Excessive rf driver current Heathkit Apache

[WB4AQL]

Shut down the transmitter after panel meter reading upscale (at times full scale) in the driver position. When using the frequency spotting button, grid current will cause the meter to flutter accompanied by sizzling sound under the chassis and a flickering of the neon bulb in the keying circuit.

Just pulled it out of the cabinet tonight and will begin checking resistances and pin voltages later in the week. I was just curious if anyone else has had a similar problem.
At this point I`m suspecting some sort of bias voltage problem. The 5763 driver tube is a new one.

Thanks to all

[Pete, WA2CWA]

You said: "accompanied by sizzling sound under the chassis and a flickering of the neon bulb in the keying circuit"

I would also check for a cold solder joint and/or a cracked resistor. Check the 6CL6 too. That compartment is always fun to work in.

[K6IC]

Hi Michael,

I have no experience restoring or using the TX-1.  Believe that it is fairly similar to the Dixie 100s in the RF department.

I  wonder about the Drive Pot.  On the DX-100,  the Drive Pot controls the screen voltage of the driver - the 5763.  This pot  is a very common problem in the 100s,  as it is a 5-ish watt wirewound job,  altho,  it this pot is bad,  there would probably be some emitted magis smoke and magic stench. 

If the bias resistor string is similar to the 100,  it  is very sensitive to resistor value changes.  Sorry I have nothing specific to say about this prob in the TX-1.
Good Luck,    Vic

[w1vtp]

I'm putting my money on the drive pot opening up on the ground side.  Maybe even a bad solder joint that connects the pot to ground.  That cause the pot not to be a divider.  All you would have is a dropping resistor function and that probably would allow excessive screen voltage which in turn would drive the pants off the final

Al

[WB4AQL]

Guys, the Apache is being plagued by gremlins. I traced the occasional erratic grid drive and excessive driver current to the 1.1 mh choke at pin 8 of the 5763 driver. This was unusual in that the windings of the coil were contacting just slightly an adjacent ground lug. At first I thought I had a cold solder joint. All resistance and pin voltage checks were good. Meter shunt resistor is good.

I moved the choke windings away from the ground lug and everything worked perfectly. Put the tx thru it`s paces into the dummy load and was happy.

Put the tx back in the cabinet and NO grid current reading at all. What the hey??
I can hear the oscillator in the receiver so it`s working. I get driver current on the panel meter, but not a trace of grid current. I`ll go back through it tomorrow, maybe.

Anybody ever troubleshot no grid current?

[Pete, WA2CWA]

You need to determine: no grid current or just no grid current reading on the meter.

[WB4AQL]

Pete,
The meter tests OK. I swapped the PA tubes thinking the grids had been damaged tipping the tx on it`s side, but no difference. I can hear the VFO and I get a driver current reading. It is not affected by the driver control, which makes sense as I apparently have no grid current.

Something must have opened up somewhere, just bizarre the way it quit after going back into the cabinet. This Apache is very well built with good workmanship. Got to be some logic in here somewhere...

73,

Michael

[WQ9E]

Michael,

cycle your mode switch to make sure that it is making good contact (and to make sure it isn't accidentally in the SSB position).

Otherwise, maybe C-75 was damaged (now shorting) during repair or C-36 has opened.

[WB4AQL]

In rechecking voltages, I have 0 volts at pin 6 of the 5763 driver. Should be 140 volts. I`ve got 370 volts of the B+ line. On the other side of R19 I have 0 volts. If I`m reading the schematic correctly, this resistor and the drive pot determine the voltage at pin 6. Continuity is good in the wires going to the drive pot and the pot itself seems to be functioning OK. Continuity across R19 doesn`t test open, but it is still in the circuit.

Shouldn`t I have voltage directly on the other side of R19? Less than 370, but closer to 140? I guess I`ll have to clip it out to be sure, but maybe it just picked a strange time to open up. It`s in a tight spot to access. Hi!

Thanks to all,

Michael

[WQ9E]

Check resistance to ground on the pot side of R-19, it should be close to 25K  If so R-19 is probably open.  If it shows low resistance then there is a problem and R-19 is probably cooked.  If R-19 is open, pull it out of the circuit and remeasure resistance to ground from the non-grounded end of the pot.  If it is significantly less than 25K (more than the tolerance of the pot) then correct the problem before another R-19 turns to toast.

[WB4AQL]

Rodger,

The resistance to ground on the pot side of R-19 25 ohms; way less than 25K. Could this mean the pot is bad and led to failure of R-19? R-19 looks kinda sweaty. Guess I`ll go ahead and clip one end and see if it is open. If that is the case, as you say, something had to overload it and I guess that could well be the pot not doing it`s job.

73,

Michael

[WQ9E]

Michael, try rotating the pot control and see if the resistance to ground changes (measured at the pot end of R-19).  Any chance you reversed a couple of wires to the pot during the repair process?

Otherwise it sounds like the pot has an internal short.

[WB4AQL]

Rodger,
No change. None of the wiring was disturbed. It seems the R-19 side (orange wire) is essentially shorted to ground. Have you any leads on a replacement source for the pot?

I sure appreciate your help with this problem. This Apache has been a fine working transmitter.

73,

Michael

[WQ9E]

Michael,

That is a difficult part to find these days.  Take a look on the tech section of the AM window for a replacement scheme for this pot in the DX-100 using a small pot along with a TV horizontal output transistor; you can use this scheme for the TX-1 also. 

I guess with the move to LCD television those replacement transistors will soon be as difficult to find as the old high wattage wirewound pots.

You could take a look at the pot to see if the short can be cleared.  Definitely replace the 68K resistor after what it has been through.

[WB4AQL]

OK Rodger,

Thanks again for your help and everyone else. Great community these AM`ers!

73,

Michael

[WB4AQL]

Rodger,

One last thing. Do you know offhand the wattage rating for that 25K pot?

Michael

[WQ9E]

Michael,

It is rated at 4 watts and is the same part used in a lot of the Johnson (Viking 1 and 2, Ranger, Valiant, etc.) rigs.  There are some modern parts supposedly rated at 4 watts but the one I looked at in detail were carbon units that had to be significantly de-rated for the amount of heat normally present in a vintage rig.  You really need to use a wire wound unit like the original if you don't go with the modification.

R1 and R19 form the bleeder for the LV supply and there is quite a bit of heat developed in them. 

[KC2ZFA]

I've seen new 20K Clarostats http://cgi.ebay.com/CLAROSTAT-20K-OHM-4-WATT-WW-POT-1-2-SHAFT-/400074625881?pt=LH_DefaultDomain_0&hash=item5d264e5359#ht_500wt_922 (I have no connection with this
seller). Could be adapted.

Peter

[WA2ROC]

Digikey has a 5 watt wire wound 25K pot for about $4.00 plus shipping

CT2160-ND

But I have seen the transistor/pot modification and I would chose that one over the ww pot

[WQ9E]

Dick,

Thank you for posting the part!  That is a big improvement over the carbon pot they were selling as a high wattage unit a few years ago.

Digikey has a 5 watt wire wound 25K pot for about $4.00 plus shipping

CT2160-ND

But I have seen the transistor/pot modification and I would chose that one over the ww pot