Hello all,
I am restoring a Hammerlund HQ129X that I was gifted. The unit itself is in very good cosmetic condition and has all period parts installed. However, the person who originally owned this unit made a couple “mods� to it. I have removed them and am attempting to bring it back to factory spec. I am about half done with the recapping process, and have also determined that 95% of the “roundies� are beyond tolerance so they will also get replaced. As I was trying to verify the location of higher wattage resistors I ran into an issue. Pin 2 (plate of first triode) of the 6SN7 is supposed to have 2 connections according to the schematic. One connection to a 0.02 cap which exists, and a (assuming a wire) that goes into the Man/AVC/BFO wafer switch in the can only top of the chassis. However on this unit this wire does not exist, and in its place there was a 100k resistor that was tied to the same destination as the 0.02 cap (which is pin 5/Grid of the 6V6) . See pictures attached . The mods that were present were a 100kHz crystal oscillator as a separate unit, and some other device that appears to have been homemade. There is also a toggle switch and a pot on the face that have been added. The pot does not appear to have any function.. one side of it ties to a 0.01 cap that connects also to pin 5 of the 6v6.. the wiper and other side of the pot are floating.
Is it possible any of you could provide me with a good resolution picture of this section on the 129x and a possible explanation as to the thinking behind this 100k resistor??

My guess is the front panel knob is a tone control. With only one connection I'm assuming the center wiper is grounded when the pot was mounted.

Your description of the resistor in parallel with the capacitor and the missing connection to the AVC switch makes no sense at all. See below.If things are connected like this it would leave the B+ disconnected from the 6SN7 and no way to get audio from the 6SN7 to the 6V6.

The only place a parallel resistor and capacitor should be seen connected to pin 5 of the 6V6 is C61 and R42 which go to ground.

It looks like the 100K was supposed to be replacing R45 (24K) but it was hooked to the wrong place. Why it is 100K is also a mystery. I can't see a 24K R45 anyplace in the picture so it kind of makes sense that they may have removed it and then lost track of where the replacement was supposed to be installed. Why they would use a 100K is anyone's guess (the value is not critical and 100K probably would work fine).

For what it's worth, I found several other pictures of this area online and located R45 (24K) installed "just about" where your mystery resistor is installed with one slight exception. Instead of being soldered to pin 5 of the 6V6 it is connected to Pin 4 of the 6V6. This makes sense because your pin 4 solder joint is not in original condition. Note, this disagrees with the schematic but would work and it may be that the schematic does not show a production change.

Other HQ-129X. And this one.CCurtis Eickerman

Thank you for the replies, and especially the pictures of this section. There was indeed no 24k resistor anywhere in the unit. From the attached schematic you modified, you have an X on the line from the Manual/BFO/AVC switch to the plate of the 6SN7. From what I can see there is no line coming from this this switch to either the 6SN7 or the 6V6. Forgive my ignorance in this regards, but doesnt this indicate the switch would not function? All the wires from the can are going other places, and none go to the 6SN7 or the 6V6. Adding the 24k resistor to the right place it would still not have connectivity to the switch, is this accurate or am I still missing something? Could this also be an update that does not show up on the schematic?

Thanks again
Brian

I suspect it was discovered that it was physically easier to get B+ through the 24K resistor from pin 4 of the 6V6 than it was to get it from the terminal strip that feeds the S5 switch. So a manufacturing production change was made that does not appear on the schematic. I am including a modified schematic below with notes about what is going on.There are two possible sources of B+ entering the schematic from the lower left. One is from the input of an inductor (L22) and the other is from the output of that inductor. This just makes one a little more hum filtered than the other and largely makes no observable performance difference.

The voltage to pin 5 of the 6SN7 is present at all times either way and is totally unaffected by the switch position. The 6V6 obtains to power via the switch either and is powered all the time from the other source of B+. The only B+ switching that takes place is the power to the S-meter resistors (470K) in the AVC position and the BFO when the CW position is selected.

Curtis Eickerman

Thank you so much for the information, and explanation of the other possible sources of B+. Re-capping (including the elec. can) is now complete, and I have a whole mess of 1% tol resistors on order to fix up the rest this beast. Really looking forward to it glowing and receiving on my bench!

Cheers
Brian

The HQ-129X is a good receiver. One was my daily driver as a ham radio operator for about 5 years. For the last 2 or 3 years mine was modified to have AVC action while on Sideband (80 through 10 meters) while I was using it in conjunction with a Heathkit Apache TX-1 with SB-10 Sideband adapter.

If you decide to try an alignment, take note... DO NOT attempt to align it to exactly 455 KHz just because it is a 455 KHz IF. These radios MUST be aligned to match the crystal filter at EXACTLY the crystal filter frequency no matter what that turns out to be (could be +/- a couple of KHz).

Alignments are best done with a sweep generator and scope, or with a sweep generator and spectrum analyzer. Alignments CAN be done with a regular signal generator and VTVM, but you need to know how to do that with a crystal filter radio so as not to end up with disappointing performance. It's not difficult, you just have to understand how it is done.

Curtis Eickerman

http://vtenn.com/Blog/?p=1200
Probably the best write-up I have seen.

John

Agreed and a very good reference to have.

Curtis Eickerman