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 H.M.V. V8-AM needs new H.O.T.
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 Return to top of page · Post #: 1 · Written at 2:29:51 PM on 11 July 2017.
Sirwin's avatar
 Location: QLD
 Member since 10 April 2009
 Member #: 465
 Postcount: 59

Hi all.

I have a friend who has a H.M.V. Windsor television model V8-AM, which was working until recently. He has diagnosed a faulty horizontal output transformer. Would anyone here have such a thing for sale? Unfortunately, I don't have the part number.

Cheers, Stuart


 
 Return to top of page · Post #: 2 · Written at 7:54:39 PM on 11 July 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 543

Actually Stuart, it's possible but very unlikely that the transformer is bad in that model. It's a very common misconception though.

The transformer is made by MSP (AWA) and it is quite normal for it to drip lots of wax, if that's why your friend thinks it's bad. But in more years in the service business than I care to recall I have NEVER encountered a bad one of those transformers, they are actually very reliable. Because of that there will be very few of them around.

Time to measure some voltages I suspect. Start with 6CM5 G1, G2 and B+ boost voltages. Boost cap is most likely culprit.


 
 Return to top of page · Post #: 3 · Written at 8:38:56 PM on 11 July 2017.
Johnny's Gravatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 141

I agree with Ian,
Don't recall a faulty LOPT(FBT) , used in this chassis ever.(45 years in the service industry).
Thought I had one once and was badly chastised by the boss when proven wrong!
JJ


 
 Return to top of page · Post #: 4 · Written at 9:15:44 AM on 3 August 2017.
Sirwin's avatar
 Location: QLD
 Member since 10 April 2009
 Member #: 465
 Postcount: 59

Hi Ian and Johnny. Thanks for your reply. Sorry for not replying earlier but I had completely forgotten that I asked this question in this forum! Wouldn't it be great if we received an e-mail alert when one of our threads was answered.

I'll let the owner know of your experiences.

Cheers, Stuart


 
 Return to top of page · Post #: 5 · Written at 9:40:39 AM on 3 August 2017.
Sirwin's avatar
 Location: QLD
 Member since 10 April 2009
 Member #: 465
 Postcount: 59

Hello again.

Well I assumed that because this was a later set the H.O.T. wouldn't be wax coated, but plastic encapsulated as I have seen on late model valve TVs. The problem may have been solved.
Years ago, I restored an HMV M1-A4 in which the H.O.T. was overheating. I checked everything, but the B.H.T. would fall as the transformer warmed and the picture would shrink horizontally. I changed all resistors and capacitors at least twice as well as the valves in the horizontal output section. I had a spare H.O.T. and tried that, but had the same fault. Eventually I twigged that it was the wax coating itself that had failed on both transformers. I melted it completely away in the oven and sprayed it liberally with circuit board lacquer. That was a complete cure! The set works well to this day and that was 8 years ago.

Cheers, Stuart


 
 Return to top of page · Post #: 6 · Written at 10:15:06 AM on 3 August 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 543

Hi Stuart

I've never known one of those MSP transformers to fail, and I certainly saw a lot of them, in HMV and AWA TVs. Certainly not a failure of the wax coating. In the HMV P1 models - they were similar to the M1 but with Philips deflection parts - the EHT winding would regularly go shorted turns, and that was an epoxy sealed winding with polyester insulation. There was a reason for that which I won't go into here, it'd take too long.

Although......

Back in the late 60s I vaguely remember seeing an HMV chassis in the workshop - probably an M1 - in which there had been a fire in the cage. So I'm willing to believe it could happen.

I doubt it was a failure of the wax, I can't see the mechanism for this happening. A more likely explanation would be hygroscopic uptake in the inter-winding-layer insulation. It is paper after all. Your heating it would have driven the water out and the lacquer would have sealed off any damage.

The HMV M1 dates from 1959 and that is getting on a bit. In my experience a S/C S correction cap was the most common cause of loss of width in that model. Oh, and the width coil would go shorted turns and burn up.


 
 Return to top of page · Post #: 7 · Written at 9:29:12 AM on 4 August 2017.
Sirwin's avatar
 Location: QLD
 Member since 10 April 2009
 Member #: 465
 Postcount: 59

Hello Ian.

