Heathkit Mohawk RX-1 Drift

[VE3DDY]

Over the past few months I have torn my hair out trying to stabilize the drift on a Mohawk receiver.
Having done the mods as described in Electric radio, I still had more than my fair share of drift.
I also replaced the rectifier and VR tube with solid state equivalent designs and regulated the filament voltage to the 3 front end tubes.
 I have found a lot of drift problems with the main tuning capacitor as well.
I found that the small wipers that provide ground continuity on the rotor of the tuning capacitor were poor conductivity at best having oxidized over the years. Also, the Whole Rf deck was sub contracted out by Heath and there seems to bhe a great lack of quality control in that section.
 Check on your tuning capacitor for small tags on the plates between each section of the capacitor and see if there are ground straps that are connected to chassis. If they are missing , there is a great deal of the drift problem.
I also found that the bearins at each end of the tuning capacitor need bypassing with a small length of tiny coax braid.  At the front end of the capacitor, the braid can be soldered to the shaft and taken in a one turn loop around the shaft and then soldered or bolted to the main capacitor frame.
Remove the rotor wipers from the frame, clean them and the groves they run in and re-install .
Apply lube oil to each but don't use 3 in 1 oil as it dries out and gets sticky after a while.
Any other ideas from anyone?
Jim VE3DDY

[WD5JKO]

I also replaced the rectifier and VR tube with solid state equivalent designs and regulated the filament voltage to the 3 front end tubes.

Jim,

  Please describe the drift you are experiencing. Is it erratic, or just a slow moving target? Does a 10% change in line voltage have a big impact? Is it influenced by AGC voltage? Is it BFO drift too?

  Realize that HV zener diodes have a steep positive temp coefficient whereas a tube VR tube like a OB2 does not. A HOT (spit sizzling) 5W HV zener might drift 10% or more higher from when cold. In order to dump 3 watts heat in say a 5W 110v zener diode, you need to heatsink the heck out of it or it will get too hot. The leads conduct the heat to fat PCB traces, which if absent dictates de-rating the Zener a bunch. Better to use several smaller voltage units in series, or go back to the VR tube.

  You said you regulated the filaments on the first 3 stages. Does that include the HF oscillator tube, and the BFO?

   So do you use the Mohawk for AM reception, or for listening to other modes where 100hz drift is very noticeable?

Jim
JKO

[VE3DDY]

I also replaced the rectifier and VR tube with solid state equivalent designs and regulated the filament voltage to the 3 front end tubes.

Jim,

  Please describe the drift you are experiencing. Is it erratic, or just a slow moving target? Does a 10% change in line voltage have a big impact? Is it influenced by AGC voltage? Is it BFO drift too?

  Realize that HV zener diodes have a steep positive temp coefficient whereas a tube VR tube like a OB2 does not. A HOT (spit sizzling) 5W HV zener might drift 10% or more higher from when cold. In order to dump 3 watts heat in say a 5W 110v zener diode, you need to heatsink the heck out of it or it will get too hot. The leads conduct the heat to fat PCB traces, which if absent dictates de-rating the Zener a bunch. Better to use several smaller voltage units in series, or go back to the VR tube.

  You said you regulated the filaments on the first 3 stages. Does that include the HF oscillator tube, and the BFO?

   So do you use the Mohawk for AM reception, or for listening to other modes where 100hz drift is very noticeable?

Jim
JKO

I'm using the Mohawk for SSB comms. The drift is now a slow(er) moving target but still not sfable enough as say a Yaesu FT-101
 I used a TIP50 HV TAB type transistor controlled by a string of 3-50 volt zeners in series to regulate the voltage to 150 volts. The transistor is bolted to chassis on top of an insulating washer. It doesn't get more than barely warm
The regulated voltage controlled the Oscillator , mixer and RF front end. In the wiring of the set it also controls the BFO and other mixer.
Strong AVC does not affect the frequency.
The filament regulator is a LM 340? (maybe a 339)TO3 package that just regs the fils on the RF mixer and OSC tubes. It doesn't feed the BFO. I noted that heat from the audio output tube next to the BFO can caused drift as I put a copper screen between them and the drift slowed away down.
The band calibrate small variable capacitor  is a major contributer to instability. It does not have a decent shaft panel bearing and depends on a wiper to ground the rotor. I think the BFO tuning cap is of the same  type construction. I replaced it but not the BFO unit yet.
 When I grounded the various sections of the main tuning capacitor the drift went from going positive to going negative. I think there were circulating currents or poor contacts there. Once I did that grounding the calibration changed by 50 kc. Sudden little jumps in frequency stopped then.
 I have the regulated fils and front end HT on all the time to negate warm up drift  and yet I still get about 250 to 300 cycles per hr  for at least an hour after switch on (Remember the front end of RF, Mixer and OSc are always ON) as the rest of the set comes up to temperature.
I/m wondering if a FET Oscillator would benefit .
Jim VE3DDY

[w3jn]

You gotta figure out which of the 3 oscillators is drifting - the first LO, second LO, or the BFO.   I don't remember if the second LO is crystal controlled, but if it's one of those slug tuned oscillators make sure it's not contributing.  All 3 oscillators drifting in the same direction will multiply the drift.  Actually depending upon the conversion scheme the 2nd LO drift could actually cancel out the first LO drift...

