1975 22" HMV 2725: Thorn 8800
Where has the Time Machine dropped me off this time? Well its 1975 and colour television take up in the UK is in full swing. David Essex held the number one singles position with "Hold Me Close". We were all flocking to see the new block buster Jaws, swimming in the sea just wouldn't ever be the same would it. On TV we were all glued to the new gritty British cops series The Sweeney. I digress, so what's up on the bench this time I hear you all asking.
Well a rather nice example of an HMV 2725, the chassis employed in this set being the Thorn 8800 series a variant of the 8000. Its not one I see often turning and a few years ago on UKVRRR it was one of the sets considered to be on the TV equivalent endangered species list, therefore I thought it would be an interesting diversion from the usual 3000/3500.
OK I know the groans of, "OH God not another Thorn pleeeeeease......" will be calling for something different and something different will be coming for the winter project, for now its the 8800. Some folk hated them, they consider them best forgotten. I say they were part of the tapestry of the CTV evolution in this country therefore deserve their place and to be preserved as part of that record.
The 8800 differs from the 8000/8500 as it employs the 22" Mullard A56-120X picture tube. The receiver is similar to the 8500 series models using the PC651 I/F-Chroma-video panel but with the following important differences.
A new convergence panel PC802, including pincushion correction, is fitted into metal brackets at the top of the cabinet. Releasing two clips allows the whole unit pull out and swing upwards allowing convergence adjustments to be made from the front of the set.
The panel used in the 8800 is PC819, it differs only slightly from the PC651 due to the use of a varicap tuner with AGC. The old set tuner gain control is no longer fitted.
A new timebase panel PC650 now includes an additional transistor, this is a FET and is VT412. It is used as a buffer stage to prevent line pulses interferring with the operation of the frame oscillator.
A worthwhile note: This panel must not be used in the 8000 or 8500 as it will cause problems with interlace problems.
Set Base-Line Evaluation
Overall it looks to be all present and pretty much unmolested. When I swung the I/F-Chroma-Video panel out a large core dropped to the floor. I've no idea where this came from, its certainly not from the signals board or the convergence above, we shall have to wait and see.
The power supply side at the bottom seems to have a choc-bloc fitted with a VDR/Thermistor fitted, certainly not Thorn standard so quite why it was fitted in this manner remains to be seen.
I spied that the cap across the line was a RIFA and although I could not see it condition, there was no way that was staying there. Upon removal it was as suspected, cracked. I've no idea why there is a 1meg strapped across it, this does not appear on the circuit. The cap fitted was a .1 uF but the circuit shows .22uF as its the filter cap no matter but I would think the original popped and then this was fitted at a later date. I will replace with a modern .1uF X2.
The smoothers look to be in pretty good condition with no bulges, splits or leaks, fingers crossed they should reform.
The rest of the receivers chassis around the high voltage circuits especially the LOPT area, is covered and smells of nicotine, lots of dangly dust bunnies hanging around. I will test the CRT before going any further to see what I've got to play with. Assuming that it has some life I will replace that filter cap and introduce it to some power.
For those that like to play along I have the service data for this, as I said above its basically the 8000 with a few minor modifications. The 8000/8500 service data is already available up top and I will upload the additional 8800 sheet later this evening.
For now some additional photos.
Re:1 meg resistor across the filter cap, some of the TX range of sets had 10meg slapped across the L and N of the mains switch, not all sets had this and TBH I cannot recall if it was shown in the chassis manual or not, JC might have better info though.
Not sure what's going on with the CRT or if the results are to be believed. If they are then the set is for the bin and spares.
Connected up the B&K in the usual manner setting the htr and then the G1. All ok so far, heaters glowing and no indication of any shorts. Next set up the G2 cut-off, unable to do this on red or blue but green I could. Set the gain to high, try again, same result. Switch to emission test not a sausage on Red or Blue Green reads very low.
Out with the leader CRT tester, set up on that has the same problem, unable to set G2 cut-off for Red & Blue but Green I can. Again run emission test on the leader shows Red and Blue dead with green showing low.
So either the CRT is screwed beyond belief or there is something amiss. I note its a Mullard colourex, I could be wrong but I believe these are Mullard re-guns? Perhaps this has something to do with the odd behaviour I'm getting on two separate CRT tester. I guess the only real test is to get the set powered up with the minimal amount of effort an components to be really sure.
I've tested other A56 120X CRT on both testers so I know its not them, not the start I was hoping for.
Without further ado the set was introduced to full power. The thump of the PSU starting up was followed by plumes of the magic smoke escaping. leaving it running for a few seconds so as to locate the source, there seemed to be tonnes of smoke escaping from IC.3 on the signals panel. A finger placed on the chip revealed nothing getting hot. Turned off as by now there was a fog descending in the workshop.
Upon further investigation I could see C203 above was looking very crusty on the edges. Yep! that looks bad and a finger showed it was quite warm, a ghostly voice could be heard, "beware the Blue Tants. Yes LLJ you're right, the pesky tant was the smoking culprit. I swiftly removed it from circuit then powered on, again swoosh thump and up it came, sound and a raster.
It does look like the CRT testers were telling the truth, all I have is green.
I fed in a signal but soon noticed I didn't have any snow, the I/F side is dead so no amount of tuning will sort that. I will have to go and see what C203 does on the circuit, it may well be what's causing the I/F fault.
However my main concern is the CRT, it would appear to be toast. I swapped the video drives around on the signals board to see if red or blue are there. I did this as follows, as we know green is OK I disconnected the red and then blue then in turn putting them onto greens drive. Nothing! Well if I had the brightness up at max there was just a tiny bit of lightness detected for each. I then put Green onto the Red & blue drives, green worked on both so I've eliminated the signals panel. I believe that's it game over for this set unless someone has a suggestion. Iit looks like the CRT really is duff, perhaps this is why the set was retired.
A nice set....good if you like green, have you tried giving the red and blue a good kick from the B&K ?
There is very likely nothing left to kick Frank,
These Tube were flat on there arse back in the 1980s, probably already been kicked before too.
Well I don't normally like blasting but as I had nothing to lose then why not. There was a lot of fireworks and the G1 to cathode short light came on for red and blue. Red & Blue, they're back but for how long remains to be seen. Unfortunately another fault has now arisen at whish is loss of line lock.
I had adjusted the drives so it all a bit messy colour wise but you can see the raster is much brighter with Red and Blue now in the mix. Also the decoder is working as I can detect colour change when the colour control is adjusted.
Now to investigate the lock problem.
I'm impressed !! A good kick woke 'em up a bit.
Being ever the pessimist, like Chris I wonder how long the guns stay up, having said that the older tubes did stay up reasonably well, and especially if Chris is not using the set day in, day out, by the time we tried the A56/66 540X series in the Tx10/100 it could be measured in hours, the FST in minutes
The delta guns lasted months.
OK the line just needed readjusting, probably component drift. Now the I/F tuning is starting to play up so I'm loosing the signal. Looks like the longer this set stays on the more parts are starting to creak under the strain.
Still we're back in the fight and I will now tackle the tuning/IF fault that's developing.
All good fun.
All good fun.
It is fun, and I for one am really interested in the progress of this set as it has the same chassis as the Ultra Bermuda that my parents had as our first colour set.
Like this white one, but in teak effect.
Moderator Note: When quoting a previous members post its not necessary to include the photo in that quoted comment. Photo removed from your Chris quote.