Mechanical instability HQ-129X

[k7mdo]

I am helping a friend resurrect a couple Hammarlund HQ-129X receivers and we are both baffled by the fact that once they are up and running they are so prone to frequency shift from physical vibration, in particular when the BFO is on and you are listening to anything.

Is this normal or is this an anomaly (happens in both sets)?  They are re-capped, attached to a decent antenna and warmed up.... but if you rap the table or adjust things like the antenna trimmer the resultant audio warbles all over the place.  Otherwise the sets seems "normal". If you keep your hands back they are quite stable.

These two sets and one I have had before all do the same thing....  anyone have a comment or idea?  I have poked around inside the set a bit and there seems no place you can't induce some frequency shift with any contact.

73, and thanks, Tom

[kb2vxa]

A friend had an HQ180 IIRC, just look at it sideways and the station he was listening to ran for cover. Maybe it's a Hammer thing? Can't touch this?

[w4bfs]

it doesn't have to be that way .... rx uses a octal pentagrid converter ( 6k8 ?) and these are sometimes microphonic ...try a replacement .... also relube multi-section variable cap with particular attention to rotor grounding springs ...a little deoxit juice added there will help .... check all chassis grounds carefully... solder bond with the BIG GUN in a parallel thread

[WB2RJR]

Tom,

I had one since 1969 and it has no vibration problems. I bought it from the Southtowns Amateur Radio Society for $30. When I first got it I changed out the variable cap in the BFO. I can not recall exactly why but it could have been a stability problem. The metal cover for the BFO was gone so they might have been having problems with it.

Didn't have an exact replacement so I just used a cap I had around. It's been in there since and this receiver has never broken once. It had been recapped before I got it.

Probably worked a good 270 countries on 20 CW with it so I never had any drifting or mechanical problems. It was a hell of a lot better than the SX-110 I had during high school.

Best of luck with it.

73, Marty

[W7TFO]

I have several, all are pretty stable.

Do what the others recommend, starting with the 6K8.

73DG

[k7mdo]

Tried the 6k8 right away with no improvement, then went to other tubes with little or no difference.

Spent time with a thin wooden dowel poking around while listening to a carrier but so far can't localize the culprit in either of the two 129's.  One is recapped while the other awaits new parts but plays enough to exhibit same issues.

When both sets had the problem I thought it would be that others would have solved it...

Will report back when and if I can find the problem... unfortunately the wife is dragging me off on a holiday so the answer will take a few weeks to report.

Tom

[Carl WA1KPD]

Tom,
This may not be the exact situation but a friend of mine encountered a similar problem restoring an SX-28. All was well until the final alignment. THe FM'ng from any mechanical inducement was so bad as to render the radio useless. He spent weeks on it. This was all with the sig generator. It was bad enough that he could modulate the sig by yelling at the tuning cap. Seriously.

Finally one day he just hooked it up to an antenna and it performed like a champ. No FM'ing was noticed on the  signal. Even a good rap on the table or rig did not seem to impact a received signal.

Final conclusion was that the sig generator, dead on and unmodulated, was so strong and specific to allow this to occur. The radio lasted many more years without any evidence of the problem.
Hope that helps
Carl
WA1KPD

[k7mdo]

Thanks, I will see if that is an issue as well, Tom

[KA0HCP]

I would suspect a tube problem first.  Next, it is possible for ceramic capacitors be microphonic too.

[k7mdo]

Arrived in Bergen Norway today so solutions to the Hammarlund will have to wait a while...  still I can think about it.  Thanks for the support, Tom

[k7mdo]

Finally back from Norway and Iceland.... great trip, no injuries, etc....  did see the attached photo of some equipment in Sund Norway... Wayyyyy up north.  Some is probably Loran navigation equipment some I did not have any certain idea.

Finally ready to tackle the Hq-129X instability.... I have two on the bench, both work reasonable well but both are unstable as all get out when you touch the knobs or even lean on them slightly...  73, Tom

[k7mdo]

OK, spent a lot of time cleaning the variable capacitor contact points on the main and fine tuning sections.  They were, admittedly, filthy and they required removal cleaning and re installation to get them to good contact condition.  (This all done on one of the 129X's only so far)

Here are the results, the instability is greatly improved, ie much better.  Was it gone? No, but I did learn a lot about the problem. On AM there is no issue anymore, and only on 20 meters and higher in frequency with BFO "ON" is it still apparent though greatly reduced.  If the BFO is on I could press on the upper corners of the cabinet and get a 1 or 200 cps change directly related to the pressure.  It then dawned on me that part of the current problem was that the chassis was out of the cabinet and that once installed the "flexure" would be nearly impossible to achieve. The cabinet supplies a serious amount of physical stability.  Huge difference.  Also, someone else suggested the BFO itself could be a problem but I did not find that to be the case.

There is still a tiny frequency shift when pressing inward on the ANTENNA TRIM capacitor shaft as this distorts the metal supporting the big variable capacitors.... but now it is very acceptable.

Tickled pink with the radio now.

About ready to tackle the second 129X..... worse than that someone dropped off a 3rd one (horrible shape) but it was free so I could not complain.

There are 23 capacitors that should be replaced and I can now tell you from experience that takes about 8 hours of bench time....  and the wife wonders what goes on in the shack all day!

Great fun!

73, Tom