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Author Topic: Hammarlund HQ170A Fading  (Read 10944 times)
WB5LXZ
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Posts: 11




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« on: November 23, 2014, 08:03:11 PM »

I recently purchased a near mint HQ170A and after a 20minute warm-up experiences signal fading to near loss of signal, then after several seconds will gradually regain the signal strength.  Checked all and replaced four tubes.  Can't seem to come up with a solution.  Suggestions would be most appreciated.
Thanks,
  WB5LXZ
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AD4U
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Posts: 2543




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« Reply #1 on: November 24, 2014, 05:41:13 AM »

Since this is a 50 year old receiver, the problem could be many different things.  First do not rely on a tube checker.  It may be expensive, but I would replace all tubes with new ones, especially if you plan to keep the receiver.  If this solves your problem - good.  If not then I would replace all electrolytic caps since they are probably 50 years old too.  Next 50 year old carbon resistors often go way high in value.  I would check them and replace as needed.  Then the receiver would probably benefit from an alignment.

Keeping boat anchors going is labor of love.

Dick  AD4U
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K5RT
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Posts: 332




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« Reply #2 on: November 24, 2014, 06:59:48 AM »

Do you have a manual and schematic?
Do you have a good multi-meter? An O'scope and counter?

Paul
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W9ALD
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Posts: 44




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« Reply #3 on: November 24, 2014, 07:43:31 AM »

Given what you are reporting I would look at the Automatic Gain Control.  It sounds to me as though the AGC is doing what is commonly known as "pumping", this cycling of the AGC is rather common in old tube receivers.  You may have a resistor that has gone high or a capacitor that is bad in the AGC system.  Since most receivers utilize AGC voltage to show changes in signal level via the S-meter, if the S-meter is working oddly it is a good indicator of faulty AGC function.  Many of these faults raise their ugly heads after the parts have warmed up.

Bill

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The mere presence of a signal is more important than it's magnitude.
WB6BYU
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Posts: 18533




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« Reply #4 on: November 24, 2014, 11:00:27 AM »

Indeed - the AGC system usually operates at high impedance, varying the grid bias.  A leaky
capacitor can easily drain enough current to disrupt the function.  Such leakage can be
a function of temperature and can vary over time.

A quick check might be to apply a signal generator to the AGC detector and see how stable
the S-meter reading is. 
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G3RZP
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Posts: 1321




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« Reply #5 on: November 24, 2014, 12:56:31 PM »

Or even how stable with time the S meter reading is in the CAL position.
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KH2G
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Posts: 349




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« Reply #6 on: November 24, 2014, 03:08:41 PM »

You may well have a tube filament opening, cooling down and closing again. I'll lay my bets on something heat related.
I've had tubes do that to me more than once.
Dick KH2G
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N6BIZ
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Posts: 32




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« Reply #7 on: November 24, 2014, 05:23:11 PM »

6c4
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G3RZP
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Posts: 1321




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« Reply #8 on: November 24, 2014, 11:39:17 PM »

By the time my father's HQ170A was about 8 years old, it was, I remember, rather 'picky' about the 6C4 - not all new ones worked.
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W9GB
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Posts: 3380




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« Reply #9 on: November 25, 2014, 03:53:47 AM »

Paul Pinyot, KB3LZP, (SK, july 2013) has documented the rebuilding of IF transformers,
due to tarnish and silver migration.
http://www.ppinyot.com/if_transformers.htm

Paul Pinyot, KB3LZP - Tube Radio Restoration web page
http://www.ppinyot.com/
« Last Edit: November 25, 2014, 03:57:59 AM by W9GB » Logged
KD1I
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Posts: 468




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« Reply #10 on: November 30, 2014, 07:47:11 PM »

WOW ! Lots of exotic advice but I think you need to do some basics first.   You can download schematics and a manual from on line. I have done this. Next, measure power supplies, then the AGC voltage with a constant input.  Look for anything that changes when the sensitivity nosedives.    It's not a bad idea to use Deoxit on all of the switch contacts and tube pins. Have you tried rocking the various switches when this happens? It could be nothing more than a dirty switch. How about using the internal calibrator?   This is a good signal source.  Don't change any tubes or caps or IF cans until you have done this. My HQ 170 works just fine after a very good cleaning and alignment.    73, Jim
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KD1I
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Posts: 468




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« Reply #11 on: December 04, 2014, 11:33:25 AM »

So, tell us, James, have you done any further work?     We're always interested in what the solution was.       73, Jim
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