The 145X is fully recovered from bandswitch fireworks. Radio works very nice, except I have noticed that the RF Gain control only works on the top 5% of its rotation. As soon as I turn it anti-clockwise 5%, the signal disappears. No problem when it is turned fully clockwise.

It appears to me to be miswired.

First photo is of the RF Gain control.

Second image is factory schematic.

Third image is how it is wired.

I would like to hear some knowledge input and/or opinions re: wiring, is it correct? And should I go ahead and rewire it from the schematic? Is the symptom consistent with the images?

I notice that I have my drawing backwards, I'm left handed.

Thanks for helping,

Bill

First thing I would do is verify that the pot doesn't have a bad carbon track.

Put an ohmmeter across it, and see if the resistance varies from nearly
0 ohms to about 10K as you turn the control.

Pete

Thanks, Pete, I always value your help.

The pot was checked early on, it was OK, tested both 6BA6s, V4 and V5, both strong tubes, tried swapping them around, cleaned and DeOxit the tube pins, sprayed R/S cleaner in the pot.

I think if I cut the wire jumper on the pot and jumper the center to the other side, it will work.

Bill

Edit: If I do that and it works, not sure if the control will work in the right direction. The section where it does work is the low reading on the ohmmeter.

That worked, and the control does work backwards. Switching the connections on the two outer lugs will fix that.

Thanks for looking,

Bill

Hi Bill

The pot will be at minimum resistance for the most gain. Normally
full CW is full gain by convention.

Pete

Hi, Pete,

Thank you for staying on and helping.

My last idea did NOT work. I was surprised.

The first change worked OK. The RF Gain now worked normally, but in the wrong direction. I had to turn it CCW to get more signal.

When I reversed the outer lugs trying to get increase in CW (normal) rotation, the RF disappeared after 1/20th of a turn to CCW. I reversed them again, and it is working again, I can turn the RF Gain quite a ways down and still hear the signal. I have little experience with pots other than they work or are broken, but I figure this one has some sort of taper, maybe in the wrong direction. Is that possible?

I am not sure if a previous owner has been in there. Mostly it looks original, but there are little hints of rework. If so, someone did a pretty neat job.

Thanks, Bill

I was going to suggest the taper is the opposite of what it should be. I think you
nailed it. I'd guess it should be a linear or reverse log taper pot. Someone may have
used an audio taper as a quick fix.

Nothing wrong with using the control backwards until you find a good replacement.

Pete

http://tubes_tubes_tubes.tripod.com/id155.html

Gary has a control pulled from a set he parted out. Four bucks.

Pete

Thanks, Pete,

If I can remember that it is backwards. I will leave it on full. I don't use RF Gain much as I also don't listen to CW or SSB much, and I have other radios, and not a ham, just SWL. Now I guess I have to find out what is the proper pot.

Thanks,

Bill

Edit: Come to think of it, I have replaced two RF Gain controls in National Radios with new 10k wire wound pots, same as originals.

Well, that didn't work. I bought two new 10k wirewound pots and installed one of them. The RF gain control works in the right direction, but the original problem is still there. It only works on the top 5% of the rotation. When fully CW the signal is strong enough. After some checking, it appears to be wired according to the schematic.

Any input appreciated,

Bill

Edit: The wirewounds work in my two National radios. I may have to get the original pot from Gary. It might have a taper.

I am looking for a calibrator for the HQ-145X, someone removed it before selling it.

R40, the 100K resistor, has it changed value? If it burnt and drifted lower that would
cause the stages to cut of more quickly when the pot resistance is increased.

Is the S-Meter working normally, and does the set seem to have normal gain?
This also could be an AGC bus problem.

Can you give us some idea of the voltage on the pot arm at the point where the
receiver goes dead?

Pete

Hi, Peter, Thanks for coming in and helping.

R40 (100k ohms) is 108K out of circuit. R16 (180 ohm) is 186 ohms out of circuit.

The S meter appears to work quite well, and the radio has lots of gain, and WWV and CHU come in dead on the dial.

Max CW on the original pot reads 0.648 vdc on the wiper to chassis. At the point where the signal disappears, the wiper is right near 11.0 vdc, AVC on. With AVC off, 8.94 vdc.

I dismantled the original pot and cleaned it. It is a carbon pot reading 8.8k ohms. It seems to work OK. The part number for R18, the gain pot, in my manual is K-26219-5. The actual pot is stamped 26218-5. I have no idea if this is of any consequence.

On the broadcast band at 1380, the RF gain control has now developed a squeal just before the signal disappears.

Hope this is some help, and thanks,

Bill

Based on the photo you include, the pot is not wired according to the schematic. The end lug with the green wire and the 100K should be strapped to the wiper and the other end lug goes to chassis without being tied to the wiper. Not sure that matters in this application but thought I would mention it.

Thanks, Geoff, for help.

I had wired it according to the schematic with no change in the problem. After your post, I rewired it according to the photo just to see what happened. The gain knob works in the right direction, but only the top 5% has any effect. After that the signal goes away. If I am very careful, the top 5% diminishes gradually, so it is useable.

Thanks,

Bill

I thought about this overnight, then found somebody had the same idea - is the taper correct?

Thanks majoco. Taper correct? Who knows? I posted that the existing pot is one digit off from the factory part number. If this changes the taper, that could well be the difficulty. Apparently this pot is tapered, since if the outer two terminals are switched, it goes deeper into the rotation before the signal disappears. If I buy an original pot, maybe I will find out.

I have replaced two 10K wirewound RF gain pots in National boatanchors which were buggy and those began acting normally. The National radios use a somewhat different RF gain.

In the meantime, thanks for all your help. It is greatly appreciated and a learning experience. The RF gain is effectively functional, and will eventually be solved.

Bill

Bill, I have to throw this in: When you reversed the connections
on the outer two terminals, did you also move the jumper to the
opposite terminal?
-just a thought, and I may have missed that part.
Nelson

Thanks, Nelson, for your input. I did move the jumper to the other terminal. I swapped the outer terminals from side to side to see the change in rotation of the control. I also wanted to try to find out something about the taper.

Luckily I can remove the pot easily, as Hammarlund put skirted knobs on this one and put a hole in the panel large enough to get a 1/2" nut driver in and remove the nut holding the pot. Until I discovered this, I thought I would have to remove the front panel, or make a skinny wrench, as some members have suggested.

Bill

From what you are experiencing it seems the pot resistance is much higher than what the
schematic calls for. I'd imagine you tried another tube by now as well. You can fake this
by adding a resistor across the pot to give it full range, but that is a real cheat and only
masks the problem. Screen and plate resistors are good? Voltages are in spec? I'm
grasping at straws.

Pete

Hi, Peter, thanks for helping.

Some previous posts I made describe the pots. The original carbon pot reads 8.8k ohms, and I had tried replacing this one with a new 10k wirewound pot, which reads 9.99k. I put the original back in after dismantling and cleaning it and verifying correct operation. I did note that the part number on the original one is one digit off from the manual.

You observation that this might be an AVC issue is possible. Just before the signal disappears on the control, there is a squeal that wasn't there before. The broadcast band below 900 has also disappeared. This radio was working pretty well for about a year after I fixed the bandswitch. The dial is still very accurate.

Thanks again,

Bill