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Old 06-07-2015, 06:23 PM
ghjkl67 ghjkl67 is offline
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Philco Mystery Control Receiver

Anybody here ever work with a Philco mystery control set? Mine is a 39-116. Everything is recapped, but the mystery control receiver/relay section is not functioning normally. Upon activating remotely, the thyratron seems to get stuck in the activated position, magnetizing both relays continuously. Sometimes, the thryraton works normally, but the stations/volume change at random.

Another problem is that the chassis gets hot through 2 line bypass capacitors. I recently disconnected these caps in the meantime since the radio operates the same with or without them, but unusually, the chassis still has a potential of about 70V to the round outlet ground. However, if this hot chassis problem has nothing to do with the mystery control, I don’t want to worry about it.

I’ve been at this thing for over two months. The radio took a couple days to recap and get running, but I have no clue how to go about troubleshooting the remote section. I read the article at philcorepairbench.com but it doesn’t make much sense to me. I’d appreciate it if someone could give me an idea where to go from here.
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Old 06-07-2015, 10:08 PM
Olorin67 Olorin67 is offline
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They are no picnic to get working right, The one at the Pavek musuem took a long time to get working right, if I recall right, and they had someone really experienced in radio restoration working on it. I have one too, but I haven't dug into it yet, I only recently acquired the remote control.
I'd get the hot chassis issue sorted first, So you don't get zapped working on it.
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Old 06-08-2015, 12:35 AM
ghjkl67 ghjkl67 is offline
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As far as the hot chassis issue goes, I just disconnected the two caps going from AC line to ground, and now it's safe to work on. Those caps didn't seem to affect the radio performance at all. It still measures 70vac from chassis to outlet ground, but I can't feel it at all. My question is, why are line bypass caps causing the chassis to go hot in the first place? Is there something wrong here, or is the chassis supposed to be hot?

The other odd thing is that I never got shocked working on this set for a whole month. Either I was lucky this entire time, or there is some internal fault that arose.

I hope there is someone here who has restored one of these that can help. I don't intend to put it back into the cabinet until the mystery control is working, even if it takes another 10 years!
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Old 06-08-2015, 12:46 AM
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Phil Nelson Phil Nelson is offline
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Have you already seen this Mystery Control section of the Philco Repair Bench?

http://philcorepairbench.com/mystery/index.htm

It includes some technical info as well as history.

I have owned one of the control boxes for years, but I'm not crazy about the styling of the receivers that use them, so it's only a decoration here.

Phil Nelson
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http://antiqueradio.org/index.html
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Old 06-08-2015, 10:32 AM
ghjkl67 ghjkl67 is offline
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Yes, I did see the tech info page. Some of it makes sense to me, but some of it doesn't. It's hard for me to understand the thyratron operation, such as, what is the proper bias for the grid? Also, what should the grid waveforms look like during activation and during steady operation? It looks like there should be AC and DC there, but the grid vs. plate graph doesn't specify. Anyway, I was trying to scope it out. It looks like the negative portion of the AC cycle is cut off at the grid during activation. When the thyratron gets stuck, the negative portion comes back very gradually. Because of this, I thought that the control grid circuit was sticking, causing the thyratron to be stuck. Then, I tried pulling the thyratron. Instantly, the grid waveform returned to a normal AC cycle. So, it looks like the grid is being controlled rather than controlling.

These are just some testing methods I came up with off the top of my head. I don't know if there is any logic to them. I'll try any other testing methods suggested on this thread. I only have a digital meter, analog meter, and a scope. If any signal generators are needed, I can try to find or borrow some.

