Superstar 3900 problem

[Robb]

When the radio is keyed/TX in AM mode; the meter/radio output swings to full 15w w/o any modulation. Radio is otherwise fine on SSB when in TX or RX. I changed the 2SB754 transistor already thinking that was the problem; but no luck with that.

See if the gurus know about this one.

The schematic here is for a Galaxy 77 radio; but it is basically the same radio (3900 chassis):

 

[tecnicoloco]

something is shorted to ground to draw so much current,a disc cap or a resistor out of specs will be my hunch

 

[bob85]

check tr52 ssb switch is not shorted, they run pretty hot.

 

[Robb]

Replaced tr51,52,49,35,39 and d91.

tr52 had a 945 in it an the scheme called for a 471 there.

 

[bob85]

the single final rigs had 945's with a 150ohm resistor dual final used d471l with 47ohm resistor, make sure you put it in the right way round, remove it or lift one end of r243 to eliminate a possible cause.

 

[Robb]

the single final rigs had 945's with a 150ohm resistor dual final used d471l with 47ohm resistor, make sure you put it in the right way round, remove it or lift one end of r243 to eliminate a possible cause.

Tried that as you requested.
No change . . .

 

[bob85]

what was the problem that caused you to change all the transistors?

 

[Robb]

Took each one out in that circuit and checked them for shorts/opens.

Just running out of thoughts here myself . . .

 

[bob85]

was full power in all modes the original fault, have you measured the base of the 754 when switching modes, measured voltage on the bias test point card when switching modes?, the finals must be getting full voltage from somewhere if its actually making full output.

 

[unit_399]

Have you listened to the radio when it's keyed up in AM?? Is it a dead carrier, or is it squealing, warbling,whatever??

 

[Robb]

was full power in all modes the original fault, have you measured the base of the 754 when switching modes, measured voltage on the bias test point card when switching modes?, the finals must be getting full voltage from somewhere if its actually making full output.

Full power output on keydown was/is the original problem on AM; SSB works fine.

2SB754/AM:
RX/base 13.31v
TX/base 12.32v

Have you listened to the radio when it's keyed up in AM?? Is it a dead carrier, or is it squealing, warbling,whatever??

No sound; just a 15w carrier.

 

[bob85]

what voltage do you have on the collector

 

[Robb]

what voltage do you have on the collector

13.18v RX
11.39v TX

That is what I got.

For reference:
http://www.cbtricks.com/radios/galaxy/dx77hml/dx77hml_trvolt_chart.htm

 

[bob85]

something wrong there, i would lift the 754 collector to make sure thats where the voltage to the bias board is coming from and not some mickeymouse swing mod gone wrong then

use this voltage chart
http://www.m0ysu.com/SS_360FM___Cobra_148GTL_DX.bmp

and this for the transistor numbers
http://www.cbtricks.com/radios/cobra/148gtl_dx/graphics/cobra_148_gtl_dx_sm_pg33_34_sch.pdf

compare it to what you have ,

good luck its my bedtime.

 

[Robb]

TR32/2sc945/AM Shunt was removed in this radio.
I popped one in to clear up the poor SSB performance and re-connected r228.
SSB sounds much better now.

However this radio was working fine until it had this AM/FM problem.

What circuits are different from AM and SSB?
Perhaps I should concentrate on that aspect?