1946/1947 Transvision Kit
 

I purchased a 7 inch Transvision kit set (the type with the plywood board front panel, not in the factory cabinet).

Anyway, it is missing the HV transformer. I'm going to have to call Arlynn at Heyboer transformer and see if he can wind a suitable replacement. I know the primary should be 117 or 118 volts, with two secondaries, one wound for 2.5 volts at 2 amps for the 2X2 rectifier. It's the HV winding that I'm not sure what to spec. The originals I believe were wound for 2500 or 2700 volts (depending on which version of the service info you trust). But at what amperage? 1000 to 2000 microamps sounds reasonable to me, but I know that if I spec it too high, and the regulation of the transformer sucks, that the rectified HV could shoot up.

I believe the 7EP4 has a max anode voltage rating of 3000 volts (design center) as opposed to the 5BP4 from which it was derived (which has a max anode voltage of only 2000 volts).

Any ideas? I suppose if I have it wound and the HV is too high, I can tack in a string of Zeners to regulate the HV at whatever voltage I want but I'd like to avoid going that route if possible.

Another question for the Transvision gurus... what woods were available for the front panels of the kits? I've seen a blonde kit at the ETF, two mahogany sets in the flesh, two walnut, and I have a photo of one with what appears to be burled American Chesnut. I'm trying to determine which were original and which were modifications from the builders.

I'm a moron. I managed to find the specs for the transformer in a VK post... from myself. I posted about another Tranvision kit I had, and sold, to Don because frankly, it was uglier than sin and very poorly constructed. After doing a search based on the specs I had posted in the other thread, I was able to find the source of my info: an advertisement from mid-1947 from Transvision. If you need a Transvision HV tranny, call up your winder and spec the following: 117 rms VAC (or whatever line voltage) to 1700 rms VAC at 4 mA, with another secondary of 2.5 rms VAC at 2 amps.

I know what I did wrong... I was assuming that the "2500 volt" rating given in the parts list was a rating of the transformer. It would have been more accurate to call it a "transformer for the production of 2500 volts". The peak DC voltage after the 2X2 rectifier does come out to 2700 volts in a simulation, and the average DC value after the rectifier and filter cap is 2500 volts.

My mistake when trying to work backward from the schematic to determine the voltage rating of the secondary was neglecting to use the peak value of the line voltage, which would be 117 volts times the square root of 2, or 165.463 volts, instead I was using 117 volts as the peak. Been a long day...

That set used a brute force HV supply. The transformers were notorious for failing. I converted 2 Transvisions to RF high voltage, using HV unit from junked 7 inch Motorolas. Many of the later Transvision sets used the 7JP4 CRT, which was an improvement over the 7EP4

With all due respect, the most interesting aspect of the Transvision is, IMHO, the lethal brute force supply, in addition to the chintzy 3 and 5 channel tuners, and the AM sound circuit. It's as close to a prewar set as you can get without spending $4000+

I can certainly understand why you would replace the HV supply with a Motorola RF coil. My set will be getting a replacement from Heyboer so I can see how the kit may have worked for the original constructor.

The set arrived from Vaughn today. It is EXTREMELY rough, but as advertised. Thanks Vaughn! The front plywood (particleboard?) panel is a total loss. I'll keep it around, but I'll just use it as a template to cut a new board...

I intend to use a burled walnut or chesnut veneer over a sheet of plywood or fiberboard for a replacement.

The set is missing the electrolytics, one HV cap, one power supply choke, and the HV tranny. Nothing that can't be replaced. Otherwise it is quite rusty up top, but surprisingly clean and complete underneath.

5 channel tuner (was hoping for the rare 3 channel job), but from what I've traced of the set as wired, it uses the early 6N7-6SN7 sweep circuits, not the late 6SN7-6SN7 circuit. AM sound circuit with the 6SQ7. Not sure what combinations of tuner, sweep circuits, and sound circuits other collectors have. Photos to follow.

I borrowed Steves blonde one from the museum and had cabinet shop close to me make one . Its perfect and he also made the wooden blocks for the crt. He has the template to make more. You pick the veneer of your choice.

