Thread: HMV 905 Project
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Old 8th Nov 2018, 2:10 am   #165
beery
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Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
Posts: 988
Default Re: HMV 905 Project

Hi all,
I'm now fortunate enough to have a good set of scan coils now.
However the pictures look very soft.
I decided to do an IF sweep from the aerial input. By using an arbitary waveform generator I can accurately set the centre frequency and sweep length. This is all the more important with a digital scope as there is no X axis input, so we are reliant upon the sweep rate of the scope and the signal generator being pretty well matched in terms of linearity.

Ignoring the first and last centimieters of the graticule, the scope shows a range of 41MHz to 49MHz left to right, at 1MHz per centimeter, with 45MHz in the centre of the display. The upper waveform is the 1Hz FM sawtooth to which the scope is triggered and the lower waveform is an inverted view of the output to the CRT.

You can clearly see the unwanted sound on vision response at 41.5MHz and the limited bandwidth of the vision IF. I will need to disconnect the CRT in order to turn the chassis on its end and tweak the vision IF response. I was however able to improve the sound rejection from what you see here by tweaking the sound IF adjusters (which you can do from the top of the chassis), but not before I had added yet another extra capacitor across a sound IF coil to get it to peak properly.
I've never done IF adjustments in this way before and it seems that great instant results become quite easy to achive, indeed I purchased the waveform generator specifically to sort out the IF response of the 905.

The third photo shows the pictures as they currently are. I have a choice of poor right hand focus or if I reverse the focus coil wires, good overall focus, but ion trap shading. I'm thinking that degausing the focus coil might improve matters.
Also you can see how a well focussed picture still seems soft due to the poor IF response. Not only is the HF end limited, but the screen shot also shows a very poor LF response, which is odd as the CRT drive is DC coupled. I've ordered a spare MS4B video detector and amplifier valve to help me track down the problem, but it maybe related that the peak vision response is not centered exactly on 45MHz as it should be.

Cheers
Andy
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Last edited by beery; 8th Nov 2018 at 2:14 am. Reason: spelling
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