Marconi TF2167 RF power amp 10W

[gramofiend]

Hi. I have a Marconi rf amp which is rather unwell with blown fuses and shorted transistors and I cannot find any information about it or, more importantly, a circuit diagram.
Has anyone out there got any information on this beasty!

Mike

[Chaparal]

Here ARE some specs

Peter

[trh01uk]

Mike

I hope someone turns up some useful info on this beast. I've got one too, but no manual.

Richard

[wd40addict]

'Made in USA' is a bit odd for a Marconi product. I wonder if they bought it in from somebody like Amplifier Research?

[karesz*]

Hallo,
I found thes cyber picture over S-l225 RF Communications Model 805 Amplifier 0.05-80 MHz 47dB Gain what I belive to be the exact same RF-Amplifier , but I dont become some details over that...

[gramofiend]

Since posting I have been in contact with an old friend from my former work place who suggested I try www.telford-electronics.co.uk. I did and they have it.
So Chaparal and trh01uk that was my source for a full manual.
Job done and thanks to all who helped.

Mike

[trh01uk]

Mike,

so did you buy their one and only original copy, or did they sell you a copy of their original?

Richard

[trh01uk]

Originally Posted by karesz*

Hallo,
I found thes cyber picture over S-l225 RF Communications Model 805 Amplifier 0.05-80 MHz 47dB Gain what I belive to be the exact same RF-Amplifier , but I dont become some details over that...

Yes, I agree with you judging by the photo. Unfortunately manuals for that model are just as sparse as they are for the Marconi derivative. Tucker Electronics list one but I wouldn't buy it without knowing precisely what mods Marconi made to the original product.

Richard

[WME_bill]

Marconi TF2167 manual April 1977, with mod sheet post serial 582, April 1979..
If you are still having problems getting the manual, I have one of these units and also the manual. It appears to use more or less standard transistors apart from the two output power devices, which are Motorola PT9704 or Philips BLX94A. The intermediate transistors, type PT9703 are close to Philips BLV30 or 31. There is a note about berylium which warns do not file or cut this material, as the dust or vapour is damaging to the lungs.
There is a caution in the manual that both the power rails are floating above ground.
I attach the power supply circuits, for April 1977 and slightly modified for April 1979. wme_bill. m0wpn

[Radio Wrangler]

I think PT-prefixed RF power transistors were originally TRW parts which later moved to Motorola and appeared in Motorola data books.

David

[Jon_G4MDC]

Agree David. PT was TRW.
Wracking my brain - what's left - for the manufacturers of wideband high power amps like this.

I had one at the salt mine - 250W I think - up to 150MHz but maybe not at full output.

They once had an office in Welwyn Garden City - where I live. I guess it was just representation not tech/support.
On inspecting our dead unit they had a wideband ClassA stage good for a few Watts output and they built heroic amounts of them, all the same, between wideband combiner networks to get the power output.

[G0HZU_JMR]

I googled the datasheet for the Philips BLX94A and these parts appear to be available from ASI in the USA, presumably as a copy/clone of the original device.

If you get stuck trying to find original replacements we sometimes deal with ASI at my place of work. They make copies of obsolete power devices, eg bipolar, LDMOS etc and the Philips BLX94A is on their list of parts they can manufacture.

I can't promise anything but I have email access to one of the main members of staff there via my works email.

I looked on ebay and there are some of these BLX94A devices for sale but they might not be genuine. Even the ASI parts from the USA aren't strictly genuine and will come with a certain amount of technical risk if you expect a drop in part.

The TF2167 looks like a very useful amplifier. Wouldn't mind one of these myself...

