5W Stentor Transmitter question...

[MiXiN]

Alright necks,

Just got a 5W Stentor kit.

The kit is complete with all parts, however, I think it's supplied with Metal film Resistors as opposed to Carbon film which is what I thought should be used at RF.

Can these metal film resistors be used, or is my limited knowledge correct about "only" using Carbon resistors at anything above HF?

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[shuffy]

The resistors are fine. What you've probably been told is, don't use inductive resistors in RF circuits. This is especially true for dummy loads, which historically would have meant carbon as opposed to wire wound. So maybe that's where you're getting "carbon" from!

[teckniqs]

The metal resistors are fine, it's the wire wound ones you want to keep away from.

[MiXiN]

Thank you guys.

It's assembled now minus the inductors, but will report back sometime this weekend when I've completed it.

The kit came supplied with an undrilllled heatsink and Chinese MRF237, so I've replaced it (MRF237) with a genuine one from 90s - but apart from that it's a nice quality board.

[Undercover Neckz]

[yellowbeard]

Some people drive classic cars - if he went by you in a Duisenburg you'd be jealerous right? I liked the stentor - it was great in the 80's.

[Albert H]

Don't knock the Stentor! 5 real Watts from just three transistors! Carefully aligned (for maximum), these things were fairly clean. The only thing wrong was the thermal drift!

[MiXiN]

I've assembled it, and although the Oscillator & predriver (2N2219) are running fine, for the life of me I can't get the final stage amplifier (MRF237) going at all.

I've not melted any trimmer capacitors, there's no solder bridges, my patch leads are fine, SWR/Power meter is fine, as is the dummyload.

I've tried the several MRF237s now all to no avail; emitter to ground, base to the 68r Resistor, etc'..

The first part of the circuit is running fine which is the main bit, but I just can't get the final working at all...

[shorty]

I'm sure a lot more people would have got one of the stentors back in the 80's if they'd been advertised in the back of the electronic mags instead of the panteks, smart kits and the other crap kits available at the time, the stentor looks good for an 80's kit, the youtube video i've seen of the stentor the frequency looks stable enough compared to what most of us were trying to get on air with back then.

I think most of us when firsting starting out went through the same steps with the same crappy kits that didn't live upto expectation after the first test, even as a kid without test equipment i knew they were dirty and unstable.

If i'd known were to get hold of a stentor back then i'd have given my right arm for one, when you used to phone around the surplus electronic/radio shops back then to see if they had anything you may aswell have been asking to buy a kilo of coke, it seemed to be a dark art clouded in mystery that most people thought so bad they didn't want to get involved with, funny old world when you look back at the past.

[teckniqs]

I've assembled it, and although the Oscillator & predriver (2N2219) are running fine, for the life of me I can't get the final stage amplifier (MRF237) going at all.

I've not melted any trimmer capacitors, there's no solder bridges, my patch leads are fine, SWR/Power meter is fine, as is the dummyload.

I've tried the several MRF237s now all to no avail; emitter to ground, base to the 68r Resistor, etc'..

The first part of the circuit is running fine which is the main bit, but I just can't get the final working at all...

Sounds like you could have the 237 connected the wrong way around.

[RF-Head]

@Mixin,
This board needs 2 times 12V
fist one is for the osc and driverstage
second one is for the amplifierstage with the MRF237
In the 80s you power up your exciter a hour before you go on air and when the osc was on temparature you put the power on the final stage
Maybe this is your problem if not yes check the connections of your MRF237

[MiXiN]

Thanks for the replies necks.

Both the oscillator/predriver & final stages have 13.8V connected, and I've tried several MRF237 both ways.

I'm led to believe the MRF237 is "grounded Emitter" suggesting that the Emitter is electrically connected to the Transistor's can. However, when testing the Transistors I'm finding that the Collector is connected to the can but the emitter isn't!

I'm going to substitute the MRF237 for a 2N3924 and see how it goes as I'm wondering if my Transistor's are really fake even though I thought they were legit'.

I've looked at images of "real" MRF237s, and by looking underneath at the pins, it looks like the Collector is connected to the can.

The heatsink on this is also connected to ground, so if the Transistor is in it, it will short as it stands.

[RF-Head]

A MRF237 is 100% emitor to ground.
if not you have fake MRF237s
I have found the original stentor board with papers in my father storage yesterday and will upload some picture of teh original 80s stentor board

[Albert H]

All the MRF237s and SD1127s I have here are emitter to can. I have several brands of each. They all have differing Hfe but all have exactly the same pin-out.

[MiXiN]

A MRF237 is 100% emitor to ground.
if not you have fake MRF237s
I have found the original stentor board with papers in my father storage yesterday and will upload some picture of teh original 80s stentor board

Thank you RF-HEAD..

So, if I use a multimeter on continuity test and don't get continuity (a beep) from Emitter to case, but get continuity from Collector to case these aren't right?

As said before, I've used the MRF237 in the proper position and it doesn't work (Emitter to emitter, base to base, Collector to collector on PCB), and I've also tried it reversed with Emitter to collector, base to base, and Collector to emitter on PCB - no positions work.

I'll put a 2n3924 in its position or 2n3553 and if they amplify - I must have counterfeit MRF237s.

I'll report back soon.

[MiXiN]

All the MRF237s and SD1127s I have here are emitter to can. I have several brands of each. They all have differing Hfe but all have exactly the same pin-out.

A continuity test has been performed on all of my MRF237s, and there's no "short" from Emitter to the can - only Collector to can.

Was told by a UK supplier that these are new old stock, and even the date codes look legit; 23rd week of 1989 (8923 on can). Gold leads, same dimensions, etc'..

Pissed off isn't the word..

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[shorty]

I got some fake mrf237 and 2sc1947's aswell but the can and leads were silver, the cans are not as tall as the originals, i was able to get a refund from the seller, i'd bought from him several times before and those were genuine, so either he got conned and got a bad batch or he knew he was selling fakes as after he'd refunded me he carried on selling the same transistors.

The fake mrf237's i have k0230 on them and are the same size as the fake 2sc1947's

Did your mrf237's come from a seller in Wales?

Fake on the left..........Genuine on right

[sinus trouble]

Hello Mr Mixin!
My Mrf237 are identical to yours and work perfectly!
Ignore datasheets and pin tab markers! they are wrong!
The pin that is welded to the case is your emmiter! (connect to ground regardless)
The pin marked with tab is collector!
Central pin is base!!

Good luck sir!!

[shorty]

Thank you Sinus, i have just tried one of the mrf237's as you described and they work, it fired up and i'm getting just short of 5 watts, the tab indicates collector on these so the it's the same pin out as the sd1127. I thought i'd got fakes that didn't work, i can't test the 2sc1947's they went straight in the bin.

Thanks again Sinus.

[sinus trouble]

No worries Mr Shorty!
I had the same concerns, i have no idea how or why the info has gone wrong??
Realising the device would short out, i went with my instinct and ignored the data!