PCS-Electronics

We've replaced an antique (?) 20 watt stereo home built kit, with a Max Pro I, after the unfortunate demise of the exciter. Is anyone familiar with older (70's ??) kit transmitters? This unit is unmarked and came second hand with no supporting documentation. I'd like to know if there was another standard impedance (other than 50ohm) which may have been used between an exciter and the linear power amp. The input to the linear is something other than 50 ohm. My purpose is to design/build a network to match the linear to the 50 ohm output to the Max Pro. I'd appreciate any suggestions or advice. I don't have the time to reverse engineer the exciter, difficult anyway, as some parts are unidentifiable. I'll post pictures, as soon as I can, at:
http://www.geocities.com/awizardalso/transmitter
Thanks for your thoughts.

Hey,

where are the pics? Its impossible to say anything before we see them!

I posted two images on the link below.
http://www.geocities.com/awizardalso/transmitter

Thanks for the reply. I would have had them sooner but it seems Netscape 6 has issues with GeoCities, and I had to re-install M$ IE to access the website.

The transmitter kit was supposedly from Radio Free Berkely, but they were unable to provide much information. The unit is outdated and no info is available.

I'm hoping someone may recognize it and possibly have some advice on the amplifier (and it's input impedance). The exciter is a free running FET oscillator, with a two amp stages. The stereo MPX is derived from a BA1404 IC.
Thanks for your time and any assistance

Thanks to those who at least took a look. I know it's old, but I figured I'd give it a shot. It's a project now put on the back-burner. If anyone has any ideas, I'd be glad to hear from you. Thanks.

Wiz
(although it seems I'm not quite the Wiz I wish I wuz)

Sorry to say this, but the BA chip should never be used
for anything but in-house transmitting, which is what it was
designed to do in the first place.

Also, a free-running oscilator and a power stage as strong
as the one you have there is, in my opinion, aksing for trouble.

Thanks for the quick reply. The exciter is dead and sitting in the junk pile. What we're trying to do now is to match the 15-20 watt amp to the output of a MaxPro1. The amp has an input impedance of something other than 50 ohm. Without reverse engineering the exciter and some complicated math, I can't determine what the impedance between the two was. Was wondering if there was another common impedance used (200 maybe?)

I have a simple resistor network to divide the power output by 1/4, used to match a 6 watt CB to a low drive linear. In this case, it won't work, presumably, due to the impedance mismatch. Anyway, thanks for your input.

Hi
In my opinion you have a misfuctional 2SC2539 on that board (or is it a 2sc1946???
The VFO-BA1404 board it reminds me something I saw somewhere on the Net.
But from the jpg of the final the input circuit looks like 50ohms. Have you try to connect a SWR bridge between Max and final amp and then adjust the 3 input trimmer caps for lower VSWR reading???
Regards
Flash96

[quote="A. Wizard"]Thanks for the quick reply. The exciter is dead and sitting in the junk pile. What we're trying to do now is to match the 15-20 watt amp to the output of a MaxPro1. The amp has an input impedance of something other than 50 ohm. Without reverse engineering the exciter and some complicated math, I can't determine what the impedance between the two was. Was wondering if there was another common impedance used (200 maybe?)

I have a simple resistor network to divide the power output by 1/4, used to match a 6 watt CB to a low drive linear. In this case, it won't work, presumably, due to the impedance mismatch. Anyway, thanks for your input.[/quote]

The SWR between the Max and the amp was way up in the red. Somewhat less than full scale, but way up. I didn't see anything in the documentation about the Max having any protection against severe SWR, so attempts to tune the trimmers were made with short 'on' times (5-6 seconds), and almost no improvement in SWR were obtained, hence the assumption of an odd input impedance to the amp. The amp final, checked out OK with diode junction tests, and it does operate, so I thought it was safe to assume it's OK. ???
Unfortunately, I don't have anything suitable to drive the amp with for testing/tuning other than the station transmitter which is in use 24/7 in another location. All my equipment here in the shop is HF. Once again, thanks for your help. Any more comments, suggestions, advice is still welcomed. I really hate to see it collect dust on the shelf.

Wizard

awizardalso wrote:The SWR between the Max and the amp was way up in the red. Somewhat less than full scale, but way up. I didn't see anything in the documentation about the Max having any protection against severe SWR, so attempts to tune the trimmers were made with short 'on' times (5-6 seconds), and almost no improvement in SWR were obtained, hence the assumption of an odd input impedance to the amp. The amp final, checked out OK with diode junction tests, and it does operate, so I thought it was safe to assume it's OK. ???
Unfortunately, I don't have anything suitable to drive the amp with for testing/tuning other than the station transmitter which is in use 24/7 in another location. All my equipment here in the shop is HF. Once again, thanks for your help. Any more comments, suggestions, advice is still welcomed. I really hate to see it collect dust on the shelf.

Wizard

First.
diode check on 2sc2539 or any other planar RF type transistor will give some "strange" results.There is a double(right & opposite) reading on the one junction that means the trans is OK.If you don't read this on test transistior is out of work at base juction.Also I believe you check it outside the circuit!!! Or at least with the Base bias circuit removed!
there is a possibility of a short circuit capacitor.
Regards
Flash96

flash96 wrote:Hi
In my opinion you have a misfuctional 2SC2539 on that board (or is it a 2sc1946???
The VFO-BA1404 board it reminds me something I saw somewhere on the Net.
But from the jpg of the final the input circuit looks like 50ohms. Have you try to connect a SWR bridge between Max and final amp and then adjust the 3 input trimmer caps for lower VSWR reading???
Regards
Flash96

A. Wizard wrote:Thanks for the quick reply. The exciter is dead and sitting in the junk pile. What we're trying to do now is to match the 15-20 watt amp to the output of a MaxPro1. The amp has an input impedance of something other than 50 ohm. Without reverse engineering the exciter and some complicated math, I can't determine what the impedance between the two was. Was wondering if there was another common impedance used (200 maybe?)

I have a simple resistor network to divide the power output by 1/4, used to match a 6 watt CB to a low drive linear. In this case, it won't work, presumably, due to the impedance mismatch. Anyway, thanks for your input.

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