To fault find on the chroma board the set needs to be placed on its side. I am unable to lug it about due to its size. If it were a 19 inch version i would not have had any problem with either lifting it or repairing the chroma fault.
I left both sets running with the 2701 displaying its decoder fault whilst reading up on the chroma section of the manual and seeing what waveforms needed checking first. I got the scope ready, nipped around the front to see how things were and to my amazement the decoder fault has cleared.
I suspect it won't last so will leave them both on soak to see.
With the G6 still on the main bench (just see it in the background) awaiting convergence and these two 2K's, it gets positively tropical in there
The 19" version has a "Parked position" for the regulator board where it mounts vertically against the IF, this gives access to the component side of the chroma for setting up but its still awkward due to the close proximity of the CRT neck components. Agreed, it's much better to do the fault finding in the 25" if you have the option.
I'm fairly confident Chris that the intermittent chroma problem is most probably down to a poor wiper connection on one of those awful skeleton pre-set pots.
John.
Status Update - The Yo- Yo progress continues
After a long session on soak-test last weekend, I had a break during the week. A subsequent switch on this afternoon to see where I at and I'm greeted with a very dark test card. Brightness is not quite at max but pretty much all the way up, I also note when monitoring the picture, it seems to be trying to fluctuate/vary by itself to a higher brightness level but not quite getting there. So a new fault to investigate now.
That was a quick find, the 2701 EHT generator has developed a fault.
With the workshop lights out I turned the set on, I noticed the EHT TX where the nipple is and the lead-out to the tripler, sparkled quite brightly behind its perspex cover, only for a few seconds then settles down.
I swapped the known good EHT generator board from the 3700 into the 2701. Powering up no issues at all and the brightness is returned to normal with a good test card F, so this proved the 2701 EHT generator board has a fault. It could be the callins or other electrolytics that are present or the EHT TX is faulty.
That's where the next investigation will commence.
Any of the Thorn guys know of EHT regulator board variants and or mods?
I have a spare EHT regulator board ( board 2), I put it in the 2701 and it works fine. I decided to replace C2 & C8 (large Callins) as they have a tendency to explode.
Once that was done I noticed a few mods and wondered if you might know what they are and why. Ignoring the heat-sink variant
Board 1 is out of the Fergy 3700
Board 2 is the one I had as a spare( works fine when I installed it in the 2700)and has the additional mods
Board 3 is a junker for spares.
Board 2 mods appear as follows:-
C10 seems to have a series resistor
R6 seems to have inherited an electrolytic which I think is in parallel
Then between R21 and R22 we have a diode ( track cut to accommodate) and the addition of a coil.
Interestingly board 3 has the same component layout as board 1, but has a yellow circle where board 2 has its additional coil.
Mystery
Thanks Graham,
I for one thank your brain for its retention of obscure facts relating to these sets.
Interesting read even without any conclusions, so it is highly probable that I have a board with the Thorn mods fitted.
As to the chaps query over on UKVRRR as to why is boards seem to have had the mods reversed, I think that it might be as simple as this. I was about to reverse them on mine whilst I was recapping, I thought I might return it to the same as the working one in the 3700 but before doing so posted the query here. Perhaps others in the past having not found any documentation, not knowing what they were returned theirs to original schematics.
I think I will leave it now and retain the as an example of the mod Thorn suggested. I should probably document it and post it here so its details are captured for posterity.
I've mapped the modified EHT regulator circuit below, the additional mods are in Red.
VT6 & VT7 vary on the modded board from the others, adding weight to this being the Thorn mod discussed on UKVRRR.
I'm was not aware of that technical bulletin mentioned on UKVRR as 2000 boards were supposed to be central repair at North Shields for our area but this is my take on it.
The modifications appear to be applicable only to boards fitted with the later type of transistors used in VT6 & 7 positions.
From the pictures the board with the yellow spot (Thorn loved their coloured spots) appears to have been fitted with the original types hence the modifications not being required, although only VT6 is fitted having blue markings it looks like a D1693 to me. Certainly the resistor in series with C10 on the middle board will slightly modify the line drive waveform fed to later type VT7.
The 400uf across VT1 the protection transistor is to delay its operation at switch on possibly allowing a longer window for VT6 to start conducting and the feedback loop to become established.
The heatsinks I suspect were were a request from the bean counters. The original types were machined or possibly diecast with fins on the rear which protruded through the cutouts in the PCB material. The later types are just pressed and bent flat metal which would have been much cheaper and not required the cutouts making the pcb less susceptible to damage in transit.
Update on the 2701.
I put the modded EHT regulator to one side as I want to keep the set as near original as I can.
So set about tackling the original board which is an early model. I replaced the Callins and a couple of resistors that end caps had detached, I noticed however the sparking in the EHT TX became progressively worse to the point it was now constant. I had one really duff scrapper board that had all manner of parts robbed, the TX looked good so I decided to transplant that over. This has now cured the boards fault and EHT is sitting at around 22kV.
After that diversion its back to addressing the other issues, which can be clearly seen on the Test card. First task though is to replace the RGB 25K video bias skeleton pre-sets on the video board. These are flakey and are preventing me from establishing grey-scale tracking.
Doesn't look too bad Chris. Bit of convergence and general setting up. People looked at much worse pictures than that! One customer I recall said how good the colour was on her Philips....but there was no blue! When I sorted out the blue drive pot and set it up, she complained that the picture wasn't as good as before......!
This is the second Thorn EHT TX I've had fail, the other one was on the Baird 3000 rebuild here. As you say though pretty rare but as these sets age, I guess the rate may start to increase.
I have the official thorn mod if it's still a problem after all this time, I'm restoring one presently and will be for the foreseeable due to its condition.
Greg
I would like to see the mod sheet if possible Greg to see if it matches the changes Chris added to his diagram posted on the previous page?
Hi john
No problem, as soon as I can dig it out it's yours, it's slightly larger than I can attach here, do you have an email address.
Greg.
Thanks Greg, I've sent you a PM.
