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Old 1st Mar 2018, 5:36 pm   #1
Jon_G4MDC
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Default Swan 100MX, 102BX etc Solid State Power Amps

Does anyone have any experience of the above please?
I have several sets and they share the same 100W Push Pull Power Amp using a pair of MRF458 in the final stage. The design dates from around 1978.

It seems to come in two versions which differ in the early driver stages which boost preselector output (about 0dBm - guess) to suitable level to drive the push pull stages. 2xMRF533 then 2x MRF458. I believe in both cases the transistor used as the last single ended driver is 2N3553.

I have memories of using these radios and finding it very hard to control output power. The gain of the PA assy rises significantly with heatsink temperature. For example start at 50W when cold and after 1-2minutes TX power has risen to full output and ALC comes into operation. I never like running like that, one hand for the mic or key the other to ride the drive control.

I have only the early schematic using an MPSA10 and 2N3866 to drive the 2N3553. Does anyone know what that became in the later version? I seem to recall it might have been the 2N3553 alone but I could be wrong.

I'm curious to know which stage is responsible for the increase in gain. It would be even better if there is a known mod to cure it.

Looking for previous Swan stuff in search turned up only the 500C, kettles and teasmaids.

Last edited by Jon_G4MDC; 1st Mar 2018 at 5:51 pm.
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Old 1st Mar 2018, 6:09 pm   #2
G6Tanuki
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Default Re: Swan 100MX, 102BX etc Solid State Power Amps

Not familiar with the kit you speak of but it sounds to me as if the problem might be drift in the bias regulators (regulators not adequately thermally-coupled to the output devices).

Swan stuff was also sold under the "Astro" and "Cubic" brands so it may be worth searching for your model numbers and those names as well as just Swan.
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Old 2nd Mar 2018, 10:03 am   #3
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Default Re: Swan 100MX, 102BX etc Solid State Power Amps

Thanks G6T, yes I know about Astro and Cubic.

One day I promised myself I would try to get to the bottom of it - that could be 20years ago. Haven't made a start yet

It could of course be bias, but I do know the 2N3553 runs damn hot.
In one it lives in a heatsink about 1" dia, don't touch that one with your finger, you will lose your fingerprints. In the other design I think it is in a gripping finger type heatsink bolted to the main heatsink with an insulation kit. I need to find another PA of that type to be certain. It makes me think maybe they knew something.

As the weather is the way it is I thought I would see who knows what before venturing to the shed to play with it.
Another few years won't make much difference...
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Old 2nd Mar 2018, 10:53 pm   #4
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Default Re: Swan 100MX, 102BX etc Solid State Power Amps

A stroke of luck to find the other Swan 100W PA circuit on-line today - in a manual for the Swan Astro 102 BX.

I was wrong about it being a 2N3553 as the single ended driver.
It is a 2N2866 but mounted as I described in the foregoing...

It looks like the 100MX version of the PA probably had more gain than the 102 version.

There are some reports that the 100MX went unstable on 10m, maybe this was an attempt to fix that. There are quite a few differences. They exported some of the low level stages back to the exciter board instead.

I doubt then that these two are interchangeable. The biasing circuits also have significant differences. As I have used rigs with both types I am not sure one is superior to the other. Both had drifting gain as I recall.

I have a Swan 102 which had a duff original PA. I substituted the 20W Cirkit kit PA - that was great. The rig was mainly for TopBand at the time and so 20W PEP was fine. I could set the drive and forget it.

This thread was partly about putting that right one day. Still too cold for any practical stuff... I will post the two circuits when I have time.

Last edited by Jon_G4MDC; 2nd Mar 2018 at 11:16 pm.
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Old 3rd Mar 2018, 10:04 am   #5
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Default Re: Swan 100MX, 102BX etc Solid State Power Amps

Interesting that they changed the design; the substitution or redesign of kit using older HF transistors is always a bit of a 'suck it and see' game: while the MRF-series are generally quite predictable some of the earlier HF power bipolars are rather more fun to deal with, specially if you have a difference in pinout pattern/lead routing as well as different electrical characteristics to consider. "redesign" part-way through a production-run, to fix issues that show up in-service, is rather common in HF solid-state gear.

