Heathkit VFO drift fix

[RolandSWL]

Hi all,

It has been reported that the Heathkit Seneca (VHF-1) suffers from VFO drift. The easy fix is just leave it on for a couple of hours. I'm not having any luck with finding the actual electrical fix.

Does anyone out there know of this fix?

Thanks, Roland.......

[Pete, WA2CWA]

Drift in the Seneca has always been a problem.
You can probably reduce it by:
Adding a separate filament transformer for the VFO and wired so that the filament is always on.
Change the 6AU6 tube to a 6AH6.
Add a OB2 regulator tube to regulate just the screen voltage of the VFO tube.
Due to component aging, you could consider replacing all the capacitors and resistors in the VFO.

[Tom WA3KLR]

I found the schematic and parts of the manual on the web and I see that the VFO tank inductor is permeability-tuned.  I did not find the illustration for the VFO section assembly (for space consideration).  I found some photos of the Seneca and in a top view I can see the 3 permeability-tuned slug shafts (VFO and 2 band plate tanks).  The VFO is a sub-chassis buried behind the front panel half-way down; not a nice situation for tinkering.

The circuit description mentions that a heavy ceramic slug-tuned form is used, heavily doped and baked.  One temperature compensating capacitor employed.  Pete says they were never stable, sounds like they did a lot to try to get it reasonably stable.  The screen voltage is 150 volts from a 0A2 just for the VFO. 

Most of the frequency drift in circuits like this is from the inductor.  Having a permeability- tuned core is not good as the permeability of the core is highly temperature dependent.  I am guessing this is where the problem comes from.  It did make it easy to align the frequency range by having the inductor tunable this way though.

A suggestion but tough exercise, would be to take out the original inductor and put in a fixed air-core coil of heavy wire free-standing or on same old form without slug? more turns.  Perhaps a VFO sub-chassis from a junker is available for experimentation.  This is not a project for faint-hearted, but for those who like to really dig in to a problem  Just a suggestion from one who will not be doing the work! HI.

[W3GMS]

All good ideas, but maybe the easiest thing is to have an outboard DDS VFO driving the XTAL socket input.  N3ZI and others make nice DDS VFO's.  You will need to build a gain stage but many circuits out there to do that. 

Joe-W3GMS

[Pete, WA2CWA]

Remember, this is a 6 and 2 meter rig.
For 2 meters, the VFO tunes 8.0 to 8.222 MHz.
For 6 meters, the VFO tunes 8.333 to 9.0 MHz.

So having an outboard DDS VFO probably would be a real pain since you can only read the fundamental output of the DDS VFO. You would have to build multiplier stages to drive the readout to the actual frequency. For all practical purposes, adding a DDS VFO would be overkill for this rig. Also, one needs to consider that the oscillator/buffer/multiplier stages may also contribute to the drift.

[Steve - K4HX]

It's likely that the freq multiplication could be programmed into the DDS so it would display the correct frequency.

[W3GMS]

It's likely that the freq multiplication could be programmed into the DDS so it would display the correct frequency.

Some if not all will do that Steve.  A lot of guys have been using these with the early 10A and 20A Central Electronics vintage SSB rigs.  You can often program in an offset frequency to allow for those rigs that don't use the fundamental frequencies due to the internal mixing scheme.   

Joe-GMS

[Pete, WA2CWA]

Considering the transmitter in discussion, and the bands that it covers, using two or three crystals probably would be sufficient. Adding a DDS VFO to this rig is like adding a spoiler to my mini-van. Adds cost and little gain.

[WD5JKO]

Considering the transmitter in discussion, and the bands that it covers, using two or three crystals probably would be sufficient. Adding a DDS VFO to this rig is like adding a spoiler to my mini-van. Adds cost and little gain.

I see a lot of impractical add ons put on cars. They at least are a form of expression by the car owner. We can do the same with a ham rig, and don't need to justify it to anyone but ourselves.

For example, look at my example of what I attempted on my Knight V44 vfo:

http://www.cumbriadesigns.co.uk/x-lock_solutions.htm

Perhaps some or all of the ideas done on the V44 can be applied to this Heath Kit vfo.