[WME_bill]

TF2167. manual / circuit.
I have been looking inside my version as the power supply is giving trouble.
It differs somewhat from the manual which I have, which uses two Motorola PT4574=MRF511 or 2N5947. And three PT9703 = MRF321. And two PT9704 ~=MRF323 or Philips BLX94A. All ceramic flat pack on stud with 4 connections
Incidentally data sheets for these are all on the usual DataSheetArchive.
But my version uses two groups of three 2N5109 (all T039 style), apparently instead of the PT4574 and PT9703, and other parts seem different. Apart from the power supply happily.
Does any one else have a manual with circuits which use 2N5109 in the main amplifier, and another additional PCB for ?overload protection. I suspect a later variation.
I would be very grateful for a copy please.
wme_bill

[G0HZU_JMR]

I bought Bill's TF2167 amp last week as this looks to be a very useful piece of kit (according to the specs)

I've not tried to power it up via the mains as it has a PSU fault so I powered it from an external 25V lab PSU with a 3A current limit. The good news is that it seems to work and it works pretty much as per the specs although I've only run it up briefly. The cooling fan is mains powered so I have to watch to see if it gets too hot. But the frequency response and the linearity is very impressive. It takes about 2.5A from +25V and all stages look to be using active bias. So it always draws 2.5A from the PSU with or without a 10W output.

See below for an IP3 test at 14MHz. I'm using a decent (8GHz high power lab) 40dB attenuator externally so the top of the screen is approx 40dBm (10W) so the PEP is 10W in the test below. The IMD terms are well suppressed so this amplifier can be used for a variety of test waveforms up to 10W PEP.

If I run it at 10W cw it gives very low harmonic distortion although I need to do more tests on it. I plan to take it apart over Christmas and check that all the stages are biased and behaving properly. I'll also have a go at repairing the PSU although I think I'm going to have to replace all the mains wiring. It looks old and feels a bit brittle. But I'm very pleased with it and this amplifier will hopefully prove to be a useful piece of test gear as it covers 50kHz to 80MHz at 10W PEP and has 47dB gain so only needs a sniff of drive from a typical sig gen to get it to full power.

[Skywave]

TF2167: many years ago, I had one of these. Unfortunately, a few bead tantalums went s/c and destroyed the O/P transistors and their drivers. Replacement devices could not be found, so regretfully it was dismantled for salvage. I still have the mains transformer and the outer case, should anyone need them.

Al.

[G0HZU_JMR]

I had a quick go through the PSU system this evening and found several issues. I've stuck a new mains plug on it and also the mains 115/230 voltage selector switch was dirty and intermittent. The output 2N3055 looks to be blown so I'll have to order one. I would recommend that owners of the same variant as mine should check the mains selector switch as per the service manual procedure with an ohmmeter. By following this procedure I was able to spot the dodgy switch and clean it.

The PSU is a slightly odd design, I simulated it on SPICE to make sure I understood how it works and along the way I found several issues in the PSU documentation wrt incorrect resistor values, a resistor missing from the schematic and an NPN described as a PNP in the BOM. There are quite a few changes between the schematic, the BOM and also what I have in my PSU. My PSU PCB looks clean so I think it is original but I'm not impressed with the documentation or the way this thing is wired up. It reminds me of the early Thurlby PL320 PSUs.

But if I put the resistor values from my PSU into the simulation it does work so I'll have a go at checking out the real thing once I get a new 2N3055.

One other thing... I set the simulation up to measure Pdiss in the various devices and the main 2N3055 does seem to dissipate a lot of power. If the raw PSU is 40V and the output is 25V then the heat in the 2N3055 is nearly 40W. The heatsink is a reasonable one (1.5degC/W??)but it isn't directly in the airflow from the fan. The Tj to case for the 3055 is 1.5degC/W

So if the box got to 50deg internally then the Tj could get quite hot. I suspect that the PSU has been designed the way it has so the 3055 can bolt direct to the heatsink with a smear of thermal grease. So no mica washer needed as the 2N3055 collector is at chassis ground. This will save on an extra thermal gradient that would otherwise appear if there was a mica washer.

[G0HZU_JMR]

I had a look at the PSU today and it turned out to have several faults/issues. However, I've managed to get the regulator board and the pass transistors to regulate correctly into a dummy resistive load and so I think I've got the PSU working. But I need to order some correctly rated parts before I can test it out properly with the amp as the load. At the minute I'm using parts that are a bit marginal on the voltage rating.