Good news and bad new: The good news is that the tuner is actually a 3 channel tuner, so the set is likely late 46 or very, very early 47. Yay! :banana:

The bad news? The tuner is missing two of the three grid coils, all three antenna trimmers, and the bandpass trim capacitor. The capacitors I should be able to find replacements for, the coils will be impossible... :tears:

Photos as promised... it's a basket case, but any of these Transvision sets are worth saving in my opinion. At $200, the price was right too! They're just plain hard to find. I'll do my damnedest to find the missing parts or have suitable replacements built. This set deserves to live again.

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Wow!!!

Real piece of shit, ain't she? :stupid: Worthy challenge for my abilities... while Nick is working on the Westinghouse I'll fight with the Transvision.

I mean... is there no way to know the value of the coils? If they're not known... couldn't they be guessed?

Tunable coils with a slug. It is possible to find a reasonable replacement, but I would need some measurements from actual coils in a surviving 3 channel tuner. I doubt I'll find another owner willing to measure for me :tears: :tears: :tears:

rip

My plan for the moment with the tuner is to replace the missing caps, align the tuner for the one channel I have a grid coil for (channel 2 or 5 I believe) and simply leave the set on that channel. Then, if that works, try to repair the remnants of the one coil form that is still present, giving two channels. If I can get that to work, then I'll try to make a wild ass guess as to final, completely missing, coil and tack in a replacement of a reasonable looking inductance.

If I can get the tuner to work on one channel at least I can restore the set. If all three coils were missing I would be up a creek.

Not if it uses a standard IF frequency. If it uses a standard IF frequency you can just grab a tuner off a junker chassis and hook it into the 1st IF until you can figure out the original...Ain't pretty but it is an option.

I had thought about that. The frequency is low, 21.9 MHz I believe. I'd prefer to keep the set as original as possible, but if the original three channel tuner can't be salvaged I can try an early Inductuner (if my memory is correct they use the same IF frequencies).

I may have found a CRT for this set. I really don't like the look of the getter flash, but I'm desperate for a 7EP4 to at least display the set with. Hopefully the owner will accept my offer for the jug (I think $150 is pretty fair) and I can stop the CRT hunt and focus on other missing parts.

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Unless you demand original look coils, with only three channels its easy.

Get some forms, new or used, even with wire on them, that
mechanically fit and are for VHF. Install them and play with new windings
and a grid dip meter until they are right.
Stray inductances are so big that calculations are useless.

I wish I had had a dip meter when I was working on my hacked TRK-12 RF
area that had been hacked apparently TWICE by the RCA engineers themselves
during the pre and post-WWII channel moves. I did use the dip meter
on my Pilot prewar FM when I redirected it to (part of) the modern band.

Third person today to recommend using the dip meter. I'll probably go that route.

I'm going to assume that since the set was found near NYC that the tuner was built and aligned for channels 2, 4, and 5.

Well the CRT is mine. I'm fairly certain it is a goner, but I would still like to find someone in South-East Michigan with a working pre-war 5 inch chassis to test the tube on. Can anyone help me there?

If you have an older oscilloscope , like a Sylvania 400 you could swap crts. Another idea would be to wire up a jumper harness with with the necessary plug and socket connections to a newer oscilloscope, like a Heathkit that uses a 5UP1. You would just have to be sure the 7EP4 anode voltage is not exceeded.
The picture I included is one I made up for repairing Philco split chassis sets.
Ed

That's a nice idea actually. I may buy a cheap 5BP1 based scope just for testing 7EP4s and 5BP4s. My current scope is a modern PC based digital scope. I also have a late model Tektronix for backup. Nothing old enough to sub the 7EP4 into. In any case, a collector in the Detroit area has graciously offered to help me test the CRT in a working chassis when it arrives from Pennsylvania.

I've come up with a solution to the "use" vs "display" question I've been wrestling with. 7EP4s do not grow on trees, and if I should happen to find a good one, I won't want to use it too much. There is a pin for pin compatible tube that would work in the set with no modifications: the 5BP4. The only real differences between them are screen size, 5 vs 7 inch diameter, and max ultor voltage (which I'll ignore; running a 5BP4 at 2500 isn't going to bring about the end of days).

If the set works well enough to warrant daily driver status, I'll cut and finish two new front boards, one with an opening sized for a 5BP4, the second for the 7EP4. On special occasions I'll swap in the 7 inch board and CRT, the rest of the time I'll just use the 5 inch board with a 5BP4. I can have my cake and eat it too...