9th Jul 2018, 8:11 pm | #181 |
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Re: HMV 1804 Television.
Great stuff David.
I guess you can now fit the replacement speaker and put the set back together. What is next on the bench? Cheers Andy
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9th Jul 2018, 8:49 pm | #182 |
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Re: HMV 1804 Television.
Good evening everyone,
Many thanks for your kind comments. Hi Andy, Lot's more projects to come up but before that there is still a number of jobs to do on the HMV 1804. Now that the sound is perfect Robin's loudspeaker is now mounted on the chassis. Been looking through various 1950s circuit diagrams of the series diode sound interference limiters employed in TV sets. The Ultra V17-60 of 1957 has those extra decoupling components in series with HT supply to the anode resistor of the noise limiter diode, C58 and R131. Following Argus25 comments I notice the interstage coupling capacitor C80 is 0.01mfd and the grid resistor R82 220Kohms, the values of these components will limit the bass response. See attachment. Those other jobs. The receiver needs an RF gain control which it had when I first took possession of the set. It was removed during the course of the restoration. The first stage RF amplifier employs an EF54 which evidently doesn't take well to having it's cathode bias altered, the engineer who modified the set knew this and chose an EF50 for the second RF stage which can be gain controlled. A neat solution is to utilise the former sound noise limiter control 5Kohm potentiometer. This is now redundant because a series noise limited is employed instead of the original EMI noise canceller. Next I must do something about the sync separator. there some pulling on the whites caused by active video component among the sync pulses at the anode of the KTZ63 sync separator valve. I checked the video waveform on the grid of the sync separator pentode and all is in order. The positive going sync tips are clamped to 0V ground potential. The whole negative going waveform is in effect below the zero line. After all this work is done there is the cabinet to attend to. DFWB. Last edited by FERNSEH; 9th Jul 2018 at 9:01 pm. |
14th Jul 2018, 9:06 pm | #183 |
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Re: HMV 1804 Television.
Perfect line and frame timebase synchronising achieved.
Adjustment of the video amplifier bias control VR2 has effected a real improvement to synchronising. According to the HMV 1804 service manual VR2 is adjusted for a reading of 220volts on the anode of the video amplifier valve V7. This corresponds to a negative grid bias of 3.5volts. The bias capacitor C23 (50mfd) had been clipped out by the engineer who had done the receiver section modifications. Refitting this component has also improved the performance of the video amplifier. When the contrast control is set to 3volts peak white video on the grid of the video amplifier the picture is fully contrasted. Advancing the contrast control further causes peak white clipping. DFWB. |
27th Oct 2019, 8:17 pm | #184 |
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HMV 1804 revisited.
Remember the mad HMV 1804? Well this evening it was switched on after at least six months lying dormant in a corner of the workshop.
The picture appeared within a minute but displayed lack of height. The focus control needed slight adjustment and the sound came on after giving the cabinet a bang. The original Emiscope 3/4 CRT is holding up well. Must get around to doing up the cabinet. DFWB. |
28th Oct 2019, 1:32 pm | #185 | |
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Re: HMV 1804 revisited.
Quote:
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30th Oct 2019, 12:31 am | #186 | |
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Re: HMV 1804 revisited.
Quote:
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30th Oct 2019, 1:12 am | #187 |
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Re: HMV 1804 Television.
radiograham wrote:
"I like it,have you been watching onslow on keeping up appearances" That's right, the HMV responds in a similar manner. Give the cabinet a bang! This is an amazing set, the gain from the two stage RF amplifier is like no other post-war set I know of. Who ever did the modifications to the signal circuits knew what he was doing. The Mullard EF54 pentode was as far as I know was never used in any commercially made TV receiver. Did notice the other evening the interlace is not very good. The frame sync was returned to the original EMI single diode arrangement. Reasonable interlace can only be achieved at certain critical frame hold settings. The Pye type of interlace filter module was removed during the restoration. It should be a simple enough task to rewire the base of the D63/6H6 interlace valve to the Pye "positive interlace filter" arrangement. It's the frame sync circuit that was used in the LV20 and FV1 models. This is certainly an unique set and yet for all the excellent work done by the mystery engineer to turn it into a high performance fringe area receiver wouldn't it have been easier to have made a high gain pre-amplifier instead? DFWB. Last edited by FERNSEH; 30th Oct 2019 at 1:18 am. |
31st Oct 2019, 3:45 am | #188 |
Hexode
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Re: HMV 1804 Television.
Hi. It's interesting about the mystery engineer, I wish we could find out who he was, without a time machine we will never know. I used to have a Ferguson Videostar top loader that would only playback if you placed a tin of beans on the top right hand corner and then rewind was in play/rewind mode, used it like that for about five years.
Graham. |
31st Oct 2019, 10:36 am | #189 |
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Re: HMV 1804 Television.
Perhaps he just did it because it was there - it was about Everest time...
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31st Oct 2019, 6:01 pm | #190 |
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Re: HMV 1804 Television.
I have thought the pre amp idea would have been a better bet David. He must have had a massive aerial back then. Of course he may have gone down that path with little success. Cambridge was a real fringe area back then. I wonder if he worked at PYE but thinking about it, why bother with the HMV 1804 if he could have obtained a trade sale PYE? J.
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31st Oct 2019, 7:18 pm | #191 |
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Re: HMV 1804 Television.
HKS wrote: "I wonder if he worked at PYE but thinking about it, why bother with the HMV 1804 if he could have obtained a trade sale PYE? J."
And Pye made high performance sets like the FV1 series which had three stage vision and sound IF amplifiers although it has to be said this modified 1804 has even better weak signals performance. One of the reasons why the HMV is so good can be attributed to the two EF54 RF pentodes employed in the RF and mixer stages. This valve has four cathode pins. G3 is connected internally to the cathode. It could be said that the EF54 was the forerunner of the EF80 which has two cathode connections. Info from the Radiomuseum: https://www.radiomuseum.org/tubes/tube_ef54.html More info: http://www.r-type.org/exhib/aaa1093.htm DFWB. |
4th Nov 2019, 7:41 am | #192 |
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Re: HMV 1804 Television.
In the same way RCA had the 6AC7 RF pentode and Mullard had the EF50, it was obvious for a video output stage it required better parameters with a better figure of merit. RCA came up with the 6AG7 to serve this purpose and Mullard/Phillips came up with the EF54, both very similar for video output stages.
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4th Nov 2019, 1:23 pm | #193 |
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Re: HMV 1804 Television.
For video amplifier service Mullard or Philips developed the EF55.
http://www.r-type.org/exhib/aad0115.htm I've no experience of this valve but it is said to consist of two parallel EF50 electrode assemblies. Gm is 12mA/V @ Ia 40ma. Mazda 6F28 has similar Gm @ Ia 10mA. DFWB. |