OK guys, good news, but bad temperatures.

I was going to use a 2N4427 for missing Transistor TR5 as suggested but have misplaced them so used a 2N3553 instead, and an 18pF Capacitor for C28.

I'm getting 24 Watts at 13.8V, but TR5 (2N3553) is too hot to touch for more than a few seconds, and the MRF240 is inadequately heatsinked for this sort of power. The heatsink is only about 7 x 1 x 1 inch - length, width, and height.

Apart from dropping the supply voltage, can anyone suggest a solution to excessively reducing the power and the high temperatures?

Here is the finished product, but I'm willing to alter things to get this running at a decent temperature.

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You need to use the black heatsink in the last photo for the 3553. What voltage are you running it at? If it's 13v8 try regulating the rest of the board to 12v or even 10v and keep the final stage at 13v8.

The piece of metal on the back is not a heatsink, it's used to transfer the heat from the MRF240 to a heatsink (or side of a box). I would buy a proper heatsink for it, or box it up.

teckniqs wrote:You need to use the black heatsink in the last photo for the 3553. What voltage are you running it at? If it's 13v8 try regulating the rest of the board to 12v or even 10v and keep the final stage at 13v8.

I tried using the black heatsink pictured, but there just wasn't enough clearance around TR5 (the 4 turn coil is the main culprit) to fit it flush. Pity, as those black heatsinks do a great job of cooling the finals on the NRG PLL boards.

I used a 13v8 10A power supply to run both the VFO and PA stage, but will use my variable supply on the next test and reduce the voltage to 12V to see what happens.

Maybe you could change the 4 turn coil to a small fixed inductor and then it might fit.

....Only other thing I can suggest is use a fan, which with all respect you should really be doing anyway.

Analyser wrote:The piece of metal on the back is not a heatsink, it's used to transfer the heat from the MRF240 to a heatsink (or side of a box). I would buy a proper heatsink for it, or box it up.

Doh, it goes to show that I'm a novice with things like this.

I just noticed there's 3 screws on the side of the aluminium to bolt it down to something more substantial, but failed to see it due to fleetingly testing the unit.

I'll try to find a suitable heatsink, but if you know of anything ideal in the UK, please do link me up.

teckniqs wrote:Maybe you could change the 4 turn coil to a small fixed inductor and then it might fit.

....Only other thing I can suggest is use a fan, which with all respect you should really be doing anyway.

To be honest mate, I don't really intend to use this on air - but I would've used a fan if the rig had have been switched on for more than about 2 min's. I was closely monitoring the temperatures and turned off power well before any imminent high temperatures.

Is it viable to use a fixed inductor in place of that 4 turn coil, because if it is I think there'd be no issues fitting that tall heatsink?

So sorry about the flurry of questions, but this is all a learning curve for me.

For the TO-39 devices, Stephen Moss often used heatsinks made out of sheet Ali. If you have some knocking around, you just need to form it around a round object; slightly smaller than the body of the transistor. Should fit in the available space.

Hi mate,

There's already one of those home made sheet Aluminium heatsinks fitted to this, as I had a couple knocking around. It can be seen about 7 posts up on the 2nd picture.

Even with this fitted, the heatsink is far too hot to touch for more than a few seconds - and this is with the rig only switched on for a couple of minutes!

I'll reduce the supply voltage down to 11 > 12V in the hope temps can be significantly reduced, but other than that I'm stumped.

I notice near TR5 that a trimmer Capacitor has been replaced with a fixed ceramic, so could this be anything to do with TR5 getting too hot?

Sorry, I missed that, but saw photos without. Common sense should have told me it had a heatsink. Having a fixed cap in place of a trimmer may not be helping. It's quite likely there's a bit of a mismatch there. The 2N3553 is notoriously inefficient, and you may get just as much out of a 4427 in that application; possibly with less heat.

Shedbuilt wrote:Sorry, I missed that, but saw photos without. Common sense should have told me it had a heatsink. Having a fixed cap in place of a trimmer may not be helping. It's quite likely there's a bit of a mismatch there. The 2N3553 is notoriously inefficient, and you may get just as much out of a 4427 in that application; possibly with less heat.

If the reduced voltage doesn't pay dividends, I'll order myself a trimmer Capacitor and try to locate my stash of Motorola & ST 2N4427 Transistors and substitute them.

I think you're spot on when you say the 2N3553 is inefficient, as every time I've used them in similar applications they've always generated lots of heat - just not to this degree.

What value would you suggest for the Trimmer Capacitor by the way? I'll get on eBay and order myself a couple.

Just thinking out aloud - for the 2N3553, perhaps if you used a tube without an open side seam, but instead drilled a series of holes slightly above the transistor you could get a chimney cooling effect. Possibly with a fan running as well the chimney effect might be stronger.

To be fair, to be getting 24W out of the MRF240, you must be getting a good bit out of the 3553, and the match is probably not too bad. I didn't think the 3553 "did" much more on 12 - 15v than the 4427; especially when they get hot and fall off, but I wouldn't expect that much output with a 4427 driving. I think it's worth trying a trimmer though. I'd probably go with the 22pF (green if it's the old Philips type). You can always add a cap in parallel - if it ends up fully meshed. A fan can only help. Or, if you went with a MRF237 or SD1127 as driver, you could probably solder it to the tracks under the board, and couple the grounded / can emitter to the heatsink...

Sounds like you're doing as good with that 2N3553 as you're going to do. I'm with Shed on the capacitor (green) but I doubt you'll get much more out of the driver. The last time I used 3553's in a TX was in the mid 90s. Even with the black crinkly heatsinks on, they got so hot that they desoldered themselves without a fan right on them. That was the last time. I'd get yourself an MRF237, it's more efficient and you'll get more power out.

Agreed, I've never really liked the 2N3553. I would stick an MRF237 in there, plus a trimmer, and you'll probably do as well as you can with that board.

With some luck, an MRF237 with a good heatsink and a big sink for the MRF240 you should be able to get around 40W!

Thanks for all the input guys.

I have 4 x genuine Motorola MRF237 Transistors here, so will put one in when my 22pF trimmer Cap arrives.

I'm having trouble finding a suitable heatsink for the MRF240 final though, so if anyone can direct me to one it'd be great - as it's going to be essential if I run this at anything over about 10W.

I've got an original RDVV driver I got from Henk about 11 years ago which has a 2N3553 and it does just under 3 watts but it runs hot and the power drops with the heat.

....I'm not sure if a 22pF will give you enough swing for the whole band. I'd be thinking more of a purple trimmer which I think goes up to 40pF.

Dutchrfshop has decent cheap heatsinks - get it while you can because you all gonna cry when brexit kicks in...

yellowbeard wrote:... get it while you can because you all gonna cry when brexit kicks in...

Heh, I'm already keeping an eye on the GBP/EUR exchange rate waiting to snag something at the right price.