Mystery Tx. Anyone know if any good?

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3metrejim
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Mystery Tx. Anyone know if any good?

Post by 3metrejim » Wed Sep 05, 2018 12:41 am

I came by this TX that someone wanted repairing. Can anybody tell me if these are any good when working. It's in a bit of a state at the moment (if you look closely),as the final transistor MRF9180 had blown one of it's devices and gone short circuit through it's gate. Everything is disconnected from the amplifier so that the bias can be set up with a new transistor - I measured it at 4.2v once the transistor had been removed, seemed a little bit on the high side (unless someone had messed with it previously).

Is there anything I need to know about handling the new transistor? I've seen reference to soldering it in with a disconnected soldering iron. Is this necessary or is there another way to handle it like somehow shorting the leads until installed?

Technical details:

s-360-24 power supply
JPL995 driver
eBay 100W amplifier module

It's also the only TX that I've seen that has a fan that lights up blue(! :-o ) and blows air in, rather than sucking it out.

Here are a couple of pics - sorry about the bluriness...
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Re: Mystery Tx. Anyone know if any good?

Post by MC Spanner » Wed Sep 05, 2018 2:18 pm

Was there another pic, I can only see one on mine. What I can see of it doesn't really look like the work of any London builders I know ;-)

Presumably the FET which popped was the one supplied with the PA kit. Assuming the circuit was built correctly it's probably overbiasing that did it as he's forcing air onto it and only driving it with a Watt. I don't need to tell you to check the output side for shorts or breaks before you replace the FET and make sure you can actually vary the gate bias voltage smoothly from zero. Oh... and if that PSU has a preset, make sure the PA voltage is no more than 26V to start with, preferably less.

Every time I solder a PA FET in, I always think about the best way to do it! As far as I know I've never killed one when installing it. I usually follow my common sense (although I'd be the first to admit that I have none) and earth the board, bolt the FET in making sure the source is connected to ground. You could ground the gate pads on the PCB before you solder them - in fact I suppose you could ground the drain pads too if you're paranoid - and then solder the tabs down.

I've never heard of anyone soldering a FET in with a switched off soldering iron. If the tip of your iron is earthed, as it should be, then you should be OK.

Tempted to agree with you about Vgs in this case but the relationship between Vgs and Idq varies significantly for different FETs and different technologies. You should use Idq for setting the optimum bias point and only use Vgs as a rough guide for obtaining the recommended Idq. In my copy of the MRF9180 datasheet for example, there isn't even a graph relating Vgs to Idq! The implication seems to be that Idq for that device is typically between 1100mA and 1400mA, which is higher than most FETs I've used in VHF PAs of this sort of size. As this FET isn't really intended to be used at VHF, my gut feeling would be to err on the low side. Once you've replaced the FET, measuring the drain current and with no RF drive, start from zero Vgs until you get to about an Amp. Then apply RF. Good luck, if you haven't already done the job.

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Re: Mystery Tx. Anyone know if any good?

Post by 3metrejim » Wed Sep 05, 2018 2:52 pm

Thanks for your help MC Spanner. The unplugged soldering iron thing came from here: http://g4fev.atspace.com/mrf186linear.htm which was linked in the 'Thought I'd try one of these...' thread elsewhere on here.

I have the bias turned right down in preparation, the power supply measured at 21.9V (I make a note of everything before anything gets re-adjusted). The new FET is an MRF186 but I've read on here that they are a bit fragile. The linked article mentions 250ma per device, 500ma total so I will aim for that.

I was concerned about damaging the new FET with static or soldering iron leakage, as the device doesn't belong to me. I'm only used to working with the 'olde fashioned' and obsolete bi-polar power devices that didn't really have that problem, the only other FET I've (ab)used as a VHF amplifier was a VN66AF, quite some years ago.

Don't know why the other picture didn't come out, so here it is anyway:
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Re: Mystery Tx. Anyone know if any good?

Post by 3metrejim » Wed Sep 05, 2018 9:46 pm

I could do with a bit of help! I changed the damaged MRF9180 for what appeared to be a new MRF186.
What a nightmare! Had to remove the amplifier pcb from the heatsink to drill new mounting holes for the MRF186 as it's case was shorter. Once everything was put back together the MRF186 was soldered in, and I proceeded to set the bias current. Everything was going well until the current reached around 170mA then suddenly the current would increase to 800mA without warning. Turning the bias down doesn't reduce the current, but if turned down and then power is removed, the current is low again.

