Author Topic: HV PSU resto, metal rectifiers  (Read 13136 times)

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Offline GK

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Re: HV PSU resto, metal rectifiers
« Reply #25 on: March 02, 2015, 09:25:15 am »
Yes, the crank is only 3 main bearings but it is heavily over-engineered and the bearing journals are huge. The little 2.25L has a solid roller camshaft (and advance never seen on a Holden 6) and delivers a good spread of torque and peak power for its capacity and vintage. It was rated at 70bhp D.I.N at only 4000 rpm. That is a genuine, installed, at-the-flywheel figure. Like many other manufacturers Holden specified their engines (before the compliance laws and regulations were tightened up) bhp S.A.E. which was often inflated by anything up to 30%. The 186 (3L) from memory was specified at ~120 bhp, but 80 bhp is a more realistic figure.
Really, the bigger 186 was only marginally more powerful than the 2.25L and it had to be revved just as hard (if not a little more) to deliver. A modified 186 would be a different story though. They were a popular swap for a clapped out 2.25L back in the day down here simply because they were common as dirt and cheap.
 
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« Last Edit: March 02, 2015, 09:29:42 am by GK »
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Online tautech

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Re: HV PSU resto, metal rectifiers
« Reply #26 on: March 02, 2015, 10:37:57 am »
Yes, the crank is only 3 main bearings but it is heavily over-engineered and the bearing journals are huge.
Yep, they even used the same crank & block for the diesel engine, but it IIRC was only ~65 hp, but you wouldn't want to mod that for more hp with only 3 journals.
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Offline GK

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Re: HV PSU resto, metal rectifiers
« Reply #27 on: March 02, 2015, 12:42:58 pm »
Okayyyyyy............. back on topic. This PSU restoration project actually began over 15 years ago when I was still living at home with my parents. That is when I acquired the first of the Marconi units. I didn't have a manual for it then and I still can't locate one on the web, so I had to reverse engineer it as I pulled it apart. After pulling it apart I confirmed that I reverse engineered it correctly by rebuilding it on a plank of wood on my bedroom floor. My original notes:



The supplies parts were then boxed up and that is how they stayed until just recently! For my own records, I am now in the middle of redoing my circuit diagrams. Here is the bulk of the circuitry for the Marconi chassis:



I initially thought that the voltage feedback arrangement was a little odd, but once I studied the circuit a little more I realized that it is actually quite a clever circuit. The current for the VR tube (V3) is sourced from the clean, regulated side of the supply, but irrespective of the setting of the regulated output voltage, the current through the VR tube is essentially held constant.

V4 and V5 form a long-tail pair error amplifier. The output voltage stabilizes when the grid of V5 is at the ~same potential as the grid of V4 (which is at the fixed 85V reference potential derived from the VR tube). For the grid of V5 to reach 85V, the screen of V5 is maintained (regulated) at a little under 200V due to the voltage divider formed by R17 and R12. So a constant current of ~ [(200V-85V) / (R18+R19)] flows through the VR tube. This constant current is sourced from the regulated output via the H.T. Volts setting pot and associated range-switched resistors. So the output voltage is simply ~200V + the constant current times the variable voltage setting resistance. C3 is a Miller feedback compensation capacitor that incorporates the series pass tubes. C6 bootstraps the resistive anode load supply for V5, making it approximate a constant current source at frequencies somewhat above DC.
 


 
« Last Edit: March 02, 2015, 12:55:31 pm by GK »
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Offline GK

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Re: HV PSU resto, metal rectifiers
« Reply #28 on: March 21, 2015, 06:06:57 am »
The other one is a 3kV PSU (Fluke 415b)  with  2uF 3kV oil caps that are not that easy/cheap  to find.
Somewhat similar ones can be found
(277$ each (by 8 ) at Mouser) (see attached pict)


I can't think of many situations where I would seek out an expensive new oil cap to replace an old oil cap. High voltage film or electrolytic capacitors in series if necessary with parallel voltage-balancing resistors are the most practical way to go. In this case I didn't need to resort to series-connected caps. I replaced the 8uF/1000V oil caps with relatively inexpensive (~$14 each) 8uF/1100V film caps that were readily available.     


« Last Edit: March 21, 2015, 06:10:14 am by GK »
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Offline GK

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Re: HV PSU resto, metal rectifiers
« Reply #29 on: March 23, 2015, 09:06:43 am »
One of the chokes for one of the Marconi supplies unfortunately pooped itself, shorting the +843V unregulated supply rail to chassis. All was fine until I powered it up again with my CRO lead ground clip connected to the circuit ground then sitting at-843V. The inductor sizzled for a second and then the mains fuse went.

