One possibility, since Q995 collector is 0v, is that all 3 transistors are shorted. That situation would result in maximum screen brightness. Tektronix scope designers feel free to jump in here.

At this point, replace at least 2 zeners and 1 diode in d.c. restoration. Remember to discharge the HV first. Check parts in the HV regulation circuit (3 transistors and related components) before turning the scope on.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

What's best way to discharge Hv?

[Agent24]

Re: Replacement caps for Tektronix 434 Power supply board.

Slowly - with a high voltage, high value bleeder resistor. There's less chance of damage to the CRT, drive circuitry and sensitive ICs.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

All three transistors test good out of circuit.
Q989, 990, 995.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

What's best way to discharge Hv?

See post #59. Discharging is very easy. Then remove the tube socket and inspect 1) for carbon tracks inside the socket 2) take a resistance reading from the cathode to G1 on the tube itself (pin 3 to pin 2). This should be infinite. If the 3 transistors can be removed from sockets, then test all the zeners and big megohm resistors. Mostly, get the faulty diodes replaced.

Waveform 3 and 4 are pretty important. It should be easy to get a stable trace, since the pulses are 40v p-p! Set the trigger to minus (negative edge) and set sweep to the time base indicated....d.c. coupled scope input. Your d.c. zero voltage reference should be the bottom of the scope screen.

After a few more things are running, you can try page 5-8 adjustment #5 grid bias. No HV probe necessary for this step. This step should succeed if things are working well. We will see.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Can I switch on with cr931 and vr935 open circuit (legs cut) or will this cause a problem.

I can resolder them if required.

Was going to check waveform 3/4.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Can I switch on with cr931 and vr935 open circuit (legs cut)

Living dangerously! NO. Get some diodes and replace them. Measure resistance between Cathode to G1 first thing with socket removed to eliminate the possibility of a shorted tube. Pay particular attention to C924, C925 (might be shorted), and confirm that every diode in that circuit checks OK. (Obviously you have already found 2 faulty ones, which you are replacing as we speak.) It looks like all the non-zener diodes are the same. BAV21 looks like a reasonable modern substitute, but there should be many others that work. I would replace them all with the diode type you decide upon. That's why you see the parts list says: "selected". They should all have nearly the same characteristics. Fine! Let's get this thing running!

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Ordered parts be 2/3 days.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Parts arrived for making high voltage probe 400meg x 100k devider.
Where exactly does this probe have to be inserted I can't see the normal rubber suction pad like on TV's. Hopefully other parts will arrive tomorrow.

[Agent24]

Re: Replacement caps for Tektronix 434 Power supply board.

Stupid question perhaps, but are the parts you just got for a divider actually rated for the voltage you intend to probe?

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

I would NOT build a HV probe without a lot of careful consideration. Normally, they consist of a 100Meg and 1 meg divider for a 100X probe. Your reading of 4KV would show 40 volts d.c. on your 10 Megohm input meter. Here's the scary part. The 100Meg resistor HAS to be about a foot long in order to prevent arcing and be housed in a safe plastic tube. 400Meg and 100K just are not convenient values, which calculate out to a 4000X reading. I doubt whether your 400Meg resistor is the HV foot long unit that is needed. Also, consider the fact that if your ground strap gives way or is not connected, the divider will have the FULL HV at the junction of the resistors. Anyway, there are a number of things to be done in replacing the zeners and diodes in the d.c. restoration circuit. Check for short circuit at the tube base, and then replace the diodes.

There. I gave you the straight story.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

My TV HV probe has a 1,440 MEG resistor about 5" long, rated for 50kV I think.

For a home-made HV probe to fix the 'scope, you don't need anything so big.
I've even used a string of 10MEG 1/2W resistors, limiting to about 750V each, inside a bic pen, to measure HV. For measuring up to ~10kV.

You can poke a wire or probe under the CRT second anode suction cup, as an access point. First, short that to GND to dissipate any charge on the CRT or you will get bit.
Careful the probe tip does not land close to the chassis or aquadag or you can draw an arc.
Don't forget to include scope/multimeter loading (10MEG// your 100k) when calculating the divider ratio. Also consider an AC volts measurement too.