1975 HMV 2725

BRC/Thorn 8800 Chassis

Model: HMV 2725

Year:1975

System:625 Line

Original List Price : £000.00 ( to be ascertained)

Valves: None

Transistors:Yes

CRT:  Mullard A56 120X

The set has now arrived and its a nice example. Its fully coated in nicotine infused loveliness, If you’re in need of a hit then you could suck on this for hours. The control fascia was yellow and sticky as was the cabinet so before handling that was all cleaned off. The chassis is in good condition and again nicotine infused so will need the same treatment.

The 8800

This version of the 8000 series chassis employs a 22″ CRT. The chassis is similar to the 8500 series model using the PC651 signals board (I.F./Chroma/video), but these are the following differences between the two.

Convergence

On the 8880 you will find a new convergence panel (PC802), it now includes pincushion correction. It is also now fitted into metal brackets at the top of the cabinet ( see photo below)

hmv2725-11

I.F. – Video – Chroma

The 8800 signals panel (PC819) differs only slightly from the PC651 used in the 8500 series in that it uses the varicap UHF tuner with AGC. The set tuner gain control is not fitted.

hmv2725-09

TimeBase

Finally the 8800 sports a new timebase panel (PC650), it now includes an additional transistor. This is a FET, VT412 and is used as a buffer stage to prevent line pulses interfering with the operation of the field oscillator. Unlike the 8000/8500 whose TMB panels are interchangeable, this new TMB panel can only be used in the 8800 and not the 8000 or 8500. This is due to the use of the transductor for the pin cushion correction.

hmv2725-07

Now the rest of the pictures. You may find you recognise the cabinet, HMV( Thorn) used this on many models and even on B&W sets as it was a very popular style with customers.

hmv2725-01 hmv2725-02 hmv2725-03 hmv2725-04 hmv2725-05 hmv2725-06  hmv2725-08  hmv2725-10

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.

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.

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, perhaps this is why the set was retired.

The culprit for all the smoke was a tant C203. I cannot find it listed on the circuit. Hard to make out but I think its a 1uF 35V. I fixed the I/F fault which tuned out to be a loose plug and installed a replacement 1uF cap for C203. The result below

Decided as I had nothing to lose I would zap the tube with the B&K. 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.

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 I was back in the fight and I will now tackle the tuning/IF fault that’s developing.

Replaced a few poorly tants and things were improving but the tuner is flakey. Still got wobbly verts, low gain, rolling up or down take your pick as I cannot stop it by adjustment. Got the brightness cranked all the way up but I suspect that’s down to all the problems around IC.3. This ones going to be a long haul. Still glass half full, the decoders working and the colours look nice and rich.

I don’t yet know why but if I tap the UHF tuner can quite sharply with a screwdriver, I can make the picture appear as you see above. Then for some reason and the timing varies, it will eventually slip back, lock starts to go, verticals start going wobbly picture breaks up and then the mess. Sometimes the first tap does not work, sometimes it improves a little, a couple more taps will either bring in a good picture or the mess.

So this I believe locates it to the Tuner PCB, what I don’t know is what the cause is. Assumptions: Dry joint/s, cracked print, failing component on the board, the 30V stabiliser TAA550, the UHF can its self, components in the can (God I hope not), coax connections to the can etc. Going to be fun chasing it down. I was informed by an ex TV engineer the ELC1043/05 tuner was not the best and very prone to internal joint problems and highly likely the problem.

I removed the UHF tuner can and to my eye cannot see anything obvious, all the joints in the can look solid, obviously something is not right. The PCB looks to be OK with regards to no cracks. I think as a start I’m going to resolder all the joints on the PCB just in case the can whacking is not the fault and the vibration is carrying through to some other component. Once that’s done if it still fails then it must be something in the can.

Paid a visit to the premier Colour telly spares shop in the North East, namely Malc’s Speedy Spares. Who cares if CHS has closed, they couldn’t begin to come close to the stock or speed of service  MSS offer. I collected a spanking new Thorn TX100 UHF tuner module also had a good chin wag about TV’s as you do whilst watching the torrential rain we’re having up here.

Back to the workshop and I installed the module in the 8800, as soon as I powered up I knew I would onto a winner. The snow was really snow if you know what I mean, before hand it was very lacking. I tuned in and well, I will let the pictures do the talking. Needless to say I hurled the the old module and its now firmly embedded in the back fence of the garden.

I went through the Grey scale procedure you solder .1uF ceramic to the top of TP1 pins. Have the brightness as max, remove the Red A1 drive to the signals panel and with and AVO set R176 ( preset brightness) to 150V. Then with low ambient light set Red, Blue and Green to near cut-off. Now set R176 to 125V. set brightness to mid, feed in a testcard with colour off and trim the RGB drives for good B&W test card.

I noticed the other day the green static was very temperamental but it slipped my mind. I was doing some final tweaks to the convergence as I had noticed some errors on the TCF and when using the pattern gen. Whilst performing the static the green knob/magnet decided it was not going to turn but not before it turned enough to put it all out leaving no ability to rectify. The problem, the magnet had come unstuck from the knob. Little else for it but to dismantle remove, glue. It worked a treat.

The final Shot

The CRT for how I found it, is giving in my opinion superb grey-scale and colour too for that matter. All in all not too bad for an example of the rarely seen 8800, a set a lot of the trade said should be forgotten. Its worth remembering this set has not been restored just repaired, its running with 90% of the original components including many 10’s of tants.

The body count

 

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