The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

Radio Make and Model: GE A3-5825A
PCB Number: PTBMO58COX
PLL Number: 02A
Test gear available: Dmm,Frequency counters,Leader LSG-215A signal generator,Tektronix 2235 AN/USM 488 scope,audio/function generators,80 watt 50 ohm dummy load with rf "sniffer" port.etc
Symptoms: When channel selector placed between channels TX indicator lights

What is working: The unit now receives and transmits on frequency 27mhz signal at base of Q7 havnt tested for transmitted modulation as yet will get to that although Public Address function operational.RF gain,squelch,NB/ANL,Tone,Signal meter on receive and channel display operational.
It did have open AVR Q44, D50 zener and L19 ok,shorted c127 tantalum and burnt r112 replaced,D2 zener ok, blown open audio ic TA7205P replaced and overheated Dc power input choke replaced as well as reverse polarity diode D51 replaced,was ok but had a broken lead connection and not in circuit.Have replaced every component in the RF PA within the shield except the 2166 driver and 1969 final which i have left out of circuit at this stage.
Synthesizer and oscillator frequencies ok

What has been tried: Repaired above plus replaced T1 and resoldered VCO for pll lock now ok,resoldered every joint on board.
I suspect i have a switching problem with the tx led and am asking advice of those very knowledgeable folk out there who have a better understanding of troubleshooting than i do as no doubt other cuircuits will be affected.

Did it suddenly happen ?: Dont Know
Anything else that may be of help?:This unit was acquired not working and has been sitting in a box "of i'll get around to it someday" for some 25 years and had a totally burnt out rf power section from a mrf 497 being fitted without pinout reassignment and possibly reverse polarity of Dc input power to radio.I dont know wether the reverse polarity diode was in circuit at the time of damage to the rest of radio mentioned.
This radio is very clean externally and internally apart from the repaired rf pa section which now you would never know.No burnt tracks or frequency mods apart from addition of 40 channel switch.I have a schematic for a PTBMO48AOX and ther are some slight differences.I do have a Sam,s photofacts on the way for a 3-5825A.

This is my first post so "Hello to all" and hope someone out there can help me.

Cheers,Craig.

When channel selector placed between channels TX indicator lights

Is this an 18 channel unit that has been converted to 40?
If so, remove the dark blue wire that goes from near the channel switch to the centre of the board (near the crystal filter, it connects to a diode at this end).
No more TX indicator light flicking on when you change channels....

Sue

G'day Sue and thanks for the reply.
Yes,it has had an 18 to 40 switch conversion.
Wire removed and problem fixed
What was the purpose of this wire connection originally
when 18 ch switch fitted ?
I see in a couple of schematics for the 48AOX boards this circuit feeds + dc to either pin 1 or pin 2 of the mixer depending on configuration when channel selector between detents.
When the Sams manual turns up for this board i "hope" i'm going to be much better off,unless there is a major difference between the aussie 18 ch GE A3-5825A 58COX board and the 40 ch GE 3-5825A 58COX board.


Thanks again Sue.

Cheers,
Craig.

What was the purpose of this wire connection originally

It's there to stop illegal frequencies being synthesized if the channel selector is placed "in between" channels.
It disables the transmit mixer (and sometimes the RX, courtesy of some hand soldered components on the print side of the board, on some models only).

It is also used on the 23 channel version to blank out the "spare" channel between 23 and 1.
All 23 channel sets had an extra position - 18, 24, and 40 all divide without remainder into 360 (degrees of rotation), but 23 does not - mechanical engineers prefer to work with whole degrees, hence the extra position.

When a 40 channel switch is fitted, the gap is too small to hit illegal codes easily / deliberately, so no need for the blue wire.
In some versions of that chassis (the GE A model was one of these) it flickered the tx/mod light which drove people batty, so whenever we converted them to 40 we removed the blue wire to prevent complaints...

Cheers

Thanks Sue,
No more questions on that one,you have answered/explained everything.

Cheers.