Tony G0MQG NARC resident radio repair club member has done some articles from recent repairs.

Tony G0MQG Tips 1 BNOS PSU
Tony G0MQG Tips 2 FT1000
Tony G0MQG Tips 3 FT290 ctcss
Tony G0MQG Tips 4 FT290 PA
Tony G0MQG Tips 5 CB Display
Tony G0MQG Tips 6 MX295
Tony G0MQG Tips 7 RF Slab
Tony G0MQG Tips 8 PSU Repair

A Power supply repair....

A friend gave me some miscellaneous equipment amongst which was a 13.8 volt power supply of the type used on cb radios.
On trying it for the first time after a quick look inside to ensure it was safe to plug in i found that there was no output and the LED for output indication was out.
I removed the control board to investigate, the rectifier is 4 off 3 amp diodes (in 2 parallel pairs) in a bi-phase configuration using a transformer with a centre tapped secondary, 2 ofthe diodes were open circuit.
The circuit was quite simple, the dc from the rectifiers and smoothing capacitors feed the collector of a TO220 type transistor, the emitter of which feeds the base of an (unmarked)TO3 type output device the collector of which is also fed from the dc output from the rectifiers, the emitter of which becomes the regulated output. Clearly the transistors were NPN.
The TO220 transistor has a 15 volt zener diode connected to it's base which is fed via a resistor from the rectifier output, the 1.2 volts dropped across the base – emitter junction ofboth transistors (0.6 volts across each) gives 13.8 volts output.
Further investigation showed that both transistors and the zener diode had also failed
I replaced the diodes with 1N5401's, the zener diode with a 1N5352, the TO220 transistor with a TIP 31 and the TO3 output transistor with a 2N3055, all now worked fine.
It should be noted that these supplies are, in my opinion "built to a price" and can fail with the output device going short circuit applying unregulated output (which on this one isabout 25 volts) to an (expensive) radio so i would hesitate to use one on an amateur transceiver but should be ok on auxiliary equipment.
                                                                                      
Tony g0mqg

P.A.Slab failure on VHF radios.

Sometime ago a club member brought a dual band (2 metre/70 cms) FM radio with no transmit output on 2 metres but ok on 70 cms.This was an Alinco radio with seperate PA modules for each band.Most of the PA slabs used in VHF/UHF radios have 4 terminals plus theheatsink which is ground.These are (L-R) rf input, 12 volt to driver stage, this is usually varieddepending on power setting, 12 volt to output stage and finally rf output so to check these you need an rf voltmeter of some description as well as a multimeter.On putting the radio on 2 metres while connected to a power meter and dummy load and keying to transmit no rf output was seen.I opened up the radio and connected my rf millivoltmeter to the rf input of the 2 metre PA slab, rf was seen and a frequency counter confirmed it was on the correct frequency, also it could be heard on a nearby radio.12volts was available on both the 12 volt terminals so I then put my rf millivoltmeter to the output terminal, no output at all so it looked as if theslab had failed, this one was an “SAV17” which is 50watts output.These at the time were about £70 each (ouch!!) a search of the 'net produced an article about repairing them so I removed the slab and opened it (the plastic cover on this one comes off quite easily) on careful inspection I noticed the substrate was cracked removing the 12 volt supply to the PA transistor so I repaired it by bridging the crack. On reassembling the radio (with heatsink compound under the slab which wasn't there before!) it now worked fine.I've since had another radio with the same problem, different make of radio but the same slab and another club member has since repaired one with the same problem (different make of slab) so it seems it's worth checking this before condeming a slab which may be repairable.
Tony g0mqg

Editors note: A 'Slab' is a kind of hybrid of several conventional components soldered to a circuit board or ceramic circuit and then the whole thing encapsulated in resin or similar to make a complete module, commonly referred to in radio circles as a Slab.
73 de Tony G0MQG

MX295 2 metre conversion.

Some years ago, having read an article in “ham radio today” ( I think that was the mag the article was in) I decided to have a go at converting a Philips MX295 to 2 metres. The modification went quite well and I found that when rewinding the coils in the receiver front end, if this was done carefully to get these correct the receiver was extremely sensitive, probably because of the dual gate mosfet front end. I done several of these radios and some I replaced the prom with a UV eprom which I programmed myself with all repeater and simplex channels. However, because these radios were designed for band 3 (approx 200 mhz) I found the PA would oscillate when retuned to 2 metres and spent many an hour trying to tame it ith only partial success. The last one I converted I decided to have a different approach with the transmit PA. I removed the heatsink from the rear of the radio and removed the PA and driver transistors and associated components leaving only the rf in to the PA compartment, the pin diode switching and the 12 volt supplies to the PA and driver transistors. As the heatsink has a cast finish in the PA compartment I took it to a toolmaker friend and got him to machine over it for me to give a good finish. I then fitted a slab pa module (which I got from ebay) to the heatsink and wired this to the rf in, the 12 volt supplys as neccesary and the rf out after rebuilding the filter stage (copying how the MX 294 was done) this worked quite well and was stable and produced about 12 watts output – the maximum the slab I used could provide. This worked very well on 2 metres and even had ctcss fitted as it was used commercially so only had to be set to 94.8 hz for GB3NB, I used this radio for quite a while on 2 metres.

Tony g0mqg