HQ-170A dead on all but 7 and 28 MHz

[K3STX]

All of a sudden, my HQ-170A is now dead on all bands EXCEPT 7 and 28 MHz. I can not even hear my 100 Hz crystal calibrator on the dead bands. I know it sounds like a bandswitch, I sprayed de-ox it all over the contacts, worked the switch, LOOKED at the contacts (they LOOK good) and no joy.

Any ideas if it could be anything OTHER than the bandswitch, and any idea why onlyy 7 and 28 MHz work?

paul

[KA5N]

The only thing that makes sense is that something has gone awry with the bandswitch.  As you know the switch has many wafers and the most likely thing that has happened is that something has slipped or broken and the wafers are no longer correctly aligned.   That two bands still work (note that they are rotationally far apart) is also an indicator of physical misalignment.  Are there any couplers in the bandswitch shaft that have come loose so that some of the wafers are not indexing?  If so this would just require aligning the wafers and retightening the set screws that hold the coupler.  If something
(like a wafer) is broken that is a whole different matter and very difficult to repair.
Using the schematic as a quide visually check the alignment of the wafers from band to band and check the integrety of the wafers and contacts.
Good luck
Allen

[G3RZP]

Check that the oscillator is working. The quick way is to put an RF Choke  (220 microhenries or more) in series with your multimeter and check the volts on the 6C4 grid. If it's working, it will have a negative voltage  -  I can't remember how many but probably between 1 and 10 volts.

Or look with an oscilloscope that has a suitable bandwidth and a X10 probe.

[K3STX]

If the oscillator was not working wouldn't it be dead on ALL bands?
I studied this bandswitch, everything SEEMS fine. I'll look at the schematic more. No calibrator signal on the dead bands really stumps me, sure SEEMS like the switch.

paul

[WA0ZZG]

Paul;

These receivers ae getting really old and some strange problems can pop up.  Suggest you check the voltages around V12 as you move the band switch.  Also, make sure the voltage regulator, V14 is still working.
Dave   WA0ZZG

[KA5N]

The oscillator may well not be working on the dead bands, but surely not because of a bad capacitor.
On the dead bands some part of the necessary circuit may be disconnected.  If one has an oscilloscope and is acquainted with its operation, it would be an excellent troubleshooting aid.  Most posters on this forum are usually somewhat inexperienced, have a lack of test equipment, and don't have 40-60 years experience troubleshooting and repairing electronic equipment.
That sometimes mica capacitors fail doesn't mean that every problem is due to a bad mica capacitor, or for that matter while many resistors that have been used a long time are now off-value doesn't mean that every one is, etc.  
What most new posters don't understand is that any malfunction or symptom can have many varied  causes and the notion that someone knows exactly which part will cure the problem is just a sometimes
thing.
That said, we have a receiver that works on two bands only, and several bands that are dead.  Usually one would find some mechanical fault such as a broken connection a misaligned switch, etc.  But the repairer may well have to measure all the voltages on all the tubes on a working band and then on a non-working band to find where the problem occurs (yeah it will be time consuming) and then find the fault that is causing the malfunction.  If that doesn't work then ask for more help.
Good luck
Allen

[WA1RNE]

 Well said, Allen. That is how it is.

[K3STX]

Most posters on this forum are usually somewhat inexperienced, have a lack of test equipment, and don't have 40-60 years experience troubleshooting and repairing electronic equipment.

You've just described me.

This will take some time, I'll get back to you in a month or so. I have never had to troubleshoot a bandswitch, I'll try to figure out how to read a schematic for the multiple positions on the switch.

paul

[G3RZP]

The amount of feedback in the oscillator is not necessarily constant from band to band. So you can have an oscillator stage which is low on gain, but some bands having higher amounts of feedback see it working. Although from memory, the HQ170A (at least, the one my father had) was more marginal on 10 than on the other bands: we had to select 6C4s. Mil surplus were good: cheap imports from who knows where were somewhat dodgy.

The approach you use is soemwhat dependent on the test gear to hand: for example, I'd use a spectrum analyser and FET probe to check the oscillator and the gains while feeding from a signal generator, because I have all that test gear. With a multimeter and few components, I'd look for oscillator grid voltage and so on.

