Tektronix 545B Restoration EHT PSU.

[Diabolical Artificer]

I've after restoring one of the above, after giving it a deep clean removing all the mouse leavings and having tested every valve and checking every big electrolytic for leakage, I powered it up - no trace.

So, to measuring voltages. first off I have all the "low" voltage rails, -150v, 225v, 500v etc then checked to see if the heater is working on the CRT, can't see a glow but 6.3v AC is present all the way to the CRT neck cap.

Next checked to see if I have -1700 and 8.5kv, I read -200v and 2.5kv. I subsequently changed out the EHT PSU module twice and got worse readings. I also checked I had oscillation on V800's anode = yes. I also checked the CRT cathode/grid voltage = +200v, should be -1870 minus -1700 = -170 ish.

So to my question, it's well known fault on these scopes that the EHT tfmr goes kaput due to moisture ingress or similar, not noted on the schematic is the AC OP of the EHT tfmr, any idea what it should be? It says +100v DEC which I take to mean +100v DC, this is present, but what's the AC out of the EHT secondary tfmr? Can't find this in the manual.

There are other issues with the scope, I suspect the TB isn't running or the HOR amp isn't working as the HOR indicator arrow indicator on the front panel is lit RHS only one also on the HOR TB only one neon is lit and not flashing. However I suspect the EHT PSU is the main issue.

TFL, Andy.

[Radio Wrangler]

Until you've got the right EHT, then there's no point looking at anything further.

Are you sure your EHT measuring gear isn't loading what you're trying to measure? It's worth a double check as people sometimes get sent round in circles by this.

Tek manuals of the period are rich with waveforms (it's their business!) so you can check the grid and anode of the oscillator bottle. Note also the feedback path. Is that pulling back the oscillator power?

Any of the rectifier valves gone to air? But you'd normally see a bit of a light show.

Note there are TWO EHT secondaries on the inverter transformer. One does the grid of the CRT, the other does the cathode. So there are two supplies floating relative to each other. This allows an amplifier running near ground potential to vary the grid/cathode volts almost 2kV away. This does retrace blanking and Z-mod.

David

[IanG3XYV]

Hi Andy,

Being the TEK man I know you are, I imagine you are aware of the TEKWIKI resource. I've grappled with 500 series power supplies and they are all very similar and a lot of them share the same EHT transformer part 120 0308, the spec for which I've attached.

My 544 suffers from the failing EHT problem due to overheating of the transformer. All is fine for an hour or so then the screen disappears. I don't have an EHT voltmeter so measure the -1700v using the 2500v range on my Avo but it reflects the full voltage and I see it drop. There are lots of thoughts on the TEK Groups io and TEKWIKI including fitting heat sinks and fans to keep the temperature down. I've thought of replacing the thermionic 5642 rectifiers with EHT silicon diodes which are cheap and easy to obtain. That would save a few watts. Also the capacitors in the multiplier circuits can go leaky.

The 100 DEC is I think a decoupled version of the 100v rail. Simply a series resistor and a parallel cap down to ground to give some decoupling from the main 100v rail.

Is the 325v rail present and correct? I think some 500 series have that rather than 500v but I'm a bit hazy to be honest. But it sounds as if the timebase might not be running which may not be related.

Ian

[Radio Wrangler]

That heat sensitivity could either be a fault in the windings, or it could be the core exceeding its Curie temperature somewhere in its deepest depths. As this happens part of the core loses its permeability and the inductance drops, causing inefficiency and more heat so you get a sort of thermal runaway effect.

David

[Diabolical Artificer]

"Are you sure your EHT measuring gear isn't loading what you're trying to measure?" Yes, I've checked it with other meters, it's about 20v out, but that's all.

The copy iof the Tek 545B manual I have isn't brilliant, the waveforms are faint and hard to see but I've included a pic of the primary - 150v PP, clipped a little at the bottom. I presume the FB path is through V814 (12AU7) onto V800's g2?

Measured voltages - shown in brackets, some are off, but they would if the EHT section is faulty. First off supply voltages from L - R - 350v (i measure 345v), 325v - (334v) 100v (100v). Next V800 g2 (33.6v) V814 grid 1 (pin1) -2.6v (0.6v), grid 2 (pin 7) -160v (+112v). This latter will be off if V at bottom of R847 is low. I have +350v at the top of the potential divider formed by R841/2/3 etc but only -377v at bottom of R853.

Aside from checking DC voltages I scoped half the EHT tfmr secondary from CT to anode of V832 and got 250v P-P. On another tfmr I have I got 1200v P-P, these taken using a differential IP on a HV scope. I disconnected the EHT tfmr and diodes to see if something was pulling the voltage down, voltage went up, but still short.

