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Author Topic: HQ-160 suddenly quiet  (Read 8162 times)
K7WXK
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Posts: 68




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« on: September 13, 2012, 04:04:47 PM »

My recently acquired HQ-160 was working well shortly after receiving it, but suddenly during an attempt to check the alignment, it went quiet.  There is hardly any signal on any band.  In fact the only way I can hear anything is with the Function switch in the CW-SSB mode, both Audio and Sensitivity fully clockwise, and a local AM broadcast station selected.  Here is what I know:

1.  The receiver went quiet when turned on its side in preparation for a routine alignment.  The signal generator and vtvm were hooked up exactly per the owner's manual.

2.  All tubes have been subsequently swapped with presumably good replacements (I don't have a tube tester).

3.  All tube voltages have been checked against the socket voltage table and are within limits except the 6U8 BFO/CW/Det.  Pin 1 has 78v vice 100v, pin 2 has 0.5 AC vice 2.4 AC, pin 3 has 130v vice 108v.  Also, the 12AX7 AF amp/q-mult pin 8 has only 0.5v vice 6v.  Note:  RF, IF's, mixer, converter, osc, output, are all within normal limits.

4.  There is no negative bias on the RF grid (pin 7), but there is nothing in the manual to suggest that there should be.  The table leaves it blank.

5.  All transformers (IF, and others) have been checked for continuity and all have no "opens".

6.  When I attempted to adjust the IF transformers, the only one that showed any deviation was T-8 (coupling transformer between 2nd IF and Det/Lim/AVC).  All other transformers behaved as though there was no signal to adjust.  (As it was the signal generator amplitude was extremely low even with all gains advanced.)

7.  All rotary switches (Band switch, and Function) have been De-oxited and visually and electrically inspected with no apparent compromises.  There were some contacts that I could not reach in order to test with the vtvm.

8.  There are four RC network assemblies that are difficult to test and MAY be suspect.

9.  As yet I have not changed any coupling capacitors.

With this information do any of you Hammarlund experts have a game plan to solve this?  I feel it is something simple that catastrophically failed, but I am lacking in a systematic troubleshooting flow.  I have a VTVM, a signal generator, and an oscilloscope (the scope I am a complete novis, so its usefulness in my hands is suspect).

Hope this helps.

Marc


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KD0REQ
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Posts: 836




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« Reply #1 on: September 13, 2012, 04:53:07 PM »

I'd start walking backwards from the grid of the AF voltage amp, putting signal on each grid, and see if you see anything on the scope.

for instance, use the AF generator on the signal generator straight into the scope vertical input, set the timebase for a smooth line,not that blip-blip-blip slowest trace.  adjust fixed and variable attenuators until you see the AF signal halfway across the tube face, up and down.  that's basically how you work the scope... and if the frequency gets higher, you have to adjust the timebase to see sine waves, and if the trace goes away, adjust the attenuators on the vertical input.  use the AC input selection on vertical.  end of scope lesson 1.

for each type of testing, put the scope on the grid of the next tube in the chain, put the signal source on the grid of the tube ahead of it, and see if the signal goes through.  when it does, move the probes on both generator and scope ahead one stage.  adjust the signal generator to appropriate frequencies for each stage (for instance, 3.5 MHz is not appropriate for an audio stage, and it's not right for the BFO, either.)

you are going to find dead stages.  at that point, you need to lightly poke with an insulated tool like a chopstick for intermittent connections.  finding none, you will have to check the components.

sudden death in a working radio with only a physical change of the cabinet is most likely a loose connection, but it could also be an open wire on a coil.  be careful with coil wires.  when checking resistance (power off, right?) use an Rx10 or Rx100 setting on the multimeter so you don't accidentally burn up a litz qire winding.
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N8CMQ
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« Reply #2 on: September 13, 2012, 05:20:18 PM »

I'm with REQ, injecting a signal at the audio amp and working to the input will tell you where the chain is broken.
I am thinking you have a loose component that shorted to ground, or has an open, and when you tilted the unit, it failed.
Good luck and let us know if you get it!
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K8AXW
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« Reply #3 on: September 13, 2012, 08:49:14 PM »

Old timers suggest the old "divide and conquer" approach.  Start in the middle with an injected signal.  If you hear it in the speaker, work toward the antenna.  If you don't hear anything work toward the speaker.