Can anyone enlighten me as to what I'm doing wrong?

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Re: Mystery Tx. Anyone know if any good?

Post by MC Spanner » Thu Sep 06, 2018 12:36 am

My first thought was it's the crappy bias pot they supply with those boards, but looks like you've got a proper multiturn on it and since the current didn't drop again when you backed off the bias, it sounds like a case of the dreaded self oscillations - turning the bias down once it's hooting doesn't fix matters because the oscillation is self sustaining once it starts. Have you done this procedure with the power meter on the output, or a spectrum analyser, just to confirm that's what's going on.

If it's self oscillating, it can be a pain to track down and fix the cause and no use me asking loads of questions because it's always something different depending on the individual application. These are painfully obvious/stupid points but make sure the exciter's powered down completely, you've bolted the PA board down properly again to the heatsink (i.e. not just the FET) and you're measuring the current on the PSU side of the supply choke, you've not changed anything near the FET drains to allow you to measure the current. Not trying to insult you by asking these. Also if there's a chance it's hooting, make sure you've got your dummy load connected to the output before going any further.

So, the rest of this assumes the problem is self oscillation. Ignoring for now what the cause is, it's possible that it's being kick started when the amplifier is operating at its least linear, when the bias voltage is low - you're starting the oscillation by turning the bias up slowly through the point where the FET first starts to conduct. Normally a FET is biased ABOVE this point from power up, so it's operating on a more stable part of its characteristic. If you can achieve this without it taking off then fine, but ideally you should be able to change the bias all the way without this happening, so something's not perfect.

When you turn the bias up and it starts to hoot, don't turn it down again, power off immediately and then power back up. If it's still obviously hooting (you'll see RF power out, nasties on the analyser and unexpected drain current), power off and turn the pot up a TINY bit and power up again. You don't want to overbias the FET so be careful when doing this. As a guide, my data sheet says the Idq will be 500mA at Vgs of around 4.4V and 1A at around 4.7V, but this varies per FET and anyway you might not be able to measure the gate voltage with the FET in circuit if it's oscillating. Don't let it draw more than 1A with no RF drive connected, whether it's oscillating or not!

If you can get to a point where it behaves at power up, start to increase Vgs again until you get to the theoretical optimum Idq (data sheet says 800mA but I've a vague memory of 500mA working for me with an MRF186).

If you can reliably power on at the optimal Idq without it hooting, you might get away with it but I think I'd want to find the cause of the oscillation. Hope that helps some, sorry if it's a bit muddled.

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Re: Mystery Tx. Anyone know if any good?

Post by MC Spanner » Thu Sep 06, 2018 12:44 am

3metrejim wrote:
Wed Sep 05, 2018 2:52 pm
the only other FET I've (ab)used as a VHF amplifier was a VN66AF, quite some years ago.
I've blown a few of those up, they used to fail quite spectacularly, usually launching grey plastic into the air like a firework. I used to use them on medium wave. Devices are far more robust these days, and now especially on HF you can get great results with all kinds of cheap MOSFETs and actually things are getting even better with SiC and GaN devices becoming cheaper and more widely available.

Back on topic then... on the 2nd pic, it looks like the coax coming from the exciter board is connected to the small board on top of the heatsink, (DC converter?) That's just an optical illusion right - the exciter is disconnected?

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Re: Mystery Tx. Anyone know if any good?

Post by 3metrejim » Thu Sep 06, 2018 1:32 am

Thanks MC Spanner. I tried your suggestions, but the amplifier is not behaving at all. In the end I got the spectrum analyser out (only a cheap AT5010) and the amplifier is indeed 'hooting' at around 247Mhz with it's harmonics.

I thought it could be meter lead resistance, but it was still the same when connected direct to the supply (seen on the analyser).

The exciter board coax had a 47 ohm resistor soldered across it just to load it while it was disconnected from the amplifier input. The small board on top of the heatsink is a buck converter for powering the cooling fan. I ended up removing power to the exciter, just in case it's output was somehow still getting into the amplifier.

Could I get the instability problem if the circuit ground is not too good? Ground comes through the case and then heatsink via a couple of small self tapping screws as well as the output co-ax cable outer (which almost looks a 1/4 wave at 247Mhz). The FET grounds to the heatsink via 2 more self tapping screws and the board by 2 similar screws, in diagonally opposite corners. I've never seen a power supply ground connected directly to the case either, which is what is done in this transmitter.

