Transvision 7 inch kit project

[dtvmcdonald]

I got a Transvision 7 inch CRT kit at the ETF auction for what I thought was a high price, but turns out to middling. That's because it seems to be an unmodified "early" one with a three channel tuner and a 6N7 as one sweep oscillator. Its in good shape cosmetically, with a good plywood front and good speaker grille. The holes for the wood screws that don't go through the plywood are stripped and will need the "plastic wood" treatment.

There are a few electrical problems. The speaker is the field coil sort, but it needs a permanent magnet one, and the voice coil scrapes to boot. If anybody knows where I can get a good correct look permanent magnet one, let me know. Its 6" round, as vanilla as they get. The 7ep4 was necked, but I got one for $20 at the consignment sale, and my Sencore tests it as very good. The Sencore is reliable electrically but the tube could have an ion burn or have a generally tired phosphor. All small tubes are there.

I took the front off for moving it, but pictures will follow, before I start restoring. I found suitable replacements for the HV oil caps at Mouser, not cheap. This set has 60Hz "widowmaker" HV. I will put a 1/4 or so amp fuse in the AC line to the HV transformer.
I will not restuff these caps: one for sure is "Vitamin Q" which is PCB and poison. Otherwise I will use axial replacement caps as I need to order all these ... somehow they managed to use only the few values I do not stock.
I have not decided on the electrolytics. I may or may not restuff, maybe just put modern radials below chasses, or maybe use
Hayseed Hamfest or other premade cans
(expensive.)

Keep tuned.

Doug

[dtvmcdonald]

Today I inspected it and found it in good shape underneath. Except the interlock was defeated and the 117 AC connected in the air and not insulated!

I removed both rectifiers and slowly turned it up on a variac. All AC voltages looked ok on a scope (I have scope probes up to 50 kV.) Not all tubes lit or got warm.
Inspection showed tubes in wrong sockets and only one 6N7 ... the socket wirings are both clearly that and not 6SN7. Correcting that (for some reason I can't recall I stock 6n7s!) resulted in all tubes lit or warm. I then inserted the 2V2 and again turned it up very slowly on the variac with dim bulb. The HV got to 2.3 kV with no distress after about 20 minutes, so I removed it and ran the HV up to 3kV, again with no distress.
I then disconnected the HV at the 117 line.

I powered it up very slowly with a 5U4, variac and a 300 watt dim bulb, getting it to 85 vac in a couple of hours. No cap or resistor got hot. I used a scope to verify that the horizontal and vertical oscillators were working. The RF oscillator didn't. I replaced the dim bulb with a kill-a-watt and 3 amp fuse. I slowly raised it to 117v with no distress. At 89 volts the RF oscillator started.

I then investigated both sweep output stages. The horizontal was OK but the vertical was not. It had both phase outputs, but one was a bit low voltage and the other was very very low. The set set could never have worked completely
OK vertically in this state. Inspection showed the problem instantly: the stage is miswired. That is enough for today so I went out for a burnt ends BBQ sandwich.

[dtvmcdonald]

The only replacement part in the set appeared to be a red 0.1 uF cap in the vertical, replacing a paper wrapped one. It was hooked to the wrong spot. It also measured about 10K resistance. The other 0.1 cap next to it measured 35 megohms and was left in as its going to be replaced. Installing a new 0.1 made the vertical work. The red cap was only a 200 volt one: the manual lists the voltage as just below that ... but it has AC peaks far above 200 volts, so no wonder it failed.

The sync circuit looked odd. It eventually turns out that this is an "old" chassis except sync which is "new". It was wired OK.

I then tried to get a signal generator through the IF. It went through
IF3 OK, but IF two was dead. This took a while to fix as it had three problems: The tube had a grid-screen short. The "contrast" (cathode resistor for the first two IF stages) control was bad. The bypass cap for the screen was about 5Kohms. The first IF and mixer were OK.

I tried feeding RF from an RF sweep generator. This showed that the three channels were 2,3,4 and all worked. The IF response is abysmally bad. I fed a real signal in and it produced nothing until i wiggled the channel switch. Deoxit fixed that.
I then got obvious slope-detected audio at the detector. Adjusting the oscillator trimmer fixed that and I got video nice and clear (on a scope ... no CRT and no HV at the moment.) There was no video at the feed for the CRT. This was a dead short screen bypass cap. I'm surprised that had been no sign of magic smoke.
Fixing that gave good video. I have not tried to see if sync works yet.

Edit: vertical sync seems to work "well enough" to use. Horizontal doesn't. If the output stage 6SN7
is removed it can be made to sync, poorly. Something's wrong.

[dtvmcdonald]

Swapping 6SN7s made it work passably. I attached a CRT, plugged in the 2x2 and reenabled the HV. It produces a very dim in sync poor contrast picture.

[dtvmcdonald]

It was out of alignment worse than any TV
I've ever seen. Their signal generator alignment produced a better waveform (the HV being disabled again). A sweep alignment test showed it still be rather poor.
A full sweep alignment made it look exactly waht a cheapie 3 IF stage design should look like, plenty good enough for a 7 inch screen.

Edit: installing a good 5BP4 resulted in a quite good picture. Its off center as I am awaiting insulated shaft extender for the shafts.
Brightness is average for 5BP4s. It does suffer from focus shift with large brightness changes.
Pictures will follow before a recap. Only one change is visible, the red cap replacement as the new pot looks almost the same.

*******

After a break I discovered that all that was a line voltage of 90 vac. I was measuring at the input to the dim bulb.
Turning up the line slowly with the HV off resulted in better video signal and better lock. Until, at about 111 volts there suddenly was a puff of magic smoke.
This was the plate isolation resistor for the audio IF
.
It was instantly burned to a crisp: 37 ohms from 2.2k. Since I literally had my hand on the variac it only lasted
about 2 seconds. At low voltage the caps to ground test good. The new 630 volt ones have3 not arrived.

