Galaxy 88HL Bias Adjust

[LeapFrog]

Trying to set the Driver and Finals Bias on my 88HL, no npc on this one.
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I pulled the mirror board (jumper board) Attatched my d.m.m. inline between TP9 and TP8, Key TX adjust for 50-75mA. When i adjust vr10 the meter reads open circuit untill I adjust some more and finally I get a reading only I can set the bias between 0 mA and 40mA if I try to bring it up to at least 50mA the meter reads open circuit again.

When I set the bias for the finals I don't have this issue, one thing I did notice is that for any transistor while setting the bias my reading will steadily decrease the longer i key the DUT, example I start @ 40mA and the longer it's keyed up the number slowly starts to drop and will not hold steady, I don't see why the device draws less current the longer it's keyed up, I was expecting the opposite maybe it's just where i'm at in the circuit..

Any advice would be greatly appreciated, I would like to be able to set the driver bias high enough to meet FSM specs. I'm not sure what to look for (i.e. bad cap/burnt resistor/failing transistor).

Thanks
-Leap

 

[sonoma]

have you tried changing the VR ? if it gets a open circuit I would be changing the VR to make sure it is good. as old as the 88 is it might be bad

 

[Robb]

Yep, those variable resistors can fail.
Had to change one out in an 88 a few weeks ago.

 

[LeapFrog]

Changed the pot for a known good one, same phenomena observed any time I get right to 40mA or over it goes open circuit, and when I re-key the radio the bias reading does not show up unless I bring the pot back down to the range where i measured between 37-39.9 mA, i'm stuck on this one guys.

 

[sonoma]

well I do not have any idea on this one. never seen this after the VR was changed to a good one. have you checked the bias diode. off the top of my head can not remember if it has one or not for the 88. have not seen one in a while. I just checked and it does have the bias diodes. if one of them is bad can make the adjustment act up.

 

[Robb]

Solder joints; never underestimate that point.

1) Test in USB mode.

2) Modulation zeroed.

3) Select "mA" scale on DVM.

4) VR-11 is for the driver; so TP9 is connected to the positive DVM lead and TP8 is connected to the negative DVM lead.

5) VR-20 and VR-10 are for the dual finals. TP9 is connected to the positive DVM lead and TP7 is connected to the negative DVM lead. Those final bias VR's have to be set to zero - first. Then adjust VR-20 to 50mA first and then adjust VR-10 to 100mA.

That is what and how I do it.

 

[LeapFrog]

Machine also shows downward modulation on high power setting.

That's the procedure I followed "to the T", thank you I will check again for solder joint problems.

 

[LeapFrog]

I just checked and it does have the bias diodes. if one of them is bad can make the adjustment act up.

The one for the driver was coming loose from the "plastic" that it was "glued" to, I need to order some of these as I have none here.

Thank You Sonoma for the suggestion, this could be the issue!

 

[Robb]

I agree with sonoma; check the biasing diodes.

 

[sonoma]

as Robb mentioned check the bias with no modulation at all and in ssb mode. just thought about you saying the mod problem. all you need to do is check the diode on the DVM meter. it either is good or bad.

 

[LeapFrog]

as Robb mentioned check the bias with no modulation at all and in ssb mode. just thought about you saying the mod problem. all you need to do is check the diode on the DVM meter. it either is good or bad.

Should check-out similar to a 1n4148, if I am reading things correctly elsewhere?

 

[sonoma]

you did not say and I did not ask, are the finals still bipolar? if they have been changed to mosfets can make checking the bias a little different. mosfets are checked for voltage.

 

[LeapFrog]

Oh, no-doubt i'm still running bi-polars in this rig "fo sho"!

I appreciate the question though, it seems that many people have now converted these older galaxy radios to use the more modern mosfets, just not me.
(I do own a DX66v that came w/ an irf520 conversion installed before I got it though )
Earlier today I was transmitting into a dummy load with the 88HL (while testing), it turns out that I was transmitting for about thirty seconds with no load/antenna and the transistors survived this serious abuse (seemingly without problems), I doubt the IRF520's would have lasted through such an abuse; I prefer the "oldschool" parts for this very reason.


Edit: Going to Pull the Diode (from circuit) In The Morning For a Simple Test, WIll Post Results Here

Best Regards
-Leap

 

[Robb]

Never done that before; but I've heard that it can work. It will need to be mounted in such a way that it can transfer heat to that diode to get it to track.

But the first question is: Did you first test that bias diode yet?

 

[LeapFrog]

Never done that before; but I've heard that it can work. It will need to be mounted in such a way that it can transfer heat to that diode to get it to track.

But the first question is: Did you first test that bias diode yet?

The radio in question is powered up and running on the bench as of now for an "rx-all-night-&-will-it-burn-out-test?", in the morning I will pull one leg of the diode out of circuit and perform a standard test, I will post the results .

I am Just going to buy the n.o.s. mvy1 (or w/e) instead of "glueing" a standard 1N4148 to plastic.
If I get desperate enough though I will just make one
Solid Builds = Solid Results

-LeapFrog