Does anyone know what kind of varnish these things are potted in? Is it shellac? I've had it sitting in acetone for while but so far it looks like it will need to soak for a week before I can extract the individual lamination's.
 
« Last Edit: March 23, 2015, 09:11:03 am by GK »
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Offline T3sl4co1l

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Re: HV PSU resto, metal rectifiers
« Reply #30 on: March 23, 2015, 01:10:41 pm »
If it is shellac, it should dissolve in ethanol I think... should think acetone as well.  Don't know that shellac has ever been used in such thick layers, though?

I would suspect varnish, but if that's what it is, you aren't getting it off with simple solvents.  Suggest fire?

Curious that it seems to be flaking away.

I'd suggest methylene chloride, but if you were hoping to reuse some of the wire, that wouldn't work out so well... :P

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Offline SeanB

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Re: HV PSU resto, metal rectifiers
« Reply #31 on: March 23, 2015, 06:18:03 pm »
That would be a dipped shellac, which is pretty much going to ignore acetone. Dissolves in methanol quite nicely, and as you are going to be making a new pressboard former for it in any case I would suggest a very simple method, which is place it in a small tin box and make a fire with it inside for about 5 hours. Then open the tin after it is cool, and dunk the charred mess in methanol overnight, then wire brush the char off till you can undo the bolts, then take it apart.

The paper insulation has gotten moisture in it, and then carbonised to create the ground short. Use high temperature wire on the new one, and at least 5 layers of kapton tape between the first winding and the paper former, then a single layer of the kapton tape per layer. First turn use a PTFE sleeve for extra insulation, as it likely arced there first. Bonus is that it will be a lot less bulky with the kapton tape, and the number of windings is not as critical, provided you fill most of the core. Just note where the air gaps are and put back the same way. You can varnish it after winding, but with modern tape and wire it will not be needed.

The common field fix for these shorted chokes was simply to undo the 4 bolts and place the whole choke on 4 20mm long plastic spacers so that the live case would not touch ground. Works, but the choke case is then part of the B+ circuit and is at high voltage.
« Last Edit: March 23, 2015, 06:20:12 pm by SeanB »
 

Offline T3sl4co1l

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Re: HV PSU resto, metal rectifiers
« Reply #32 on: March 23, 2015, 07:49:30 pm »
If you knew which end is shorted to ground, you could possibly move it to the ground side of the rectifier / transformer CT! :P  I forget offhand if that's even possible in the circuit...

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Offline SeanB

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Re: HV PSU resto, metal rectifiers
« Reply #33 on: March 23, 2015, 07:57:12 pm »
It is a HT choke, only one winding and 2 connections. Part of a LC filter on the HT supply.
 

Offline GK

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Re: HV PSU resto, metal rectifiers
« Reply #34 on: March 24, 2015, 09:04:35 am »
Actually the acetone appears to be working its magic - it just takes time. 24 Hours later and I can see bare lamination's. The acetone I had it sitting in has evaporated but I now have the choke fully submerged in an enclosed container. I'll just let it soak for a couple of nights and see what happens.

« Last Edit: March 24, 2015, 09:08:42 am by GK »
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Offline GK

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Re: HV PSU resto, metal rectifiers
« Reply #35 on: March 28, 2015, 09:17:41 am »
The acetone soak did the job, dissolving the shellac (I presume that is what it was). There was no saving the original bobbin, however. I had to hacksaw through the windings and then pull them out with vise grips with the core gripped in the bench vice. The core was gaped with a sheet of that phenolic-like stuff that the bobbin is made of. I think I'll make a replacement bobbin with rectangles of etched FR4 bonded with Araldite.
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Offline SeanB

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Re: HV PSU resto, metal rectifiers
« Reply #36 on: March 28, 2015, 09:55:31 am »
Make extended end cheeks for the bobbin, so that the leadout wire does not come into contact with the core lessening the insulation requirements. Lots of tape as well for the rewind, and at least for a choke you do not need an exact number of turns, just enough to mostly fill the winding area, then a generous layer of tape to cover and then an overcoat of varnish.
 

Offline GK

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Re: HV PSU resto, metal rectifiers
« Reply #37 on: March 29, 2015, 09:36:48 am »
Yeah, I'll certainly make some end plates to the bobbin. That will also make it ten times easier to wind.
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Offline SeanB

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Re: HV PSU resto, metal rectifiers
« Reply #38 on: March 29, 2015, 09:49:49 am »
Yeah, I'll certainly make some end plates to the bobbin. That will also make it ten times easier to wind.

Make it a coreless self supporting pancake winding, without anything other than varnish holding the windings together. That will be a challenge.  >:D
 

Offline Richard Head

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Re: HV PSU resto, metal rectifiers
« Reply #39 on: March 30, 2015, 05:39:07 am »
I wonder if you were able to count the number of turns? I suspect not looking at the windings!
 

