SkyArrow 250 CB Radio

[SiriusHardware]

So basically, it's an old-style 80 channel set with the 40 EU / CEPT / FCC channels 26.965 to 27.405 on 'Low' and another 450Khz block of channels immediately above that, 27.415 to 27.855. That is standard for 80-channel radios of a certain age.

You'll notice that there is a 20KHz jump between channels 3-4, 7-8, 11-12, 15-16 and 19-20. The frequencies 'jumped over' are the 27MHz radio control frequencies. The corresponding channels in the 41-80 block also have these odd jumps because the steps are built into the code table in the PLL IC. In earlier radios those occasional jumps were encoded in the channel switch.

There's also a bit of craziness around channels 22-25 - originally there were only 23 channels in the USA system (from which almost all worldwide CB systems copied their frequencies) and then when the additional frequencies were added on they first filled in a couple of gaps and then continued in 10Khz ascending order.

The 'UK' switch is doing nothing but pulling the channel frequencies ~3.75Khz lower than normal in the hope that some of the ones in the 'high' band land on UK channel frequencies with no actual relationship between the channel numbers on the display and the corresponding UK channel numbers. It's about as crude as it gets. It won't even cover the whole UK band, on which channel 40 is 27.99125 MHz.

[Sparky67]

Thanks very much for all the info. My only only experience of CB was in the early 1980s with 3rd harmonic interference to the emergency services’ 81MHz receivers, so you have nicely clarified what the mod to this radio is all about. Not exactly what I was expecting though! I also hadn’t realised just how many CB radio manufacturers and models there were until I looked at the Dutch web site linked above. Wow!

I will have another read through the Transmission1 forum info and then decide what to do with this set.

Thanks again.

Martin
G4NCE

[SiriusHardware]

The 'Low' band (only) on that radio is actually now legal to use in the UK on any of the modes AM, FM, SSB. The 40 CEPT / EU channels were added to the already existing 40 UK channels years ago, originally on FM only. AM and SSB were added as permitted modes quite recently.

Unfortunately any equipment used on that band (on any mode) has to be type approved, and of course the 'historic' (and often more interesting, repairable and collectable) old sets fall outside of that category.

There are older sets which have freely programmable PLL ICs (you could hypothetically feed them any binary code and generate any 10Khz stepped frequency within their operating bandwidth, within reason). In those cases the channel codes (including those jumps I mentioned) were hardware-encoded into the channel switch wafer. However, it was possible to break the parallel data between the channel switch and the PLL IC and insert an eprom to translate the codes output by the switch into a straight ascending run of 40 alternative codes. Using this method combined with the -3.75Khz 'pull' seen on your radio, it was actually possible to add the UK channels with the frequency and the channel display all correct on that older class of radio.

[Sparky67]

Thanks for the info. Makes sense.

The other curious thing about this radio is it doesn’t receive or transmit from a cold switch-on. It needs a few minutes for the synth area to warm up and then it starts. After which the Tx power slowly creeps up to over 4W. Never come across a slow-start radio before. It’s a shame there isn’t a legible copy of the circuit available as I would like to look into the reason for that... The carrier balance on SSB could do with looking at too...

Having said that I have decided to part with it as I can’t see me ever using it, so it’s on eBay as-is. Thanks again for all the advice and interest.

Cheers

Martin
G4NCE

[SiriusHardware]

At one time I would have offered to take it off your hands as it is an unusually small size for an all-mode radio from that period so quite quirky and interesting, but I just gave away about 6-7 of my radios to a colleague yesterday. I am trying to thin the collection down to only my absolute favourites, mostly ones which I have owned from new in the early 1980s.

To be honest that radio of yours sounds like it needs to be someone else's problem, no doubt it will eventually find its way into the hands of someone with the time and the patience (and a good enough circuit diagram) to get it going properly.

[Sparky67]

Just a quick update on this radio... It sold on eBay with the known warm-up issue and the buyer asked me to try to improve the AM mod depth, as it was quite low, and remove the FCC mod. Both a simple job. So I took it out of its case again, put it back on the bench, did the work - and it's now not transmitting or receiving, even after a lengthy warm-up. It makes all the right noises as the mode is changed and the channel display is working. The xtal involved in the mod is running OK. The PLL chip (LC7131) is running and the reference divider input is there, but the 1st VCO / mixer IC (TA7310P) output is very unstable and I don't see a lock output on the PLL pin. Pressing on the PCB gently, or sometimes even touching the case affects the 1st VCO freq and can stop it running, although there is no obvious dry joint area I can locate by tapping around the board and the xtal input stays solid. All the xtal VCO inputs seem to be running at the right freqs when switched between AM/FM and SSB. Frustratingly it does transmit OK from time to time, switch it off and back on - and it won't.

Then, when I was working on it last night, the audio output slowly reduced to zero... Without a service manual I'm thinking this is now going to be a bit of a challenge to get running. The buyer has been very decent about it, even sent me a legible copy of the cct, and I have refunded his payment. But I would like to get it working again if at all possible for him.

So, a thought - is there any known issue with wax affecting the life or performance of components buried in it, or soldered joints perhaps? The VCOs and their xtals have been liberally covered with the stuff for possibly 30 years plus...

Also, does a 1st mixer injection frequency stepping in 10kHz steps around 38MHz, and an IF of 11.275MHz sound right for a CB radio?

And any thoughts on what the VCO control voltage should be approximately mid-band please?

Cheers,

Martin

[SiriusHardware]

With so many things seemingly going weird on that radio it might be an idea to go back to fundamentals and check whether the constant regulated supply voltage is healthy and whether the switched regulated TX supply and switched regulated RX supply are OK.

A -general- answer to the question about the VCO control voltage is that it should be roughly about half of the PLL circuit supply voltage when the PLL is locked on the middle channel (19 or 20). This is so that it can shift upwards and downwards by equal amounts to reach channel 1 and channel 40 without getting close to the maximum or minimum control voltage limit.

The I.F frequency you state (and which I remember was scribbled on that diagram) is not the orthodox one for the LC7131 PLL when used in SSB radios, but you can probably verify it by looking at the frequency marked on the crystal filter.

I've never had a problem with wax except in my ears - the wax on the radio PCB is there for a purpose, it is to deaden vibrations and stop the VCO components from jolting around, otherwise the VCO would probably be very 'microphonic'. There certainly has been a problem with a sort of glue sometimes used to 'seal' trimmer capacitors, the glue often damages the capacitors.