Last week I picked up a Midland 78-976 40 Channel AM/SSB Cybernet base station on Ebay in "unknown" condition.
It arrived a couple of days ago and when I started testing it, I found that the receive was working well, but I had no transmit.
I was able to cross-reference it with the 79-892 Midland. Sams #180. I don't have the Sams manual yet, but I've been trying to determine why I am not getting any output wattage.
When I test the radio, I originally saw no power out on my Watt meter (Not the internal S/RF Meter) but my frequency counter was showing it on the correct frequency during testing.
I started going over the schematic and testing my transistor voltages. Both the final and the driver have the correct collector voltage during RX (13.8v) but on transmit the voltage is only dropping down to 12.8-13.2v The schematic I am looking at says I should see somewhere in the neighborhood of 6.4v at the collector.
As I continued testing voltages, I got back to Q7, Q8; the RF Pre-Amp transistors. The voltages appeared to be correct.
Something strange happened while measuring the voltages on Q7. I had my test lead running across the board and the probe touching the base of Q7. I happened to look up and saw that I was now transmitting at full power on the watt meter. I moved the probe and once again was down to 0 watts.
Thinking cold solder joint, I began probing around the area with a non-metallic probe, but could not get the power to jump back up. I put the test lead probe back to the same point, but again saw no power. I was able to get the condition to repeat by snaking the test probe lead across the board in the area of the driver/final, but moving the wire away from the board while keeping the mic keyed and the probe on the correct point on the board makes the transmit drop out.
I believe at this point I should be looking for something that is stopping the signal from getting from Q7 to the driver, and that the test lead is somehow working as an inductor to get the signal where it needs to go. Just never seen this happen before and with the lack of service information available on this radio I'm working semi-blind.
It arrived a couple of days ago and when I started testing it, I found that the receive was working well, but I had no transmit.
I was able to cross-reference it with the 79-892 Midland. Sams #180. I don't have the Sams manual yet, but I've been trying to determine why I am not getting any output wattage.
When I test the radio, I originally saw no power out on my Watt meter (Not the internal S/RF Meter) but my frequency counter was showing it on the correct frequency during testing.
I started going over the schematic and testing my transistor voltages. Both the final and the driver have the correct collector voltage during RX (13.8v) but on transmit the voltage is only dropping down to 12.8-13.2v The schematic I am looking at says I should see somewhere in the neighborhood of 6.4v at the collector.
As I continued testing voltages, I got back to Q7, Q8; the RF Pre-Amp transistors. The voltages appeared to be correct.
Something strange happened while measuring the voltages on Q7. I had my test lead running across the board and the probe touching the base of Q7. I happened to look up and saw that I was now transmitting at full power on the watt meter. I moved the probe and once again was down to 0 watts.
Thinking cold solder joint, I began probing around the area with a non-metallic probe, but could not get the power to jump back up. I put the test lead probe back to the same point, but again saw no power. I was able to get the condition to repeat by snaking the test probe lead across the board in the area of the driver/final, but moving the wire away from the board while keeping the mic keyed and the probe on the correct point on the board makes the transmit drop out.
I believe at this point I should be looking for something that is stopping the signal from getting from Q7 to the driver, and that the test lead is somehow working as an inductor to get the signal where it needs to go. Just never seen this happen before and with the lack of service information available on this radio I'm working semi-blind.