Phil, when you talk about the styling of receivers, do you mean the cabinet style? If so, I'll have to agree. The only reason I bought the set was for the mystery control. That's why I'm trying to so hard to get it working. Without it, the set looks like a dishwasher cluttering up space. However, as far as performance and reception goes, this is one of the best sets I have.
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Old 06-08-2015, 05:30 PM
RJMiranda RJMiranda is offline
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Thyratron proper bias

Hi, ghjkl67. Looks that the thyratron´s curve is puzzling you. ¨Proper bias¨ is not a fixed single value, but depends on the plate voltage. Look at the 2A4G´s curve. The conducting zone is above the curve, the cut-off zone is below it.
At -2V on the grid, the thyratron will start conducting (and will stay conducting) if the plate voltage is 25V or greater. But, if the grid is at -8V, you will need 175 or more plate volts to make the tube conductive. It most likely will not conduct.
If the grid is about -9V, the tube will not conduct at any value of plate voltage (up to the 200 V limit).
When the thyratron has fired (is conducting), on the following negative cycle of the AC the tube will be cut off, and on the next positive cycle will start conducting again if the grid voltage is not negative enough (that is, if enough RF keeps coming out of the transformer after the 6J7G and the 6ZY5G). The brief 60 Hz interruptions will not de-energize the relay coil(s).
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Old 06-08-2015, 05:45 PM
RJMiranda RJMiranda is offline
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AC condensers and (maybe) alignment method

I think the two condensers on the primary are important. Look at the schematic in the link suggested by Phil. The power to the control receiver circuit comes from the primary of the main transformer (it is not isolated). It looks that Philco wanted the chassis at "half-way" potential between both AC wires. I would use an isolating transformer to test this receiver, to be safe.
Check whether your thyratron is defective: short it´s grid coil. The thyratron should NOT ignite. If it keeps dark, it looks OK. Remove the short and try lowering the sensitivity control on the 78´s cathode.
If you have a signal generator, put .01mF 600V condensers in series with BOTH the signal and earth connections, set the generator at the max. amplitude, supply the signal at the 6J7G´s plate and sweep the freq. between 350 and 400 kHz. Maybe the signal will not be strong enough to fire the 2A4G. Supply the signal at the 6J7G´s grid (maybe at a lower level). Lower the generator output as much as you can, and the correct frequency will be the one when the thyratron still fires.
Supply the found frequency (at much lower level) to the input by using a coil coupled to the "secondary inductor" and adjust the interstage and first transformers and the trimmer so the thyratron reacts at the lowest possible level.

Last edited by RJMiranda; 06-08-2015 at 06:00 PM. Reason: Correcting redaction mistakes
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Old 06-08-2015, 05:54 PM
RJMiranda RJMiranda is offline
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Or is it oscillating?

Maybe the control receiver is oscillating and so keeps the thyratron happily fired. Could be that any of the .05mF condensers on the plate, screen and grid circuits is (are) open? Even a high-resistance ground on the primary or secondary of the transformer between the "secondary inductor" and the 78´s grid could be of importance. I am not very good figuring out ground return paths, but still think the condensers between the AC line and chassis are there for some reason. (Are they good also?)
Sorry for writing so much. I had never heard about the Mystery Control to this day, and had a very good time reading the info Phil suggested, including the schematic.
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Old 06-08-2015, 06:36 PM
ghjkl67 ghjkl67 is offline
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Don't apologize for writing so much. Keep the thoughts coming. You're a great help. I had no idea what the thyratron curve meant until now. I'll reconnect the bypass caps for testing.

I attached an image of the grid waveform in both activated and deactivated position (The scope is for a vertical scale of 2v per division). The deactivated waveform is just a basic sine wave coming from the step down transformer. Plate voltage is 125vac. The activated waveform cuts off the bottom of the sine wave. Activated, the plate is about 75vac.

What I'm having trouble understanding is how to measure the grid voltage? It is a combination of AC and DC through the grid coil. Can I set the meter for DC and measure the grid voltage accurately? Maybe the scope image is enough to determine this info, but I don't know enough to understand it. The cut off mark looks like it is at -1v. Possibly, the grid voltage is -1 vdc (multiplied by 1.41 to account for RMS)? Just my thoughts.

As for the grid coil, I did try disconnecting it from the thyratron a while back. Still the thyratron remained activated. Just now, I tried activating the tube and then shorting the coil. The tube remained activated. The tube, however, will not activate if the coil is already shorted.
Attached Images
File Type: jpg 39-116 010.jpg (140.2 KB, 9 views)
File Type: jpg 39-116 011.jpg (29.0 KB, 10 views)

Last edited by ghjkl67; 06-08-2015 at 08:01 PM.
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Old 06-08-2015, 08:20 PM
ghjkl67 ghjkl67 is offline
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Update. I just put the bypass caps back in. You're right, the chassis stays at 60v above ground (or half way between AC line). If the set is plugged in the correct way, it is safe in both on and off positions.

