I had a non-working Patcomm PC-500 come across my workbench.
The Patcomm PC-500 is a 2000-vintage CW/SSB 2-band HF radio with about 15 watts out. The company looks to have gone out of business. The user manual is mostly useless. Schematics (in TIFF format) are available on the usual sites. There is no service manual.
The problem reported was that the band modules were not reliably detected.
The two band modules use two 0.1" headers per module to connect to the main board for RF and control signals. Band detection is done via four bits hardwired on the module itself and bussed to four pins on the CPU. After following the signal path on the schematics I was able to test continuity between the plugin slots and the CPU. Three good lines -- one bad. Closer inspection revealed damage to the trace on the board.
This particular trace runs along the edge of the motherboard and was damaged by the chassis screws being driven into the board. I suspect a lot, if not most, of the PC-500 radios have this damage.
The solution in my case was to clean up the board edge and jumper the broken trace with a short loop of wire-wrap wire positioned out of harm's way.
Fixed! Right? Nope.
No receive. With the band module issue solved, the radio was discovered to be completely deaf. There was a rushing noise in the speaker and DDS hash (yuck...we've come a long way...) but nothing on the bands.
This was a perfect opportunity to use my AD9850 DDS synthesizer to generate a continuous signal for tracing. With that module plugged into the antenna port and oscillating at 7.100MHz I was able to follow the 7.100MHz signal with my oscilloscope through the various filters, mixers and relays to verify the level.
The signal completely dropped off after the first mixer at Q7, a 2N5109 small signal amplifier. A little more probing shows that there was no bias voltage via "VRX" on the transistor. A little more probing revealed that Q10, a 2N3906 (SOT-23 size) transistor wasn't doing anything.
I happened to have a few of those in my parts inventory. The radio started
working properly after that transistor was replaced.
This page last modified Mon Dec 1 12:10:19 CST 2014 by timc!