RF PA transistor test rig?

[SiriusHardware]

I'm thinking really of CB RF PA transistors such as the 2SC1307, 2SC1969, 2SC1909,2SC2166, MRF475 etc. Testing them under DC conditions is one thing, but really they need to be tested under RF conditions and handling whatever continuous power they would be expected to be able to handle under in-application conditions.

Can anyone suggest or point to a source for a minimal circuit which could be used to do a basic RF test on these types of transistor? As is probably widely known, there is a big problem with fakes and even normally reputable suppliers selling them may not have the means to verify their authenticity, although most will probably accept the parts back for a refund if they are returned in the condition in which they were supplied - almost impossible if they have been soldered into an actual radio before being found to be defective.

For simplicity, assume for the time being that the (27MHz) input to the circuit will come from a stable or synthesised signal generator with variable-output level and the output will be to a dummy load and a power meter or spectrum analyser, whichever is available. Most likely the To220 or To126 device would have to be plugged into a single inline socket and have a temporary heatsink bolted to it. (Other suggestions for making solderless connections to the device under test would be welcome).

[Trevor]

For me get hold of an old cb set fit the device and test
used to do it all the time
Having said tha tI did not have to deal with the fakes plague
replaced 100,s in my time
Trev

[crackle]

Its interesting you have started this thread.
You remember the 2SA1012 I replaced in the SS 360FM, well I did a little DC test on it and it seemed to work OK, the largest resistor that would still turn the 3 watt test bulb on more or less fully, worked out at about 6.8k, so the current gain as far as my calcs went was about 14. I have not tried it back in the radio, still waiting to hear back from a couple of suppliers. You may remember the transistor did show up as working on my Chinese tester.

One other interesting thing I have learnt recently is RF power transistors have what I believe is a protection diode. I have noticed that a few that I have tested recently show as PNP one way round and if you reverse them they show up as NPN.
Can that be easily explained as a result of the protection diode.

I think Trev's idea of an old CB chassis is a good one, you could adapt it to be fitted with a socket for the transistor.

Mike

[SiriusHardware]

Problem there is that you will need the minimum of a 'muppet' box to test 4-5W PAs like the 2166 / 1909 / 1306 and a Multimode to test the 1969 / 1307 etc.

If by 'fitting' them Trev meant soldering them in, that creates a possible problem when trying to return the devices if they prove to be bad. How does the supplier know you didn't just blow it up by doing something stupid?

[Jon_G4MDC]

Many have desired this and at much higher frequencies (GHz?).

I think it doesn't exist except that if you are a big buyer of one particular device type the manufacturer will give or sell you a test fixture.
I could be proved wrong but....

I did go down this road with some crystal filters. They supplied a nice test fixture which had a very clever sneaky ground path.
It meant that their filters were self neutralising in the test fixture - wonderful measurements.

Put them in your real world box and they were rubbish. Names witheld to protect the charlatans.

[SiriusHardware]

Quite often in RF transistor databooks there would be a 'test bed' circuit including a PCB layout.

All that's really wanted here is a go / no go test circuit to prove that the device under investigation really is a suitably power rated RF device and not a rebranded audio transistor (which will pass on a DC transistor tester).

There is no need for the circuit to be wideband as these devices are typically used in 27MHz / 28MHz applications, so testing at a fixed frequency (say 27.500MHz) would be fine.

[crackle]

Regarding temporary connection to the transistor one of these would do it. Not sure where it came from, maybe a PC fan, I tend to cut these things off scrap items before throwing them away. Its a perfect tight fit.


Mike

[SiriusHardware]

Top tip!

Although, for RF use the leads would have to be extremely short. A compromise would be Trev's idea of using a working chassis combined with your idea of fitting a socket.

You could maybe drill a new hole higher up the heatsink so that the pins could be inserted into the (PCB mounted) socket without cutting them, and then the device bolted to the heatsink (with appropriate insulators) to keep it firmly in place and cool during testing.

[G6Tanuki]

I'd be worried about the inter-lead capacitance in the example above.

Testing RF transistors, like testing RF valves - you really need to be aware of the whole 'parasitic oscillation' issue. A RF power-transistor with good gain at 28MHz has really-loads-of-gain at 1500KHz, and will happily launch into oscillation - potentially destructively - if your test-circuit isn't designed to manage this.

Always remember: every gain-device aspires to one-day become an Oscillator.

[Mattylad]

Can you take the sockets out of the connector housing and solder them direct to a PCB?

[SiriusHardware]

The contacts probably only press on one side of the inserted pin, which is otherwise held close to the contact by the other (plastic) sides of the housing.

You can get single-inline PCB mount sockets although most of the ones I know are made to accept narrower (square) pins rather than flat ones like those found on a typical To220 device.

[Techman]

All of these transistors have the internal diode collector to emitter, that's why I always use an AVO for testing them.

I seem to think that the guy who sells the Eleflow transistors demonstrates testing them under working conditions using a CB radio chassis with the appropriate sockets on the board, so doesn't actually need to solder them into the circuit - as shown on his YouTube channel, although it's been a few years since I last viewed it.

I've found the Chinese fake 2SC1969 transistors seem to work. I've used several, all from the same supplier and had one failure, but it had done a couple of years or more of hard labour before giving up, so who knows!

I'm told the Eleflow ones are made in China, but again, whether that's true, who knows!

[Steve G4WCS]

2sc1969 application note here with a test circuit.

http://www.alphacron.de/download/hardware/2SC1969.pdf

or chop a PA strip out of a working CB.

if you could get one for the right price, the Belcom LS102L has a separate PA strip on the rear hetsink which couls be removed and used free standing

[SiriusHardware]

Well done Steve, can you also use your Google-Fu to find something similar for the lower power PA devices (2SC2078 / 2SC1909 / 2SC2166, etc)? I tried and failed.

I wouldn't normally consider breaking up a complete, especially working, unit for one particular part, but maybe that's just me.

[Steve G4WCS]

2sc2078 with test circuit https://www.digchip.com/datasheets/d...number=2SC2078

2sc2166 https://www.alldatasheet.com/datashe...I/2SC2166.html

failed on the 1909 but there is some data on the web. I just googled "2sc**** application note "

[SiriusHardware]

...And that's how it's done. All those devices are generally considered interchangeable in typical four watt CB radios so with any luck one test circuit would serve them all.