Offline GK

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Re: HV PSU resto, metal rectifiers
« Reply #40 on: March 30, 2015, 10:51:31 am »
It was impossible to unwrap the windings but fortunately the value of the inductance isn't critical at all so I'll just fill the winding window with the same gauge wire.
If I was paranoid I could wind a layer onto the new bobbin, assemble and then measure the inductance to compute the cores AL value. Then I could compute the number of turns for the original inductance.


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Offline Richard Head

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Re: HV PSU resto, metal rectifiers
« Reply #41 on: March 31, 2015, 07:36:27 pm »
If I was paranoid I could wind a layer onto the new bobbin, assemble and then measure the inductance to compute the cores AL value. Then I could compute the number of turns for the original inductance.
I would do that. If you put on too many turns you may well saturate the core.
 

Offline T3sl4co1l

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Re: HV PSU resto, metal rectifiers
« Reply #42 on: March 31, 2015, 07:43:35 pm »
DOIT! Swinging choke... ;)

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Offline GK

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Re: HV PSU resto, metal rectifiers
« Reply #43 on: March 31, 2015, 11:04:18 pm »
I can't see myself getting substantially more turns onto the core than it originally had. That bobbin was packed so damn tight it's a miracle they managed to slide it onto the core.

Also, saturation is not an abrupt phenomenon in an inductor like this and a large value of variation in the inductance is easily tolerable. And in addition to that even if I did somehow wind up too many turns and have an excessive degree of saturation all I'd have to do is slightly increase the core gap. So then I'd have the original value of inductance but with a little higher DCR than original. That's not remotely critical either.
« Last Edit: May 09, 2015, 01:05:40 pm by GK »
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Offline GK

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Re: HV PSU resto, metal rectifiers
« Reply #44 on: May 09, 2015, 12:55:37 pm »
Well I haven't found the motivation to hand wind this inductor yet, but this afternoon I finally knocked off the one of the two Marconi supplies whose inductors are still OK. Here it is in all its glory. The scope screen shot shows the regulated output ripple and noise with the output voltage set to 400V and a load current of 130mA. Most of the HF component is courtesy of the local AM radio broadcast station.

This is a power supply that needs to be treated with respect. On the highest voltage range the potential at the "UNREG" binding post is in the vicinity of 800VDC.
You don't want to accidentally grab hold of that binding post instead of one of the 6.3VAC ones.









« Last Edit: May 09, 2015, 01:08:13 pm by GK »
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Offline JacquesBBB

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Re: HV PSU resto, metal rectifiers
« Reply #45 on: May 09, 2015, 01:54:06 pm »
Very impressive !!

You put the level quite high. I will certainly not try to reach this degree of perfection when I will try to restore my HV power supplies.
 

Offline GK

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Re: HV PSU resto, metal rectifiers
« Reply #46 on: May 09, 2015, 11:20:42 pm »
Thanks, though I thought that my wiring was actually a little messy.  ;D

One thing that I knew about, but forgot when finishing this thing off (d'oh) is the supplies behavior when warming up and when switching ranges. The power output tubes warm up before the pentodes in the regulator circuit do. This causes the output V to over shoot / saturate towards the unregulated potential for few seconds until the pentodes come up to temperature. The same happens briefly when switching ranges, as the range switch that modifies the feedback resistance is not a make-before-break type. As a consequence feedback is temporarily lost when the switch wiper is in motion and the output voltmeter pegs briefly to the right.

The original electrolytic output capacitor had a marked rating of 450V "WKG" (working). The surge voltage capability must have been substantial however, as on the highest voltage range the unregulated potential is about 800V and the temporary spike in the output voltage is only limited by the saturated series resistance of the two 21E1 triode-connected series-pass power tetrodes in parallel.

A modern 450V rated electrolytic doesn't doesn't seem to fare too well in comparison, however. On my first range change it went out in spectacular fashion, filling my workshop with that stinky white smoke. For the output capacitor I ended up soldering two 450V electros in series with voltage balancing resistors.


 
« Last Edit: May 09, 2015, 11:24:59 pm by GK »
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Offline SeanB

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Re: HV PSU resto, metal rectifiers
« Reply #47 on: May 10, 2015, 07:27:57 am »
I have some old tantalum capacitors with a working voltage of 35V, but they also are rated for a 70v surge. Others were rated for 40V but have a 130VDC surge rating.  Likely the old 450V unit would survive 700V with only an increased leakage, as the thicker foil and paper would survive the short overvoltage for a few seconds with only some extra oxide forming.
 


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