Help with a HMV TV

[TriMan66]

Hi All,

I have been hunting for a tube for a HMV TV for a while now. See the thread

https://www.vintage-radio.net/forum/...ad.php?t=63808

It is now in place after re-caping the waxies and the electrolytics. I brought it up slowly on the Variac and the HT supplies are fine. There is a healthy amount of static from the speaker and the line whistle is present. It is an Australian set, so although it is 1959 it is 625 line.
All tranformers/chokes measure reasonably including the LOPT.
I have a 2-3mm spark from the 1S2 (DY86) but no life from the tube.
The boost voltage shown in the circuit below is low at 375V instead of 650V.
All the valves in the EHT section are NOS. I haven't yet replaced the disc ceramic C86, 150pF across the LOPT.
Any thoughts on how to test\check if the tube is OK and the low boost voltage would be apprectiated.

regards
Craig

[oldticktock]

You could try shorting the Grid and Cathode of the crt together very briefly you should see a bright raster.

Chris

[murphyv310]

Are you drawing the spark from the top cap of the DY86? If so and you are using a decently sized screwdriver and only getting a 3mm spark, I would think the line stage is in distress!
Check the HT, the line drive preferably with a scope, one thing comes to mind. How old are you?
If like me you have a few grey hairs I would doubt you should hear a 625 line whistle so if its way low and sounding like 405 that will possibly be your problem!
Let us know your findings.
That 21 inch 110 degree tube will need about 16 kv EHT a spark of 10-12 mm is what I would be looking for.

[Danny]

Hi whats the voltage on the line output valves grid and screen feed ? check the connections to the scan coils and asociated components. Danny

[AndrewM]

As mentioned above, with a low boost voltage I would be concerned about the Line Output stage operating conditions.

Check that the screen, grid and cathode voltages are within spec for the 6CM5 output valve. If they are close to correct then you can safely leave the set running for more in depth troubleshooting on the CRT.
It is most important that the 0.14 volts on the cathode not be much above 0.18 or so volts (measured with an analogue multimeter) or the Line Output transformer will be overly stressed. If voltages are near correct, the Horizontal Oscillator may be running off frequency,causing boost voltage to drop a lot but the CRT should still light up.

If the Line Out stage conditions allow extended running, then check the CRT pin voltages. It may be biased off despite the low boost volts.

[Heatercathodeshort]

Check the H.T. as Trevor suggests. Then check line coupling capacitor to grid of line output valve. Next capacitor is boost storage that usually has a value of around .25uf-.5uf. It is connected between the H.T. rail and boost rail. If line output valve is stressed, remove top cap from boost diode and see if stage livens up. If it does, this suggests the fault lies in the boost capacitor mentioned. The LOPT could be faulty and may have been the reason the receiver was set aside originally. The EHT rectifier should be lit if there is EHT present. It should be a noticable glow.
The CRT will be fine. The Philips/Mullard AW53-88 is indestructible and will give 100% emission when the set is sorted. John.

[TriMan66]

Hi All,

Thanks for the comments, it is always encouraging to get advice.
I used the CRO on the oscillator. It was an interesting comment about my age, 44. The oscilator was working at 12kHz instead of 15.625kHz. I replaced C81 and the horizontal hold pot which was broken. It now oscilates at 15kHz and I can't hear the whistle!
The boost voltage is up to 530V and I can draw a spark to approx 5mm now. Still no life from the tube though.
There is no glow in the 1S2 (DY86) though.
I have replaced all the waxies and electros, so this includes the boost cap.

regards
Craig

[murphyv310]

The DY86 heater is fed from usually a loop of EHT cable around the core of the LOPT, so check this is ok and there isnt an o/c resistor or choke in the valve base of the DY86.
530V boost is getting there and with that level of boost you really should have a lit up screen, have you tried another DY86? A DY87 or DY802 will do as well.

[TriMan66]

Hi All,
So, I'm making some progress now. I have my first raster. There was a wiring mistake around the T7. Also the capacitor on the LOPT was between pins 2 and 8 not pin 3 and 7. With a new DY86 I now have something to work with on the screen. The boost volts are up to 570V now, not quite the required 650V.
Also pin 3 on the picture tube is 300V instead of 400V.
There is an anying "pip" in the speaker once every 1-2seconds which I need to trace too.

regards
Craig

[TriMan66]

I'm a little stuck again and would appreciate your thoughts.
After making some good progress with the line oscilator which was running at 3/4 speed, I now have a stable picture.
But the boost volts are still low.
If I adjust the contrast, the picture rapidly expands and distorts.
The boot volts are at 450V, not 650V. There is practically no smoothing on this line with almost 100V ripple. The boost capacitor has been replaced.
I turned all the lights off in the room and there is not even a faint glow form the DY86.
I replaced the single loop for the heater for the DY86 which was open circuit originally. I used an EHT lead form another TV. This lead had a center core and an outer sheild. I didn't connect the sheild braid, but I wonder if that is limiting the effect of the loop around the LOPT.
Also, the aqudag on the new tube is in a really bad state of repair. If this is the smoothing cap for the EHT ot the tube, then this might effect the EHT level?

[michamoo]

That looks like the HT is still too low

[HamishBoxer]

Poor coating on the crt would cause a kink in the picture not what you have at present.
David

[HamishBoxer]

Certainly worth checking those high meg reses in the linestage as well as coupling cap from Line Osc.

[Boom]

Originally Posted by michamoo

That looks like the HT is still too low

It would make sense to measure the HT before going any further.

[TriMan66]

Hi,
I don't have an EHT meter. so apart from drawing a spark, I'm not sure how to measure it.
I do assume you are talking about the EHT, 16kV. The two HT rails are fine 210V and 234V, compared with 215V and 240V.

regards
Craig

[Boom]

No I meant the HT rail itself. I'd have thought the HT was near enough then and that the problems lie with the boost HT circuit. High value resistors going higher are a favorite.

[HamishBoxer]

As i stated earlier ,high meg reses in line stage also any waxy caps excluding small silver mica.

[TriMan66]

Hi All,

Thanks for the comments.
Still no luck. All high value resistors are within 5%.
All wax caps have been replaced.
I fixed some wiring and resistors around the vertical height section which is now working.
But its still low on EHT I fear and the boost volts are quite low at around 450V.
I did have them up to 530V and brighter picture, but that was before I moved C86 on the LOPT from pins 4/8 to pins 3/7 as it says on the circuit diagram.
The small spark I can draw from the DY86, the low boost volts and the picture brightness, seem to imply low EHT, but I still haven't found the cause.
I'm going to try another cable loop for the DY86 heater. One without a sheilding braid.
Any other comments are welcome.

regards
Craig

[oldticktock]

I've nothing much to add but perhaps an observation, I thought that if there was low EHT this would result in a overly large raster and that you do not have. Your raster is reduced in both directions, perhaps voltage checks throughout the circuit should be made and verifed against service data. Most likely one of those misleading faults that provide so much frustration.

Do you have a copy of the whole circuit you could post, it may help us all to visualise the problem better.

Chris

[TriMan66]

Hi Chris,

Here is the circuit.
I moved the C86 back to where it was on the LOPT and boost volts are up a little to 480Vand the picture is more stable as I adjust the contrast.
I'm just trying another heater loop for the DY86 - 1S2 now.
Hope the circuit quality is OK.

regards
Craig