[Hammarlund] HQ-180A 60 KHz IF Problem - Suggestions?

[Steven Reed]

Excellent points, Bob.  And it's true that we sometimes get to the point with these old boat anchors where you have to stop circling the drain and just begin replacing parts.  I'm probably at that point -- the behavior of the IF section is just too systemic to isolate.  I'll replace the capacitors on the switch assembly first -- those are much more accessible.  

Ran into a logically similar situation with a HRO-60:  I finally threw in the diagnosis towel and started replacing parts in the affected circuit.  It got fixed.  It's not very elegant of a technique, though.

Unfortunately, I have no idea what the history of this HQ-180A is or how it was stored.  It's cosmetically in pretty good shape so I don't think it was tossed in the back of a hot barn somewhere.  It appears to be a later production model, probably built around 1970, so that's a good 52 years old.  1970 doesn't seem so long ago until you put the math to it!

Steve.

On 2/2/22, 9:41 AM, "Bob kb8tq" <kb8tq at n1k.org> wrote:

    Hi

    Coils *do* age. They are a mechanical gizmo and they can / do shrink. It
    is not surprising that a batch that all came off the coil winder on the same 
    day and all lived in the same environment did pretty much the same thing. 

    Did the radio get stored out in a shed that routinely got *really* hot in the 
    sun? Did that happen day after day for years?  Some radios do get stored
    that way. You just never quite know ….

    Again, this is only a guess. There is no way to be 100% sure ….. 

    You could spend some “quality time” with a circuit analysis program and 
    maybe work out the original circuit. If you have never done that stuff before, 
    plan on spending more than a bit of time at it …..

    Bob

    > On Feb 2, 2022, at 10:55 AM, Steven Reed <reedsteve59 at gmail.com> wrote:
    > 
    > Thanks for the reply, Bob.  There are no padder caps and judging by the soldering work in the IF section, no one has touched it since the radio was built.  I have a parts donor that's from around the same timeframe and pulled a 60 KHz IF can (they're all the same) to run some measurements.  The capacitors are all El Menco dipped micas and measured correct on the LCR meter (that's no guarantee of course).  The coil (at least in the one I pulled) measured between 3.4 mH at one extreme, to 5.6 mH the other.  This puts the "native" resonant frequency of the IF can between 74 KHz and 94 KHz.  However, Hammarlund's design switches various additional capacitances in and out of the circuit by way of the bandwidth and sideband selectors.  Quite complex.  I've evaluated every capacitor and resistor on the switch array and so far have found nothing out of spec.  I also cleaned and checked the operation of that switch array under a magnifying glass.  
    > 
    > Pulling all six IF transformers and recapping them is something that can absolutely be done.  But I'm restraining myself until more is known -- I have a hard time digesting the idea that all of those transformers have simultaneously drifted.
    > 
    > Steve.
    > 
    > On 2/2/22, 8:36 AM, "Bob kb8tq" <kb8tq at n1k.org> wrote:
    > 
    >    Hi
    > 
    >    A couple “simple” answers to why the drift:
    > 
    >    1) Somebody had a faulty signal generator and re-capped the IF transformers. 
    >    If you are lucky, they did it by adding padder caps that are easy to remove.
    > 
    >    2) The coils have “tightened up” ( and thus increased in inductance) as the radio
    >    has been through this or that combination of heat / cold / humidity.
    > 
    >    3) The caps have all drifted due to the same sort of issue.
    > 
    >    Yes, there are other possibilities, but the solutions would still fall into those three
    >    categories. The only practical fix (if there aren’t suspicious pad caps) is to re-cap
    >    the transformers. 
    > 
    >    Bob
    > 
    >> On Feb 2, 2022, at 10:28 AM, Steven Reed <reedsteve59 at gmail.com> wrote:
    >> 
    >> I’ve been busy restoring a beautiful old HQ-180A.  Previous owners have struggled with the IF of this radio and I localized and fixed the major smoking gun:  V7’s cathode resistor had drifted from 68 ohms to over 400 ohms.  I’m now left with the aftermath of all the fiddling and prodding that’s happened to the IF section over the years.  The problem I’m now facing is that the 60 KHz IF section isn’t resonant at 60 KHz and won’t align correctly – I can’t find a peak at 60 KHz in any of the IF transformers (T6 through T11).  The radio is correctly set according to the manual for this procedure.  However, if I vary the signal generator some, the transformers will find strong peaks around 54 – 56 KHz.  Been through a lot of steps trying to isolate this but keep coming up empty.  The entire 60 KHz IF chain appears to be affected – injecting signals further down the line produces nothing different.  What would drive the resonant frequency of the ENTIRE 60 KHz IF chain down like this?
    >> 
    >> 73 – Steve, KW4H
    >> 
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