Highly confusing, but get at it with a couple of freq counters and see.

[KM1H]

I didnt read anything about trying several oscillator tubes and pick the best one.....NOS works better than junk box usually. Is it identical drift on all bands?

You have a 12AT7 1st LO and a 6CS6 Prod det/free running BFO at 50KHz which should be very stable. The 2nd LO is xtal and I'll have to assume either sideband acts the same.

Never owned a Mohawk, just looking at the manual.

Carl

[VE3DDY]

You gotta figure out which of the 3 oscillators is drifting - the first LO, second LO, or the BFO.   I don't remember if the second LO is crystal controlled, but if it's one of those slug tuned oscillators make sure it's not contributing.  All 3 oscillators drifting in the same direction will multiply the drift.  Actually depending upon the conversion scheme the 2nd LO drift could actually cancel out the first LO drift...

Highly confusing, but get at it with a couple of freq counters and see.

Hi, Your right.
Unfortunately I'm away from home for the next while.
I did most of the mods described in the Electronic radio article  where the author soldered neg 2200 caps across the oscillator  to cut drift and that slowed thins down considerably but those capacitors are subject to changes in air temp and that is quite evident.
I put an insulated box over the main tuning cap to see what effect that had on the drift and it took off upward at a fast rate and seemed to have no end in sight.
I blasted the oscillator coils with heat figuring there may be moisture in there and once they had cooled the drift was slightly less but not enough to matter. Each coil had beeswax filled in there but the wax had shrunk and the slugs of wax were loose and falling out so I removed them.
One of the main problems is that the main oscillator is a Hartley where cathode current flows through the coil tap to ground. This cathode current flow causes the coil to heat ever so slightly and drift results. I temporarily changed it to a Colpits type whith a tapped capacitor divider  but that screwed up the calibration on the drum dial. It still drifted anyway :-)
I read somewhere that HEathkit fired all the engineers associated with this project.
 I haven't really looked at the 2,nd osc and that may be where the problem lies The IF at that point is only 50 kcs .
The whole front end is in a enclosure that I have not got into. It was pre manufactured by some sub contractor to HEathkit. It may be that some resistors on the oscillator need replacing too. It looks like a major project to strip that out as there are multiple connections going through the chassis of the RF deck.
I'll get back to it soonest
Jim VE3DDY

[VE3DDY]

I didnt read anything about trying several oscillator tubes and pick the best one.....NOS works better than junk box usually. Is it identical drift on all bands?

You have a 12AT7 1st LO and a 6CS6 Prod det/free running BFO at 50KHz which should be very stable. The 2nd LO is xtal and I'll have to assume either sideband acts the same.

Never owned a Mohawk, just looking at the manual.

Carl

CArl,
I' know about that . I tried several tubes in the oscillator which is a 12AT7 . Some were worse than others. MAybe the drift I now have is reasonable at 250 to 300 cycles per hour but it doesnt stop going.
Years ago a company called OXLEY made a tempatrimmer which was soldered across the oscillator coil. It had both pos and neg characteristics in a split stator trimmer so the net result was always the same but the appropiate anmount of neg or positive could be dialed in to stop the drift from progressing further.
The oscillator is always highter than the incoming freq by the IF of 1620 or some such amount so on 28 mhz the osc is running at 29.6 which is terrible for drift. This set should have had hetrodyne crystal oscillators and the main oscillator kept at a low frequency.
 There is a lot of heat produced by some of the circuits in this set  and that may be most of the problem.
Jim

[KM1H]

Try different very low value TC caps right from the tuning cap to ground and at least determine which direction is needed.

It worked for the old WW2 Super Pros to a good degree.

[Todd, KA1KAQ]

I'd suggest contacting WA2PJP as he has a Mohawk that he reworked. Much less involved than the ER mods, no three-legged fuses involved or anything drastic. Having used it I can say that it's stable. Then again, I didn't try using it for SSB.

Joe is a member here so you could send him a PM for further information. He also addressed a couple other issues like the S meter and AGC.

[Tom WA3KLR]

Jim,

I cringe a little bit when I see an r.f.-hot cathode on an indirectly-heated tube, especially an oscillator.  This suggestion may or may not be a help but if it is, you would otherwise be chasing your tail around for a long time.  What I am thinking about is the capacitance from the cathode to ground via the filaments.

Add a common-mode choke at the tube socket filament pins for the first l.o. section and see if the drift lowers.  At first, don't bother to re-calibrate, just tune somewhere and listen for the drift.

Get a small toroid (1/4 - 3/8" o.d.) and magnet wire (24 - 28 AWG) and wind a bifilar winding on the core.  Hopefully you can get enough turns and core permeability to make a significant reactance.

* Make sure that the choke wire going to pin 9 connects to ground.

