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Old 9th Feb 2007, 6:01 pm   #1
TNC
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Default EHT problem

Mains EHT on my Baird T18 is too low (2kv ish) - should be nearer 5...so given its a simple circuit it is either the transformer, the HV cap or the HVR2 rectifier...the tranny is running cool so hopefully is OK so that leaves the cap and rectifier...given my respect for mains EHT any comment would be greatly appreciated before I start to investigate!!
Could the rectifier suddenly lose emission or is the cap the more likely culprit?
Grateful for any thoughts,

Trevor.
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Old 9th Feb 2007, 6:22 pm   #2
Dave Moll
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Default Re: EHT problem

My inclination would be to start by pointing the finger (metaphorically speaking) at the capacitor. If it's a Visconol, then it's almost certainly draining off most of your EHT to chassis - and stressing the HVR2. Although I have heard rumours of such things, I have not yet met a non-leaky Visconol EHT capacitor.
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Old 9th Feb 2007, 7:26 pm   #3
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Default Re: EHT problem

The only argument against Dave Moll's suggestion is that the transformer isn't showing any signs of distress. If the cap is a Visconol Cathodray it should be replaced anyway.

If you disconnect a leaky cap altogether, the voltage should rise though there will be a lot of ripple. The rectifier can be checked by substituting a suitable EHT silicon diode. At a pinch, half a dozen 1N4007 in series, each with a high value resistor (>1M) across to equalise the reverse voltages. Don't forget a surge limiter in series, perhaps 4K7, though the transformer winding resistance will help.

I don't think I need to remind you of the safety precautions.

PS: Just realised that for a half wave rectifier like this the reverse voltage rating must be twice the peak voltage. So more than 10KV for this set. That means 10 off 1N4007 if you use that approach and a rather lethal string of bits that will need careful insulation.
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Old 9th Feb 2007, 7:29 pm   #4
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Default Re: EHT problem

Look on my site for those - www.oldtellys.co.uk

First section on the Line/EHT section.

Cheers,

Steve P
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Old 9th Feb 2007, 8:38 pm   #5
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Default Re: EHT problem

Hello Trevor,
I think Jeffrey has got it on the spot. With mains eht the usual problem is breakdown of the transformer. Usually this is spectacularm with arcing and smoke together with wax or tar bubbling from the windings. Small breaks in the eht winding will induce flashover with again the fireworks.

If the Visconol is leaking it will get quite warm and will fail completely after a short period. Mains eht takes no prisoners. It could go O/C but I have never known this, always S/C or leaky. HVR2's are a very good valve and most mains eht sets using them still have their originals in good condition. Failure usually results in its appearance changing to that of a neon lamp....

It's worth checking any series smoothing resistor that may be between the two EHT caps, if it uses two, that is.

You must be very careful and after unplugging the mains; short circuit the eht supply [both caps] and leave the shorting links in place while doing your tests. Only remove the links when you are sure that NO PART of the eht system retains a charge. There is usually a bleeder chain of resistors across the supply that will discharge the capacitors after a short period, but faults in these or the circuit will prevent this.

You will have to take voltage checks with an EHT meter but I don't know how familiar you are with these circuits, so would advise against this until you know the score!

Regards

John.

Last edited by Mike Phelan; 3rd Nov 2007 at 11:47 am. Reason: Breathless!
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Old 9th Feb 2007, 8:45 pm   #6
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Default Re: EHT problem

Hi Trevor, if you replace the HVR2 with a silicon diode (try the RS focus diode types, which I think are rated to 18KV). This will lift the EHT by about 5% as the rectification drops are less. PM me with details if you need a new cap as I have some ex-gov ones that can be tested to see if they are leaking, and should be OK (but are quite big)

Ed
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Old 9th Feb 2007, 10:28 pm   #7
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Default Re: EHT problem

Quote:
Originally Posted by ppppenguin View Post
The only argument against Dave Moll's suggestion is that the transformer isn't showing any signs of distress.
I'm sure your right, Jeffrey, but the transformer in my Regentone TR20 was showing no visible sign of distress at the time when the Visconol was leaking the EHT to ground. The HVR2 on the other hand was giving a most impressive light show. It would appear though that the EHT winding had sustained hidden damage, as it later (some considerable time after having substituted a non-leaky EHT capacitor) gave the sort of firework display described earlier - resulting in a back voltage from the capacitor that (as I understand it) vaporised the cathode of the HVR2.
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Old 10th Feb 2007, 1:46 am   #8
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Default Re: EHT problem

Thanks guys for all your helpful comments. I think it's turned out to be less of a problem than first appearances suggested.
Whilst I was checking the EHT level, I noticed a slight "growling" from the speaker and wondering what that might be I suddenly considered it might be the result of leakage from the original EHT cable - . I therefore re-routed it as best I could - particularly away from the frame-coil "cradle" - and that restored EHT to just under 5kv and removed the growl.

