Tektronix 545B Restoration EHT PSU.

[Station X]

Thread reopened at OP's request.

[Diabolical Artificer]

Thanks Graham, much appreciated.

I've another 545B on the bench, was working ok then started loosing the display after a while, classic dud EHT tfmr symptoms. However pretty certain EHT tfmr ok, checked LV all good , checked -1700v = very low.

Replaced V800 & V814 with new valves, checked all components on oscillator primary, EG C806 & 8 all R's, am only getting about 25 RMS 60v P-P, note distorted waveform, see attached. Also checked to see if anything is loading the secondary by doing resistance measurements to chassis, all look ok.

Voltage measurements V800 Va = 309v, Vg1 = -13v, Vg2 = 148v. V814 - Va + Vg1 -140v (way off should be -2.6v) So V800 Va bit low, Vg1 bit low, Vg2 nearly double, V814 way off. Also of note voltage adjust does nothing.

Lastly I changed the EHT tfmr assembly for a known good one, nada.

So, how does the primary oscillator work? How can you get negative DC at V800 g1, there's a blocking cap? Any ideas?

TFL, Andy.

[dave cox]

C806 & C808 ?

V800 conducts, pulling the anode down, C808 then yanks C806 down by a similar amount (until they charges up!). One end of C806 is biased to gnd so when it gets pulled down that end goes -ve and cuts the valve off.

dc

[Diabolical Artificer]

Thanks Dave, makes complete sense when you describe it. One query on thr TEk 545B manuals I have V800's grid is shown as -5.5v or -55v, bit hard to see. Minus 55v sounds a bit over the top, most small pentodes cut off way before that, see attached.

Ok, so V800 is sort of working but why? What I can't get my head round is that for V800 and associated circuitry to work, there must be -1780v at the CRT cathode, but if there isn't, it doesn't, and if it doesn't there won't be -1780v! : ) The old chicken and egg.

Therefore how does the circuit start up? One would think at startup V814 turns on V800 harder V800, as cathode voltage steadily rises V814 throttles back V800 by lowering V800 g2. Ok,as is V800 Vg2 is high, meaning V800 Va should be high, but it isn't.... ehm, head scratcher, going to have to think about this, but it has to be C806, C808, R806 or R807, but I've checked them or replaced them. Back to square, here we go round the ****dy mulberry bush... again.

Thanks Dave, Andy.

[dave cox]

To me, V814 is the feedback for the HV inverter, V800.

The divider chain, R840, 841, 842, 843, 846, 847 & 853 ( running from +350V to -1700V - if everything is working ) is tapped between R841 & R842 to feedback and regulate the inverter V800. I calculate ~ -67V at R841/842 for no load but the diagram suggests -160 ?

I assume if the HT (-1700V) line goes too -ve this will pull the junction of R841 / R842 more -ve and turn V800 off (my knowledge of glassware is a bit thin at this point!). Conversely, if HT is less than -1700V R841 / R842 goes more +ve turning V800 on. Initially, there is no -1700V so V800 is running flat out

dc

PS:
I calculated for R840 at maximum value but I don't think it would ever get to -160 to cut V814 off

PPS:
Double oops, I missed that the 5M pot is in parallel with 3M3 (so add R845 to the list of R's). Ignore my dodgy maths!

[dave cox]

Doing the calculations again, at +350V and -1700V, the R841 / R842 junction would swing between -117 and -200V by adjusting R840. Putting the claimed -160V bang in the middle

[Diabolical Artificer]

Thanks Dave, bit of a delayed response, however I've dropped back onto this.

This morning I disconnected all the secondarys of the EHT tfmr and powered the screen grid of the 6AU5 with a high voltage bench PSU. Chris says I should see

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

in post #6, the most I got was 150v RMS (today), so, unless Chris is wrong I'm still all at sea.

About the 350v - 450v RMS figure Chris gives, the supply to the EHT tfmr is 325v, so how can you get more than that out? Answer the inductance of the EHT tfmr; is this how this oscillator is working.

