Crusty and the Rusty one.......
The PSU and LTB modules were borderline Junk but looks as we've all been told aren't everything. True they had suffered over the years and had been badly maintained, in fact they had to be the worst to have crossed my bench. It involved several hours repairing to get them back to working condition. First of all some major surgery to the PCB which was cracked in two places. This involved drilling a few holes and stitching with strong wire twists, finally soldering these points. Not pretty I grant you but an effective, strong mechanical repair. I found fair level of past bodging for example, the modules edge connector had been soldered due to the loss of copper, I replaced this with a NOS one. The main chassis trip had a single strand of wire strapped around the terminals as did the chopper fuse, the other fuse holders were appalling and need replacing . The trip was replaced from stock and the wire from the chopper fuse holder removed, we would see what effect these have later.
I was aware the CRT was poorly and this was confirmed with the B&K tester. Red and Green responded to a clean and balance Blue did not . Blue was given a bop and now read as good as Red and Green. After a visual inspection for anything untoward, the set was introduced to power. The 'in-line' EHT meter was connected so I could monitor if it was low but more importantly if it was too high. No dramatics and 22-24kV was obtained, feeding a 625 signal, a rather poorly test card was obtained. This was when I found that someone in its chequered past had replaced the tuner and decided to just twist together the output to A.E input. I replaced this with correct F-types to join the two sections, still a poor picture. I had my suspicion with regards to the I.F. panel so for a quick test a known good panel was installed. This proved to be correct as a test card was presented.
From this point I could see colour was running through suggesting the reference oscillator was off but before long that went leaving just a B&W picture. Then all of a sudden the PSU died. Voltage checks showed I had the 240V rail and 30V present. A few checks revealed at this precise time R607 which is a section of the main dropper, went high and I mean high, from 100R to 10M. Replaced the dropper, powered on still nothing! Found R645 on the base of VT601 had also decided to go o/c. Replaced both of these and this time I was back in business. However now I had squealing and squeegeeing sounds emanating from the top of the chassis. Immediate thoughts were “oh lord here goes the line output transformer”. A look at the EHT showed it was fluctuating and Test card F had wriggly verticals.
Powered down sharply and installed another LTB to confirm if this was indeed LOPT trouble, the squealing was still present. Two faulty line time-bases? Unlikely. Sticking my ear closer to the PSU module and half expecting the chassis to grab me by the throat for disturbing its slumber, I could hear the ringing/squealing sound much more prominent. A quick shufty at the cct for likely candidates for the ringing Chopper tx and I noticed the sound had stopped. The 58-65 rail had now died. Time to remind what happens at switch on
The 240V supply rail
- The half-wave rectifier W601 produces the 240V rail.
- As a result, the 30V zener W605 comes into operation, biased by R608.
- The 30V series regulator transistor VT601 can then come into operation.
- Once the 30V rail has been established, the line oscillator will start up supplying the trigger pulses to drive the mono-stable circuit. So the last supply to appear is the 'Chopper' regulated rail.
- No 30V rail means no 'Chopper' drive, therefore the delay switch transistor VT602 makes this doubly certain. Unless W605 conducts, VT602 remains 'cut-off' and the mono-stable cannot operate since VT603's emitter is virtually open circuit. Therefore if the 30V zener or VT602 or VT601 are o/c or C609 s/c there will be no 'Chopper' drive, no 'EHT', no 'Field scan' or 'sound'
W601, C602, C603, R601, R602 (fusible), R608 All these bar the last one create the 240V supply. R608 feeds the zener that makes the 30V supplyThe 30V supply rail
R606, R615, VT601, C627, R608, W605, VT602, C607, C609 All these create the 30V supplyThe 58-65V Supply rail
- The 58-65 supply is achieved via the chopper transistor VT604 with its inductive load reservoir L603
- VT604 is switched on and off at line frequency by a square-wave generated by a mono-stable multi-vibrator VT603/VT606
- When VT604 is switched 'ON', energy is stored in L603
- When VT604 is switched 'OFF', the efficiency diode W616 switches on and the current flow in the load is maintained.