I therefore think it would be wise to replace the cable - it's black so I need to source a couple of feet...is that something that CPC might stock? Alternatively I guess I could black-sleeve some modern EHT cable??

Last comment...just to say that this EHT arrangement boasts no bleeder chain so remembering to discharge the cap is important!!

Thanks again,

Trevor.
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Old 10th Feb 2007, 9:58 am   #9
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Default Re: EHT problem

Hello again Trevor,
Car ignition cable makes a 'thicker' replacement and can be bought by the foot. J.
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Old 10th Feb 2007, 11:35 am   #10
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Default Re: EHT problem

Trev
It seems you can only get modern eht cable in red, so you would have to sleeve it; better to use John's suggestion of car HT cable. It's also easier to buy in small quantities; next time I need some this is what I shall use.
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Last edited by Mike Phelan; 3rd Nov 2007 at 11:50 am. Reason: Punctuation needed
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Old 10th Feb 2007, 11:59 am   #11
Sean Williams
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Default Re: EHT problem

I have some EHT cable here somewhere - Black, red and white.........

Cheers
Sean
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Old 11th Feb 2007, 1:41 pm   #12
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Default Re: EHT problem

If you're going for car ignition cable, make sure you get the kind with copper inner, and not the type sometimes sold which seems to have a sort of rubberised string inner - done for ignition interference suppression reasons.

Alternatively, if you're prepared to sleeve it for aesthetic reasons, how about using the inner plus dielectric of a length of, say, UR43 co-ax cable with the outer and braided screen removed? At the sort of voltages you're talking about, this should work fine.
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Old 11th Feb 2007, 7:43 pm   #13
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Default Re: EHT problem

Hi Ray...now that sounds like an excellent idea...if I can ask what UR43 is?
I presume it's a "standard" - so can you tell me where it might be available?

Many thanks,

Trevor.
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Old 11th Feb 2007, 8:03 pm   #14
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Default Re: EHT problem

TNC:
Quote:
...if I can ask what UR43 is?
I presume it's a "standard" - so can you tell me where it might be available?
It's a standard RF cable (made by UniRadio, amongst others, I think, hence the type number) which is broadly equivalent to RG58.

Both types, if I remember correctly, designed to fit into type TNC connectors

But don't feel constrained to use that type: almost any decent (non-subminiature) RF cable would do, so long as its got solid polyethylene-type dielectric of a reasonable diameter around its inner conductor. You might even be able to use TV co-ax downlead - but make sure that it's not the foam-dielectric type since this might absorb water and go leaky on you. Also, most TV downleads have solid inner conductors, which make them rather less flexible and any jointing at the ends more likely to fracture with movement and time.

I mentioned UR43 since I happened to have previously used it successfully at EHT. It has a stranded inner conductor. The equivalent RG58 is obtainable at Maplins - code XS51F - cost 0.89 per metre. Comes covered in a PVC sheath, so if you were lucky you might be able to pull the inner+dielectric out from the outer+sheath, tease out the braided sheath and replace the PVC outer as a covering.
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Old 12th Feb 2007, 3:43 pm   #15
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Default Re: EHT problem

Hi Ray and Trevor,
I used RG58 on my HMV 1804 and did the trick that Ray suggested of removing the screen and putting the outer sheath back on. It looks quite good.
Many early flyback EHT sets simply used the RG58 inner conductor on its own for the EHT cable and line output valve anode connections.
Pye were quite fond of using the stuff.

Cheers
Andy

Last edited by Mike Phelan; 3rd Nov 2007 at 11:51 am. Reason: Typos
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Old 13th Feb 2007, 1:34 am   #16
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Default Re: EHT problem

Thanks to all for suggestions...eventually I sauntered down Leigh Vintage Motorbikes (Leigh-on-Sea) - a real treasure trove of bike parts (and very pleasant owner) and he provided a length of HT cable which is black, not too fat and does the job perfectly....70p a foot.

My next problem requires a fresh thread!

Trevor.
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