Next I tried replacing V822 and V862 valve diodes with silicon diodes, voltage still short. I also checked the EHT caps for leakage at their wkg V, EG 10kv, checked ok.

I also checked all components around the primary oscillation circuit, a few carbon R's were high, but not overly so, EG 2m2 = 200k high.

All this leads me to the conclusion that I have three duff EHT tfmrs. How would one test them? I can't really just bung a high voltage sine on the primary or can I?

Andy.

Posts crossed. Thanks Ian for the PDF, I need to do some more reading up on the subject, but it sounds like my tfmrs are lacking from power up, not failing as they warm.

[Chris55000]

Afternoon Andy!

This type of circuit is sine–wave drive, so the three 5842 rectifiers across the main e.h.t. secondary are a voltage tripled, so I'd expect the following amplitudes:–

a) Anode pin of V800 (6AU5) – 350–450V r.m.s.,

b) Grid of V800 (6AU5) – 100 V r.m.s.,

c) Filament of V862 (–EHT rectifier) – 1300–1350V r.m.s. to earth;

d) Anode of V832 (1st stage in tripler) – 2750–2800V r.m.s. to earth.

Before condemning the transformer, make sure that the grid feedback factor C806 and the anode tuning capacitor C808 are both o.k., there aren't any shorts in the e.h.t./tripler reservoir capacitors C831–C834, and you have controlled G2 voltage (+78V according to the diagram I have) on the 6AU5, and there's not any other problems with the main +325V H.T. supply!

I have a transformer assembly at your disposal to try if it becomes "nil desperandum" – PM me if you need it!

Chris Williams

[Chris55000]

PS!

Have you tried connecting a dropping resistor of about 33K from the +100V h.t. line direct to the 6AU5 screen–grid in lieu of R803?

It does seem strange that you've got three transformers suspect with the same low output fault – your cellar isn't over damp etc., is it?

Chris Williams

[cmjones01]

Originally Posted by Radio Wrangler

That heat sensitivity could either be a fault in the windings, or it could be the core exceeding its Curie temperature somewhere in its deepest depths. As this happens part of the core loses its permeability and the inductance drops, causing inefficiency and more heat so you get a sort of thermal runaway effect.

These later Tek 500 series EHT transformers are potted in a brownish epoxy compound which becomes lossy with age, and the effect worsens with temperature. They will typically run for 20 minutes or so before the primary side is running at full chat and can no longer sustain full EHT voltage. The only fix is to replace the transformer with one wound without the lossy epoxy.

I had a 549 which suffered from this, but fortunately someone else had rewound the transformer on a modern core. It didn't work because there were a few too many turns on the grid winding, which kept its voltage too far negative and no electrons made it to the screen. I took some turns off and it was fine.

My workshop 535A is just old enough not to suffer this problem, fortunately. I think it's the newer 545B, 547 and 549 which are most commonly afflicted with it.

Chris

[Chris55000]

PPS!

10V r.m.s applied @ about 50 kHz across the grid and h.t. taps of the transformer primary from a function generator will be adequate enough to see what the step–up ratios between the windings are – this should step up to about 275V r.m.s. across the e.h.t. secondary if your generator has enough output, and about 50V r.m.s across the anode–to–h.t. section, rough guess!

There's a reasonably priced e.h.t. transformer assembly from a 585 here:–

https://www.ebay.com/itm/224141138297

. . .and the circuit is identical, so hopefully it will meet your needs!

Chris Williams

[Chris55000]

PPPS!

Tektronix warranted their transformers "for the lifetime of the instrument" in those days but I fear they're not expecting anyone to be restoring 60 year old 545Bs!

Chris Williams

[woodchips]

I have just found several of the EHT units from the 76xx scopes. No idea if any use, no idea if I will be keeping them, no idea if they work, bitzed many years ago.

One I have in front of me is marked 12kV, if that is any guide. Part 337-1538-02.

[Diabolical Artificer]

Thanks all for those suggestions, I'll respond to them tomorrow. Just before posting this I got one EHT tfmr and caps, checked the caps for leakage albeit not at full V then changed all but one of the EHT diodes for silicon, albeit a ragtag collection of. Popped it back in and got 2kv ish on the junction of R836/C836* and about 500v on the cathode. *I'll check C836 tomorrow for leakage.

Desoldered R836 and all connections out to cathode etc, powered up and got 6kv at R836 and 1.1kv at where the cathode etc connects. Therefore somethings really loading this down. Is it possible a duff CRT would do this? Just a thought.

3 dead EHT tfmrs does seem hard to swallow, BTW these are all open frame tfmrs, no potting, see pics.

Had enough for today will drop back onto it tomorow after re-reading all your posts. Thanks for all you IP and help, A.