- Stabilisation is achieved by varying the on/off time of the chopper transistor, varying the mark-space ratio of the waveform produced by the mono-stable
- The output voltage is sensed by the feedback amp VT608, whose collector voltage is used to adjust the time-constant of the mono-stable circuit
Time for the scope to see what was happening around the mono-stable multi-vibrator VT603/VT606 and the chopper drive VT605. Upon testing VT605 I found there was a short between Emitter & Collector, once replaced the 58-65 rail was restored.
With power restored it was on to the decoder for further inspection. As you all will be aware by now this set prior to my ownership, had been poorly stored. As with the other sets from that batch corrosion issues are playing a big part in the faults this set presents, let alone the usual age related and normal stock faults. Previously I had discovered W322 (OA91) the pulse clipper o/c, it was replaced. On further inspection all the presets look to have lead-out corrosion, one in particular R354 which controls the pulse width, just fell apart when tapped.
This preset is normally left fully anti-clockwise as if it is too far clockwise, the effect will be colour lost so R365 was replaced. The Chroma board was also cleaned, rinsed and left to dry. Still no colour when powered up so the next step was to disable the colour killer to ascertain what the colour fault actually was. To disable the CK you need to connect an 82K resistor from the junction of C323/C324 to chassis. There was still nothing when overriding the colour killer. This suggested either the chrominance signal or the reference sub-carrier was missing. I scoped the chroma board and found it was shut down, no gated burst, no ½ line frequency component, no ref oscillator. I replaced the 4.43 crystal and with colour bar fed in I had barber-pole, "unlocked colour". So the oscillator was running but free running, not locked to the transmitted burst.
Armed with the scope I looked for the burst waveform at the collector of VT302, it was not present. I thought this might be due to the gated burst amplifier VT301 not working, so checked for the gating pulse waveforms in and out of the polarity splitter VT308, again nothing. VT308 provides the anti-phase pulses to drive the burst blanking diodes and to supply the burst gating, so if this transistor has failed there’s no burst, no ident and no colour. Upon removal VT308 had indeed failed, with it replaced I now had burst at the collector of VT302 and the gating pulse going in and out of the polarity splitter. I was however still unable to resolve colour bar.
I removed and tested VT303 which was found to be OK. R315, another of those rusty looking skeleton presets was replaced. I then decided to check the phase discriminator diodes W302 and W303. Wow! they fell apart when probed despite looking OK, more rust issues! R312 also with rusty lead-outs fell apart when checked so was replaced. With these changes I now obtained colour car lock albeit incorrect colour. A quick decoder set-up followed using a DMM, this gave me correct colour bar. feeding in Test Card F, a pleasant result was presented.
The cabinet wood had suffered from the damp storage and was water stained in many places. The 'ColourMaster' badge was badly rusted too. I lightly sanded the case as I did not want to go through the very thin veneer. The water stains were too ingrained so I applied a very weak stain to try and equalise the overall look and disguise the water marks, which after light coat of Danish oil, turned out quite well. I had a NOS 'ColourMaster' badge so that was fitted. Just need to figure what to do with some of the missing chrome from some of the decorative trim and touch up the black control panel in places.
The current picture results are far from perfect and the final tasks remain to be done such as slight rotate the scan coils, then a full chassis set-up, grey-scale static and dynamic convergence. As I've stated many times before, I'm not restoring my sets but repairing them. I fixed a couple of broken coils on the convergence board as well as one of its arms that had totally rusted away. The Field and sound or video boards are untouched, there's many tants and a few callins and almost all the components it came with still present but I have many sets like this. I do however ensure mainly they are in low voltage positions, any HV ones are replaced as a matter of caution. I've addressed only what was needed to get the set up and running and will fix-on-fail anything that decides to give up as that is where I get the enjoyment, fault finding and the education it provides.
Sorry to blather on and this account of the repair likely makes it sound like a simple breeze but all diagnosis and fault finding was over a couple of broken weeks, In total time I guess a few days, plenty of head scratching and lots of scope use. For those interested in the blow by blow account you can read it by following the link provided below. For some it may prove of use, others hopefully interesting and I maybe some mildly irritating don't worry it only runs to 4 pages.Link to the blow by blow account