Saves a lot of time.
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WB4SPT
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« Reply #4 on: September 14, 2012, 05:24:27 AM »

  be careful with coil wires.  when checking resistance (power off, right?) use an Rx10 or Rx100 setting on the multimeter so you don't accidentally burn up a litz qire winding.

That's something I had not yet heard.  You mean, if I use a ratty 5k ohm/volt meter, I can actually melt out a coil?
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N8CMQ
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« Reply #5 on: September 14, 2012, 07:12:20 AM »

Some of the old VTVM's used higher voltage/current than a battery VOM.
A Fluke uses microamps for most ohm meter of the readings.
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K1DA
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Posts: 460




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« Reply #6 on: September 14, 2012, 07:38:51 AM »

Does it hear the calibrator signal?  Find out where THAT is injected. 
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AC5UP
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Posts: 3812




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« Reply #7 on: September 14, 2012, 09:01:55 AM »

That's something I had not yet heard.  You mean, if I use a ratty 5k ohm/volt meter, I can actually melt out a coil?

Not likely. Modern super-mini coils are wound in some cases with wire so thin it amazes me they can be built reliably. But don't forget that back in the day when the HQ was new & shiny the typical ham used a 2,000 Ohms / volt VOM with an Ohmmeter that could pop a modern small signal FET. The VOM was much less expensive than a VTVM and some folks could fix an old radio with a test lamp / continuity checker. Test equipment is factors of 100 more accurate and less expensive today so even a 5K voltmeter is better than what might have been used back when the president was dating Marilyn Monroe...

BTW: I can't see, feel, or smell the radio so this is total speculation, but the mode of failure you described would guide me to make a very close examination under the chassis with both eyes open, clean glasses, and a good flashlight. Something either touched, was stressed, or became bent in an unfortunate way.

Quote from: In another thread WB6BYU wrote:
...I verified all of the tube voltages and the voltage drops across the resistors:  everything was normal.  I then focused on the feedback network: the coil and capacitors.  I couldn't test for a shorted tuning capacitor until I unsoldered the wire to the coil across it:  DEAD SHORT!  I carefully checked all the plates and slipped paper between them to clean out any debris.  Still no luck.  Then I noticed the trimmer capacitor on the side of the main tuning capacitor:  the solder lug for it was bent over and shorting out to the head of the adjustment screw, shorting it and the whole tuned circuit!  A small tweak with my probe and the receive burst into life, after 4 to 6 weeks of concerted effort.

Your situation could be that simple. And, yes, you don't have to wait four to six weeks before you find it. I'd pay close attention to any uninsulated wiring with two or more conductors in close proximity like you find near tube sockets. 7 pin Noval's in particular. Even a brief contact can smoke a 1/2 watt resistor and it has been my experience that resistors in the 100k range and higher can open without a hint of darkening or visible stress. Some tubular condensers (like waxed paper) can be damaged if the leads are pulled too hard. The old plastic "bumblebee" & postage stamp mica's can also soften with age and become fragile.

BTW: Any plastic condenser that's "soft" needs to go. Gently squeeze a corner with a needlenose to see if it deforms easily. I always replace waxed paper tubulars (be gone, spawn of Satan!) on sight with modern "yellow" poly jobbies.

If all else fails check the book for a tube socket voltage / resistance chart and run through the list. That might seem like the long way home, but chasing your tail for weeks on end is no shortcut to Valhalla...  Tongue
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K7WXK
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« Reply #8 on: September 14, 2012, 09:20:29 AM »

Thanks, everyone for your helpful hints.  I haven't considered the shorted trimmer.  I'll get back to basics and then work toward the more complex.  Thanks for the hints on the use of the scope.  So far, it has been gathering dust at an astonishing rate.  I like the idea of "divide and conquer".   All the caps are relatively modern, with no black beauties, paper, or postage stamp, but I will remain suspicious of them nonetheless.  With all this ammo, I should be able to lick this little monster.  I'll be in touch.

Keep the cards and letters coming.  No suggestion will be scoffed at.