[Diabolical Artificer]

Progress. Tried Chris's idea of a 33k from the +100v to g2 V800 which increases Vg2 from 33v ish to 74v. EHT now 7kv and and 1.4kv, low, but I do have a glow on the CRT.

I checked all the components around V800 C806/8 check ok, so the FB path must be wrong, the thing is I can't figure out how it works/starts. V814's B grid is fed from a potential divider with +350v at one end and -1700v at the other. The voltage on the grid dictates the voltage on the anode, which in turn effects the voltage on V814 A grid, which dictates V814 A anode, which in turn dictates the voltage at V800 g2. However if the negative voltage (-1700v) at one end of the potential divider is low, therefore by consequence V800 g2 will be low..... haha, is V814 B inverting, therefore low Vg1 = Hi Va. Ehm, need to check out the FB path as David said way back.

More later, Andy.

Edit. After taking some voltages with Chris's hack I then put the circuit back, had a look at the schematic again for ideas, then gave the high voltage adjust pot a good twiddle, it was stiff as hell. Powered up, jobs a good un. The 8.5kv is still low - 7.5kv, but the -1700v has come good and adjusts as t should. One other possible reason things might have come good - when taking voltages one pin on V814 shot up from low V to hi after I put pressure on it - dirty contact/valve pin?

Still no sweep though, that next.

[Diabolical Artificer]

I'll break these up into individual sections if I may & start a thread on the TB else this thread will go on for years. Could the mods please re-title this - Tektronix 545B restoration - Part 1 EHT PSU.

Thanks, Andy.

[Chris55000]

Hi!

Well done on sorting the e.h.t. supply out and I'm pleased my suggestion helped you!

I suspect the 5842s may be a bit down on emission or there's a bit of surface leakage, etc., knocking out the top bit of the 8.5kV p.d.a. supply, the assembly I linked in a previous post will probably be cheaper, for a set of valves, or you could fit 5 X 2CL77 diodes and remove the five filament loops from the transformer – this would probably reduce the overall load enough to get you the +8.5kV needed!

The only point to note is, that with the e.h.t. lower than the specified figure, the vertical and horizontal calibration will be affected, but you can come back to that later when the rest of it is fixed!

Chris Williams

[Joe_Lorenz]

Seems to demand long breath to get all issues sorted out. I still have got a 585A and a 565 on the bench awaiting restoration. Both out of service for more than 20 years now.

The 545 is a fine instrument when at good health! In early days I had a Systron Donner 9550 curve tracer plug in for the 545: I badly wish I had kept it!

Good luck!
Joe

[factory]

Originally Posted by Chris55000

Hi!

Well done on sorting the e.h.t. supply out and I'm pleased my suggestion helped you!

I suspect the 5842s may be a bit down on emission or there's a bit of surface leakage, etc., knocking out the top bit of the 8.5kV p.d.a. supply, the assembly I linked in a previous post will probably be cheaper, for a set of valves, or you could fit 5 X 2CL77 diodes and remove the five filament loops from the transformer – this would probably reduce the overall load enough to get you the +8.5kV needed!

The only point to note is, that with the e.h.t. lower than the specified figure, the vertical and horizontal calibration will be affected, but you can come back to that later when the rest of it is fixed!

Chris Williams

They are 5642 rectifiers (also used in a few HP scopes), the 5842 is a triode.

David

[Diabolical Artificer]

Thanks Chris. I replaced all but one valve diode with three 2kv diodes per 5642, they're all I had. I'll get some better replacements when I'm flush. That EHT "module" is the same as the one in the Hickok 545B rippoff Chris, I did pull it out and try and wire it up but the connections are slightly different. I spent half a day trying to work it out comparing the Hickok and Tek schematics. That 585 module is different in that it only has one secondary I think, the wiring a circuitry around the intensity control is different. In the end I gave up. Anyhoo, it's working ish for now, thanks again Chris, don't know what I'd do without your help.

"Seems to demand long breath to get all issues sorted out" Your right there Joe especially as I don't have much clue as to what I'm doing but I've had three 545B's sitting in the shed for years, they're heavy, take up loads of room in my small workshop but are things of beauty, to not fix them ( or try to) would be a crime, hope you get yours working Joe. "In early days I had a Systron Donner 9550 curve tracer plug in for the 545" that was a big boo boo there, a very useful plugin.

Cheers all, Andy.

[Radio Wrangler]

If all goes well, you should have a fine room heater in time for winter!

David

[Diabolical Artificer]

"If all goes well, you should have a fine room heater in time for winter!" There's that, it gets very cold in my workshop, two jumpers and a coat job, one of these big Tek scopes just keeps the edge off till the suns overhead.

Andy.