RCI 69 Hi power key issue

[Hawkeye351]

Got an RCI 69 Base in today for an output issue:

RF Power wide open = 3w key, swings 75w peak.

RF Power all the way down = 0w key, swings 75w peak.

High power VR will only adjust up to 3w max, all the way down to 0w (but shows 13.34v test on rail no matter where I turn it).

Owner stated it used to key 10w all the way up on RF Power, then started dropping and settling down to 3w.

I also noticed, once you key up, the key will slowly drop .5w within 5 seconds and settle. Seems like a bias issue.

What could cause the high power key drop? Passthrough?

Everything else on radio works flawlessly, although both sidebands only do 50w whereas am does 75w.

No mods whatsoever in this radio.

Thanks in advance...

 

[Eldorado828]

That radio has rt1's, have you checked bias and finals?

 

[undertaker]

Just Ranger now a days…

 

[Hawkeye351]

Gate on both drivers are 3.43v
Gate on both finals are 3.32v

Voltage from ground to TP for high power key adjustment stays over 13.3v no matter where you turn it. I normally set it for 5.45v to 5.65v. But can't, cause it's always 13.3+ v.

Brand new radio, very clean, therefore don't want to start replacing stuff on a board with such small parts, lol...

 

[nomadradio]

Bias voltage needs to be set by measuring the current draw of the driver transistor with no drive, and the same for the final transistors. There is too much variation in the exact turn-on voltage for this kind of MOSFET to do this with a voltmeter alone. Whenever I buy a batch of these MOSFETs for stock, we plug each one into a tester that displays the transistor's abrupt "turn on" gate voltage. Using pairs with the same Vth reading improves matching. We see changes in this voltage from one brand of transistor to the next, and from one part in a batch to another. Makes it clear you have to set those trimpots for the particular part that's installed. Not a "one size fits all" proposition.

The "mirror board" gets unplugged. The inboard-most fork contact takes the positive lead of the current meter. Negative lead goes to the outboard-most pin. Select either sideband, mike gain at ZERO, then key the mike. The trimpot alongside the driver transistor gets set for 40 mA. Now the negative meter lead gets moved to the center fork contact. Same setup, but first turn both trimpots to zero reading. Now advance one of them (won't matter which) to 50 mA. Then advance the other final trimpot for a 100 mA reading. When you're done, put the mirror board back. Never hurts to gently squeeze the ends of each fork first, just to insure good contact.

Odds are this will improve the carrier power you can obtain. Usually makes sideband sound a lot clearer, as well.

73

 

[Hawkeye351]

Thank you nomad. I'll give that a shot.
It has only 1 vr for drivers a 1 for finals.

1 rt1 is mounted on the side,

Another (with marking scraped off) mounted where drivers are normally mounted.

Then 2 more mounted beside that one (also with markings scraped off).

4 output transistors, only 2 vrs.

 

[AudioShockwav]

Thank you nomad. I'll give that a shot.
It has only 1 vr for drivers a 1 for finals.

1 rt1 is mounted on the side,

Another (with marking scraped off) mounted where drivers are normally mounted.

Then 2 more mounted beside that one (also with markings scraped off).

4 output transistors, only 2 vrs.

1 pre driver, 1 driver and 2 finals in this radio.
The predriver has no bias adjustments, only the single driver and two finals.
Thus the 40 ma for the driver and 50 ma each for the two finals.

73
Jeff

 

[AudioShockwav]

Picture from Nomad.

73
Jeff

 

[Hawkeye351]

Shockwave, Yes, that's exactly how this one is.

So, I remove that little black jumper, unsolder the 2 jumpers and adjust both vrs accordingly for current draw in either sideband with no audio, right?