If the radio is completely dead, I would suspect the oscillator; if it's very low gain, the RF amp or mixer.

[W5RKL]

The band switch selects the appropriate band's oscillator coil and appropriate trimmer piston variable capacitor through the band switch wafers S2C and S2E. Each band has it's own oscillator coil contained in transformer like cans, T23, T24, or T25 can directly behind the front panel. The variable piston type capacitors are located between T23, T24, and T25 and the front panel. Band switch wafers S2C and S2E are located directly below the oscillator coil cans and piston capacitors.

Each oscillator coil can has "2" oscillator coils inside and they do interact with each other to some degree.

I suspect all but 40 and 10 meters band oscillators are not oscillating. This could be caused by a very dirty oscillator coil band switch wafer S2C and S2E or simply those dead band oscillator coils and piston variable capacitors are improperly adjusted resulting in the dead band oscillators not oscillating.

I don't suspect a bad V12 (local HF oscillator) as the source of the problem. I also don't suspect a broken wire to or from the band switch wafer or a broken band switch wafer as the source of the problem. I do, as mentioned above, suspect the dead band oscillators are simply not oscillating.

You will need the HQ-170A manual. If you do not have it then download it from bama's website.

73s
Mike

[KA5N]

"All of a sudden, my HQ-170A is now dead on all bands EXCEPT 7 and 28 MHz. I can not even hear my 100 Hz crystal calibrator on the dead bands. I know it sounds like a bandswitch, I sprayed de-ox it all over the contacts, worked the switch, LOOKED at the contacts (they LOOK good) and no joy."

This part of the initial posting made me believe that the receiver hadn't had the "Golden Screwdriver"
treatment.  He has cleaned the bandswitch two or three times.  Most oscillator problems that affect some bands are more likely to occur on the high bands, so working on 10 meters makes those problems unlikely.  As for as a weak oscillator tube, it would be nice to sub in a known good 6C4 if the op has one.  A bad voltage regulator would probably result in higher than normal plate voltage or erratic plate supply voltage.

On SB Heathkits I have often found bad connections in their bandswitches because the switch was depending on rivets through the switch wafer for connections.

Was working, now only works on two bands sounds like a mechanical failure of some kind.
I sure hope it gets fixed and we can all learn what the problem was.

Allen

[G3RZP]

we'll prbably find all our theorising based on our experience was totally wrong!

[K3STX]

Well, this is wierd. I turned on my HQ-170A again and STILL only works now on 40 and 10 meters. Then I noticed THIS thread here:

http://www.eham.net/ehamforum/smf/index.php/topic,4246.0.html

How strange, ANOTHER owner of an HQ-170A whose only functioning bands are 40 and 10 meters (and no, it is not the same rig)! In the fall I might work on this again, but not now. But how strange another guy had same problem.

paul

[K3STX]

PROBLEM SOLVED!!

I emailed Dwain, W0CL who started the previous thread I just referred to, and asked if he solved his problem. He said it was a bad Oscillator, V12, a 6C4 tube. I replaced mine and BAM, all is good!!!!  I am not electronics wiz, but it surprises me that it was NOT the bandswitch. It was TOTALLY dead on all but 40 and 10 meters. I can imagine how some bands have more or less gain from the Oscillator, but NOTHING is nothing. Not even my Calibration Crystal. I will study the schematic and try to learn, but I am very happy now. As I told Dwain, I am kind of sick make QSOs with my TS-850S; it is too easy. I am SO happy, this HQ-170A is such a beautiful rig. It is a shame to use it with a DX60B and VFO: I think I should get the matching Hammarlund TX for this gem.

A happy ending, years later.

paul

[G3RZP]

It all depends on the amount of feedback: it is a careful compromise in design between having enough feedback to reliably oscillate with all the variations in tubes, and not having so much it is squegging. Plus the fact that is more than possible that the Q of the coli could have degraded over time - what are the formers made off? Many of the cheap formers can slowly absorb moisture over time and degrade Q.