Marc
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WB4SPT
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« Reply #9 on: September 14, 2012, 02:18:13 PM »

I like the idea of "divide and conquer".  If you divide the unknown by a factor of two for every test, the complex system gets small fast.  It's that geometric vs algebraic relationship.  So, you can either SNIFF at the stage points, or INJECT at the stage points.  I've done both.  If you have a general coverage (modern) receiver, build up a little sniffer out of a couple turns of insulated wire.  Tune the Rx to the expected signal, don't forget the injection freqs.  Xtal osc, VFO's can be easily analyzed for basic op this way.
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K7WXK
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Posts: 68




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« Reply #10 on: September 15, 2012, 06:48:28 PM »

Update:  Well, I dragged out the O-scope and began to give myself a refresher in signal tracing.  Here is what I found.

As I said earlier, I was able to faintly hear a local AM station, so I opted to see where it would lead me.  I was able to follow a signal to the output side of the converter (6BE6) and into T-3 (for those of you who may have the schematic).  However, the signal was absent on the output side of T-3 with no appreciable signal at T-4 or the grid of the first IF and beyond.

Before I proceed with tearing into the T-3 transformer, I wanted to bounce this off you experts and see if I am on track.  My thoughts are that there is a faulty capacitor inside of T-3 (or one of the .01uf caps at the base of T-3).  The coil windings are not broken, so that pretty much leaves the internal capacitor or surrounding capacitors.

Your thoughts?

Marc
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AC5UP
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« Reply #11 on: September 15, 2012, 08:34:11 PM »

...next step is to remove the 1st IF can and open it up.

If you find a postage stamp type mica or two inside I'd replace them just because there's no better time. If it isn't a bad condenser, either a shorted winding on one of the Pi sections or a chafed spot that's finding a path to ground.

Could also be miniature squirrels inside T-3 building their winter nests, but that's a long shot.   Tongue
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W9GB
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« Reply #12 on: September 15, 2012, 09:03:37 PM »

Marc -

It can be a coil wire that has come loose, or is broken (easy repair).
http://forums.qrz.com/archive/index.php/t-331612.html

It could also be Silver Mica disease, (Silver Migration).
http://www.radiomuseum.org/forum/if_transformer_repair.html
It is a frequently reported problem with many antique radios, as they are aging (50+ years).
http://www.antiqueradios.com/forums/viewtopic.php?t=90591

The HQ-145, 145A, and the HQ-160 were double conversion receivers.
HQ-160 IF=455KHz/3.035MHz

HQ-160 also had a crystal controlled second oscillator. All were single conversion below 10 MHz.
HQ-140, 145, 150, and 160 all tuned from .540 to 31 or 30 MHz.
===
START, with RESEARCH FIRST --- 80% of your time.

Very good web page with detailed photos of the silver mica issue
with various mfg IF can models.
http://www.ppinyot.com/if_transformers.htm

SOMETIMES, you can clean and burnish for repair.
http://home.roadrunner.com/~boatanchors1/zenith%20if%20can.html

More suggestions:
http://antiqueradios.com/forums/viewtopic.php?t=83971

THEN, pull HQ-160 schematic and get properly sized (new) silver mica capacitor (~ 500 V rating).
You can install in the can OR at IF can lugs inside the chassis.  
Replacement approach depends on form factor of capacitor and IF can size.
« Last Edit: September 15, 2012, 09:34:06 PM by W9GB » Logged
K7WXK
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Posts: 68




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« Reply #13 on: September 16, 2012, 07:29:33 AM »

Thanks for the latest tips and encouragement.  Also, thanks for the links, GB.  Those pics and text are invaluable.  I plan to tear into the can today, as the surrounding bypass caps were fine.  I really hope this solves the problem.  I can find no other signs of failure so far.  I'll let you kow when I'm done.

Marc
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K7WXK
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Posts: 68




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« Reply #14 on: September 16, 2012, 11:56:15 AM »

Here are the mica sheets that I removed.  I really don't know if they were bad,  but they sure don't look too good.

 
http://smg.photobucket.com/albums/v237/caneseeker/?action=view&current=010-1.jpg

Don't really know how best to post pics, but this should work for now.

Marc
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