It didn't affect the performance that much though. The only difference is that sometimes the thyratron engages automatically upon warmup, rather than remotely (I repeat, sometimes). Since disconnecting or shorting the grid coil doesn't turn the tube off, I think we can eliminate the preliminary receiving/amplification stages as the source of the problem. Sometimes the tube will engage with the coil disconnected simply by poking at the socket with a meter probe.
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Old 06-09-2015, 03:09 PM
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Phil Nelson Phil Nelson is offline
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Quote:
Originally Posted by ghjkl67 View Post
Phil, when you talk about the styling of receivers, do you mean the cabinet style?
Yes, the cabinet style. We only have room for a couple of consoles, so appearance is a big factor, along with performance.

Phil Nelson
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Old 06-09-2015, 03:25 PM
RJMiranda RJMiranda is offline
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The best way to assure the amplifier is not working/oscillating is to remove the 6J7G. That would isolate the thyratron, that should not fire without signal from the amplifier.
I wonder if the 2A3G is bad... suppose you don´t have another to test it.
Don´t worry to measure the grid voltage. Remember that the thyratron starts conducting if the instantaneous relationship between the grid and anode voltages falls on the "conducting zone". And the grid instanteneous voltage is the sum of the RF coming from the amplifier, superimposed on the AC coming thru the 4000 ohm resistor, the filament xfrmr secondary and the AC line. The plate voltage comes thru the relay coils from the opposite end of the AC line. (See the schematic on http://philcorepairbench.com/mystery/operation.htm) The 2A3G does not uses a filtered B+ supply, nor its grid voltage is from a steady DC level.
Is there a possibility that sometime in the past the two cables between the filament transformer and the power line were swapped, or maybe the filament transformer replaced? This would invert the phase relationship between plate and grid voltages, and so the thyratron circuit would not work even with a good tube.
I can´t say that the connections were reversed in the past without looking at the chassis, but maybe you can make the following test.
Lets say that the upper power line (the one with the On-Off Sw) is "A", and the lower one is "B". Could you unsolder JUST the two wires to the primary of the filament transformer and reverse them between "A" and "B"? (but keep undisturbed the conductor from the "A" line to the tap on the xfrmr secondary, and the one between the "B" line to one end of the 150 ohm R and the relay coil) If after that the thyratron does not fire, you can leave the cables reversed and put back the 6J7G to check if the amplifier works.
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Old 06-09-2015, 04:28 PM
ghjkl67 ghjkl67 is offline
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Just to clarify, I think you mean 2A4G, not 2A3G.

I looked at the wires going to the filament transformer. It appears to be a factory soldering job. I'm going to leave it there until the new 2A4G thyratron comes in that I ordered. The one that's in there now is already NOS, but it still could be defective.

I already tried disconnecting the thyratron grid. Wouldn't that isolate it? I don't see the need to remove the 6J7G for testing, but I'll do it anyway. Again, when I disconnected the thyratron grid from circuit and left it hanging, it remained activated. Sometimes, poking at the tube socket terminals would cause the tube to activate after cold start. I assume it should remain off the entire time the grid is disconnected. Is that correct?

Last edited by ghjkl67; 06-09-2015 at 04:45 PM.
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Old 06-09-2015, 04:55 PM
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If the grid is floating it could easily gain enough charge to turn it's self on.
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Old 06-09-2015, 06:35 PM
ghjkl67 ghjkl67 is offline
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Putting a meter lead up to the floating grid turns the tube on instantly, so that makes sense. The question is, what can I do to make it turn off??? (besides switching off the primary)

I tried unplugging the 6J7G. Same response, except that it cannot be turned on remotely. Activating the thyratron remotely and then unplugging the 6J7G, the thyratron still remains on. I think when activated, both the stepping and holding relays are magnetized continuously.

A few hours later, I tried fiddling around with it some more. For no reason at all, the stepper is now trying to work with the mystery control. Sometimes the thyratron gets stuck, but sometimes in turns off like it should. Who knows how long this will last?
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