Looking for Hotair TX35FM info

[Kdw32]

Hi does anyone know anything about a mono Hotair TX32FM? I have given one and I can't find any info on it online. Apart from Future FM used one in the early 90s there's a picture of the rear of there's with a 50w sticker.

There's a flat blade adjustable Freq pot under the audio knob is that for selecting your FM mhz or something else?

There's a Pre EMP switch which I presume is to stop Freq drifting when device is hot or cold ??

The unit came with a brick PSU and v long coax cable, no Ariel.






If anyone can help with any info or point me in the right direction, manual pdf etc. I would be most grateful. Thanks for reading.

[King Croccy]

Wow nice item, I'm not sure on this but I've seen a few similiar units on Ebay and one of the sellers mentioned Roger (originally broadcast warehouse but sadly deceased now).

"Pre Emp" is for audio pre emphasis. In the UK you need 50uS pre emphasis on your audio. If you get that neat little beast working, it will sound much better with it on!

[shuffy]

If anyone can help with any info or point me in the right direction, manual pdf etc. I would be most grateful. Thanks for reading.

Yeah, there's more info a little closer to home, I expect others will chip in. See the "SONIA" on the PCB, that should jog a few memories.

viewtopic.php?t=2117&start=80#p26318

[Kdw32]

Thank You both. This is amazing info, really grateful. I'll take a better photo of the board and put it that other thread

The plan is to get it up and running. I have a mate who can put it a work bench and 'run a dummy load'.

[Kdw32]

Decided to power it up quickly. Couldn't see any bulgy caps or anything. The PSU had to have a new fuse and plug rewire due to loose wires.

Yellow + - 75 KHz flashed on off very quickly then steady green rf light.

[shuffy]

It's nice and clean inside, a good example of the era. Shouldn't be too hard to add a PLL should you be inclined. Please tell me you had some kind of load on the output when you fired it up?

[Frequent Lee]

Steve who designed and built these is active in a group on Facebook.

https://www.facebook.com/groups/1106378 ... ?ref=share

He will be happy to see this is out in the wild again and will probably be prepared to answer any questions you have about it.

[Albert H]

That's awful - I'm really glad that I don't bother with FarceBook! Looking at the first couple of dozen posts on there, most of the posters are ignorant moron wannabes - and that's being generous!

The "Hotair" rigs were "at frequency", using a dual-gate FET oscillator (in a vain attempt to make the VFO stable). Much of the design was quite competent, but there are a few glaring errors. The capacitors in the VFO need to be NPO types for stability, and there needs to be thermal isolation between the exciter and the PA. The attraction of putting it all on one PCB means that they're quick and easy to throw together, but the performance will really suffer.

It always struck me as a bit silly that the builder of these efforts bothered to detect audio level, but just flashed an LED with the output of the detector. If he'd pointed the LED at an LDR, he could have had a crude audio limiter that would go some way to preventing over modulation!

The other silly thing is that he's gone to a lot of trouble to try to get stability, but didn't include a PLL. Back when these things were current, a simple four or five chip PLL would have cost about £4.50 - £5 in parts, and would have kept the thing on frequency. It would also have been sensible to screen off the output filter, to prevent RF coupling back into earlier stages of the rig. This is a real beginner mistake, and is trivially easy to fix. It will cause the rig to be less stable than it should be, and would run the risk of generating all sorts of additional products - probably on very inconvenient frequencies!

[radionortheast]

yeah I thought this didn’t have a pll when I looked at, if the rf amplifier is on when your trying to retune it with the trimmer on the front I don’t think it would be good.

[Shedbuilt]

The capacitors in the VFO need to be NPO types for stability,

In fairness, with VFOs in general (not talking about this design in particular), I always found that using all NPO capacitors, would always result in significant downward drift, with increasing temperature. I put this down to the inductors (especially ferrite cored inductors), having positive thermal coefficient, device internal capacitances increasing with temperature, and probably reactances of the PCB. I played with this a lot, and always found that at least some NTC capacitors were needed for balance; sometimes used to swap for aluminium cored inductors too. Yes, PLL was a lot easier, and cheap to implement.

[Albert H]

many years ago, just for the sake of amusement, Roger, Myers, Pyers, Nige, AB and I all built VFOs to see who could achieve the best stability. The joint winners were me and Myers. We both used similar VFO circuits, based on dual-gate FETs. I used temperature compensation which made use of the change in Vf with temperature across a diode and varied the varicap bias. Myers did something similar, but used a thermister. My VFO moved about 4 kHz at 100 MHz with a temperature swing from 0C to 30C. Myers got very similar results. The others all drifted by several 10s of kHz!

