Philco Mystery Control Receiver - Videokarma.org TV - Video

ghjkl67 · #1 06-07-2015, 06:23 PM

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.

Olorin67 · #2 06-07-2015, 10:08 PM

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.

ghjkl67 · #3 06-08-2015, 12:35 AM

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!

Phil Nelson · #4 06-08-2015, 12:46 AM

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

ghjkl67 · #5 06-08-2015, 10:32 AM

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.

RJMiranda · #6 06-08-2015, 05:30 PM

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).

RJMiranda · #7 06-08-2015, 05:45 PM

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.

Phil Nelson · #8 06-09-2015, 03:09 PM

Quote:

Originally Posted by ghjkl67

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

Electronic M · #9 06-09-2015, 04:55 PM

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

ghjkl67 · #10 06-09-2015, 06:35 PM

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?

RJMiranda · #11 06-10-2015, 04:45 PM

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.

ghjkl67 · #12 06-10-2015, 05:45 PM

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.

dtvmcdonald · #13 06-09-2015, 08:22 PM

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.

ghjkl67 · #14 06-10-2015, 10:22 AM

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.

Electronic M · #15 06-10-2015, 03:40 PM

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.