Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Help repairing Motorola GM340 thats been reverse polarity

Not open for further replies.


Newbie level 3
Oct 26, 2007
Reaction score
Trophy points
Activity points

Hi can any body help. I am trying to repair a GM340 that someone has connected to a battery the wrong way around with no fuse! I have replaced the protection diode and Q3411 & Q0661. The radio now switches on and receives OK however it does not Tx (Tx led on and current draw 500mA). My question is what is the chance of the PA device u/s as I can’t do any meaningful test because when the cover is removed the radio cannot Tx anyway! All voltages seem OK and resistance measurements all look sensible. I fix radios all day but this one has me flummoxed is their a special jig to simulate the cover being on in order to fault find?

look for a burned track
look for +12v supply to p.a

its more likely the driver transistor has gone
look for anything common to a 12v supply and not further back in the mixer
a quick test using a scope will reviel if there is drive locked frequency post mixer
to the driver transistor

the 12v usualy is common to both the pa and the driver
and often there is a series diode in the p.a somewhere to previent this type of damage

ive found this problem to be a burned choke usualy a white box or blob type
with two dots 2pin choke in series with the 12v supply to the driver and or pA.
and it when fried dumps any amplified rf to the 12v rail and not to the pa
so there is no drive and no output

you can pull the top cover off the white box type chokes and check the colour of the copper coil see if its black or gone pale gold colour then its fried

very often most transistors will survive reverse polarity
and youll get a burned track or chock or a regulator

again check the 12v rail at the pa and press TX to see if it dips
this will show that there is not enough avalible current and a regulator
usualy used to control power output is damaged

let me know here how you get on :D

i cant find the diagram or i would help some more

motorola gm340 service manual

Thanks for your help. 12 volt supply to PA is OK. No burned tracks (apart from where the reverse diode got very hot). As it’s a multi layer board there could be a track gone that I can’t see.
I can’t make any RF measurements because the radio will no longer work in Tx when the cover is removed (I have proved this with a good one) I think that it may cause the VCO to unlock. The driver has its own 9 volt regulated supply and that’s OK.
I could probably identify the offending component if I could work on it with cover off by tracing the drive with my diode probe.
Most of the chokes are of the open coil type or wire with a ferrite bead and they are all OK. I am quite surprised at the small amount of physical damage given the abuse it’s taken
I have the service manual buts that’s as far as I have got

service manual motorola gm 340

Thanks for your help. I have all of that & was wondering if somebody has any experience of what typically gets damaged when power is applied to the radio the wrong way round or how to make it work with the cover off.

motorola gm340

I use to fix car stereos that were reversed. The first thing I looked for was a diode that protected them from reverse polarity. It would blow and in turn blow the fuse.

gm340 service manual

ok looking at the schematics now i got a copy from above thanks !!! for the links

as there is two different p.a modules

i see a resistor .01R 2watt R2471 bottom schematic
R2371 top schematic of .05R

they come from the battery connection and diode etc decoupling caps
and form safety resistors for the pa and perhaps a little current limit for the sense system

i would check these depending what pa
if they are metal oxide or film type they will definatly go open or high resistance

and as the radio draws .5amps in Tx mode
its safe to assume that the predrive and driver transistors in the PA
are both working to provide a 1WATT drive level to the final transistor mrf1946

if the main pa transistor worked it would draw more like
45watts / 13.8 {~3-4amps } as you know

so perhaps the final transistor is gone as you assumed
if these resistors are aok assuming this then
the pll will be aok and locked and tx predrive to the module will be there

maybe you can take a cable out of the interconnection between the pa module and its TX drive rf whilst still being able to close the case maybe you can temp remove a socket from the rear and feed one thru as a quick test of pre drive

i used to use many such extension leeds to service yeasu and kenwood radios
although the ham radios are better thought out you do need them in some
i worked with pmr and ham radio for 20 years here in glasgow
usualy i would remove the whole pa module and work backwards first checking predrive
then knowing its aok i would further test the p.a

looking at the main power regulation and audio sections
i see a transistor Q452 it is perhaps open circuit
or has no emitter junction left..

i assume the audio output is there as you say Rx works
?did you have to replace it?
if not
and as its connected fully to the unswitched +12V
and it survived this is a good indication that the fault lyes in the PA
and not the main pcb's as it would have vaporised if the reversed voltage had made it back to it

so this leaves just those two resistors i mentioned above and the final RF stages

a good quick test it to set the meter to 20R range
and test between the battery connection and the collector pin of the final
pa transistors
to make sure there is continuity of +12v to the final

finaly its worth checking R2370 top diagram R2470 bottom diagram

summing up

in this power rail configuration
its highly unlikely anything other than the pa has an issue
if the audio chip survived for sure...!!!

Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to