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Old 26th May 2024, 8:14 pm   #1
high_vacuum_house
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Default Ultra VR17-52 very poor video

Good afternoon,
I picked up this television right at the end of the Retrotech event. This was the one with the EM81 magic eye in the front.

Restoration is going well with the chassis cleaned up and most of the wax capacitors replaced. The only previous signs of work was a replacement for the HT metal rectifier and a repair to the voltage dropper, possibly as a result of the metal rectifier failing!

I have now got most of the set working, the timebases need a bit of tweaking as well as square up the scan coils, and adjusting the magnetic shuffle plate to centralise the picture.

I have reasonable sound, but I cannot get the video looking much better at the moment even with the fine tuner, contrast and brightness adjusted. This is on channel 8, Channel 1 has just about detectable vision. Could this be caused by a weak IF input from the tuner if the valves are tired?

I have swapped over the common IF and vision IF 30F5 valves with the sound IF one as the sound seems ok with absolutely no change. Also I have swapped over the video amplifier and sync separator 30FL1's with no change. leading me to believe that the problem is possibly deaf tuner valves.

Christopher Capener
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Old 26th May 2024, 8:52 pm   #2
Graham G3ZVT
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Default Re: Ultra VR17-52 very poor video

I don't suppose this set uses a germanium vision detector diode?
I would check that if it does.
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Old 26th May 2024, 9:23 pm   #3
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Default Re: Ultra VR17-52 very poor video

Good evening,
It does have a germanium diode for the vision detector. I swapped that for another germanium diode but that had no change to the video quality. It did measure correctly as a germanium diode out of circuit.

Christopher Capener
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Old 28th May 2024, 7:23 pm   #4
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Default Re: Ultra VR17-52 very poor video

Hi Christopher.

Pretty smeary……! What effect does the Contrast control have on the picture?


Cheers, SimonT.
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Old 28th May 2024, 10:03 pm   #5
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Default Re: Ultra VR17-52 very poor video

The contrast control has not very much effect. The picture goes from completely blank anti-clockwise to the picture posted fully clockwise. The sound also fades away as you rotate towards the anti clockwise position. I replaced the vision detector diode which had no change. Putting it in the other way round as the picture looked negative, lost all the video completely so was replaced correctly.

Christopher Capener
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Old 29th May 2024, 11:47 am   #6
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Default Re: Ultra VR17-52 very poor video

These are very reliable receivers and give an excellent picture. Check L25/L26 to the grid of the videos amp and C99 the video coupling to the CRT. I presume the brightness control operates normally. If it has very little effect the CRT CRM172 may be O/C Cathode. Check the interference limiter circuit, Regards, John.
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Old 29th May 2024, 1:53 pm   #7
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Default Re: Ultra VR17-52 very poor video

Circuit diagram of the video and CRT arrangements in the Ultra VR17-52.
Note the AC coupling between the video amplifier and CRT cathode.
Capacitor C40. An oscilloscope will determine the condition of the video at the grid of V5A which should be about 3volts of positive going signal.

DFWB.
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Old 31st May 2024, 3:47 pm   #8
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Default Re: Ultra VR17-52 very poor video

Quote:
Originally Posted by high_vacuum_house View Post
Good afternoon,
I picked up this television right at the end of the Retrotech event. This was the one with the EM81 magic eye in the front.

Restoration is going well with the chassis cleaned up and most of the wax capacitors replaced. The only previous signs of work was a replacement for the HT metal rectifier and a repair to the voltage dropper, possibly as a result of the metal rectifier failing!

I have now got most of the set working, the timebases need a bit of tweaking as well as square up the scan coils, and adjusting the magnetic shuffle plate to centralise the picture.

I have reasonable sound, but I cannot get the video looking much better at the moment even with the fine tuner, contrast and brightness adjusted. This is on channel 8, Channel 1 has just about detectable vision. Could this be caused by a weak IF input from the tuner if the valves are tired?

I have swapped over the common IF and vision IF 30F5 valves with the sound IF one as the sound seems ok with absolutely no change. Also I have swapped over the video amplifier and sync separator 30FL1's with no change. leading me to believe that the problem is possibly deaf tuner valves.

Christopher Capener
My money, for what it’s worth, is on the turret tuner valves. Had a similar issue with my BT302 & was kindly pointed in that direction on here with complete success.
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Old 31st May 2024, 5:00 pm   #9
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Default Re: Ultra VR17-52 very poor video

Looks like severe lack of LF response somewhere in the video stages. With a step grey scale applied, a scope should show up the problem area.
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Old 31st May 2024, 10:35 pm   #10
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Default Re: Ultra VR17-52 very poor video

Check the vision AGC decoupling capacitors C62 and C92 for leaks, both are 1microfarad.
Also, with a signal supplied to the aerial socket measure the negative voltage at the grid of the sync separator valve. -20V when receiving a strong signal.
An overloaded 405 TV signal will result in crushed video and stretched sync pulses. A 'scope will determine that condition.
Try reducing the signal input amplitude and observe the results.
The video detector crystal XL1 is not beyond suspicion.

DFWB.
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Old 1st Jun 2024, 7:55 pm   #11
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Default Re: Ultra VR17-52 very poor video

Good evening,
Sorry for the delay in responding. I had a short walking holiday.

So far I replaced the tuner valves with new ones which made no difference. All of the small inductors around the detector and video amplifier are ok. The 2 1uF AGC capacitors have been replaced.

