Houston, I got a problem.

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QRZ-934
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Houston, I got a problem.

Post by QRZ-934 »

Ok, so I received my Loop antenna for 934mhz, looks to be well contructed but is pretty old, has never been assembled though.

Also in the box was a brand new old stock stabo prs 933/934 radio, in mint condition still in its box and packing.
Looks to have been made around 1983 i think so its knocking on a bit but looks the dogs danglies.
So After seeing what I had in the box, I assembled the power cord and microphone to the radio, checked the polarity to the power supply and switched on.
The first thing I noticed was I could not see the display, it didnt come on which isnt what should happen, the other stabo shows either the channel number, hi/lo block indicator or the prs channel codes, nothing showed up at all which was a tad disappointing.
I could hear the static on receive but no display, the channel selection button did work as did the beeper.
I then thought I saw the vaguest wisp of smoke coming from the front of the set, immediately I switched off.
That was most concerning to me, so I decided to remove the covers and see if anything obvious stood out.
Its been probably 25 years or more since I had my nose inside anything electronic and these radios are fairly compact with a lot of smd components so I was a little worried to say the least!
I couldnt see anything obvious on the pcb, no tracks burned, nothing shorted, the radio was keeping its secret.
Sniffing the pcb gave a clue, I could detect the smell of something that had become heated but not burned.
Hmmm, where to look?
Thankfully the circuit diagram provided some help.
I had noticed that a small ceramic coated inductor appeared to be discoloured behind the front panel, so I looked at the location on the diagram.
Its connected to pin 1 of a transistor array which drives the display and from there it jumps off onto a 470 uf 10 volt electrolytic capacitor, the other side of which goes to ground.
Tracing backwards on the diagram, shows that the inductor is fed by an 8 volt line that comes directly from an AN 7808 voltage regulator.
Establishing that the regulator is the source of the power for the IC I turned my attention to the capacitor.
Using a needle type multimeter, set to ohms range x10 I touched the leads and found a direct short. The diagram shows the capacitor in series to the power feed so it means the 8 volts is flowing directly to ground and turning the inductor into an electric heater.
A quick check of the capacitor in reverse showed the same result-shorted.
As you know, a quick and dirty check of electrolytics can be done using the charge/reverse charge method where the needle will flick up momentarily as the capacitor charges then drops back down to zero, same in reverse.
What I think has happened is over time the capacitor has broken down internally due to the electrolyte corroding the internal structure and allowing the short to occur.
Since the 8 volts has been directed to ground through the capacitor, it also means the transistor array has no voltage to drive the display, I do feel happy to have found the issue so quickly!

Im quite pleased with myself so far ( hopefully no other issues will be found) as its been a long time since I had anything to do with electronics of any kind.
I have ordered up a load of capacitors ( Rubycon from Japan) to replace all the other electrolytics inside the radio also. I shall also replace all the capaictors in the other Stabo radio too as its of a similar age.
I will report my findings when I have replaced the defective component!
Tomorrow, I may assemble the loop yagi and take a few pictures and post up the insides of the stabo and the offending component! :D
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Re: Houston, I got a problem.

Post by Yamanx »

Wow good luck with it, I hope you've got a friend local with a similar radio :)
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Re: Houston, I got a problem.

Post by Oggy »

Thanks for sharing this about your 934 MHz radio......I have a few old ones myself
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Re: Houston, I got a problem.

Post by cb4ever104 »

Might as well change all those electrolytics while you're in there .
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QRZ-934
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Re: Houston, I got a problem.

Post by QRZ-934 »

Thanks for the replies guys.
Oggy, yes Ive got a couple of deltas, jti npr900 handhelds and two of these stabos. Have to start a pirate net eh. ;)

Ok, capacitors arrived very fast indeed so first order of business was to release the suspect component from the board. Quite fiddly as its a through hole component but double sided with those little "vias" that join each side.
Using a very small tip and a desolder pump (not ideal, I need a proper desoldering station) I released the little bleeder.
Quick check with the multimeter showed it was shorted.
I actually took it apart and after unwinding 4 ish turns the short disappeared...could reuse it....er not! :)
The new component went it easily and was soldered in position.
After clipping the front panel back on and checking nothing was amiss, like trapped wires etc, I held my breath as the power went on.
Neednt have worried, I had a fully operational display and best of all no smoke! :)
Ive had a closer look at the diagram since and found another electrolytic thats even more critical than the failed item.
Its located on the 13.8volt feed directly from the switch and is again connected between the ground and power line, so if that ever shorts out its possibly going to destroy part of the pcb and let out lots of expensive magic smoke so that will get changed along with all the others before I dare apply power again.
Heres a couple of pictures inside the radio, I didnt get a chance to assemble the antenna today unfortunately.

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The diagram shows the capacitor that failed on the left arrow, followed by the choke that heated up, then over on the right the voltage regulator and finally the capacitor thats connected to 13.8volts that might do some nasty damage if it ever goes the same way as the other one.

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Failed item.

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Replaced item.

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And it works!

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Re: Houston, I got a problem.

Post by The Collector »

I've got the 3 934mhz mobile radios that the UK had back in the day. Need a Cybernet mic' with the funny plug though for one of them.

There was a Reftec 934 homebase on sale on ebay a few weeks back but didn't bother watching it to see how much it sold for.

There's also a foreign seller that makes aerials (co-linear?) for these types of radios for about £30ish on ebay every now and again.
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Re: Houston, I got a problem.

