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 What to do if you can't get a part - a case study
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 Return to top of page · Post #: 1 · Written at 1:55:31 PM on 5 July 2016.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 578

I'm currently restoring an HMV E1.

This TV has (had) so many faults, if the CRT was bad you just wouldn't do it.

When you can't get things like a vertical blocking oscillator transformer or the special pots this TV uses for the minor controls, some ingenuity is needed. In the case of the pots I found I could fit a PCB mounting pot into the metal trays that held the worm driver slider pots. Altronics R1910 fits nicely and almost matches the look of the original knobs. See pic:

HMV E1 Pot Replacement


When I found that one winding on the pitch-impregnated original TR3 was open, I first thought we were stuffed! But then, recalling that many TV designs don't use a blocking oscillator transformer, instead running the triode-pentode as an astable with feedback from the pentode anode circuit to the triode grid, I thought this would be worth a try. See the PDF of my modified vertical circuit:

HMV E1 Vertical Circuit Diagram

What's a little unusual here is the injection of the sync into the grid of the output stage, because the sync is negative-going in the E1. It took a little fiddling with the vertical integrator circuit to get a good solid lock. See the result:

HMV E1 Showing Philips 5544 Test Card


It looks promising, doesn't it? The CRT is a no-ion-trap regun of the original AWV 17HP4B. The aquadag is painted over the original transfer on the glass!

Despite tracking down many intermittent faults, replacing all paper caps and many well-out-of-tolerance resistors (a long story for another time) there is still more to do. As with all E1s and E2s, the EHT transformer was replaced very early in the set's life. This one has a Radar part (most were Rolas) and I'm not absolutely sure it is wired correctly. Note the crushing at the 2nd white castellation (this gets much worse if you wind up the width). It is unaffected by the linearity coil setting which effectively functions as a width control. The drive waveforms are (now) spot-on, with many bad parts replaced and a new 6BL8 fitted. Tried new 6CM5 and 6AL3 - no difference. And the "crinkles" on the left hand side - this is the best result obtainable by adjusting the variable cap on the yoke.

Does anyone have a copy of the original EHT replacement service bulletin from HMV? Or any clues as to what's going on?


 
 Return to top of page · Post #: 2 · Written at 8:11:05 PM on 5 July 2016.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 4321

Files uploaded.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
Brad.

A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 3 · Written at 8:16:51 PM on 8 July 2016.
TV Collector's Gravatar
 Location: Ballarat, VIC
 Member since 4 January 2011
 Member #: 803
 Postcount: 456

HMV released numerous modifications for the E1 chassis which makes any restoration of these sets more difficult than usual. Add the potential for unofficial mods by various repairers over the years, the end result can be a long way from when it left the factory.

I have some mod sheets including the one for replacing the short lived EMI 5/3T CRT with a 17HP4B. None of the sheets I have cover replacement EHT transformers though I am missing several of the early bulletins.

I also had a look though whatever old documentation I have for these sets and there is nothing relating to replacement EHT transformers on the E1 chassis.

The mod sheets do refer to problems with the sync separator and horizontal AFC circuits so these may be a possible area to investigate. There was also mention of parasitic oscillations in the horizontal output stage. Chokes needed to be fitted to all receivers to make sure they were eliminated.


 
 Return to top of page · Post #: 4 · Written at 1:46:21 PM on 10 July 2016.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 578

Thanks TV Collector.

This set has had the sync separator and hor AFC mods done very early in its life by the look of things. I would say the sync sep mods were intended to improve the interlace performance, which is still not perfect, which is why I guess that the E2 got separate hor and vert sync seps. Chokes have been added everywhere around the hor OP and the damper. And you can tell the CRT socket has been rewired a long time ago to take the 17HP4B.

This morning I removed the yoke to check for possible insulation or shorted turn damage. I found none but I was worried that if any possible damage proceeded further the CRT neck would be cracked. It would be a disaster to damage this CRT, it looks like it still has a lot of life ahead of it, the getter flash looks as good as the day it was made.

There is no sign of heat in any windings and the 6CM5 is drawing 65 to 70mA depending on the brightness so nothing is distressed. Everything scopes out OK. When the width is wound out the crushing is extreme. Disconnecting the width coil makes it worse, bypassing the Hor Lin coil doesn't help either. It's completely unaffected by beam current change, which is not what I'd expect.

This one's got me beat at the moment and I don't say that often! It's the last remaining fault!

The only thing I haven't yet checked are the anti-parasitic chokes that were added a long time ago.

If there was some way of getting copies of the mod sheets you have for this set I'd really appreciate it...

BTW, have a look at the resolution on the PM5544 test card. Remember this is being fed from an RF modulator on CH7! The sound also is completely clean with no sign of buzz, even with the test card up. Not at all shabby, which is why I'm so determined to fix this last remaining fault...


 
 Return to top of page · Post #: 5 · Written at 8:23:50 PM on 12 July 2016.
TV Collector's Gravatar
 Location: Ballarat, VIC
 Member since 4 January 2011
 Member #: 803
 Postcount: 456

I'll get the mod sheets organised for you in the next few days.

I did notice the chroma subcarrier pattern in the screenshot and was wondering about your video source. Not many sets will display that these days via the RF input unless they've had the alignment touched up.

The horizontal linearity control is normally set to achieve minimum current through the horizontal output valve which should coincide with good linearity. Given the scarcity of output transformers, minimum current is a desirable state to achieve to get the most life out of the transformer.

I had a look at the E1 circuit and the original transformer is quite basic with very few taps. I assume the Radar replacement is an improved copy of the original. You should be able to verify the the connections with an ohmmeter and compare to the circuit. If they match up I think it's safe to say the transformer is wired up correctly.

It does look like some ringing is occurring in horizontal output stage or deflection coils. You have checked the values and leakage of the two caps across the horizontal deflection coils?


 
 Return to top of page · Post #: 6 · Written at 11:36:22 AM on 14 July 2016.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 578

My video source is a Linux based A10 network player box, the PM5544 image is a JPG played by its photo viewer function from a USB thumbdrive.

Modulator is a Kingray with VSB filter and set to CH7.

Yes, alignment is amazingly good as it is. My AWA 209C certainly won't display the chroma like this set does! It's about 6db down at 3MHz!

I normally set hor. linearity for minimum current through the 6CM5, easy to do on this set because of the 1 ohm in the cathode. But I get the same result (and a bit more width) with the lin coil bypassed.

With width coil disconnected, width increases but crushing/foldover is much worse. Much the same as when it is in circuit, set for max width.

Yes yoke caps check out OK for capacitance and on a megger. There is only the 1 fixed and 1 variable caps in series across one half of the winding. I was thinking of trying some capacitance across both windings but I have no suitable HV parts at the moment.

I already did a resistance check of the transformer taps to confirm connections. But I think I'll do that again in case a pair of taps got reversed. It is such a simple setup as you say, it shouldn't be this hard!

I did a temperature rise test on the yoke windings while I had it off the CRT looking for shorted turns. No difference between the two windings and only 2 degrees above ambient after 1 hour running. So probably OK. Still, if I had a substitute yoke I'd be trying that!

! would appreciate very much the mod sheets! Thanks!


 
 Return to top of page · Post #: 7 · Written at 10:03:44 PM on 17 July 2016.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 578

Well I think I found it!

Did a simple ring test (using scope and ohm meter) of each hor scan winding and found one of them was more damped than the other.

Pulled the yoke apart and Voila! One of the ferrite pole piece rings is broken!

See picture below:-

E1 Pole Pieces


Will try to araldite it back together with minimum possible gap at the break. I'll report back when it's done.


 
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