[dtvmcdonald]

photo before work except disconnecting and adding radial film cap for red cap in upper right.

https://www.videokarma.org/attachmen...1&d=1778289501

Doug

[dtvmcdonald]

I'm still awaiting arrival of the shipment of the axial film caps, which are most of them.

In the meantime I rewired the AC input, adding a 3 wire cord (the original was near getting crumbly), adding an
ac line fuse, and a separate lower amp fuse just for the HV.
I rewired the interlock (which was bypassed) so it only is on the HV.

A question: where do I get a replacement old-time paper cone "6 inch" speaker? The mount holes are a 5 1/4 inch square.
Its permanent magnet type.

[dtvmcdonald]

I spent resterday recapping and checking resistors. There were very few resistors far enough out to change. A couple looked a bit overheted and I replaced with like-sized modern film ones (which of course have a much higher watt rating then equal-sized old ones.)

I first did audio, then RF and 1st and 2nd IF, then last IF and detector, then vertical, then video, then horizontal and finally sync. I checked operation after each step. Checks were by oscilloscope.

All went well until the very last step, which I did last because it looked quite unlike any schematic. It still worked but not as well as before. I will investigate today.

Then I cheated the HF interlock and tried it with a 5BP4.
It looked good except for a little bit of horizonal bending. I will work on that.

[dtvmcdonald]

I found that it also had vertical rolling on scene changes. It had very odd "tuning" of the contrast-brightness,
like no other set I have used. More than one setting gave a passable picture.
I decided to look at all the scope waveforms that they showed in their repair booklet.
It became quickly clear that only one group of settings for the brightness and contrast (RF gain) were correct,
and that the brightness of the picturwe was less than what
I had obtained before. Another symptom was that with bright picture sometimes but not always it had defocus
due to brightness dependant currect draw from the focus electrode, up to 300 uA. This was far far
over the few microamps spec.

The 5BP1 spec is for no more than 2 volts positive on the CRT grid. Keeping to that also prevented signficant drooping
of the CRT gird voltage (also, DC coupled, the video amp plate.) In any case I fixed the focus by regulation the top of
the focus put with 840 volts of Zerner diodes in series.

Setting the CRT grid drive as specced resulted in inadequate horizontal sync
drive (.2 versus .4 volt spec.) This was traced to the value of the horizontal oscillator grid that the sync
drives ... it was being reduced by the (as specced) 10K resistor.
The previous owner clearly had seen this and tried reducing some of the integrating network caps.
This was a silly situation: these networks typically have two low pass filters acting as "integrators". This set has those ...
but the specced connection from that the the oscillator grid was a high pass network that cancelled on of the two.

It looked like the grid resistor should be bigger, perhaps 100K. I tried the first thing that I saw in my pile,
82K, and so long as the above mentioned grid current dropped the size it became rock stable and had huge horizontal hold range.

So I tried a prevention for too much grid current: I put a string of 5 silicon diodes
in series from the cathode to grid of the CRT, cathode to cathode. These two changes resulted in more clipping of
the sync pulses, removing essentially all the "picture" level excursions. Both vertical and horizontal became rock
solid over a large range of adjustment. The picture still gets quite bright, just not as muach as before. The
apparent gamma was much more pleasant looking.

The method necessay to set the contrast and brightness is like no other set I have seen. One has to first set the
contrast to zero and the brightness to a medium gray screen, then turn up the contrast on a contrasty scene
until both white and black appear. Then its just small tweeks.

This resulted in four changes to the circuit:
1) regulating the top of the focus pot.
2) increasing the brightness control tap bypass cap from 0.1 uF to 1 uF
3) changing the horizontal sync grid resistor from 10K to 82K.
4) adding diodes between CRT cathode and grid

All that is probably left is to change the RF gain (contrast) pot from 50K to 5K ... 50K is far too much
range, and adding a resistor in series to prevent overdrive.

[dtvmcdonald]

I guess that I am a tweekoholic. As per the last post I had it working (almost) per spec.
But then I noticed that the video amp 6AC7 was working well beyond specs.

So I added two more Zener diodes (well, chains), one to regulates its B+ to 250 volts and one to regulate the screen to 85 volts.
This of course also adds thr reduction in B+ to the effective HV, which helps brightness a bit. Clamping the screen means that
since the black level set by the signal is clamped by the diode action at the 6AC7 grid its level at the plate (and CRT grif) is clamped too. This means full DC restoration
(at sync tip level, not black level of course. So now you can set black level and then the contrast more or less only adjusts the white level
(until it too gets clamped). The white clamp means I could remove the focus voltage clamp. So this tweek on average only adds one Zener chain.

[Electronic M]

That "Multiple correct looking brightness/contrast settings" describes my TRK-12... Granted mine had a G1-K short I had to clear and for all I know may still have some variable leakage.

[Vintage TV]

I PM'd twice, no reply. I am interested in all the Sprague caps removed and the old CRT if you want to sell them.

[dtvmcdonald]

Originally Posted by Vintage TV

I PM'd twice, no reply. I am interested in all the Sprague caps removed and the old CRT if you want to sell them.

I keep all old parts ... if I ever sell the set, they go along.
Its true that the paper caps would be at least silver for the audiophools, gold for guitar folks, as they are all leaky above 40 volts.

[dtvmcdonald]

Its finished except for converting to FM, awaiting the necessary coils. I've got it aligned to give about a 3 MHz cutoff. Pictue of the bottom after recapping is attached. Note the two added fuses and the original HV interlock.

https://www.videokarma.org/attachmen...=1&d=177984815