[The Atlas 210/215 radios (intriguingly, made by a company founded by one of Swan's founders!) come to mind: I doubt there are *any* of those left still running their original CTC CD2545 output transistors]

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Old 3rd Mar 2018, 12:50 pm   #6
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Default Re: Swan 100MX, 102BX etc Solid State Power Amps

Quite. I looked at the PA of the Atlas 210 and found it interesting that only the output stage was push pull. I have another set which is Southcom, the third company in the trinity. It is 20W PEP and uses the Atlas 210 driver transistor in a push pull arrangement as the output stage.

I managed to blow up my 100MX PA killing one of the pair of MRF458s. Not a difficult fix but an expensive one at the time. I have another around here somewhere without the driver transistors. Recently G-QRP started selling MRF433 for good prices so I bought a few. That is another round-tuit job which I was going to combine with investigating the gain drift with temperature.

Still not managed to get the two PA schematics in a format where I can post them. Will do that in the next day or so.
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Old 3rd Mar 2018, 7:10 pm   #7
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Default Re: Swan 100MX, 102BX etc Solid State Power Amps

Here are the 2 schematics.
The first one is the 100MX, the second (simpler) one is the 102BX.

Strange that the 102BX design appears to have no mean of adjusting the bias. The PA Bias line is taken to ground on TX by a 2N5223 on the distribution board. That is where TX/RX and VOX / Break In are controlled.
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Old 8th Mar 2018, 7:07 pm   #8
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Default Re: Swan 100MX, 102BX etc Solid State Power Amps

Now I have found the varmint...
Missing TIP42 in the bias switch is the first thing to fix...(top right corner)
This one is the simpler 102 Variant.
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Old 9th Mar 2018, 10:23 am   #9
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Default Re: Swan 100MX, 102BX etc Solid State Power Amps

The "forward-biased diode" approach [CR101 and CR102] is quite common in these sorts of amplifiers - it does however depend on the diodes being very well thermally-coupled to the amplifier-transistors.

A far better approach uses a couple of power-transistors in the regulator, which provide a much lower source-ompedance for the bias network and - if done properly - can actually be deliberately non-linear so the bias falls-away slightly faster with rising-temperature, so minimising the chance of thermal runaway.

In the past when designing semiconductor HF PA stages I've had specially made-up copper blocks milled for the heatsinks, with counterbores in them into which a number of TO-5-cased temperature-esnsing transistors were pressed so the thermal-contact was really good. The Motorola "Application Notes" for the MRF-series transistors are a good place to look.

[Thankfully I could always achieve the required thermal-stability for my amplifiers using lots of simple copper/brass/aluminium, rather than having to get into the frightening area of machined Beryllium-Oxide coolers]
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Old 9th Mar 2018, 6:03 pm   #10
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Default Re: Swan 100MX, 102BX etc Solid State Power Amps

That is the funny thing Swan did here. They put the diodes miles away from the devices.

A very strange thing to do - what were they thinking? I have highlighted them in the attached.

I too have been down the road you describe on other PAs, LDMOS, GaAs etc.
No GaN so far though! Have done battle with CCS1 and BeO blocks isolating HT from heatsink - Pye T55AM etc.
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Old 16th Apr 2018, 7:50 pm   #11
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Default Re: Swan 100MX, 102BX etc Solid State Power Amps

I found some more time to look at these again.
Not investigated the circuit or component positions, but I did check the heatsink.

It was milled with the leg of a chair!
All the RF power transistor positions had at best 50% contact due to milling tramlines.
You could cake it up with heatsink grease but that would only make you feel better about it rather than do
any real good.

I have lapped them with wet&dry.
Pics may follow - but I have lost them for the moment.

And I forgot - the junk box turned up a TIP42.
This only lacks time for testing now.

Last edited by Jon_G4MDC; 16th Apr 2018 at 7:57 pm.
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