I wouldn't lose any sleep over the vertical shift assembly. My recollection is it was always set in the neutral position. Which means you don't need it.
I would have had my alternative solution ready by now except I have been crazy busy at work modifying designs to substitute for parts that have become unavailable or have stupidly long leadtimes. My son's ute restore project has also suffered. The only reason I'm on the computer at home tonight at 10:30pm is my driver's licence expires tomorrow!
BTW, the 6 amps is a peak to peak value. Each transistor only needs to contribute 3 amps. So not a big deal.
Implementing that circuit reliably without special transistors and thermal coupling between certain components mounted onto a common substrate is likely to be a problem. As I've indicated earlier, it's really not a very good design. There are much better ways of achieving what it is doing. Such as using a power op amp like the LM1875.
Re the 0.68 ohm resistor, it needs to be a special current sensing type, designed to handle 6 amps P-P. That is the part that senses the current through the yoke for feedback. Other designs used up to 6 or 8 resistors in parallel in this position. Example - the Philips K11. Put Current Sensing in your Farnell search.
Re the 0.68 ohm resistor, it needs to be a special current sensing type, designed to handle 6 amps P-P.
Hi Ian, you'll have to forgive, I'm not sure if I follow you. With regard to R411, are you suggesting the resistor I've bought is not suitable? In which case, I don't see why.
According to the modifications manual, part 5 in the data library, none of these resistors were special, other than they were calibrated low ohms. Well, it could be argued the first incarnation was special in as much as it was a thick-film.
The early chassis had .68R 5% 6W thick-film, this was then changed on later releases to a .68R 2% 4W wire-wound. This being fitted on the one Vertical board I do have. The changes were, as expected, to improve long term reliability.
The ones I bought from Farnell and turned up today, are .68R 5% 7W cement, I've mounted this in its own heat-sink. I thought this would be perfectly suitable as, like the ones originally used by Thorn, it's a calibrated resistance. The current flowing through R411, in the form of a voltage drop, is detected and monitored “The sensing” by the cct in TF402. See pages 30/31 of the service manual (in data library).
Or have I totally got that wrong? Sorry if I appear a bit dense, I've only got the manual to rely upon.
Resistors in this position in just about any SS CRT TV are a reliability hazard. Hence the more recent introduction of special types of low ohm resistors that can handle high peak currents without changing their value. I use them in switched mode power supplies for LED backlighting, for example.
I think the one you have chosen should do the job.
The 75pF 500V 5% arrived today, still waiting on the final two components to complete the base PCB .27uF 250V and the 2.2uF 40V Non-polarised.
I cut and shaped two matrix daughterboards, for TF401/TF402, once coated in the brown EHT silicon, I think they will look authentic. Parts for these replicated modules will be gathered next, I think I have most in stock. Work on this will then cease until I have the etched PCB. If that turns out to be a no-go, then I will solder everything in on this mock up and proceed.
Next; I will do some research about the missing TF303 on the HMV line board, see what options there are, if only there was a cct for it.
The .27uF for C411 arrived and two 470R 5W to replace the cement 470R 5W, looks neater than the cements. These acting as mid-point for missing vertical shift. All I'm waiting on now is the 2.2uF NP.
All the components for the main board are now present, I also swapped out the two 2200uF caps for high ripple ones.
John is hopefully going to try a trial run at etching the main board one evening this week.
In the meantime, I'm going to have a stab at recreating TF401. John has kindly passed me a cct to assist me with its construction. Not looking forward to this bit, I’ve got to try and fit all that, on a little 50mm x 35mm matrix board. I can see it being a rat's nest of jumpers on the rear, plenty of scope to make a mistake.
Anyway, this should keep me occupied for the coming few days. I mapped out the pin-out spacing, bent them to match the main etch and installed. Starting to offer up the components as a dry fit, to find optimum layout, before finally soldering them in.
All parts bar one have been found from my stock. The one missing at present is the zener diode, this is because I cannot tell what voltage the zener should be from the code John supplied. BZXB3XC1B ?????? That just makes no sense. Is that C18 and not CB, making it an 18V zener? Will have to wait for John to confirm.
In total, there are 27 components making up TF401, I've dry fitted, now just need to figure the optimum layout. As I suspected, the back is going to be a nightmare to wire up. I'm going to use DIYlayout to see if that helps me plan the layout and wire-up
Edit: Actually looking at the rest of the fonts on John's cct, IN4148, the 8 there looks like a B, so I think 18V is correct for the zener, which is what I've now fitted. A BZY88C18.
1) To be able to call the local Thorn supplier and get some thick films.
2) to have a machine where you insert a blank piece of PCB, import a photo of a circuit, it then etches and cuts the board to size. Leaving you just to drill the holes.
Instead....... Nurse, my brain hurts......
This is spatially taxing and slow progress. All I've managed to do is establish the two rails required on the TF connect, pins 3 & 12, and install R1, R2 & R3.
I think perhaps I was being a little optimistic, thinking I could replicate these TF's on matrix. It's been a nightmare, and I've only managed 5 resistors, 2 diodes, 1 transistor and 1 cap. This required the use of 6 jumpers. I've still got 18 components to install and no doubt more jumpers....... Yikes !!
I think perhaps I was being a little optimistic, thinking I could replicate these TF's on matrix. It's been a nightmare, and I've only managed 5 resistors, 2 diodes, 1 transistor and 1 cap. This required the use of 6 jumpers. I've still got 18 components to install and no doubt more jumpers....... Yikes !!
You've gone to the effort of capturing the schematic ... only one step away from making a PCB up!
I made a complete hash of the first one yesterday, I think that needed to happen for me to gain the experience of how not to do it.
Start Again......
Today, with a fresh approach, and knowing what not to do, and importantly how to best layout the components, it came together. Not one single jump required, and a much neater component layout topside, and a neater solder-side.
Now I know the optimum layout, I will document it, so I can replicate these, should the need ever arise.
All that remains is to add the pin-outs, plenty of space left to do those along the bottom
Once, I'm happy everything is OK, It will then get dipped in silicon, to match the original, but that's not for a while.
Today, TF402 to build, this time only 17 components to squeeze onto the little board...... Ah, the sheer luxury of having space. 😉 I'm using 1W 1%, opposed to 1/2W 5%, for the 22R's.
Noticed you have D3 missing from your parts list but I think I see it on the board.
If I can get a normal finishing time tonight I’ll try and give it a go. Just need to sneak the clothes iron into the workshop to do the toner transfer🤣🤣
Well, some glasnost from me too. I'm afraid my first attempt at making a PCB using the toner method didn't quite go to plan. The paper I used, Kodak glossy photo paper appears to have a high plastic content in the backing. The result being that after applying heat, allowing to cool then soaking in water it didn't penetrate the backing at all and the paper wouldn't come away cleanly. I've attached a pic of how it turned out.
I've just ordered some toner transfer paper and will give it another attempt when it arrives.
