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Restoring  an HMV 901

I bought this set in 1978 and it survived its 516 mile journey from Cornwall to my home in Edinburgh lying on a double airbed in the back of our Reliant Rebel Estate.  

 After some rudimentary checks I powered it up without drama but unfortunately the screen image was flecked with the tell tale signs of EHT transformer breakdown. My employer had a transformer shop back then and the manager, John Anthony very kindly offered to rewind it for me. I didn't have any design details  for it but John came up with what he thought to be about right. Unfortunately the first attempt showed breakdown between windings. We still had some space in the core so I suggested making the EHT rectifier heater winding using PTFE insulated wire. This worked and is still working  45 years later.

More recently John Wakely showed me the proper winding details that he had received from EMI the year before I was getting my transformer rewound. Their turns/volt basis was somewhat less than mine and would have made it easier to get the core back on but unfortunately I didn't know John at that time.

Other problems tended to be intermittent. The sound would disappear randomly and Steve Osler had the same problem in his 702 and Mike Barker gave me the same cure in the form of a rewind of the second IF transformer.

I did have other sound problems that I traced to the flaky outer coating of the sound frequency changer and I still need to improve this.

For the first few years I still had BBC 405 line transmissions although on channel 3 so I made a little frequency changer to convert to channel 1. I can't remember how well the 901 TRF unit was tuned in terms of gain versus bandwidth but needless to say I did tweak it for best performance at the time.

It was only many years later when I had an Aurora and also Graphics Card signals that I realised that the set could display 3MHz bars and that  after seeing Jac Janssen's beautifully restored 702. After losing the 405 line BBC transmissions my set lay silent until I built the Darius Standards Convertor. The CCD delay lines in it only gave about 2MHz bandwidth but I still use Darius' excellent modulator circuits for the Graphics Card signals.

I had various vision faults in addition to those caused by leaky timebase capacitors. I never replaced any original components if they were still operational and in the case of the focus chain this was probably a mistake as it caused me to remove the power supply chassis several times and this requires the CRT to be removed so I'm now well practiced at that.

Slightly more mysterious than the focus chain faults was an intermittent loss of brightness that I eventually traced to poor connections at the CRT heater terminals. A similar fault with loss of contrast took me ages to trace. The contrast would change at a change in picture content and recover equally mysteriously. Believe it or not this turned out to be an internal fault in one of the MSP4 RF pentodes.

One day after several years of running the 901 I was working in the back when it suddenly went bang with a very loud explosion. I had been aware that the HT electrolytics, that shared a non-original can next to the rectifiers had been running warm and these exploded violently spraying their contents everywhere. To replace the can I made some dummy TTC types with modern components inside.

The cabinet of my set had suffered from garden shed storage and had some woodworm and also a splaying of the mitred top corners. Clamping and glueing did give some improvement but far from perfect. I did re-French polish the lid and side panels but for the rest of the cabinet I just used boiled linseed oil and that did restore some clarity to the wood grain.