As I didn't put this thing together, and can't ask the original constructor for help (not knowing who it is), I may have to let the owner know that it's going to take some time to get working safely.

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Re: Mystery Tx. Anyone know if any good?

Post by Albert H » Thu Sep 06, 2018 3:47 am

The description of the flying grey plastic from the VN66s brings back memories of the same thing with my earliest FET MW efforts! I went through a whole heap of those things until I got a box of 2SK135s😁. With a bit of experimentation, I managed to get 18 Watts carrier, 75 Watts peak and that first successful rig is still in use almost 40 years later!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: Mystery Tx. Anyone know if any good?

Post by MC Spanner » Thu Sep 06, 2018 10:16 am

3metrejim wrote:
Thu Sep 06, 2018 1:32 am
Could I get the instability problem if the circuit ground is not too good?
Yes, definitely. It's really important to have a good ground, on PCBs and throughout the whole rig. These devices these days have stupid gain, right up to GHz in some cases, so any stray reactive components due to the layout of the PCB need stamping on, and that's why you'll see so much "stitching" on RF PCBs. I've done simple boards - one in particular involved an RD06 being driven by a "BFG" bipolar and and lost count of the number of iterations I had to do of the layout to get the thing unconditionally stable!

I don't like the way your board is connected to ground only at the corners, and from what you've said I'm guessing that the PCB around the FET isn't grounded properly as just the FET source is bolted to the heatsink in this area. If you look at some proper pallet amps, they usually come with the FETs bolted to a spreader under the PCB and then the PCB also bolted to the spreader in quite a few places, and loads of stitching all around. Google "WRF-350H" for an example. There's a good ground from the FET source to the rest of the PCB via the 4 screws holding the board to the spreader closest to the ends of the FET.

That Chinese board has a relatively poor top ground plane, and only 6 plated through holes for bolting down to the heatsink, none of which are that near the FET. So, what I did was to ensure a decent connection from the FET source to the PCB by passing at least one of the FET screws through a solder tag, and soldering the other end of the tag to the top ground plane of the PCB. On the chinese boards I've got here, there's a stitched through piece of ground plane at one end of the FET (bottom of the board in your picture) which is ideal and possibly there just for this purpose. If you don't get what I mean here let me know and I'll post a picture later on.

Looking at the second photo you're right, the ground could be better overall, he's used the case but it sounds like there's insufficient earthing around the PA. I always have a fat wire from the PA PCB back to the PSU to guarantee a proper DC path to ground - in this case, the pad right next to Vcc which it looks like he hasn't used. I don't like using RF earths for DC returns. I'd tidy up that positive supply wire away from the output transformer too.

So - sort out your ground, definitely make sure the FET source is connected to that ground plane on the PCB and try to bolt the board down through more of the plated through holes. I don't know if you've got the circuit for that PA, I think there's one on the net somewhere but check it and in particular check that all caps to earth are present and working - supply decoupling and decoupling at various points in the bias circuit. See how you get on with that lot.

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Re: Mystery Tx. Anyone know if any good?

Post by MC Spanner » Thu Sep 06, 2018 10:17 am

Albert H wrote:
Thu Sep 06, 2018 3:47 am
The description of the flying grey plastic from the VN66s brings back memories of the same thing with my earliest FET MW efforts! I went through a whole heap of those things until I got a box of 2SK135s😁. With a bit of experimentation, I managed to get 18 Watts carrier, 75 Watts peak and that first successful rig is still in use almost 40 years later!
Yes, there are some great devices around now and whilst they impove matters they can bring new challenges at VHF and higher - see above re: amplifier stability ;)

HF is different though. Class E PA's with PWM mod are so easy now and you can make them pretty much bombproof, something the VN66AF definitely wasn't! The only things I'm smoking these days are capacitors!

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Re: Mystery Tx. Anyone know if any good?

Post by s2000 » Thu Sep 06, 2018 2:31 pm

I am surprised you guys haven't worked out the "manufacturer" yet lol

Here's a clue.. :smoke

viewtopic.php?f=11&t=1757

All that is missing is six tons of glue and it will work perfect :lol:

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Re: Mystery Tx. Anyone know if any good?

Post by 3metrejim » Thu Sep 06, 2018 5:27 pm

Update:

Went through the entire amplifier looking for and repairing bad joints, added the solder tag ground on the fet. Found that the outer braid on the red co-ax matching transformer had been cut through almost all the way around so patched that up.

