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.

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.

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!

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
Phil's Old Radios
http://antiqueradio.org/index.html

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.

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).

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.

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.

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.

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.

Yes, the cabinet style. We only have room for a couple of consoles, so appearance is a big factor, along with performance.

Phil Nelson

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.

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?

If the grid is floating it could easily gain enough charge to turn it's self on.

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?

Have you considered that modern power line interferene is simply so
strong it fires the thyratron?

To see what it takes to keep it off, simply put a battery in series with the 50k
resistor in the grid lead. Try 2 to 15 volts, negative to grid.

Use a scope to look for noise pickup in the amplifier. Use an
isolation transformer of course.

That sounds like a good idea. Instead of a battery, I can use a regulated DC power supply and increase it gradually. That will tell me where the exact cut off point is.

Would you mind clarifying this part?:

"Use a scope to look for noise pickup in the amplifier. Use an
isolation transformer of course."

Where exactly should the scope leads be placed? Is an isolation transformer necessary for the scope primary or radio primary? I'm guessing it doesn't matter.

If you use a power supply instead of a battery, then you should plug either the supply OR the radio into an isolation transformer (but NOT both at once). Some power supplies ground a terminal, and if you connect that to the radio's hot chassis either or both devices will likely incur damage, and at best your test results may be corrupted.

(2a3)+1
 

Yes, 2A4G. My mistake.
While waiting for the new (NOS) tube, lets keep thinking. The thyratron grid needs a substantial negative voltage to keep the tube from conducting. Returning it to the cathode would not cut off the tube, because you can see in the curve that at 0 grid volts, the tube conducts with about 20 plate volts. That´s why the lower end of the grid coil goes thru the 51K/0,5MFD and the 4K res to one end of the sec of the filament xfmer.
The reactance of the 0,5MFD at 60 Hz is about 5K, so the total impedance between the grid and the filament transformer is less than 9Kohms.
The schematic does not show where the filament voltage of the amplifier tubes comes from, so I will assume that from the main receiver, that also supplies +B and -C.
So, you have part of the filament transformer´secondary supplying 2,5V to the 2A4G filament, and the other part of the secondary, that is longer in the drawing (but I can´t know its voltage), goes to the grid circuit. This is the voltage that keeps the 2A4G from firing, but it has to be in correct phase relationship with the plate voltage (that comes from the AC line at the points I arbitrarily called "A" and "B" in a earlier post).
For the circuit to work well, at the same instant the plate voltage is rising, the grid must be receiving a rising negative voltage from the transformer. So the tube won´t fire. Only the RF peaks from the amplifier, superimposed on the negative grid voltage, will make it less negative so the tube may fire.
That is why I was wondering about an hypothetical inversion of the conductors that would supply the wrong (positive) half-cycle to the grid while the plate was also being made positive.

I'm getting about 2.75v from the filament transformer for the tube and an additional 5v from the end of the winding going to the grid (7.75v all together). So, it's probably a 7.5v transformer with a tap coming out about 1/3 of the way.

With respect to the center tap, the voltage at the grid of the thyratron is 3.75vac, that's after going through the 9K impedance.

Also, how can you have negative voltage from a transformer??? It's AC.

Update: I just tried hooking up a 9v battery to the grid (in series with the 51K ohm resistor). It worked!! The battery measured 8v, and it was just enough to turn the tube off and work with the mystery control. Sometimes, it would activate momentarily, but increasing the battery voltage would probably correct that. The control does seem to be missing a pulse here and there, but it's actually working! Now for the hard part, I have to fix it correctly to work without the battery.

Have you checked the value of the resistors in the grid circuit to make sure they are within tolerance? I know you said that you recapped the radio but did you also check the resistors?

Negative AC
 

Yes, I know it is AC. When I said the voltage from the transformer to the grid was negative, I meant that during the half-wave period that the plate is made positive, the grid gets the negative-going half-cycle from the transformer, so the tube does not fire without RF input from the control box. That´s why I wrote about a possible connection reversal somewhere that sent the positive-going half-cycle to the grid.
I know it is not a likely condition, but I have found several puzzling sets when every component seems to be OK but nothing works. 30 years back got a EV-300 Sony R2R VTR that didn´t play video. I swapped everything with a good VTR: heads, PB preamplifier PCB, PB booster PCB, demod PCB; and everything worked on the other machine, and the boards from the good one wouldn´t work in the defective VTR. Crazy! (Only thing I didn´t swap were the rotary transformers, because it is very difficult to do). Eventually I found that somebody had reversed the wires that connected the PB preamplifier to the playback booster, (in this VTR there were no connectors, everything was soldered), so the ch-A head signal came thru the ch-B circuit while it was being muted, and viceversa.
I think dtvmcdonald has a very good point, and it is really very unlikely that any cables are reversed. Just say, don´t overlook that possibility.

