CTV [Sticky] 1974 HMV 2726 Coloumaster; Thorn 4000

@crustytv How is the power supply to the audio stages derived?

(I should go and look at the map, really) 

Just tested the ultrasonic remote, it does not appear to work. Whether this is due t the remote or the ultrasonic receiver, I've no idea. It's highly likely the IC ERC3064 was toasted many years ago. Perhaps some of this hash I'm getting is related to that area.

Difficult to know where to start.

I've got to go out, so I'll give it some thought while I'm out and get back to it later this afternoon

@crustytv I don't know what signal source you're using but it may be worth trying a different one. Channel 8 was, on later models, for vcr/video games. I think the tuner drawer has a micro-switch that operates the tuning afc when it is in the closed position. The tuner is plug in and might be an idea to check the pins and the socket. It's looking good though and is bound to have a few niggles.

Hi Jim, the video source is the same I've used on all my TV's, not a DVD or VCR but a dedicated pattern gen, then IMOGen test card and teletext generator.

To be sure, I just tested with my trusty old PM5509 feeding in colour bars.

I then fed in the TSG95 as an external source

Then the IMOGen as an external source.

The 4K is the common problem, I just need to get my thinking cap on and figure out where.

Now I'm feeding in tone, the hash seems to have gone!  I need to sort out why I have this poor video, so that's my next job.

Latest Test Card F via IMOGEN and Tone from TSG95

There's some unusual circuitry going on to get you the supplies to the audio stages, but I must admit when I listened to your video I thought "picture-rate content".

I'm having a break from some electronics that's driving me nuts at the moment.

Well, I just had to know, as the suspense was killing me......... Unbelievably, the replica Vertical deflection that was built over the winter of 2021/2022 works a treat. I just plonked it in with all the pre-sets at mid-point.

@crustytv That's just... brilliant! So does this mean that (eventually) there'll be two Thorn 4Ks working?

Meanwhile, can I just say I hate Parker-Kalon screws?

Indeed it does, Mike. 👍  Might possibly end up being the only two working 4000s  left in the whole of the U.K. Unless anyone else knows different.

Just to recap the story for those that may not have followed. When I purchased the two 4000's I bought them without an internal inspection. I know, silly, but it would have made no difference, I was going to buy them regardless.

When I opened up the HMV, I discovered the Vertical deflection board was missing. The Ferguson 3C03 was complete. I contacted the previous owner on the off chance his father (now in a home) might have stored it somewhere in his workshop. No board was located.

The background to the story sort of then unfolded. The chap had a dim recollection that his dad bought the HMV as a spares set for the Ferguson 3C03, which was this chaps bedroom set when he was a youngster. The Vertical board was removed from the HMV and relocated to the 3C03.

I removed the Vertical from the 3C03 and that served as a template for building the replica. Thankfully some of the critical parts to aid replication like the mounting frame come heat-sink, 2-pin socket and coil, were provided by Jim. John etched the board, and an anonymous guest made contact, and sold me a bag of NOS thick films. All the rest of the parts I had in stock.

The HMV board was returned to the HMV and was present upon the first power application. The test of the replica module, a couple of hours ago, brings the story of the replication to a close. That board will now live on in the 3C03, that TV will be tackled once the HMV is fixed.

So pleased to see you tried the new vertical board, well done. Suspect your home made TF401 and TF402 would have done the business too.

The fact the audio is reasonably OK means a good place to start is the tuner then IF strip. Start by checking the HT supplies to the tuner and RF AGC level on SKT43. Now this is where the circuit can get a bit confusing if you've never encountered a 4000 as it was essentially meant to be an export set. The tuner is in fact pluggable and it's socket, SKT43 can accept different types. In the UK an ELC1043/05 or it's VHF version the ELC1042 for VHF 625 communal aerial systems or VHF/UHF ELC2000 series for continent. There should be two 12V supplies, pin 4 which is the RF amp section and pin 8 for the mixer/osc. Pin 2 is the RF AGC and I seem to remember this should be somewhere between 4-8V depending on signal strength. If the voltages are OK plug in a new tuner but don't use any tuner without an aerial isolator unless using a mains isolation transformer. BTW, I don't think it will but has the tuner drawer got band switching capabilities?

Posted by: @jayceebee

There should be two 12V supplies, pin 4 which is the RF amp section and pin 8 for the mixer/osc. Pin 2 is the RF AGC and I seem to remember this should be somewhere between 4-8V depending on signal strength.

This will be fun, if you recall, SKT43 has multiple occurrences of those. Time to consult the circuit and board view.

Hi Chris, some of the notes I made (over 40 years ago) in the 4000 circuit desription book that I uploaded a good while ago might be worth a gander possibly. You never know.

Pretty sure it’s going to be the lower row of pins with pin 2 at the left and 10 at the right.

Posted by: @jayceebee

There should be two 12V supplies, pin 4 which is the RF amp section and pin 8 for the mixer/osc. Pin 2 is the RF AGC and I seem to remember this should be somewhere between 4-8V depending on signal strength

4=12V, 8=13V & 2=2.1V

Another interesting observation.

Video

Pin2 seems low but I can't be certain. The RF AGC control is done at pin 4 of the TCA270 but looking closer at the manual I expect it now to be at least 3V or little higher at pin 2 of the tuner. Can't remember whether this voltage rises or falls with increased signal strength 😩, have to look up past ELC1043 articles in TV mag. Still won't hurt to try a tuner to rule it out of the equation.

The "interesting observation" is that L114, the AFC coil is slightly off and may need slight realignment when the signal issue is resolved

Quick update, just checked on the 9600 I’m seeing 2.8v on the AGC pin of the tuner. Adjusting the AGC crossover pot causes the voltage to go higher which reduces the RF gain.

Looks like your TCA270 is telling the tuner to work harder🤣

I thought I had lots of ELC1043/05 as I have a large box full of NOS tuners. After looking, I discover I have tonnes of brand new unused, but the vast majority appear to be Philips. I found only one 1043/05 with isolation. No idea if it's good or bad. I did find some NSF which look pin compatible.

This is probably not the problem, but as you rightly say, it's little effort to at least eliminate from suspicion.

Posted by: @jayceebee

Still won't hurt to try a tuner to rule it out of the equation.

First thing this morning, I swapped in the spare ELC1043/05, as we all probably expected no change, and the voltages are pretty much the same. Pin-2 =2V, Pin-4 =12.6V, Pin-8 =12.6V.

Posted by: @jayceebee

BTW, I don't think it will but has the tuner drawer got band switching capabilities?

Correct, it does not.

I'll reinstall the old 1043, I guess now it's time to go a hunting in the IF module.

Just so there's no doubt, these are my test workshop test signals, monitored and as fed to RF.