Looked back through my conversations with the owner and it came from ebay, seller unique electronics. Have seen that mentioned elsewhere as not the best place. Also that user 'woodstarr' (possible builder) mentioned these amplifiers can be unstable with no input, but its honestly the first time I've seen that happen.

Looks like the build quality maybe isn't that good after all, has a 'CE' sticker in there too but the mains cable is within a few cm of an unscreened power amplifier - surely RF will be blasting out along the mains cable if the amplifier was running.

Am I flogging a dead horse trying to get this working correctly? Also looks like I've managed to answer my original question about being any good - guess it could be better...
co-ax.jpg
Wiring close to amp.jpg
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Re: Mystery Tx. Anyone know if any good?

Post by 3metrejim » Thu Sep 06, 2018 5:28 pm

co-ax picture...
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Re: Mystery Tx. Anyone know if any good?

Post by MC Spanner » Thu Sep 06, 2018 6:29 pm

s2000 wrote:
Thu Sep 06, 2018 2:31 pm
I am surprised you guys haven't worked out the "manufacturer" yet lol
Well, I was suspicious...

viewtopic.php?f=11&t=516&start=40#p7193

I had spotted a blob of glue on the PA board, right where a screw should be holding it onto the heatsink! I don't like to slag people off though, I'm only one dodgy circuit board away from hypocrisy...
3metrejim wrote:
Thu Sep 06, 2018 5:27 pm
Am I flogging a dead horse trying to get this working correctly?
If your tidying up doesn't fix the hooting, then it's probably time for a rebuild. I purposely didn't say this again above, cos I've said it that many times... but here goes... I've never had one of these self oscillate!!

Something else that interests me about that board, is the 25Ω coax matching transformers. Supplied with the kit as new, it was always the coax with the white sheath. In fact that's the first Chinese board I've ever seen a photo of without the white coax. Wonder what happened there?

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Re: Mystery Tx. Anyone know if any good?

Post by 3metrejim » Thu Sep 06, 2018 7:46 pm

Maximus posted a picture of one with this clear sheathed co-ax (after only receiving one piece of white co-ax in his kit) viewtopic.php?f=11&t=516&start=120#p17959

You have a point though. The output cable and the two pieces used for the transformer look identical in every way, but would I be able to make out the size difference betweeen 25 ohm and 50 ohm small diameter co-ax cable just by looking? I guess that would cause problems if the wrong impedance cable was used.

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Re: Mystery Tx. Anyone know if any good?

Post by MC Spanner » Thu Sep 06, 2018 11:33 pm

Problems, yes - self oscillation, I'm not so sure. I wasn't suggesting it isn't 25Ω cable on your board, just wondering why it wasn't the white stuff. It's possible that the builder wasn't supplied with any coax to make the transformer and like Maximus, had to source his own. You're right, it is hard to tell by looking - I've got some 25Ω cable here that looks just like RG316 etc and nothing useful printed on it by way of identification. There's also 50Ω cable with white sleeve on it! I do now remember having seen Maximus' pictures by the way, so apologies!

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Re: Mystery Tx. Anyone know if any good?

Post by teckniqs » Fri Sep 07, 2018 10:10 pm

Image

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Re: Mystery Tx. Anyone know if any good?

Post by 3metrejim » Wed Sep 12, 2018 5:49 pm

Managed to cure the self-oscillation problem. Seems like simply connecting the input co-ax (without any drive) allowed the setting up of the bias current without the self-oscillation occurring. The amplifier must be so sensitive that output to input feedback must have been happening without the input co-ax connected. Maybe I shouldn't have been surprised with it's supposed 20dB gain. Still seems like it could do with some (more) damping on the input though.

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Re: Mystery Tx. Anyone know if any good?

Post by radium98 » Wed Sep 12, 2018 6:20 pm

3metrejim wrote:
Wed Sep 12, 2018 5:49 pm
Managed to cure the self-oscillation problem. Seems like simply connecting the input co-ax (without any drive) allowed the setting up of the bias current without the self-oscillation occurring. The amplifier must be so sensitive that output to input feedback must have been happening without the input co-ax connected. Maybe I shouldn't have been surprised with it's supposed 20dB gain. Still seems like it could do with some (more) damping on the input though.
thats good to hear that ,in my 15watts amplifier i did not managed to acheive where could be the self oscillation occured.

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