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Old 23rd May 2017, 11:32 pm   #61
FERNSEH
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Default Re: HMV 1804 Television.

The burn up was caused by the 500pF mica coupling capacitor inside the vision IF amplifier anode coil can having gone short circuit. Rare for such capacitors to fail.

DFWB.
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Old 24th May 2017, 9:13 am   #62
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Default Re: HMV 1804 Television.

Closed circuit respiration gear, and ....... up-rated life insurance David! (Keep us all up-to-speed re. your travails .....)
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Old 24th May 2017, 10:44 am   #63
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Default Re: HMV 1804 Television.

After the signal circuits have been explored and made to work what follows is the rather scary task of testing the timebases. Normally that would cause anyone concern but as mentioned earlier the heaters for the timebase valves are derived from the EHT transformer. A good suggestion from Jeffrey on the Golborne Forum is to employ a separate 6.3volt heater transformer and leave the EHT transformer disconnected until much later.
And in fact I do have a suitable heater transformer, one removed from a 1953 Pilot TV87. But in the meantime let's find out if the RF and mixer oscillator circuits work. It all depends if those EF54 valves are any good.

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Old 25th May 2017, 11:26 am   #64
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Default Re: HMV 1804 Television.

This is exciting, that is, if you call working on this heap of junk exciting! At last I'm getting some positive results from the signal circuits.
This morning the signal generator was connected to the aerial socket and believe or not signals are getting through, there's hope for this set after all.
The sound peaks up at 50Mc/s and the vision at 54Mc/s. Those frequencies are above channel B2 which are 48.25Mc/s sound and 51.75Mc/s.
To monitor the vision signal level a voltmeter was connected across the detector load resistor.
I'm sure it will be possible to retune this set to channel B1.

DFWB.
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Old 25th May 2017, 8:20 pm   #65
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Default Re: HMV 1804 Television.

The 1804 is now supplied with channel B2 signals from an Aurora standards converter. The local oscillator has been readjusted for max video at the grid connector of the Z66 video amplifier valve. Also peaked up is the grid coil of the mixer valve.
The scope trace shows the video developed across the detector load resistor, a very impressive >12volts P - P. Even more video will be present at the detector when the aerial and first RF amplifier anode and coils have been tweeked up. To tune the receiver to channel B1 the metal tuning slugs will be replaced with iron dust cores.
Even with 12volts video across the detector load resistor there is still no sync crushing. I wish I had same amount of video to drive the CRT grid in my Marconi 702.

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Old 25th May 2017, 10:41 pm   #66
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Default Re: HMV 1804 Television.

That video signal looks really sharp, the sync pulse look excellent.
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Old 26th May 2017, 3:04 pm   #67
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Default Re: HMV 1804 Television.

The set seems to give the best performance on Channel B2, hopeless on channel B4. All this would suggest the set was modified for the Holme Moss transmitter. Was that TX receivable in Cambridgeshire?

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Old 26th May 2017, 3:36 pm   #68
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Default Re: HMV 1804 Television.

Attached map for Holme Moss ch 2 taken from:-

http://duncanwheelhouse.com/bbc/bbcmoss.pdf

Cambridge is outside the service area, you would think Ally Palace or Sutton Coldfield would have been better.

Frank
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Old 28th May 2017, 5:58 pm   #69
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Default Re: HMV 1804 Television.

Readjusting the aerial input coil increases the vision gain. With the Aurora set to channel 2 the video developed across the vision demodulator load resistor is 15 volts P - P, and that's with the contrast control set to minimum.
So the performance of the vision stages is improving but it's not so with the sound. The performance of the sound channel is absolutely hopeless! The attachment shows the sound take off from the anode of the mixer valve.
The set came with a Mazda SP61 as the sound IF amplifier, this was subsequently replaced by an MO W61 vari-mu pentode. I've now replaced this valve with a Marconi Z66, a valve that resembles the SP61, so in a sense the sound IF amplifier is now back to the original design.
Because the timebase is inoperative without any special measures the HT voltage will rise to a dangerous level, as high as 400 volts. To reduce the HT voltage to a safer level a 15Kohm resistor is inserted between the negative tag of the HT reservoir capacitor and chassis.

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Old 29th May 2017, 10:10 pm   #70
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Default Re: HMV 1804 Television.

A "safe" source of EHT to test the CRT. This unit will after adjustment supply 4000 volts for the final anode of the 10" Emiscope type 3/4 CRT. I believe it is same EHT voltage for the 15" CRT that is installed in the model 1803.

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Old 30th May 2017, 10:36 am   #71
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Default Re: HMV 1804 Television.