Having said all that I'm now getting more than the 25v RMS I was getting at V800's (6AU5) anode. I'm now going to connect each secondary in turn and see what drops the oscillation.

Andy.

[Diabolical Artificer]

Well, the fun and games continue, worked on this on and off yesterday and am non the wiser.

I reconnected the EHT tfmr secs one by one and the anode oscillation fell to 20v to 35v. So I took em all off again, hells beard, 20v RMS! Right, change every component in the oscillator, nope, swop the EHT tfmr, another nope. Put everything back together, changed V800 for another new one, checked heater voltage, checked caps, checked diodes, checked whether the wind was in the East and prayed to the scope Gods, nope, nopey, nope nope. Was the 150v RMS I "saw" a figment of my imagination?

Ok, lets try logic, does the dieletric of C806/808 matter? Don't think so,both original caps were old film caps, replaced with modern poly yellow axial jobbie's. Does the EHT tfmr need a load to work, EI reflected impedance, not sure there but don't think so. The oscillator is just two caps, an inductor (EHT tfmr) and a couple of R's, oh and a pump (V800 6AU5) so why won't the b***er work?! Or is the oscillator working but it's something else?

What do I observe? 1)Increasing V800 screen grid voltage increases OP, so sounds like the oscillator is working ok, however in the manual it says Vg2 is 84v or 545A says 94v; the 6AU5 datasheet says Vg2 max is 200v? Another disparity 545B Vg is given as -55v, 545A as Vg1 -66v, both sound odd, datasheet loadlines show -25v/-30v as cutoff, so why so high? I've measured around -10v yesterday, but I see re-reading these posts that I've measured -160v; is the manual wrong, doubt it.

2)Twiddling high voltage adjust does nowt. The EHT and OP of V800 seems to vary, I had about -950v yesterday, which then fell, and went up a bit. Something floating?

Ideas - 1) is the voltage divider to the cathode and FB open or intermittant? 2) Is the CRT toast and somehow loading the EHT down? 3) Measure V AC at the EHT tfmr secondary's, what OP do we get for X IP voltage. this will take some setting up, but I have a HV scope. The EHT turns ratio looks to be low (few turns) so should be ok there with test gear limits.

Posting this feels like I'm talking to myself, but maybe this will help others in the future and writting stuff down sometimes helps, if anyone has any ideas please shout up or just throw something, really struggling with this one.

Andy.

[Craig Sawyers]

I haven't read through the entire thread, but the 545B was prone to transformer problems like you observe. The 535 and 545A used wax impregnated transformers, and all was sweetness and light.

But for the 545B, 547 and 576 curve tracer Tek decided to use epoxy impregnation.

And the problem with this is that over years the epoxy degrades and as temperature rises and becomes lossy. So on turn on the brightness on the CRT looks fine. But over five minutes or so the transformer heats up, the epoxy becomes more lossy, and the brightness fades out.

For the 576 curve tracer, which was still a current product when the epoxy problem raised its ugly head, Tek revised the transformer design to use silicone impregnation.

The characteristic of a dying CRT is something called "double peaking". As you increase trace brightness, it fades out a bit, then comes back but out of focus.

The other problem that sometimes occurs is that the voltage multiplier develops a fault in one of the capacitors, or the final capacitor C836 from the CRT 8,300V to ground.

Craig

[Craig Sawyers]

This is the transformer winding spec.

[Diabolical Artificer]

Am pretty sure my EHT tfmr are waxed paper, but could be wrong.

Interesting developments this morning, I measured some voltages around the EHT tfmr sec taps and got - EHT tfmr "pins" 7 800v RMS, 8 575v RMS & 9 600v RMS, so a bit low I think. One thing I noticed was that the DC at V862 was around 100v DC, therefore I suspected the diode and replaced with 3 silicon diodes (2kv) in series, powered up, still no trace, EHT still low. After some more heead scratching and taking more readings I heard a sizzling and a nasty wiff - R602 was red hot.