Post by QRZ-934 »

Hiya, thats a different mic plug from both of mine. The 20 ch deltas got the 4 pin plug and the 40 channel deltas got the 8 pin type with up/down on the top of the mic ( not original tho).

Ive got two of the co-linears, the sellers called stanislav, got them out to me very quickly.

Wouldnt mind one of the reftec mobiles, quite like the looks of them.
Do you listen in on 934 at all?
I think ill take the loop yagi out on a hill someplace and see what I can hear (besides all the digital phone crap), apparently still users in holland from time to time.
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Re: Houston, I got a problem.

Post by The Collector »

I've never fired them up into an aerial as I've got nothing that size, plus it'd be very naughty! ;)

I made a couple of typo's sorry, as I've got the 4 radios and it's the Commtel that needs a mic' with the odd plug on. I also need one for the Cybernet though which as you say, is a 4-pin. I think I'd drunk too much Pernod & black at the time...hic! :)

There's also an earlier Reftec than mine with a different switch and I think possibly the earlier 934 frequencies on it...but I'm certainly no expert on them.

The blue Uniace 400 on the second picture needs a new squelch control fitting so it's knobs are off it at the mo:
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Re: Houston, I got a problem.

Post by QRZ-934 »

Wow, you got a Uniace! Very nice indeed, and the Reftec. :) Not many people know that the commtel has a cousin for the japanese market, identical in every way except for the frequency which in Japan is 902mhz iirc, That radio is called the Magnum 8300 TRU-1.
Theres one currently for sale here, was going to have a go at it but might be a bit expensive: https://www.jauce.com/auction/b258353258
As for attaching to an antenna and being "naughty", I see it this way, If loads of breakers hadnt been just as naughty on AM, cb wouldnt have happened here. ;)
Vive la Naughtiness!
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Re: Houston, I got a problem.

Post by The Collector »

Well I must admit I rarely use 27mhz, let alone go out of my way to sort an aerial for those radios.

They've got differing connectors on the rear too, just to make things more awkward. Who's daft idea was that?

I decided a few months back that as my 27/81 stuff had come to a standstill, that I'd keep a look out for some 934 stuff to collect. Luck was on my side on ebay and the Cybernet Delta was mine for a tenner on a B.I.N., the Reftec and Uniace 400 cost about £35 each (same seller, Uniace needs the new squelch knob putting in so got a discount on that) but the Kestrel NPR Commtel thing cost me £65 gulp.

They look cool though.
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Re: Houston, I got a problem.

Post by QRZ-934 »

Ok, finally replaced all the electrolytics in all the 934 mobiles.
I invested 80 quid in a desoldering station from CPC, duratool brand.
I have to report that it does everything its supposed to although I did get a blockage at one point which required a 2mm drill bit to clear the mass of solder. Likely my fault though as there was rather a lot in the collector and the vacuum had gone low due to the filter get obstructed, other than that, its made the jobs easy to do with no damage to the boards at all.

I had noticed that the stabo that had the faulty cap that lead to this thread tended to make a swooshing kind of sound when it had been on a few minutes, a kind of phasing sound out in the static.
Since the radio hadnt been opened prior to my ownership I reckoned it was likely a cap or two had drifted out of tolerance, seems I was right, after powering up again after replacement that noise has gone and the radio seems a tad more sensitive.
I also assembled the loop yagi and its currently hanging on the wall like a piece of modern art.

Intrigued by the constant noise on the band I took the delta out mobile and left it on scan.
Its quite obvious that the noise is digital in nature and definitely gets louder as you approach certain cell masts.
Up on a local hill the noise was much reduced, with channels 20+40 largely clear of it.
Back at base the co-linear once again picked up huge amounts of this garbage, but with the loop yagi in one piece im intending on taking a trip out to the sticks at some point up as high as I can ( probably cart all the gear in like a packhorse) and see what I can get whist aiming it in different directions.

I also aquired a nevada HRA 934L preamp to suit, all the advertising blurb from yesteryear prices these at around £145 new.
It does appear to make a good difference to received signals with the signal meter lighting up even on the quieter channels.
Me being me, I wanted to take a nose inside and so with care I managed to gain access to its insides. Its constructed by the famous japanese company Tokyo Hi-Power, with the model number found on the board inside reading as: Tokyo Hi-Power THP -171-1.
Ill put some photos up shortly as theres very little info regarding these kinds of items on the net at all, even 934mhz .com seems dead now.

After building a 10 watt dummy load for a couple of quid (im am NOT paying 35 plus for a resistor pack!) I could test out both the stabos on PRS mode.
They work a treat. Put the radio on channel 0 (cq channel) and just key the mike. The opposite radio locks on and they both tune to a clear channel. Considering the age of these radios Im very impressed with the way they do what they do.
I shall return! :)
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Re: Houston, I got a problem.

Post by FJF »

@QRZ-934, great job!!! I'm from Holland and I have 3 radio's, 2 are not working.
How to get that circuit diagram??? That is exactly what I need that diagram, I also have pre-amps and big real big amps 400Watts from the old ATF3 networks.
Last edited by FJF on 10 Jan 2022, 22:08, edited 1 time in total.
QRZ-934
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Re: Houston, I got a problem.

Post by QRZ-934 »

FJF wrote: 10 Jan 2022, 22:07 @QRZ-934, great job!!! I'm from Holland and I have 3 radio's, 2 are not working.
How to get that circuit diagram??? That is exactly what I need that diagram, I also have pre-amps and big real big amps 400Watts from the old ATF3 networks.
Hi sorry about the late response, check your PM's! :)
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Re: Houston, I got a problem.

Post by FJF »

Same here, pm
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