It turned out that there were several intermittent connections in the PSU and a dead tant cap. Each time I thought I'd fixed it the PSU was still unreliable. In the images below you can see that one intermittent fault was PNP transistor VT1 on the reg board. The collector leg connection looks to have corroded away over time. It was probably just touching although the photo shows that there was a lot of corrosion so I think this has been a poor connection for a long time.

In the other photo I've arrowed where VT1 is on the board and also the arrow shows a tant cap that had failed and also another intermittent connection.
Looking inside the unit there are several crimped pin connections that look to have had some sort of repair many years ago and some black sleeving put over the connection. The arrowed connection with the black sleeve had gone intermittent and once I removed the sleeve it was obvious that someone had tried to solder the wire to the top of the (crimp) pin many years ago. But today, there was just a corroded mush where once there was solder.

I'm going to replace all the PCB pins and pin connections with a soldered wire/eyelet to try and get this PSU to be more reliable over time. Each time I thought I'd repaired it the reg would fail if I tapped the board or wiggled any wires. I'm also going to replace all three transistors on the reg board and try and improve the various connections and hopefully the PSU will be more reliable over time.

[geeoboeh2s]

I remember this from back in the 1980s.
I believe that it was Marconi/Sanders, hence the made in USA.

Reliability was terrible. Let one of our young engineers even look at it and it was away for repair. I suspect that the output stage was prone to destructive oscillation.

Chris

[G0HZU_JMR]

Yes, the build quality is pretty grim inside. I'm tempted to convert mine to external +24V DC operation and swap the fan for a 24V DC fan because I'm scared that the PSU will fail again and maybe damage the amplifier.

Also, most of the time I won't need 10W so I will try and use it with a 3dB or 6dB attenuator on the output as much as possible. Hopefully this will keep the PA stage stable and reasonably reliable.

[G0HZU_JMR]

I've been doing some soak testing of this unit now that the PSU has been repaired and I've found a few things that may be of interest or concern.

The first soak test failed with a hot smell from the PSU area and this turned out to be the fuseholder for the 25V 3A supply. This uses a couple of metal springy contacts that you press the fuse into. But it was getting hot. Very hot! Over 120degC on the thermal camera within a minute from a cold start and probably much hotter than this on that first soak test.

I think the extra ESR in the fuseholder was in the rivet contacts for the solder tags. These have corroded over time and the I^2*R losses rocketed. I need to make a new fuseholder for it but for now I've soldered the fuse inline. It now runs without any hotspots in the fuseholder.

The other thing to note is that the PSU will run very hot unless you run it with all the chassis covers on. It absolutely relies on the residual 'turbulence' from the fan to keep the PSU cool even though it looks like the fan can't get any air to the PSU area as there is the PA section directly in the way. If the PSU is run with the covers off the heatsink quickly climbs to 85degC and will probably keep rising towards 100degC if you left it long enough. Scary stuff!

But it does manage to get some 'turbulent' air to the PSU heatsink with the covers on and the PSU heatsink runs at a steady 56degC on the thermal camera with the covers on. I use a dab of thermal compound as a thermal marker for the camera to measure and I slide back the cover briefly to take a reading.

I've fitted a 2N3055H to this PSU as it has a 200degC junction temp spec as opposed to 150degC for a standard 3055. The PSU has a very slow loop so no worries fitting the slightly slower 3055H here. I've also used thermal grease between the PSU heatsink and the chassis baseplate to try and distribute heat away from the heatsink.

There's (40V-25V) x 2.6A in the 2N3055/heatsink to dissipate = approx 40W.

If there's 40W to dissipate and 1.5degC/W J/case then if the heatsink ever gets to 90degC and there is maybe a 10deg gradient in the thermal grease the the Tj inside the 3055 could easily hit 90 + (1.5x40) + 10 = 160degC which is beyond the 150degC limit. I've got some extra margin with the 2N3055H but you definitely don't want to run the PSU for more than a few minutes with the covers off.

So far it's survived two long sessions running a 2 tone test at 10W PEP and the PSU heatsink is typically at 56degC with the covers on. However, I can't help thinking that the unit would benefit from being run from a variac as this would reduce the 40V to 25V voltage drop across the 2N3055 in the PSU.