Silly Faults on HMV901

[peter_scott]

For some time now I have been ignoring a couple faults, namely a screeching instability on the sound and a jittery picture height. Accessing the sound chassis requires removal of the CRT, the timebase chassis and the loudspeaker and the first of these always worries me.

However, the picture completely disappeared although EHT was still present so I figured there must be a failure in the focus chain and access to that requires removal of all the above plus the very heavy power supply chassis so I ordered up some suitable resistors and also a stock of capacitors for the sound chassis to do some stuffing (thinking that might cure the instability.)

Sure enough there was a couple of resistors in the focus chain that I had not previously replaced and one of them had gone open circuit. Although these resistors are completely out of sight without major disassembly I don't like replacing originals if they still work. These are large dog bones and not easy to make replicas of so my whole chain is now in modern parts.

I then went through the sound chassis stuffing all the original paper capacitors and one or two electrolytics. After reassembly in the cabinet the instability was if anything worse than before so I figured I'd have to power it on the bench as it's not really possible to probe it when installed with the TV power supply chassis. After much messing about I traced the problem to a break between the conductive coating on the IF valve and the little wire that connects into the base.

Next I tried squirting Servisol into the height pot to try and resolve the jitter.
This had no effect and the lid appeared impossible to remove so I looked for a replacement pot but couldn't find one so back for a more serious attempt at lid removal. It was very tight but eventually I got a bit of movement and off it came. Much to my surprise the innards were a beautifully engineered wirewound with no obvious reason for noisiness. In years past I had drilled a little hole in the lid and squirted oil into it so I figured I might improve matters by giving it an ultrasonic bath.

After reassembly I was very pleased to find all three faults were fixed and whilst I had the timebase chassis on the bench I took the opportunity to fit a 405/240 line standards switch. My set left the factory without the 240 line components but having now got a source of 240 (that has line syncs during the frame sync) it was a fairly simple matter to add two resistors and two capacitors and switch.

I didn't want to remove the blanking plate for the extra valve that maintained line frequency during frame sync as this was now superfluous anyway and I didn't want to remove the blanking plate on the control panel where the standards switch was located on earlier sets so I just attached it to a small metal bracket on the timebase chassis.

Peter