A cheap VHF radio, worth the effort?

[Andrew B]

I picked this MAXIM multiband up in a skip and wodered if it was worth a lot of effort to set up / repair.

It is 3 band AM/FM covering 26-28 Mhz (CB), 54-108 MHz (FM) and 108-176 MHz (AIR), i suspect the 26-28MHz is the AM portion.

It consists of 6 Transistors, a TA7640P IF strip and an AN7112E audio amp. It seems insensitive on the FM/Air band, it pulls in FM stations but is horrendously noisy, is this noise a function of it's cheapness or may thre be something worth dragging the signal generator out for?

[SiriusHardware]

I can't really advise whether this is worth saving except as a curiosity (CB band is a relatively uncommon feature). You are right that it will be the AM detector and not the FM one which is engaged in CB mode, this model would have been aimed at the USA market originally.

For strong local signals you will be able to resolve FM CB signals by tuning about 2-3Khz off frequency, which will result in 'slope detection' of the FM signal. Crude, it does work, but of course intentionally tuning off-signal also weakens it, so that's why it is only good for strongish signals, especially given that the set is only likely to have a short telescopic aerial.

The other thing you could do is add a BFO at the I.F frequency, you might be able to receive 10M/11M CW/SSB signals that way although you would really need an extended aerial, I think. Since you have a sig-gen you could test this quickly by using the gen to inject a BFO signal.

Just now it's a little bit too early for the usual spring / summer sporadic-E activity on 10/11, the band should get a bit more interesting from mid-May onwards.

[Andrew B]

Thanks Sirius, it does have a "107M5" crystal filter, so I assume it's dual conversion 10.7/455. Everything worth seeing is gooped in wax, sponge rubber in the coils (and gooped in wax). I can easily attach an external antenna as the original rubber duck unscrews and has an M6 thread. I can't identify the RF amp or OSC transistors but probably common NPN silicon . It just seems that with a nice Toshiba IF amp/det chip the radio MIGHT be capable of decent 2M FM, but will probably have little selectivity, so I could listen to all of the 2M band at the same time.

[electronicskip]

Certainly worth a bash if you have some spare time .
Unusual set up , do you have a picture of the front?

[TonyDuell]

Wouldn't 'Air Band' be likely to be AM too?

[SiriusHardware]

You haven't mentioned whether it has AM / FM narrow / FM wide modes freely selectable.

I would have thought that would be necessary if it is designed to tune FM broadcast (WFM, 88-108), and VHF low PMR (NFM, 54-88) and VHF high PMR (NFM, ~140-176) 2m), and airband (AM, ~108-140).

It would have to have switching between those modes either manually or automatically by band, like a scanner. If mode selection is auto then it probably does switch to AM on the CB band, but maybe you can arrange to manually switch the mode with a small slide switch.

[Andrew B]

I'll get a warm air gun and get rid of the wax, then try some better photos. I will take a look at the switch, it's a 3 position sliding band switch, so probably accomodates AM/FM switching. The PCB is phenolic single sided "brown" board and if I drop the board out, I'll be able to see how the switch / IF strip are connected. No makers name on any of the RF/IF cans, it probably does have switched RF amps / Osc coils and uses a variable capacitor for tuning. Apart from the ceramic 10.7MHz IF filter, then I can't imagine that it has much in the way of bandwith control, maybe 110KHz 3db "filter"?? Vol control and switch are "iffy", I don't know how long China post takes these days.

[Terry_VK5TM]

Unless you need to get in and repair something (or do some mods) under the wax, I would leave it where it is.

Those circuits get very microphonic when you remove the wax.

[andrewn]

I've got a very similar radio but it doesn't have a makers name on it. Its model DE-379-A and I was given it about 20 years ago. Its missing its aerial but I screw in one from a RC car transmitter. The PCB looks very similar but the band switch is on the edge of the board. It also has TA7640P and 6 transistors. I've just powered it up and the volume control was so bad I had to spray it with servisol. With my radio it sounds like its FM on all three bands, the hiss is loud and doesn't reduce when switching to air band. I was able to hear a local amateur on 2M and broadcast stations come in well although the tuning is a bit tricky as the scale is compressed.

[Andrew B]

Thanks andrewn, I haven't done anything further to mine. I think the 6 transistors are an RF amp and an OSC for each band, i have the ta7640ap datasheet and appears that AM/FM is switch on pin 3, but audio quality is so rough that it impossible to tell the difference, hopefully i will tune in to local radio club net on 6M FM GB3WY, i'm going o keep toying with it

[Andrew B]

I decided that probably the IF filter (unknown ceramic) is very broadband to accommodate broadcast FM stations, so as a first trial modification I have ordered a 10.7 MHz 15KHz B/W filter and see if that gets rid of some of the noise. I will probably use the radio for the 6/4/2 metre bands, tuning stations may be more problematic

[Pfraser]

I had one of these. Mine was badged as Academy.
https://www.ebay.co.uk/itm/Academy-R.../293538777190?

I wouldn't expect too much in terms of bandwidth switching. Mine certainly wasn't very selective on Airband or 2m. It was functional for the stronger signals on the FM broadcast band.

The FM mode remained engaged for Airband, so tuning was...interesting! I had the impression it was a domestic radio circuit with expanded tuning, much like the earlier 'Joysonic' sets which were all-transistor.

Good luck with your experiments!

[Andrew B]

Hi Pfraser, looks almost the same, just enough gubbins inside to do the job. I will pull the board out later and examine how the bands are switched, maybe pull the filter and found out if it's IF amp noise or mixer/front end noise. It has a squelch control that seems to do very little. I will foto the front and post it, also some better quality ones of the "gubbins"