[TMC] GPR-90/GSB-1 CW note

[Roy Morgan]

On Mar 4, 2016, at 8:24 PM, Chris Kepus <ckepus at comcast.net> wrote:

> I've  been enjoying my GPR-90 / GSB-1 combo ...
> However, this time,  I detect some minor distortion in the CW note 

Here are some observations about the GPR-90 and the GSB-1, and a couple of suggestions:

First on the GSB-1 (not related to your symptom):  The change in the local oscillator frequency from USB to LSB depends on the capacitance of the front panel to the main chassis - so if you have “refinished” the front panel, and repainted the rear side,  you must  clean the contact between the panel and the chassis well, or the shift will not be right.

(It does not sound as if the GSB is causing your symptom.)

- You may have noticed that a strong signal will pull the BFO - that is tuning through a strong signal, near zero beat, the tone gets “stuck" at zero beat on either side of zero until you tune away a little bit. This is caused by some sort of feedback or leakage path front the IF section into the BFO oscillator.  I don’t know how to fix this.  It also sounds as if this quirk is not your problem, either.

- The distortion may well be caused by bad grounding.  I suggest you loosen and retighten all tube socket mounting hardware, and the same at all terminal strip ground lugs.  if you do this while listening to the distortion on a signal from your signal generator, and this is the cause, you’ll know when you’ve found it.  I wonder if the distortion you hear has 60- or 12- cycle hum on it.  If 60-cycle, it points toward a ground that is carrying filament current.  If 120 cycle, it may point to bad B+ filtering.

- To test for insufficient B+ filering, bridge a new filter cap across the main B+ filter caps and listen for a change in the distortion.  A type of distortion called “tunable hum” happens when a signal gets modulated by hum in the plate or screen supply, but is not audible with no RF or IF signal. It would be a good idea to test the B+ for humans noise while you are at this (Scope in AC mode, ensuring your scope can stand the B+ level).

- You may be able to isolate the trouble by feeding an IF signal into the last mixer grid at the IF frequency, modulated moderately (say 30 percent) and noting if the distortion is present.  If it is there, it points to trouble in the IF section.  If not, it points to a source ahead of the mixer, that is in the RF sections.

- Changing out the tubes one at a time (replacing the original tube back in its socket) may well reveal that one tube has cathode to heater leakage, or some other fault.  It's a good approach, and may be the least troublesome to carry out.  (You don’t have to get the set out of its case and on its side to probe the tube socket pins along the way.)

- If you can cause the distortion by introducing a strong modulated signal at the RF input, perhaps not enough to overload the final stages, you may be able to follow the signal along through the set with the scope.  Seeing the distortion in the modulated signal will tell you where it is being introduced.    If you have 7- and 9- pin tube socket extenders, it makes this easier.  This may need careful grounding of  your scope probe and high gain in the scope in the early stages of the radio.


Good luck, and do report what you find.

Roy

Roy Morgan
k1lky68 at gmail.com
K1LKY Since 1958