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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 4th Apr 2017, 8:54 pm   #1
SiriusHardware
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Default Fidelity CB3000FM CB under repair...

You may justifiably ask: Why?

These not unpleasant looking CB base stations had a pretty grim reputation for wide open receivers and limp RF output power. A friend is reliving his youth by buying an example of every type of set which passed through his hands, or those he just liked the look of but never had, bought this set as-seen and found it had no output power so it has been passed to me, despite the look on my face.

One RF power transistor later the transmitter is producing output again, but as usual for these radios and all the ones made by the same manufacturer (like the Fidelity 1000) the output power is very poor, a mere 2.5W on this example when fully peaked.

However, I find that the first coil can in the transmitter chain (L6) doesn't seem to have a noticeable peak and it looks as though it should have, so it may be that the parallel internal capacitor (usually somewhere down in the base of the can) has been broken. One possibility is just to try a ball-park selection of fixed capacitors or a trimmer across the pins of L6 on the underside of the board to see if I can get a peak that way.

The PLL synthesiser which is based on the otherwise standard LC7137 PLL seems far more complex than usual with the addition of an IC buffer for the TX output from the VCO and a transistor buffer for the RX output from the VCO.

Circuit diagram is here:

http://www.cbtricks.com/radios/fidel...00fm/index.htm
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Old 10th Apr 2017, 6:25 pm   #2
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Default Re: Fidelity CB3000FM CB under repair...

Update on this: Apart from some tweaks here and there the only real problem with this was the output transistor (2SC2166). I subsequently found that it had already been diagnosed as faulty and changed by the owner, so the part fitted when it was passed to me was a new part, perfectly correctly fitted.

The only difference between the part I subsequently fitted and the part he had fitted was that his was recently bought from a fairly long established component source in Ireland, and the one I fitted was one from my original 1980s hoard of never-used components.

Changing the output transistor on this particular radio is not routine, much more difficult than most because there are soldered RF shields on the underside of the PCB and tinplate boxes (with lids) over the component side of the transmitter, then to make it even more difficult the sloping front panel overhangs the transmitter strip and has to be dismounted and pulled away from the PCB to allow access, ideally without snapping the numerous wires going to the front panel pots, indicators and so on... so, having fitted a device which did work, I didn't feel like pulling the whole thing apart again to try another 'modern' device which might not work.

What I plan to do is take a known working more 'normal', standard mobile radio with easy access to the RF PA transistor and try one of these 'modern' 2SC2166s in that to see if they actually work. They check out in DC terms - they would pass on a transistor tester.
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Old 11th Apr 2017, 11:05 pm   #3
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Default Re: Fidelity CB3000FM CB under repair...

I'm surprised that this set uses a 2166 as these are normally found as the driver for the 1969 in 'multimode' type rigs. I would have expected to find a 1307 in this particular set, so are you sure it's been fitted with the original type? The circuit you linked to doesn't seem to indicate the transistor types. I'll have to have a look to see if I've got a similar circuit somewhere that may indicate transistor types. Having said all that, I always thought that the 2166 was virtually the same as the 1307 and the 2078 and would have used any of those three myself for the same job if pushed.

I've actually just replaced a 2166 in a radio. I didn't have any new ones but managed to find a used one in my box of bits. Realising I didn't have a new spare 2166, I had noted that 'Knights' actually stock this transistor and was thinking of buying at least one the next time I pop over to the shop. However, now you've sown the seeds of doubt about the 'new generation' 2SC2166 transistors, I'm wondering if the ones at Knights are newer dodgy ones or good older stock. I noted a while back that 'Cricklewoods' actually stock the 2SC1307 and they're noted to be a reliable supplier.

I've just fitted one of those cheap 1969 transistors from the second two that I bought in a radio and it works very well so I would think that all of that particular batch from that Chinese supplier are good. I seem to think that the same supplier sells the 1307, possibly in a pack with a 1306, but that doesn't necessarily mean that they will work as well (or work at all) as the 1969s. What looked like a stuck on label on those 1969s seems to be either part of the transistor case or is baked on, as a gentle pick with the scalpel blade wasn't going to remove it, which is probably a good thing as you wouldn't want it sliding off a hot transistor in a sticky mess. It could be that there was a last generation genuine batch of those transistors that were actually made like that, but I've never seen any that looked like those before.

Last edited by Techman; 11th Apr 2017 at 11:22 pm.
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Old 11th Apr 2017, 11:59 pm   #4
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Default Re: Fidelity CB3000FM CB under repair...

The 2166 was a pretty common choice in 'straight' 4 Watt FM radios and seems to be directly interchangeable with a few other RF PA transistors you will be familiar with, such as the 2SC1909.

I have a couple of interesting booklets here, bound compilations of a regular 'Rig Doctor' newsletter collectively entitled 'The Midnight Express'. On one page there is a handy table of interchangeable transistor types which among others suggests that the following parts

2SC1909
2SC1816
2SC1974
2SC2075
2SC2078
2SC2166

Are all directly swappable. The Cybernet 134 diagram also suggests that some of the above, including the 2166 and 1909, are interchangeable as well.

Unfortunately the diagram for the 3000 (link in earlier post) does not include part numbers for the semiconductors, but my colleague found a 2166 in the radio originally and replaced it with one. My gut feeling is that Knight's transistors will be OK because they are and always have been CB specialists. The suspect ones I've mentioned came from a large component stockist in Ireland: I doubt whether they would even have time to verify the authenticity of every component they stock.

Last edited by SiriusHardware; 12th Apr 2017 at 12:04 am.
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Old 12th Apr 2017, 1:28 am   #5
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Default Re: Fidelity CB3000FM CB under repair...

Yes, I found a 1909 as a driver in that Colonel radio that I picked up a while back. I probably never have had occasion to notice 2166 transistors particularly used in 'straight forties', perhaps because they didn't fail as much as some of the others, but more likely just coincidence.

The 2166 and 1969 that i replaced a few days ago didn't actually need replacing as it happened, but I was trying to track down a strange lack of transmission fault that occurred for just three seconds every 18 minutes, but it was a bit of an excuse to test the two transistors anyway - I did cure the fault in the end!

I've just been looking and I can't seem to find the Fidelity circuits that I thought I might have. While looking I noted an Amstrad CB900 using a 2166 final, an Audioline 341 using a 2029 and a Shogun using the 2075. I also found some of the 'Midnight Express' publications. I don't think I have the complete set and possibly not the one with the list you posted above, but it's years since I looked at them - they might even be available 'on-line' somewhere, although I haven't tried to do a search as yet.
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Old 12th Apr 2017, 8:19 am   #6
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Default Re: Fidelity CB3000FM CB under repair...

The 'Midnight Express' volumes I have here - two in total - were ring bound compilations sold in that format for a time by 'S J Tonks' who were for a time CB specialists in the way that Knights still seem to be now, offering CB books, spares, parts, accessories and add-ons and service information.

The transistor substitution list is in the bound volume called 'Midnight Express II'.

When the CB market became too small, Tonks (I believe) became 'Satcure' and moved to specialise in Satellite TV, which they may possibly still do if they are still around.
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