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  #16  
Old 06-09-2015, 08:22 PM
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dtvmcdonald dtvmcdonald is offline
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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.
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  #17  
Old 06-10-2015, 10:22 AM
ghjkl67 ghjkl67 is offline
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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.

Last edited by ghjkl67; 06-10-2015 at 10:28 AM.
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  #18  
Old 06-10-2015, 03:40 PM
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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.
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  #19  
Old 06-10-2015, 04:45 PM
RJMiranda RJMiranda is offline
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(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.
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  #20  
Old 06-10-2015, 05:45 PM
ghjkl67 ghjkl67 is offline
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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.

Last edited by ghjkl67; 06-10-2015 at 07:00 PM.
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  #21  
Old 06-11-2015, 07:42 AM
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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?
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  #22  
Old 06-11-2015, 08:10 AM
RJMiranda RJMiranda is offline
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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.
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  #23  
Old 06-11-2015, 01:30 PM
ghjkl67 ghjkl67 is offline
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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?

Last edited by ghjkl67; 06-11-2015 at 01:35 PM.
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  #24  
Old 06-11-2015, 04:44 PM
RJMiranda RJMiranda is offline
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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.
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  #25  
Old 06-11-2015, 10:07 PM
ghjkl67 ghjkl67 is offline
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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???
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  #26  
Old 06-12-2015, 12:10 PM
Olorin67 Olorin67 is offline
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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.
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  #27  
Old 06-12-2015, 03:29 PM
RJMiranda RJMiranda is offline
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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.

Last edited by RJMiranda; 06-12-2015 at 03:32 PM. Reason: Typing mistakes
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  #28  
Old 06-15-2015, 06:45 PM
ghjkl67 ghjkl67 is offline
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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
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