R602 de-couples the +325v rail and supplies the EHT tfmr primary CT, therefore either the EHT tfmr is suspect, C802 is leaky or something on secondary side of the EHT tfmr is overloading it. I suspect the latter, maybe a HV cap. I checked C802 previously for leakage, it tested ok.

Although this fault is bad, it's the first strong indication of something wrong, an indication of where to look. Lastly I measured the EHT at C836, the aquadag or final anode or whatever you call it, that was around 4000v, low but considering oscillation at the primary is low too, I think ok, indicating that part of the circuit is ok. I'm strongly suspecting something pulling down/overloading the diodes/caps that feed the cathode.

Lastly I measured the EHT diode heater voltage = 1.7v, weirdly over it's 1.25v spec.

[Diabolical Artificer]

Making progress, thank bog, after hours of testing I think I've narrowed it down to the CRT, IE a heater cathode short or an insulation breakdown of the power tfmr. If you look at the attached screenshot, theres a 27k resistor - R857 across one side of the CRT heater and cathode, I disconnected it at the cathode/C858 end = low EHT, reconnected that joint and disconnected R857 and two wires which I presume go to the heater = EHT at spec/all good.

With this in mind what would you say is the best candidate, CRT or mains tfmr? Andy.

[Diabolical Artificer]

Changed CRT, no improvement, therefore I either have two duff CRT's or one duff mains tfmr, if that's the case though one would think -1kv is either leaking to ground or another tfmr winding. I'll run an insulation check tommorrow, enough for today.

Edit attachment for previous post

[MotorBikeLes]

Andy, heater to cathode shorts are quite common in CRTs. A common fix I often did was to get a little RS mains transformer with a 6.3v secondary, but SPECIFIED high voltage insulation between prim and sec.
Fixed a few scopes that way.
Les.

[Diabolical Artificer]

Thanks Les, I suppose an insulation test from heater to cathode should reveal a fault, good idea re separate tfmr.

Andy.

[Craig Sawyers]

Odd that two CRT's exhibit precisely the same issue.

Craig

[Craig Sawyers]

Originally Posted by Diabolical Artificer

Am pretty sure my EHT tfmr are waxed paper, but could be wrong.

The 545B EHT transformer is very definitely epoxy potted.

Unless some previous owner has put in a wax impregnated transformer from a 545A. I don't have the winding spec for the 545A transformer, but it should be similar, although the 545A CRT is quite different, with 4 vertical divisions rather than 6 with the 545B.

Craig

[Diabolical Artificer]

Don't think it is the CRT's Craig, tested one this moaning between htr and k @ 1kv, seems ok, I then tested the mains tfmr between pins 24/25 (CRT htr) and pins 27/28 & pins 20/21 which the 120-0140-00 power tfmr spec sheet gives as the windings nearest, top/bottom of 24/25 @ 1kv, my tester couldn't do it, IE voltage failed to rise. Same test on a spare 585A tfmr it can.

Confusingly my tester shows about 1000meg insulation on the 545B tfmr and that winding is supposed to be insulated to 1350v.

Am doing some more insulation testing just now but the fault seems to be a will o the wisp.

Andy.

[Diabolical Artificer]

Well, got there in the end, I now have a working 545B scope again after a lengthy fault finding process. It was the mains tfmr, which means both of my 500 series scopes have had power tfmr transplants. I suppose after 70 years the insulation breaks down probably from either drying out and cracking or moisture ingress, my bets on the former, it is hot inside these scopes.

For the record V800/6AU5 oscillation voltage is 85v RMS. I swopped the EHT diodes for silicon diodes which means EHT is higher, adjusted right down this put it -1400v - 50v over spec, hopefully that's ok. The trace is brighter than it was with plenty of adjustment on the intensity and focus.

Think that about wraps it up, thanks for all your contributions, Andy.

[Craig Sawyers]

Wow. I would not have betted on the mains transformer. Well done tracking it down Andy.

Craig