The 0.1 uF capacitor on the wiper of the brightness pot had previously been replaced but the picture doesn't go all the way to black and the overall picture brightness is still quite dim with the brightness turned right up (with some flyback lines visible) even with the line stage operating correctly. C40, the CRT cathode capacitor has also been replaced.

The line and frame timebases seem stable and locked, so I think the correct information at the right level is getting to the sync separator.

I connected the scope up with the heghog in step testcard output with pictures attached which shows that the video signal around the video amplifier stage seems to look quite good (the video amplifier anode was difficult to get stable on the screen). I am starting to suspect the CRT is not in its best of health.

Christopher Capener
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Old 1st Jun 2024, 8:52 pm   #12
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Default Re: Ultra VR17-52 very poor video

Hi Christopher.

Those staircase waveforms look promising and good.

Time to check the CRT me thinks. Do you have an emission / electrode tester?

I guess you’ve checked tube base voltages already?

Best of luck.

Simon.
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Old 2nd Jun 2024, 12:12 am   #13
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Default Re: Ultra VR17-52 very poor video

Well I ran it up again, the CRT voltages don't look too bad. The boost voltage is 450V

Unfortunately, I don't have a CRT tester to check what the emission is.

I noticed that the CRT heater was at 9.5V when it should have been 12.6V The heater current was checked and was a bit down due to the heater set for 250V. Dropping it down a tag on the dropper got the heater current much closer to 300mA though the CRT heater was still under 10V. Taking it out of the heater chain and running it from a bench power supply adjusted to give 12.6V at the heater pins made the heater glow almost yellow and was drawing 500mA. The picture, although slightly brighter was still very smeary. I am wandering if the CRT heater has gone partially S/C

Interestingly, when I switched the set off before the separate supply running the CRT heater, the remaining white dot in the centre of the CRT took about 10 seconds to eventually disappear

Christopher Capener
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Old 2nd Jun 2024, 4:04 pm   #14
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Default Re: Ultra VR17-52 very poor video

"I noticed that the CRT heater was at 9.5V when it should have been 12.6V The heater current was checked and was a bit down due to the heater set for 250V. Dropping it down a tag on the dropper got the heater current much closer to 300mA though the CRT heater was still under 10V. Taking it out of the heater chain and running it from a bench power supply adjusted to give 12.6V at the heater pins made the heater glow almost yellow and was drawing 500mA. The picture, although slightly brighter was still very smeary. I am wandering if the CRT heater has gone partially S/C "

The heater chain has many decoupling capacitors some might need replacing.
The CRM172 CRT has a 12.6V heater.

DFWB.
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Old 2nd Jun 2024, 7:27 pm   #15
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Default Re: Ultra VR17-52 very poor video

Good afternoon,
The heater decoupling capacitors are all good quality ceramic disc type. I have followed them through and all appear to be ok. The valve heater brightness appears correct throughout the valve chain. I have tried the trick of shorting the CRT cathode to grid, but this made the raster slightly darker!!
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Old 2nd Jun 2024, 7:32 pm   #16
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Default Re: Ultra VR17-52 very poor video

EHT is quite a respectable 14KV at the CRT anode.
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Old 16th Jun 2024, 10:15 pm   #17
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Unhappy Re: Ultra VR17-52 very poor video

Good evening,
I think that I am going to give up on this set for the moment. I have been playing about with it and I cannot get the picture to improve at all. I have left it running for a decent period and there is no improvement or waking up in the CRT brightness. I think that there is almost no cathode current flowing as the EHT final anode takes a considerable time to decay after switching it off. I think the likelihood of finding a working Mazda CRM172 is almost nil. A real shame as the rest of the set seems to be operating nicely. Even the EM81 on the front of the set is bright!!

Christopher Capener
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Old 16th Jun 2024, 10:30 pm   #18
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Default Re: Ultra VR17-52 very poor video

There are two likely possibilities with the CRT...1 it has a partially shorted heater....sometimes a tap on the neck with a rubber handled screwdriver will temporarily remove the short. 2. It has an O/C cathode. Either way you'll need to look for a replacement CRT.
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Old 17th Jun 2024, 12:20 pm   #19
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Smile Re: Ultra VR17-52 very poor video

Good morning,
Well some good news for this television, at least for a while. I decided to run it directly off the mains and give the neck of the CRT increasingly heavier taps from the handle of a tack hammer. There was almost no change whatsoever so I raised the front up about 2" and gave the neck slightly heavier blows.

I was thinking that I was soon going to end up with an instant low vacuum CRT (!) when there was a bright flash from the screen. Another hard whack must have made the cathode connection as I instantly had a near perfect test card C appear! The brightness is also massively improved.

I have watched it for around an hour and the picture has improved a bit. The camera doesn't do the picture much justice. There are a few things still to adjust such as the scan coil rotation is a bit off and the main electrolytic could do with replacement plus the refitting of the EM91 and loudspeaker to the cabinet.

When switching off, the raster disappears almost instantly unlike before. This must be down to significantly higher cathode current collapsing the stored EHT charge between the anode and graphite coating. I now wonder how long the connection to the cathode will be maintained or weather any knock will end up opening it again.

Christopher capener
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Old 17th Jun 2024, 1:56 pm   #20
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Default Re: Ultra VR17-52 very poor video

That is good luck Christopher ! Well done with pursuing it, looks to be good emission. I think some of the "barrel" Murphy sets use the same CRT? It was often the only fault they suffered from after many years of service.
Fingers crossed the tube holds up!
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