Biscuit Tin Transmitter from 90s Pirate Documentary

[teckniqs]

biscuit tin rig 1996.png

biscuit tin rig 1996 (2).png

A few of you might be very familiar with this old 1990s biscuit tin transmitter as demonstrated by Barry Maxwell of the DTi.

....I know the pictures aren't the best but it's the clearest shots I could get, does anyone know what is going on inside this box?

There appears to be two seperate boards, the top one appears to be some kind of PLL circuit but I'm confused if it's for a Band I or III link receiver or for the exciter which appears to be located in the middle with the RF stages on a seperate board at the bottom.
I have stared at it for a good couple of minutes but I can't work out of the crystal PLL at the top is for RX or TX but I think it's just a PLL TX but I'm not 100% sure.

....And what the hell is that big black square thing at the top, it looks like a bridge rectifier but I don't think it is???

I am wondering if the whole thing is a link transmitter as it looks to be very low powered, I can only see what looks like a 2N4427/3866 as the output stage but I can't see any PL259 or similar sockets on the outside of the box.

Someone on here might even know who it belonged to, is it a PLL low powered TX designed to drive a larger power amp not shown in the video?

If anyone has any information or educated guesses please feel welcome to post them below.

(It was originally shown on a 1996 documentary but you can see a clearer shot on of it on more recently released documentary below)

[Bton-FM]

It’s marked with the initials ‘A.R’ maybe that is a clue to who built it?

[teckniqs]

Yes I did notice that and A.R doesn't ring any bells.

....But it could also stand for anything like Alice's Restaurant, Angel Radio. (who knows?)

[RF-Head]

It's looks like a B111 or even a 500MHz receiver with signal relay and a B1 VFO driverboard
Maybe a old midpoint

[duncanorak]

The top board is definitely a link RX, you can see the 3189 in the top left with the IF coil, choke and 10.7MHz filter and the audio out via the axial electrolytic. The part of the circuit in the top right is the local oscillator, you can see the crystal. In the bottom right is the tuned front end with 1.5 or 2 turns coils, with either a 10pf or 22pf trimmer, so either band 3 or 4. The big black thing is most likely a relay for carrier access driven from the 3819.

[teckniqs]

It's looks like a B111 or even a 500MHz receiver with signal relay and a B1 VFO driverboard
Maybe a old midpoint

Yes I had a feeling it might have been a midpoint of some kind and could have been band 3. It looks like there is a BNC socket for the receive antenna soldered onto the board.

....I love all the older 90s equipment and it's interesting to see there was a PLL crystal being used inside.

Back in 1996 most of the London FM stations were still using VFO transmitters.

[teckniqs]

The top board is definitely a link RX, you can see the 3189 in the top left with the IF coil, choke and 10.7MHz filter and the audio out via the axial electrolytic. The part of the circuit in the top right is the local oscillator, you can see the crystal. In the bottom right is the tuned front end with 1.5 or 2 turns coils, with either a 10pf or 22pf trimmer, so either band 3 or 4. The big black thing is most likely a relay for carrier access driven from the 3819.

Yeah I thought it had to be something like this, I have never seen anything like this before and I didn't think it was band 1 which I'm used to seeing from looking at it, because I could see a few differences to it suggestt it might be a band 3 receiver not band 1.

...But looking at the bottom it does look like a very basic VFO but you can't really see it properly on the video at the angle it's quickly shown at.

[Albert H]

Back in 1996 most of the London FM stations were still using VFO transmitters.

Not the ones I dealt with! I can't imagine how many PLL sub-boards I built back in those days. The PLLs were the four-IC diode-programmed CMOS job, with the input through a 74HC4024 for <70 MHz, and through a SAB6456 prescaler for anything higher. The PLL was clean and quiet enough for use in link receivers, transmitter strips and all the rest. They all used 4MHz reference rocks, sometimes with ovens around them for extreme stability. I started using PLLs in my gear in 1978!

I remember on set-up we did with three Band IV frequencies used, with two midpoints (the dealers who operated the station were particularly paranoid). That system had seven PLL boards! The funny thing was that when they finally did get the knock on the door it was because of their DJs smoking dope in the flat, nothing to do with the station at all!

I bought a huge number of folded aluminium boxes from H L Smith in Edgware Road, and most of the rigs and receivers went into those. Later, I bought a set of bending bars, and used to fold my own cases - very convenient for some of the non-standard sizes I wanted.