Testing the sound circuits of the HMV 1804. With 100millivolts P - P of 10.5Mc/s modulated signal supplied to the input IF transformer an audio signal of 2.5 volts is developed across the demodulator load resistor. The signal across the load resistor rises slightly when the AVC is shorted out.
When you consider the input voltage is 100mV P-P and half of the modulated signal waveform is removed by diode action of the demodulator the gain of the IF amplifier is actually 50 times. I'd reckon that's a very satisfactory figure. The IF amplifier valve is a W61, a valve that resembles the 6K7G.
But this still doesn't answer the question, why isn't the sound coming through from signals supplied to the aerial socket?

DFWB.
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Old 1st Jun 2017, 2:49 pm   #72
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Default Re: HMV 1804 Television.

I've found the reason why the sound was so weak. The inner of the co-ax cable link between the EF54 mixer anode coil and the first sound IF transformer has been found to be open circuit.

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Old 2nd Jun 2017, 10:31 am   #73
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Default Re: HMV 1804 Television.

Corrosion of the copper inner conductor of the co-ax cable was the problem. Now sorted and the sound is coming through, although sound vision spacing of the IF frequencies is way out. Nevertheless, things are starting to happen.
The audio developed across the 33Kohm demodulator load resistor is positive going, one could question why this was done because if the diode was reversed to give a negative going audio and DC component a convenient source of AVC voltage would be available and it would be unnecessary to employ a separate AVC rectifier diode
My take of this circuit is because a high slope straight pentode was employed as the sound IF amplifier it's possible that high signal levels will result in a high negative AVC voltage which in turn will cause some distortion of the negative going part of the modulated IF signals. The positive side of the modulated signal waveform will be unaffected. Once again this attention to circuit design confirms that the modifications were the work of a very competent engineer.

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Old 2nd Jun 2017, 1:34 pm   #74
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Default Re: HMV 1804 Television.

Quote:
Once again this attention to circuit design confirms that the modifications were the work of a very competent engineer.
And it takes an at least equally competent engineer to recognize the circuit specifics!
Which is the case.

Good find - the broken cable. These things can be very tricky to pinpoint.

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Old 2nd Jun 2017, 2:30 pm   #75
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Default Re: HMV 1804 Television.

Good afternoon Jac,
Thank you for the compliment. It gives me encouragement to see this restoration through.
We're coming up to the restoration of the timebases. The oscillators are known to be working and are supplying sawtooth drive waveforms to the grids of the output valves. The only modification these circuits received was the addition of the double-diode interlace filter which took the form of a separate module.
That unit was removed earlier during the initial stages of the assessment of this receiver. The interlace circuit can be reintroduced but instead of using the EB91 double-diode the existing D63/6H6 valve can be used instead.
As mentioned earlier an addition 6.3 volt transformer will supply the timebase valve heaters, much safer than employing the transformer in the receiver which also supplies the EHT. Don't want 4KV cables swishing about while working on the set.

DFWB.
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Old 2nd Jun 2017, 8:09 pm   #76
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Default Re: HMV 1804 Television.

Quote:
Originally Posted by Jac View Post
Good find - the broken cable. These things can be very tricky to pinpoint.
Seconded! You'd have made a good investigative archaeologist too David!!
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Old 4th Jun 2017, 2:29 pm   #77
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Default Re: HMV 1804 Television.

The attachment shows the waveform present at the anode of the KT61 frame output valve. Scope settings: timebase 5mS per division and Ch1 50V division.

A 100microsecond 30 volt P - P sawtooth drive is present at the control grid tag of the KT44 line output valve. The next task is supply 4 volts to the KT44 and find out if the line output stage is working.

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Old 4th Jun 2017, 3:26 pm   #78
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Default Re: HMV 1804 Television.

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Originally Posted by FERNSEH View Post
The next task is supply 4 volts to the KT44 and find out if the line output stage is working
But not with this KT44.

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Old 4th Jun 2017, 6:22 pm   #79
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Default Re: HMV 1804 Television.

Looks a bit soft to me
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Old 5th Jun 2017, 10:10 am   #80
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Default Re: HMV 1804 Television.

The gassy KT44 has been replaced by an Osram KT45, which is essentially the same valve but has better peak anode voltage ratings. The KT44 was a development of the KT66 audio output valve. The KT44 was first used in 1938 for service as the line output valve in the HMV 902A TV and radiogram combination.
Info about the KT45: http://www.r-type.org/exhib/aad0234.htm
I can confirm that the line timebase is now working and is supplying the scanning coils with a 100microsecond sawtooth waveform.
The next stage of the restoration of the 1804 will be attention to the video amplifier and sync separator stages.
The attachment shows the similarity between the KT66 and KT44 valves.

DFWB.
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