Mini-kits Amplifiers

[M0YDH]

Hello all
As a newcomer to ATV, I have to build the whole station. V1 Minitiouner and the Portsdown transmitter are up and running. Next is my 70cm amplifier from Mini-kits. Yesterday's tweet now on @BATConline showed the transistor module and bare pcb on the Pentium P4 heatsink and can per the instructions. I was happy to find this in the scrap PC under the bench.

So the unplanned activity is what to do with the fan. Who has a 3 wire fan control circuit (and software if applicable) that they would like to share please? It will have my first ever Thermistor on it. Fan says its for 12v and .17A.

In other news I have a six stage attenuator on the bench in construction. I saw it on the amazing website of S53MV as part of a DDS signal generator. The matching low pass filter from Mini-kits is to hand too.

73
David M0YDH

[g0mjw]

You can buy fan controllers for next to nothing on ebay - but maybe you just need to wire it directly to the 12V via a dropper resistor to make it run slower and account for the 13.8V vs 12 supply. You will need to experiment to work out what series resistance is best. Why do you want to use a thermistor? Best have the fan running all the time on TX.

Mike

[g8vpg]

David,
I have built the amplifier kit from Minikits and it works very well. Because it takes a fairly heavy, continuous bias current, then it does get quite hot. I leave my fans running continuously and this stops it from getting too hot. I have always run 12V computer fans from my 13.8V supply without any problems. I don't think you need to worry about dropping the extra 1.8V. I think the term 12V supply is fairly loose, implying supply from a 12V lead acid battery, which can be anything up to 14V.
73 Shaun.

[M0YDH]

Thanks Shaun. It's the first time that I have built an amplifier for above 30MHz. I built the HF Packer amp from a comprehensive kit and instructions before.

So I bring 13.8VDc into the case through a 10A quick blow fuse and branch to dpdt relay and fly back diode.

Forgive me but I am away from the circuit diagram. One service gets the full voltage direct and the other goes via the relay that's actuated by PTT from the Portsdown. This relay also turns the fan on full.
There's some voltage regulation on the circuit which at these currents is I guess is a DC-DC converter. Any good?

It's poor that for £89 (including low pass filtet) that you don't get a full typical circuit schematic - just long warnings about application of heat sink compound with which I have complied.

Thanks
David M0YDH

[g8vpg]

David,
The supply marked VDD is the high current supply to the amplifier. The supply marked VGG is the bias voltage input. The bias turns on the amplifier. With no bias, it will draw no current. When the bias is turned on, the amplifier will draw its quiescent current, which is about 3-4A. The bias input goes to a standard 5V regulator, which produces a stabilised rail to feed the bias adjust trimmer pot. The bias draws negligible current. They recommend 4.5V bias, but if you can, also monitor the current as you adjust this - I wouldn't go beyond 4A quiescent current. Before you connect the high current supply to make this setting, make sure the bias is set to zero, if it is at the other end of its travel, the amplifier will draw too much current and may be damaged. You should also make certain the input and output are terminated into 50 Ohm loads when you do this. I have built quite a few Minikits projects and they are always pretty well sorted designs. All the clever stuff is in the amplifier module and you should have no trouble with this. Make certain that you put an solder tag from each fixing screw for the module to the ground plane of the pcb. Poor earthing will cause unstable power output. Also, do not overtighten the fixing screws for the module, because this can stress the internal ceramic pcb and cause it to crack- new module £40!! Please ask here if there is anything you are uncertain about!
73 Shaun.

[M0YDH]

Hi Shaun.
Super answer. Would it be possible to put some pictures on the BATC Wiki together with wiring up the missing parts e.g. a schematic?
The only complete build that I can see is https://vk3yy.wordpress.com/2013/04/16/ ... amplifier/
I have several relays marked Axicom V23105-A5303-A201. Are they any use? Or what should I use instead?
73
David M0YDH

[M0YDH]

Hello all
I think I've reached a successful conclusion to the amplifier build. As there was no advice elsewhere to expand on the instructions I made a film of the steps I took and the results. I'd added an arbitary 20dB attentuator stage in place of the 0ohms link per the instructions so from circa 100mW out of the FT817ND through the attenuation and out of the amp at 400mW seems satisfactory for now.

The film is here:-

https://www.youtube.com/watch?v=RoLburL23VA

Any obvious errors?
Hopefully our club's new analyser will have leads, pad and adaptors after National Hamfest shopping and I can complete setup. I'm making a G4KLB controller for the PE4032 programmable attenuator that's on the slow boat from China. I hope to then see spectrum from the Portsdown and find the level of drive.

Any recommended cases or enclosures for these amps when mounted on old computer heatsinks like mine?

73
David M0YDH

[M0DHP]

Hello David

Nice build! Did you reach a conclusion about what enclosure to use? And how deep roughly is your heatsink from the plate where you've fitted the module and PCB, back to the fan?

Thanks and 73
Ray M0DHP

[g0nzo]

Excuse me hyjacking this thread....

Does anyone have any words on how good the internal LPF is on these modules?
I have a 2mtr and 70cm version of these and about to start assembling. Wondering how clean the o/p from the bare modules are?

I do have a very nice 2mtr helical BPF, all tuned up for the receive side. But this is quite lossy. So would rather keep it in the RX line after the LNA, and not use it at the antenna.

Also, what sort of drive do people use on these modules, for clean DVB-S ?
I have about 6dBm from the portsdown, and wondering is I am going to need another stage?

Jules

[g0mjw]

Excuse me hyjacking this thread....

Does anyone have any words on how good the internal LPF is on these modules?
I have a 2mtr and 70cm version of these and about to start assembling. Wondering how clean the o/p from the bare modules are
Jules

Hi Jules,

Their filtering is not quite good enough. You might consider glancing at the device data sheet it you can find time. The 60W 2m module for example presents this graph.

Capture.PNG (75.61 KiB) Viewed 5739 times

As you can see, it's the second harmonic that is only about 50dB down. The table at the start indicated -35dBc which is much worse, but perhaps that's at maximum output. If you use the 43 + 10 log P rule of thumb, at 17 dBW (50W) you need another 10 dB. Of course if you are running it at 5W then chances are, the harmonics will be low enough but its good practice to put a low pass filter on the output, a simple 3 or 5 pole LC filter ought to be sufficient and would be pretty low loss.

All the ones I have built have had more gain than the minimum and provide full output with much less than 50mW when biased up for DATV use Given you will be running at only 10% or so of maximum output I don't think you will need another gain stage from the Portsdown, more likely some attenuation, but you will need some more gain from the Pluto.

I did some tests on the 2m, 70cm and 23cm modules. The 2m 30W module was very poor indeed but got a lot better when driven through an isolator, so I think the input matching is important. On 2m you can get 3W, maybe 5W at a push from the 60W module. That's because you need to have -50dB shoulders. On 70cm and 23cm where the shoulders can be a bit higher while still remaining in the amateur band you can get 10W from an RA60H4047M1 and about 5W from the RA30H4047M1 https://flic.kr/p/s9kqu9 and 3W from RA18H1213G with -45dB to -50dB shoulders- or perhaps more depending on your conscience and regard for other band users.

Mike

PS - You will need a lot of heatsinking. I originally use the 30W 70cm module but have upgraded it to a 60W one. Wondering what to do with the old 30W module.