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Techman · 19th Dec 2018, 2:17 am

It sounds like the driver or output is perhaps going into oscillation. (I think I mentioned this before). I'm wondering if it's been doing this before and you have only just noticed it? If you remember I said that off peaking, or de-tuning the transmitter driver and output inductors (or coils) should be avoided, well this is a situation where slight 'off peaking' of one (or more) may be necessary. On AM/SSB, monitor the output with your frequency counter and watt meter and slightly adjust each of the inductors in turn and you may see it suddenly stop oscillating and return to transmitting on the correct frequency - some of these sets are prone to this. I'm still not convinced that there's not still an un-diagnosed fault somewhere in the drive, or perhaps it is just that the whole set was way out of adjustment in the first place.

You mentioned the 'swing' seems to be backwards on AM modulation - I think this is down to bad regulation and the audio level is making the supply rail 'dip' to this part of the circuit. Certainly they wouldn't think a lot to it on the old channel 6 AM, where carriers are wound down to a couple of watts and audio is wound up flat out and the whole lot fed into massive linear amplifiers! However, your set will probably sound quite nice to listen to on AM as it is. You can't really say that the audio on 6 is 'nice'. I'm sure you know about channel 6 AM, on what is now known as 'mid band', if you don't, then just look it up - there's nothing to be heard on there at the moment though.

As a note on those resistors with a '*', it's interesting to note that the same resistors, which are the same value but marked as R39 and R38 on the 059 chassis circuit diagram, are not marked with a '*'. The 059 board is not known for output stage biasing problems, whereas the 121 board is well known for issues with distortion and clipping of the output waveform.

As another note regarding R45 which is 4.7K, but without the point/dot shown on the drawing, I note that on the original Hi Gain and Concorde diagrams that the 'dot' is shown but is below the line compared with other similarly marked values, so it looks like the Intek circuit could be an early version with the 'dot' being mistakenly left out, but was corrected on later issues, having to be inserted lower due to spacing. Makes you wonder if there's any other corrections from this drawing onto the later ones - that is if they are indeed later.

19th Dec 2018, 2:17 am	#283
Techman Dekatron Join Date: Nov 2009 Location: Lincolnshire, UK. Posts: 5,000	Re: Cybernet PTBM121D4X problems It sounds like the driver or output is perhaps going into oscillation. (I think I mentioned this before). I'm wondering if it's been doing this before and you have only just noticed it? If you remember I said that off peaking, or de-tuning the transmitter driver and output inductors (or coils) should be avoided, well this is a situation where slight 'off peaking' of one (or more) may be necessary. On AM/SSB, monitor the output with your frequency counter and watt meter and slightly adjust each of the inductors in turn and you may see it suddenly stop oscillating and return to transmitting on the correct frequency - some of these sets are prone to this. I'm still not convinced that there's not still an un-diagnosed fault somewhere in the drive, or perhaps it is just that the whole set was way out of adjustment in the first place. You mentioned the 'swing' seems to be backwards on AM modulation - I think this is down to bad regulation and the audio level is making the supply rail 'dip' to this part of the circuit. Certainly they wouldn't think a lot to it on the old channel 6 AM, where carriers are wound down to a couple of watts and audio is wound up flat out and the whole lot fed into massive linear amplifiers! However, your set will probably sound quite nice to listen to on AM as it is. You can't really say that the audio on 6 is 'nice'. I'm sure you know about channel 6 AM, on what is now known as 'mid band', if you don't, then just look it up - there's nothing to be heard on there at the moment though. As a note on those resistors with a '', it's interesting to note that the same resistors, which are the same value but marked as R39 and R38 on the 059 chassis circuit diagram, are not marked with a ''. The 059 board is not known for output stage biasing problems, whereas the 121 board is well known for issues with distortion and clipping of the output waveform. As another note regarding R45 which is 4.7K, but without the point/dot shown on the drawing, I note that on the original Hi Gain and Concorde diagrams that the 'dot' is shown but is below the line compared with other similarly marked values, so it looks like the Intek circuit could be an early version with the 'dot' being mistakenly left out, but was corrected on later issues, having to be inserted lower due to spacing. Makes you wonder if there's any other corrections from this drawing onto the later ones - that is if they are indeed later.