Okay, now I'm starting to understand why the grid and plate need to be in phase. I just have to think of that 2A4G curve scale as being DC, then take the instantaneous AC voltage. Sorry for being a little slow here.

sean,
Yes, I did check resistors. The ones in the grid circuit look good. I changed one out in the control receiver circuit some time ago. It was the 99k ohm resistor going to the grid of the 6J7G. The B voltages there are still a little low, but they are slightly low in the whole set.

As long as I ordered the new tube, I'd rather wait until that comes before changing the transformer wires. They are very brittle and somewhat difficult to solder. If the NOS tube doesn't fix it, I'll try swapping the transformer wires. Meanwhile, I'd like to try scoping out both the grid and the plate to see the phase relationship. Would an isolation transformer be necessary for the scope?

Scope isolation
 

By all means use an isolating xfrmr. Connect the radio set to it (the radio is the one that may have voltage at the chassis, the scope is safe). The thyratron circuit is not fed from an isolated supply, for its plate supply comes directly from the AC line, and the tap on the secondary of the filament transformer is connected to the line also.
I would connect the scope ground to the control receiver´s chassis. As you will be measuring AC and not DC, this point is equivalent to the hot "off-center" tap of the filament transformer. It is safer to ground the scope at the chassis and not the center tap.
With one channel at the 2A4G´s plate and the other at the grid, the positive-going half-cycle at the plate must coincide in time with the negative-going half-cycle at the grid.
Some time-displacement will exist between waveforms, because of the reactances of the grid capacitors and coil, and at the plate the relay coils, RC series network in parallel with the relay, and RF choke. But I think that, generally speaking, the voltage at the plate will be positive-going while the grid is negative-going.
If this is not so, either a) reverse the primary leads of the filament transformer or b) swap the conductor from the second relay coil to one side of the AC line with the conductor from the tap on the secondary to the other side of the AC line. That way the phase will be shifted 180º.
The battery stopped the thyratron firing because it put an additional negative voltage on the grid, even if the AC phase was wrong. But if the AC phase is OK, then it may be that the thyratron performance has degraded, and so it needs extra negative bias.
The receiver-amplifier works OK, it seems, because with the battery inserted you could control the relay from the remote box.

I just hooked up the scope, and the grid plate phase relationship was 180 degrees just like you said. That would narrow the problem down to the 2A4G, except for the other possibility that dtvmcdonald mentions: modern power line interference. He mentions it along with the battery idea, but I don't see how they are related exactly. Maybe the theory is that the battery is supposed to overpower the line interference???

modern switching power supply devices and CFL bulbs can put out a lot of noise onto the power wiring of your house. The Philco remote might be picking up too much noise from that, especially if caps are bad or missing, or any shields are missing or removed from servicing. Maybe there is some way to "listen in" to the signal being picked up by the remote receiver, and identify sources of interference., or evaluate tweaks meant to reduce it.

Identifying line interference
 

interference can come by 2 ways: thru the AC line and by radio waves.
If you can´t find a suitable AC line filter, you may use a uninterrupted power supply made for computer use that has a set of AC outlets labeled "surge and noise protection" that means a line filter is inserted between the power line and these outlets.
If the monster, I mean, radio can be plugged there, you would be filtering line noise. Make sure the wiring of the building is according to the modern codes (that is, 3-pronged outlets, and the round pin of the AC plug of the UPS is being really connected to earth). APC brand UPS have a LED near where the AC cable enters the UPS. If this LED is lit, the ground is faulty and the UPS´ line filter will not protect you from interference.
For airborne RF interference, now that you have connected the scope to the radio, look at the waveforms on the receiver while you operate the remote control. (Use the AC coupling). On the 78´s plate must appear RF bursts while the remote dial is being turned. The same on the 6J7G´s plate but at a higher amplitude. Look for suspicious noise on these points with the remote control inactive. If any appears, try turning off fluorescent lights, computers and the like to isolate the source of noise.

Success! The new tube came and it's working. I probably would have never figured it out, since the other tube in there was NOS as well. The pulse skipping problem turned out to be the mystery control box itself. There are some strange mechanics in there that need very fine tweaking to get working correctly. I haven't tackled this yet, and it's going to take some time to make right. If there are any other problems with the electronics end of it (and it's very likely), I'll be back here.

Thanks