Thinking back, we seldom used blowers - they're always noisy (drawing unwanted attention to the location of the rig) and prone to clogging up with muck (the top of blocks was seldom clean!). I used to design for the biggest heatsinks that I could find, and bought a lot of very long aluminium extrusion from Redpoint - this got cut up into the various sizes that I needed.

You have to remember that in those days, the noise floor was much lower, and it was possible to get a good signal heard over a wide area with much lower power than today. Some of the most widely heard stations in the 80s (for example) only ran 40 or 50 Watts, but they used really good sites, properly constructed and installed aerials, and clean rigs. There was one block up near to Forest Hill in South London which had a really commanding view of the city - I used it for years, because I was the only one with a key for the roof access (apart from the caretaker). Fire Brigade access was by smashing a glass panel and operating a recessed door release - the pirates never smashed their way in, so my gear was always secure up there, and was left for months, with tone access control. I don't think that I ever ran more than 90 Watts up there, but the stations were always heard over a huge area.

Just about the last installation I did up a block was in '92, and the rig - without a blower - was designed to be installed into a lift shaft, where there was plenty of rapidly circulating air. It was the first of my 200W rigs that I built for use in the UK, and the station guys using it were convinced that it wouldn't survive without a blower. That rig stayed in use every weekend for almost three years!

I did a few dodgy things way back - burying rigs in walls, loading up a window frame in a tank room for a link transmit aerial, hiding gear in fire hydrant boxes, or even behind bulkhead "security" lamps (a great power source), and even screwing a rig to the underside of a water tank. However, I never built a rig in a biscuit tin or used a frying pan for a heatsink!

[teckniqs]

Not the ones I dealt with! I can't imagine how many PLL sub-boards I built back in those days.

Yeah back in the mid 90s there was about 5 or 6 (well known) builders in London but only one or two of them were doing PLL back then.

....I still remember for example Vibes FM locking on to both 93.85 and 93.90 back in 1999/2000 sort of time so there was still 'big' stations using bloody VFOs back then but a lot of stations by that time were starting to use PLL.

And from what I've been told there was still stations in the Birmingham area continuing to use VFO TX's for many years after.

[nrgkits.nz]

Eventually Gotts found that rig screwed to the underside of the water tank, he simply unbolted it and flooded the top three floors of the block.

[Albert H]

I remember one station (no names, but I'm looking at you, AKG) that started on 92.45MHz, and as the rig and the cupboard it was in warmed up, it went up to 93.1MHz over a day or so! Listeners in those days were used to tuning their analogue radios around to find their favourite station.

I didn't want to put expensive specially cut crystals or a heap of ICs inside a cheap link receiver, so most of my Band III and Band I jobs had AFC circuits that would compensate for the drift of the receiver's local oscillator. If the link rig had a PLL, the receiver would stay locked to it forever. The receivers in those days used the CA3089, a single-gate J310 FET for the local oscillator, a J310 input amplifier, a 3SK88 or BF981 mixer and a couple of BF494s to match the IF filters. The AFC control voltage came from the '3089, and the local oscillator had a varicap to tweak it on to the right frequency.

There was also space on the board for a relay (to switch the main rig on), and this could be driven in three ways - carrier detection, sub-audio tone (using an NE567 tone decoder chip) or a sub-audio pulse to toggle on / off (and NE567 and a 4013 bistable) - there were tracks on the board for all three relay control options.

I had a few hundred boards etched, and used to have a pile of them mostly populated, and would just insert the front-end and oscillator coils and a couple of caps when the link frequency was known. Those receivers cost about £6 in parts, and we used to sell them to the numpties for £60 - 90 a time.

The Band IV and V boards were similar, but were always crystal or PLL locked. They used etched Lecher Lines for the input stage filtering and local oscillator, and there were a couple of variants of the board. One version was designed to be cut in half, providing a transmitter on one part and the corresponding receiver on the other. You could leave them attached to each other if you wanted a midpoint!

The earliest versions with PLL used the separate PLL PCB, in its own little tinplate box to prevent the digital noise from the logic getting into the receiver and affecting its sensitivity. Later ones, using the TSA5511 and LMX series had the logic on the board - the PIC would just load the data into the PLL IC at start up, and then shut down, leaving the PLL IC to use the data in its registers.

About 18 months ago, I visited a commercial radio station that was still using one of my Band V links after over 30 years! They were weighing up the purchase of a new Band II transmitter since the output valve for the ones they had was getting seriously expensive!