[VE3DDY]

Jim,

I cringe a little bit when I see an r.f.-hot cathode on an indirectly-heated tube, especially an oscillator.  This suggestion may or may not be a help but if it is, you would otherwise be chasing your tail around for a long time.  What I am thinking about is the capacitance from the cathode to ground via the filaments.

Add a common-mode choke at the tube socket filament pins for the first l.o. section and see if the drift lowers.  At first, don't bother to re-calibrate, just tune somewhere and listen for the drift.

Get a small toroid (1/4 - 3/8" o.d.) and magnet wire (24 - 28 AWG) and wind a bifilar winding on the core.  Hopefully you can get enough turns and core permeability to make a significant reactance.

Thanks. Now that is practical and I'll keep it in mind.
I also came across comment on "DRIFT" on the HEathkit forum in YAhoo.
An article in 73 mag from 1961 by Jim Kyle  gave some more practical suggestions. One that is intriguing is to adjust the spacing between the Stator and Rotor plates of the main tuning capacitor.
Apparently, if the spacing is slightly off center balance the heat expansion of the plates in the capacitor can show up as drift.
Adjustment is done by adjusting the pressure screw on the back bearing. To adjust you tune in on a calibrated signal, turn on the BFO and the without touching the tuning knob or BFO again , adjust the screw tension for minimum frequency.
At that point the plates are perfectly equidistantly  spaced and heat has a minimum effect on the capacitor.
Another good idea which would go hand in glove with the dual choke on the filament of the oscillator would be to reduce the oscillator HT voltage in this case from 150 down to say 100 volts.
Going back to the Oscillator tube filaments, I think the way they are wired is the filament voltage is fed in at the center tap and the two outer legs are grounded. I would surmise that this has a balancing effect and cancells out the problems of RF effecting them. I know it sure cuts down on hum.
Another hint in that article was to install a composite 1800 ohm resistor between the cathode and everything that attaches to it. (No Bypass cap). Small interelectrode capacitance changes within  the tube produce a voltage that varies as I understand it. The added resistor diminishes the effect on stability.
Anyhow the result was to improve an old SuperPro from several KHz drift down to 40 hz and then steady.
Check the article for yourself at:
https://rapidshare.com/files/752759657/Jim_Kyle_-_Down_with_Drift.pdf

73 to all
Jim VE3DDY

[Tom WA3KLR]

Make sure that the choke wire going to pin 9 connects to ground.

[Tom WA3KLR]

Jim,

I realized that there is a problem with the bifilar filament choke while on my 1 hour drive to work this morning.   The socket pin 9 carries current for 2 filaments while pin 4 carries the current for only one filament.  So there is a large imbalance in the bifilar winding.  This scheme also starts to couple the one cathode circuit to the other.

Either going to trifilar windings to cancel out the filament current in the core (which couples the two cathodes more), or having two separate chokes for the 2 filament pins may be the thing to try.  The current for pin 4 is 150 mA, and 300 mA. for pin 9.  There should be a bypass cap to ground at the 6 v filament buss side of the choke from pin 4.

[VE3DDY]

Jim,

I realized that there is a problem with the bifilar filament choke while on my 1 hour drive to work this morning.   The socket pin 9 carries current for 2 filaments while pin 4 carries the current for only one filament.  So there is a large imbalance in the bifilar winding.  This scheme also starts to couple the one cathode circuit to the other.

Either going to trifilar windings to cancel out the filament current in the core (which couples the two cathodes more), or having two separate chokes for the 2 filament pins may be the thing to try.  The current for pin 4 is 150 mA, and 300 mA. for pin 9.  There should be a bypass cap to ground at the 6 v filament buss side of the choke from pin 4.

You have lost me Tom.
What about pin 3 which is the other filament connection?
Pin 3 and 4 are connected together and feed either side of the 12 volt filament. The center tap of the filament on pin 9 is the ground path. So either side is fed with 6.3 volts and the ground return is on pin 9.
So then, why wouldn't the one choke winding connect to the pair (pins 3 and 4) and the other wire to pin 9 but in the opposite polarity for cancellation?.
Incidentally, I was looking through some boxes today in my current workshop and found a whole packet of
5 to 45 pf trimmers N750. They are scarse as hens teeth. I'll take those home with me next week.
Jim VE3DDY

[Tom WA3KLR]

Sorry for the delay in answering Jim.  When it comes to making mods to circuits where the tubes are multi-section there usually is a complication where the ideal thing cannot be done the way you want or can't be done at all.

I would have liked for the oscillator filament to be separate from the other section, but it isn’t.  I am slightly concerned about the feedback path that will form from the buffer output back to the l.o. via the cathodes, but this may not be a problem at all.

Yes, just try the bifilar choke on pin 9 and then the other winding feeding both pins 3 and 4.  In this case, the filament currents will balance out in the core.  I doubt that a trifilar choke feeding 3, 4 and 9 separately will wind up with any better isolation between the two section's filaments.  This is an experiment to see if the drift is reduced by this mod.

The temperature-compensating capacitors are becoming rare due to the PLL and other newer circuit technologies.  I was able to buy a reel of leaded 22pf N1500 ceramic caps at a hamfest last year.