After lengthy discussion over a few beers, we all decided that simple PLLs won out every time! When you consider that a 4 MHz crystal and four cheap 74HC CMOS ICs cost less than £3 at the time, there was no reason whatsoever to have drifting VFO rigs on the air at all.

Later on, I used the MC145151 (an all-in-one PLL) and an MB3500 prescaler for a cheap, easily programmed PLL with a low noise dual op-amp for the loop filter. Again, the component cost was negligible, and I made literally hundreds of those simple exciters - many of which are still in use!

When the prescaler ICs became harder to get, I moved to the 5511 PLL with a PIC to load its data at power-on. It's a really good PLL IC, and relatively cheap. Unlike many of the rig builders, I used a single crystal to clock both the 5511 and the PIC. I used a ½f oscillator with a diode doubler to eliminate the risk of RF feedback in single-box designs.

[Kdw32]

It's nice and clean inside, a good example of the era. Shouldn't be too hard to add a PLL should you be inclined. Please tell me you had some kind of load on the output when you fired it up?

Hiya thanks for reply. yes borrowed a circular heatsink dongle thing rated 50 ohm 50w. Have since found out that was 50w peak I had it all on for less than a minute don't think I've done any damage. Will be buying a higher rated one.

[Kdw32]

Everyone who is replying this is amazing, thank you for sharing this knowledge it's giving me a lot to Google . I'll give the FB group ago and see what happens. I have decent knowledge of pro audio but this new to me. I have played on pirates in the past. One thing I'm not sure about with this hot air box is if it is fixed Freq with a adjustable pot mark Freq for a fine tune or is that pot for going between 87 and 108 ?

Is it possible to upgrade it and have a pll fitted in it?

Thanks for bearing with me.

[shuffy]

From what I can see, it's an on-frequency VFO tuned by an inductor. I can't see what's going on on the PCB under that large pot however since no-one in their right mind would have a reactive component of the oscillator that far from the actual oscillator, what I would imagine is happening is, there's a small preset pot connected back to the varicap's bias network which will be able to trim the frequency a little bit. It looks like there's a long PCB track heading back in that direction. Therefore it's "coarse" tuning with the inductor core and "fine" with the twiddler under the large pot.

As I mentioned earlier yes it's possible for someone with the relevant experience to add a PLL to this and back in the era that this rig comes from, people were coming up with add-on circuits to achieve exactly this. Here is an example of such a design:

pira.cz/enpll.htm

This uses the SAA1057, designed for receivers and obviously ancient so you might struggle to get one (I haven't used one of these for at least 20 years but if you can get one, they will work on-frequency at 100MHz and be suitable for your rig). If you want to build something like this with a different PLL, I'd recommend trying to get an older through-hole device like one of the Motorola MC14517* or a TSA5511, which I still see people on here talking about. On the same website I linked above, there's an exciter design using this. Most modern PLL ICs are not just surface mount but extremely tiny surface mount so going old-school will not only be easier to wrangle as a one-off, but won't grate against the aesthetic of your rig either

What I would then do is add a second varicap network on the input of the oscillator transistor, connected to the PLL output (yes I know you could drive the bias on existing varicap but I wouldn't do that personally) and a "sniffer" connection from the VFO output to the PLL input via a (very) small capacitor. I guess Jan will have covered the rest of it in his various articles on pira.cz.

[shuffy]

Looks like I messed up the link above, sorry. I'm sure I used to be able to edit posts too... anyway. Try this
https://pira.cz/enpll.htm

[Albert H]

Shuffy's right - that's the way to go. I'd also add screening to screen off the output filter from the rest of the board, and I'd use a small metal box for the PLL to make sure the digits don't get into the VFO. The difference in stability with these two mods will be amazing - it'll stay on frequency irrespective of temperature changes, and the output will be cleaner because there's less chance of the crud the filter should be removing getting coupled back into earlier stages.

You need to get in touch with a competent rig builder, who should be able to add the PLL easily.

[Kdw32]

Shuffy and Albert H thanks so much for taking the time to help. I will be having the mods done hopefully in April and post some dates.

[radium98]

can you post it is diagram , i want to see how the 75khz and the peak is done .What determine that 75khz is okay

[Albert H]

It's not worth worrying about - the level detector is just looking at the instantaneous modulation voltage, not measuring the deviation! The deviation sensitivity will vary across the band, so if the level indicator is right at 88 MHz, it will be completely wrong at 107 MHz!

[radium98]

Thanks Albert , true because , a freind who have a deviation meter in old day i tried to align an exciter board with 1khz fro 75 and was surprised while playing with dip switches on other fr that the deviation is lower ,and i still ask why ? the exciter was a DB sintel 80