Grant DX... floating final bias current?

[ExitThirteen]

Hiya folks!

I have a bit of an unusual problem, one I can say I've never really seen before...

I have a President Grant DX, the dual final 3 band radio. I wanted to set the driver and final bias currents, and the final bias current wants to "float" upwards, by quite a bit. Driver bias current is rock solid at 29-30mA, but final bias current starts at about 95-100mA and rapidly rises. Have a look at the video posted. Might I have a leaky coupling capacitor between the driver and final stages? Possibly a leaky junction on a final transistor? Bad thermal diode? What might cause this issue? This one admittedly has me scratching my head a little... but then again... I had a 13 hour day at work, and my mind isn't super sharp.... thanks in advance!





~Cheers~

 

[kopcicle]

I'll slip into that later tonight. I ran into that SSB 858 years ago, it fixed itself ....

 

[loosecannon]

im guessing, but leaky bias diode?
LC

 

[wavrider]

VR pot dirty or corroded, had that happen on a 2510 ended up changing the VR and stabilized the voltage.
Dead key kept drifting upwards was the initial problem
De oxit didn't work

 

[ExitThirteen]

Hmm... I'll look into that Wavrider! I did shoot the pots with De-Oxit D5, and it didn't change anything. I can install a couple of new pots and see if that changes anything. I'll post an update with the results tomorrow, I gotta hit the flat side and get some sleep. 1 more day of the grind!

LC, it could possibly be a bias diode, I'm just not super familiar with this problem, this is the first time I've encountered this issue. It's almost like the finals are going into thermal runaway.


~Cheers~

 

[loosecannon]

im used to seeing that happen to the newer mosfet rigs, as they love to oscillate, but not with bipolars.
LC

 

[bob85]

if the pots and transistors are original & barely cracked open id say its acting like you have an open circuit mv1y bias tracking diode,
turn both finals off & set just one @50ma watch the drift then turn it off & set the other to 50ma, watch again see if both do it.

 

[ExitThirteen]

Bob85, thanks for the info! I'll look into that tomorrow evening after work. I suppose that method could also tell me which one of the bias tracking diodes could be at fault as well.


~Cheers~

 

[bob85]

it will if they both turn down low enough to just set one at a time,
good luck.

 

[Handy Andy]

You know you show a DUAL final radio - if you just did this up look for a SMALL GAP between VR and the GROUND and COLLECTOR trace - there may be a bridge or "shard" (Read : CHAD) in place under the board...

 

[Handy Andy]

Remember too - that the BASE bias of the 2nd Final needs to have somewhere to go as well - so make sure the BIAS on that 2nd final is doing it's job - else what can occur is by your Test lead - it can obtain energy thru the test leads (your current shunt test) and the part turns on because your now adding more bias than you're taking away - forcing the part to switch on - feel for heat - you'll see what I mean

​

 

[ExitThirteen]

Hiya Andy!

I checked under the PCB, and everything looks clean under a magnifier, nothing bridged at all.

Nothing heats up at all during testing, everything runs cool.


~Cheers~

 

[Handy Andy]

Since the "Dual Final" is being used, are you able to see voltage on the BASE of the 2nd Final transistor climb too?

It is my fear that one of the Finals is not getting Base Bias - the Diode "tap" may be at fault. It may be a "one way" versus being swamped with a 47 ohm resistor included to give the current somewhere to go - else voltage will climb to 8 volts and pop the base...then if its' MOSFET - that simply means it's getting ready to turn on...

The Vid doesn't reveal a lot of details - just climbing Bias

 

[ExitThirteen]

Ok, from what I just tested, both finals have stable voltage on their respective bases, 1st final has .54V, while the 2nd final has .66V. However, if I turn both bias pots to minimum, then adjust the 2nd final bias pot up, the bias current starting running away, but the bias voltage stays stable. If I turn the 2nd bias pot back to minimum, and adjust the 1st bias pot up, the bias current is stable. So it seems as though there's an issue with the bias on the 2nd final. Hopefully this helps!


~Cheers~

 

[bob85]

idle current will keep rising if you apply a constant voltage to the base of a transistor high enough to turn it on, unless you have thermal tracking,


try unsoldering one leg of the mv1y on the 2312 that's running away,
should measure like a diode on dmm & not oc.