UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

SiriusHardware · 6th Dec 2018, 12:40 am

First a few voltage checks: In TX mode, AM, what do you have on wire terminal (18)? (BTA, which is the transmit-only regulated supply voltage).

In -RX- mode, AM/FM, what do you have on Q9 (RF Driver) collector and base and on Q10 (RF Output) collector and base?

In -RX- mode, USB/LSB, what are the same measurements?

The collectors should be a little over 6.8 volts on AM/FM and approximately the full supply voltage on USB/LSB.

Now, back to your scope:

Set the probe to the x10 position if it has one, and the input mode switch (AC/GND/DC) to AC. Centre the trace.

In FM TX mode, take the scope probe to the base of the driver Q9 and adjust the volts / div dial until the signal (a thick band) is roughly one division high. Because your scope is quite high-frequency capable you should be able to see the RF signal as an actual waveform if you wind the time/division dial up towards the high end, but don't worry if you can't. It's only the vertical height of the signal we are concerned with at the moment.

Now take the scope probe to the collector of the driver, the signal there should be a lot bigger (vertical size) than it was on the base.

Repeat for the output transistor Q10 - scope the signal on its base, adjust the volts / div so that the signal looks quite small (say 1 division high) and again move to the collector, once again the signal on the collector should be bigger than it was on the base.

If both these checks are OK, then the driver and output are probably OK. If either of the devices has a much smaller signal on its collector than on the base, the device may be defective.

6th Dec 2018, 12:40 am	#149
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,554	Re: Cybernet PTBM121D4X problems First a few voltage checks: In TX mode, AM, what do you have on wire terminal (18)? (BTA, which is the transmit-only regulated supply voltage). In -RX- mode, AM/FM, what do you have on Q9 (RF Driver) collector and base and on Q10 (RF Output) collector and base? In -RX- mode, USB/LSB, what are the same measurements? The collectors should be a little over 6.8 volts on AM/FM and approximately the full supply voltage on USB/LSB. Now, back to your scope: Set the probe to the x10 position if it has one, and the input mode switch (AC/GND/DC) to AC. Centre the trace. In FM TX mode, take the scope probe to the base of the driver Q9 and adjust the volts / div dial until the signal (a thick band) is roughly one division high. Because your scope is quite high-frequency capable you should be able to see the RF signal as an actual waveform if you wind the time/division dial up towards the high end, but don't worry if you can't. It's only the vertical height of the signal we are concerned with at the moment. Now take the scope probe to the collector of the driver, the signal there should be a lot bigger (vertical size) than it was on the base. Repeat for the output transistor Q10 - scope the signal on its base, adjust the volts / div so that the signal looks quite small (say 1 division high) and again move to the collector, once again the signal on the collector should be bigger than it was on the base. If both these checks are OK, then the driver and output are probably OK. If either of the devices has a much smaller signal on its collector than on the base, the device may be defective.