Replacement caps for Tektronix 434 Power supply board.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

120 mV across vr1025 not possible to measure base of Q1032 no voltage across cr1032.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

The neons flash if you look back at previous posts longbow explains what's going on with similar symptoms the fault was on the secondary side. I have checked all primary transistors all good. I will check Q1030 and Q1032 bases and emitters and let you know.
Cheers guys.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

The neon lamps DS1024, DS1020 are apparently part of a 5 second oscillator for Q1025. It looks like the 56VDC rail is briefly pulsed off when the neon lamp fires. Might be hard to see the blinks.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

What do you have on the base of Q1030 and Q1032? That will narrow down where the 325 isn't getting through.

While your measuring take a look at the emitter of Q1025 to make sure it hasn't turned on.

Just as an aside it looks like there are 2 neon lights on the schematic, are they on?

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

With 325VDC present, the 51V zener should have around 51V across it.
This then feeds Q1030 and Q1032 to make the 56V rail. I would test those transistors or measure their voltages.

If the 56V rail is overloaded, I think R1032 will cook but you measure 325VDC on Q1032 collector, so R1032 must be ok.

Q1025 is a (mains) overvoltage lockout and that transistor should be off. This is the only thing that could shut off VR1025.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

No 56vdc. Either side of cr1032.
Collector of Q1032 has 325vdc. I tested 51v Zener in circuit tests good with diode function on Dmm.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

That's fine, just means you have 325VDC primary at C1080.
What about 56VDC rail across C1034? You can't have a party without 56VDC

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

C1081 has 325vdc on each side not across it.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

Service manual for Tektronix 434 s/n BELOW B500000 (1971) is here, 17MB PDF is best. Attached snippet.
The power supply is all discretes verses the change to custom PWM IC for S/N 500000 up. A bit of extra 'fun' to figure out. May exceed alcohol budget for the month.

There is a 56V regulator (Q1032, Q1030, VR1025 51V zener) which supplies the PSU control circuitry. See if it is alive at C1034 or CR1032.

The "turn-on circuit" to initially turn on main switching transistor Q1080 (pdf pg57) is the Q1045, Q1040, Q1050 circuit. I think it (Q1050) keeps firing trying to start up the PSU. I would measure around the start-up (oops "turn-on circuit" as Tek calls it), if you have 56V present, and see if it is pulsing.

Another approach is to just check those transistors/diodes in-circuit, keeping in mind the transformers will give you false low ohms readings so you may have to lift a leg to check them.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

I can't for the life of me understand the schemeatic it's beyond me.
Redwire pass the drink quick ha ha.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Yes guys s/n BELOW 500000 at Tek wiki.
How do you think I feel guys I messed about for hours with the cal T/B sweep sorted frequency was really pleased with myself waited couple of days then decided it was good.
Put all back together and shorted. Won't repeat what I said.
Anyway at moment have 325vdc at p119 also at Q1030, Q1032 Q1080 no low voltages no front lights I am working my way through previous posts I have a feeling that the short is on the secondary board. Tested all transistors on primary board all good (out of circuit).
Have 94vdc at c1020 but no 51v Zener tests good.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

redwire - that's the schematic that I posted. John I believe has a scope with s/n below 500000. That may be the confusion in the Part #s

John - could you post the schematic you're working from? I'm getting a little older and slower and need all the help I can get....

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

I drank so much celebrating you fixing the 'scope, then this with the hangover
I am looking at service manual pdf pg192/199 primary power supply & reg. for Tek434 s/n 500000 up; Is that correct, because Q1834, Q1844 are the bridge and you are at Q1080, Q1032 ??

If I have right SCH, It's an odd SMPS design, but fortunately transformer-drive so a bit easier to work with. Use extra care because U1910 is probably a rare custom IC.

It appears to be a self-resonant design that operates full tilt until the control IC U1910 comes up and stops cycles. Start-up happens with Diac VR1831 at 32V.
Can you carefully take voltage measurements around the bridge transistors and diac, using DC bus (-) TP1844 for multimeter GND.
If the PSU does not come up I think SCR Q1846 overvoltage trip activates and may get in the way of troubleshooting.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Found cold solder near diode bridge now have 325vdc at Q1080 and collector of Q1032 but no low power supplies no lights come on mmmmm.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

John, srhoffman great to get things working. These old scopes are totally worth repairing, they are like a good friend.

I've used the latest DSO's from Agilent/Keysight some to $25K and they are generally crap. Their firmware is so buggy, I've had a blank screen on the DSO where the Tektronix showed me the true waveform. I can't trust a Keysight DSO to troubleshoot, despite the fancy touch screen, big LCD etc.

Yup, can't stand HP/Agilent/Keysight scopes(old or new), but I do like their Spectrum Analyzers and Frequency counters. Love my Tek 475A, old school analog still has a place in the digital world. Had a 7854 just too big and bulky for the workbench at home. The Tek 11800 chassis is an old friend, still holds it own against new stuff. Throw a few SD-24 tdr heads or sd-26 sampling heads in it and it's good to around 20GHz. You can actually see connector features with the TDR heads and even characterize wire bonds.

The one big thing about the new scopes is price, you get a hell of lot scope for money now a days, when it does what should.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Well left Tek434 on for few hours frequency was good at several ranges.
Decided to put back in case and turned back on yes you guessed it crackle arc shorted.
Back to square one doh!!!
No low voltages at all just ac coming in at 240vac.
Think bridge rectifier blown back to stripping down aaaarrrgghhhhh.
Can anyone recommend a replacement diode bridge.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

John, srhoffman great to get things working. These old scopes are totally worth repairing, they are like a good friend.

I've used the latest DSO's from Agilent/Keysight some to $25K and they are generally crap. Their firmware is so buggy, I've had a blank screen on the DSO where the Tektronix showed me the true waveform. I can't trust a Keysight DSO to troubleshoot, despite the fancy touch screen, big LCD etc.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

The sweep speed dial does have a x10 magnifier - labeled "MAG PUSH TURN".
Any chance it is turned on? Not to be confused with variable sweep.
I don't know the scope exactly but try that first.

Nope we looked at that. He was actually having a divide by 10 problem which is what had me stumped for a little bit. The time/division selector is something that would make a swiss watchmaker scream. John probably had something sticky in the selector and the act of getting the var adjustment pot fixed and working correctly might just have unstuck a sticky microswitch in the big assembly.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

The sweep speed dial does have a x10 magnifier - labeled "MAG PUSH TURN".
Any chance it is turned on? Not to be confused with variable sweep.
I don't know the scope exactly but try that first.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Excellent good to hear.