Magnum Alpha Force

[crackle]

I was wondering if someone can help me by explaining the function of C110 in this circuit.

It appears to be part of the IF circuits, but it has been cut out in the radio.
Thanks
Mike

[HamishBoxer]

Bias/avc cap?

[SiriusHardware]

The circuit is almost too cunning to be understood, but if it is merely the case that you think the missing capacitor is responsible for some issue or other that the radio has, just try fitting the missing capacitor and see if that has any influence over your problem?

How exactly was the removal of the original part carried out? Roughly? Neatly?

It may be that this was an 'afterthought mod' by the manufacturer to solve some issue which, once discovered, was applied to all the built PCBs already in stock.

[crackle]

It was roughly snipped out. prior to the current owner the radio was owned by someone who likes to "de-cap" radios, and I think this is an very poor misguided attempt at that.
It was reported that the radio was slightly deaf.
I guess the answer is going to be to add the 47uF cap and see if it makes any difference.

Mike

[crackle]

Here it is, or more to the point isn't, more or less centre on the main board.


"de-capping" is an expression I have heard amongst a couple of the local CB'ers, it is their attempt to improve the audio tone of the radio. Mainly on Uniden boarded radios, which I am informed are sometimes a little woolly or lacking clarity. I am assuming it is normally done in the audio stages.
There must be some website giving information on how to do this.

Mike

[SiriusHardware]

I only know of one bona fide 'decap' instance and that was in regard to the FT290R, a portable all-mode 2m amateur rig. The FM receive audio was rather 'woolly' on those so there was a widespread mod which involved 'crunching' a SM capacitor in the de-emphasis circuit, so I suppose that is along the same lines.

As you say, I can only imagine that applying to the audio side so I can't see what it was supposed to achieve in this particular situation.

[crackle]

Actually I may be mistaken, the reason for "de-capping" could have been to remove a capacitor in the microphone circuit to give louder and clearer modulation.
That makes more sense to me as over-deviation seems to be the aim of some of the CB operators around here.
The guy near to me seems to have calmed down a little now, (the one who owned this rig before my friend) I think from a bit of peer pressure.

I am just off now to see the effect of bridging a 47uF cap onto the cut wires.

Mike

[crackle]

Oh dear what a pity. Looking at the underside of the main PCB reveals some major alterations to the circuit. So it looks like the missing capacitor above the board was not just a inexpert attempt to improve the modulation.
Someone has been attempting to make many mods to the Radio.
I dont know what they are all for and don't really intend to do anything with them.
There are 3 wire links added, 11 resistors, 1 electrolytic, and a couple of diodes added to the underside. These may or may not be factory fitted but I because of the other wires and poor soldering I suspect they are not. The capacitor which was added under the board was only soldered by one leg.
C110 has been removed (does not seem to make any difference when I added it back) Q132 and Q142 have been removed.

The reported problem on receive is down to an intermittent fault, which comes and goes when I tap the board gently mainly around the centre right hand side of the board.
So I am just going to try and locate this issue and hand it back.

Below are photos of the some but not all of the mods, and a few other issues like cracked solder and stray blobs of solder.

Mike

[Techman]

The solder in the last picture looks like an accidental 'dropping' off the iron rather than bad soldering.

There's probably some original pictures of the underside of that board on the net somewhere.

I did an audio mod in the mic handset of an HF transceiver (may have been a small Yaesu) for someone a while back - worked well apparently and could have been how I got given that Radiomobile radio mentioned in the other thread, can't remember, I get given all sorts in reward for doing various 'jobs' for folk...even though I try to avoid them.

[crackle]

I have located what I believe is the capacitor that helps set the frequency, C236.
It is marked as 390pF but it also had a 92pF soldered in parallel on the back of the board.
Taking the 92pF cap off made a very small difference in the frequency so I removed the 390pF one and by experiment found that a 340pF cap (measured value) was giving me a closer frequency to what was displayed on the frequency display and I could get the frequency closer to the UK channel frequencies, if that makes sense. The cap is located here on this photo just to the side of Q127 circled in red.


Here is the 390pF cap and the 92pF cap


Prior to this change the radio had to be tuned to 27.782MHz to receive clearly on channel 19, 27,78125MHz

I have to admit to being fooled into thinking these tiny caps were resistors, (the component number on the board was obscured so I could not read it), and I went through all my meters testing them as resistors to then realise I was testing a capacitor. OOPS.

Mike