Anyone know anything about this PA?

[reverend]

Does anyone recognise this PA. It seems some of the capacitors are partially burnt out and some of the inductors have melted, just wondered if anyone had any ideas about its provenance and whether it's worth repairing.

Rev

IMG_20211005_090035.jpg

[teckniqs]

It's an old David Bowman PA from around 20 years ago, usually good for around 80 to 100w+.

That's what happens when idiots use a low powered Toko S18 on a high powered ampflier's LPF, just replace that S18 with a wound coil and it should be fine. Although it could probably do with an additional filter coil after it as that filtering is lacking....

....The die-cast box it's in is certainly not the enclosure I was used to seeing those in though.

[teckniqs]

[Albert H]

Who threw that one together? That's certainly not a "Bowman" rig! I recognise the toroidal mains transformer - they're the ones we got wound specially from Antrim Transformers.

My version of a rig like that used a single big smoothing capacitor, had the exciter in its own box, used a tuned coiled-up coax between the exciter and the PA, had a little PCB for the exciter supply regulator, had a bigger heatsink for the PA and an additional coax stub filter for the second harmonic, and used the SD1019 instead of the lower gain '1407. Mine always did a consistent (and clean) 120 Watts, and was housed in a custom-made aluminium box from H L Smith in Edgware Road. Dave K and I built about 130 of those in the mid-eighties, the first ones using a prescaler and a '145151 and the second version with a '145170 and a PIC. They were used everywhere!

[teckniqs]

Who threw that one together? That's certainly not a "Bowman" rig!

Oh yes it very distinctively is!

N also used those very same transformers that you "got wound specially".

[reverend]

Thanks for all the info. I'll replace the burnt-out components and fire it up and see how it performs. If anyone has the circuit (including the correct component values) that would be help too.

Rev

[teckniqs]

Thanks for all the info. I'll replace the burnt-out components and fire it up and see how it performs. If anyone has the circuit (including the correct component values) that would be help too.

Rev

I think it might vary slightly depending on which part of the band.

[reverend]

I think it might vary slightly depending on which part of the band.

So not a broadband design then. I'm guessing it's probably better at the bottom end of the band as the SD1407 is more of an HF device. I'll use some engineering nous to figure out what goes where

[teckniqs]

I think it might vary slightly depending on which part of the band.

So not a broadband design then. I'm guessing it's probably better at the bottom end of the band as the SD1407 is more of an HF device. I'll use some engineering nous to figure out what goes where

No not at all, the variable trimmer capacitors should be a bit of a giveaway...

[Albert H]

N also used those very same transformers that you "got wound specially"

Check the secondary voltage. It should be 21.8 V AC under load if it's one of ours. This gave a gnats over 30V DC on the PA under load, and rising to about about 33.1 V DC with the drive off. They were wound by Antrim Transformers when ILP stopped doing custom voltages (and I got fed up with wasting winding wire and having to take some turns off their 24V standard product). It was much cheaper and easier to get a custom-wound transformer than build a regulator for the supply. Also, ordering 40 at a time gave a useful discount. We found a cheap source of 22000µF 40V rated electrolytics (overkill, but they never hummed!), and used a 25 Amp rated metal-clad bridge rectifier.

It wouldn't surprise me if other people used the same transformers - they gave a useful supply voltage and were pretty stable on and off load.

[teckniqs]

I'll use some engineering nous to figure out what goes where

There is also a resistor missing to the base (input) of the 2N5642. Probably 47ohms

[Shedbuilt]

In the first pic (Reverend’s), there looks to be a pi attenuator - attenuating the input to the PA board.
The 5642 probably needs to run fairly hard - especially toward the top of the band, to get 100W out of the 1407. The SD1407’s saving grace, was it’s ruggedness (definitely not gain, linearity, or efficiency).
I did 150W with a 1407 a few times, but by that time, the already low to moderate gain, was compressing further. Changing the 4427 in the exciter, for a MRF237, generally yielded 2W or so. Feeding that into a 5643 (instead of a 5642), would - with a suitable PSU, yield about 150W out. Not the best way to get to 150W though, and the power increase (~1.76dB), is pretty meaningless (except when the user demands 150W rather than 100W); otherwise not worth the effort. Those were my experiences, but I can’t remember if those experiences were with ST branded SD1407s (25 to 30 years ago)….