Jim
Wd5JKO

[RolandSWL]

Hi All,
Thanks for responding to my question. Does anyone know how much the VFO drifts from a cold start? Does the drift continue after the radio is good and hot? I am aware that PTO's are subject to aging and may not be fixable. The suggestion of using an external VFO is worth investigating. If the option of using crystals is employed, what are the common frequencies for A.M. work?

Thanks again, Roland

http://www.manualslib.com/manual/464627/Heath-Heathkit-Vhf-1.html?page=4#manual

[WD5JKO]

If the option of using crystals is employed, what are the common frequencies for A.M. work?

Not sure about the common 2m frequencies, but 50.4 or thereabouts is commonly used on 6m AM...

From email I received today from W8EK:

Six meter crystals
8 MHz fundamental multiplies times six to give 6 meters.
8354 (50.124), 8375 (50.25), 8410 (50.46), 8475 (50.85),
8480 (50.88), 8500 (51.0), 8575 (51.45), 8600 (51.6),
8620 (51.72)

Two meter crystals
8 MHz fundamental multiplies times 18 to give 2 meters.
8006.67 (144.12), 8040 (144.72), 8060 (145.08),
8073 (145.314), 8075 (145.35)

All crystals are $8 each.

His call @ arrl.net

Jim
Wd5JKO

[Pete, WA2CWA]

Typically over the last (many many) years of operating on 6 meters and especially during band openings, you can find AM operating anywhere between 50.25 MHz and about 50.5 MHz. Rolling down into the SSB area is generally a waste of time unless you want to work cross mode contacts if you can find someone who wants to.

On 2 meters, in most areas of the U.S., you now going to find a lot of digital signals, FM signals, and repeaters starting roughly at 144.3 MHz and up. If you want to hassle with these signals, or be constantly annoyed in your receiver, you have a lot of area to choose. An "informal" push has been to keep all the typical AM, SSB, CW modes below 144.3 Mhz. I believe, a request has been made to the ARRL band-planning group to designate 144.27 MHz as the 2 meter AM calling frequency.  

[Pete, WA2CWA]

Hi All,
Thanks for responding to my question. Does anyone know how much the VFO drifts from a cold start? Does the drift continue after the radio is good and hot? I am aware that PTO's are subject to aging and may not be fixable. The suggestion of using an external VFO is worth investigating. If the option of using crystals is employed, what are the common frequencies for A.M. work?

Thanks again, Roland

http://www.manualslib.com/manual/464627/Heath-Heathkit-Vhf-1.html?page=4#manual

Obviously, it's a function of your particular Seneca. It could be zero (not likely), or it could be anything. It could drift up "X" number of cycles, drift down "X" number of cycles, or it could drift back and forth "X" number of cycles. You need to test your particular Seneca. There's no rule of thumb on how far something drifts based on a manufacturer model.

The Seneca VFO follows the typical VFO design used in the Heath VF-1, DX-100, TX-1, and other Heath tube VFO rigs. Sometimes even changing the 6AU6 VFO tube will change the drift characteristics. Heath also recommended changing the 6AU6 to a 6AH6 to reduce drift. The Apache VFO had a separate filament transformer for the VFO to keep filamant voltage ON at all times when the Apache was connected to AC mains.

And remember, when operating AM, even several hundred cycles of drift can be hardly noticeable on the other end of the contact. On 6 and 2 meters, you have 4 MHz to drift around. Given the typical level of AM activity on both bands, no one probably will care. And, the Seneca has a spotting switch, so it's real easy to test where the VFO is and get yourself back on frequency.

[Jim, W5JO]

Another thing about drift in kit built VFO sets was the abilities of the person who constructed them.  One thing I have noticed over the years is some people forgot to heat sink the Temperature Compensating capacitor(s).    Some builders simply attached them to the tie points and soldered with a soldering iron.  That iron may have been rated anywhere between 20 and 80 watts.

The TC caps are sensitive to heat and low in value so high heat would ruin them.  That meant the VFO would drift from the first turn on, you never know with kit equipment.  The bad thing today is finding a replacement capacitor that will work in those circuits.