I think is was the wax that had failed. After all,it is hygroscopic. It did appear that it was the wax over the overwind that was getting hot rather than the winding itself. I presume that the dielectric properties of the wax became lossy, thus dissipating power. Increasing heat exacerbates the losses. I don't think it could be a case of electrical leakage, as there was no sign of carbon tracking, and it would mean that the wire insulation had broken down, leading to quick destruction of the wire.

Interestingly, I have an early electrostatic deflection television (Hallicrafters 505) that displays similar symptoms, except that it is just the E.H.T. that drops with time, causing blooming and loss of brightness. The E.H.T. is derived from an R.F. oscillator. I intend to give the oscillator coil the same treatment, - one day, when I've got more time.

Cheers.


 
 Return to top of page · Post #: 8 · Written at 9:04:52 AM on 8 August 2017.
Sirwin's avatar
 Location: QLD
 Member since 10 April 2009
 Member #: 465
 Postcount: 59

Hello.

I can confirm that the H.O.T. in question is faulty. It tests bad on a H.O.T. tester. So help still required please.

Cheers, Stuart


 
 Return to top of page · Post #: 9 · Written at 3:28:37 PM on 8 August 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 543

Those HOT testers don't always get it right in my experience. I'd check it with a scope and a battery.

Very unlikely you will find an original part.

What you can use is a Philips NT3101, NT3101A or with minor modification, NT3102 which is relatively common.

If you can't find one anywhere and if the CRT is bad in a potential parts set I have, I may be able to help you with an NT 3101.


 
 Return to top of page · Post #: 10 · Written at 11:32:28 AM on 12 August 2017.
Irext's Gravatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 31

The scope and battery test works quite well. Once you set it up and observe how the transformer "Rings" on the scope when you disconnect the battery, use a quick connect lead to introduce a shorted turn by wrapping it around the ferrite core and clipping the ends together. If the ringing waveform disappears then the transformer is likely o/k. If no difference is noted then the Tranny is suspect. I made one up using a 9V battery, a 1k resistor, a momentry press switch and a small jiffy box to house it. Works well with valve audio amp output transformers also.

Cheers.


 
 Return to top of page · Post #: 11 · Written at 1:46:58 PM on 12 August 2017.
Sirwin's avatar
 Location: QLD
 Member since 10 April 2009
 Member #: 465
 Postcount: 59

Thank you Irext and Ian. What I do have here is a chassis out of a 50-00 series AWA TV. Any likelihood that that H.O.T. will work?

Cheers, Stuart


 
 Return to top of page · Post #: 12 · Written at 11:16:32 PM on 12 August 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 543

Yes, it is pretty much the same transformer, both run in stabilised mode in very similar circuits and use parallel yoke windings.

The transformers' pin IDs are different though. Look at the 2 circuits and you'll be able to work it out. If not, I could help you.

Note that the HMV circuit is drawn upside-down WRT the AWA's.

It's very important with this circuit to set the B+Boost voltage correctly with the Stab pot and to set the hor. linearity for minimum current through the 6CM5. If the transformer has really failed it is probably due to these items not being set correctly.

While you are at it, it's a good idea to fit a 1.5k 5W resistor in series with pin 9 of V6, the 6Y9 video amp. They suffer from G2 - G1 flashover which takes out the video detector diode. The change also prolongs the life of the 6Y9. You should also consider changing V4 and V5, the 6U9 and 6X9. Those Decal valves aren't known for their longevity and a low emission IF amp can cause sync problems.


 
 Return to top of page · Post #: 13 · Written at 10:42:32 AM on 18 August 2017.
Sirwin's avatar
 Location: QLD
 Member since 10 April 2009
 Member #: 465
 Postcount: 59

Update:

The transformer required has a part number of 52500. The one in my 50-00 series chassis is 52536. There do seem to be considerable differences. The letter connection designations don't seem to match. For example, the 6CM5 anode and damper cathode connect to C and D respectively in the AWA, but D and E in the HMV. I don't know whether the two TVs use different taps, or if the actual taps in the transformer are wired to different terminal positions. I'll send it to him anyway.

Thanks for your advice.

Cheers, Stuart


 
 Return to top of page · Post #: 14 · Written at 9:04:48 PM on 19 August 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 543

You need to read the two circuits to derive the correct connections.


 
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