Distorted Audio on my TS-940S after installing the upgrade
Howdy. I successfully swapped in your TS-940 power supply restoraion kit a few months ago. The instructions and the videos made it very easy. But now I’m getting reports of distorted audio from my contacts. I’ve put a current choke on the feedlone with no effect. I didn’t have this problem before. Any chance the new power supply is contributing to this condition? Any comments/suggestions for checking and correcting this? Thanks! Kerry
However, the critical voltage in the TS-940, provided by the DC-DC converter unit, is the -40V bias voltage used by the luminescent frequency display. That same voltage is tapped and returned to the Control Unit as “—C”, a.k.a. “ Negative Constant ”, where it is used to bias differential comparators (Op Amps). If this voltage has changed significantly since the last time the radio was aligned, especially way off, the affect will be seen as miss adjustment of several key radio controls, such as the S-Meter calibration, but more importantly, ALC level calibration, which in turn can produce overdrive symptoms. The user should check the ALC while transmitting, I would be willing to guess it is way too high. This issue is easy to correct with proper alignment of the Control Unit.
However, I have to comment that this is a sensitive area as most issues of overdriving the transmit audio occur through the use of the speech processor. People have been told for years to set the processor level to a point that visibly drives the ALC. This is a mistake, since any ALC is an indication of too much drive, and causes an automatic reduction of the drive signal. I am not a great fan of processors, to me they always tend to ruin an otherwise clean transmit audio in favor of a minor increase in transmit power that would be far better achieved with a better antenna of a quality RF amplifier.
Good point, the radio will operate at 24V, but not well. The DC-DC converters will continue to do their job as well, and the “—C” will definitely be off.
For years, I ran a Yaesu FT-401B, 250 Watts out, clean. It had one of the best receivers I have ever owned, and the transmitter was well known for its purity. When I operated on AFMARS, I was “accused” of running Collins. Didn’t need a processor to make people pay attention to my shouted call sign. Then again, I didn’t take part in contests, although one weekend I did find a quiet spot on 15M and started calling “CQ NO CONTEST”. With that rig, I never heard a station I could not contact, and while I had a matching FL2000B linear, I only turned it on occasionally to drive my next door neighbor nuts because it would open his garage door on 10 Meters.
Sitting on my shelf is a near pristine Yaesu FTdx-570, the young cousin of the FT-401B, identical in every was with the exception that the 401B had an AM mode. I found the 570 at an estate sale, and it looks like it has never been used. I need to replace it’s cap’s and align it, and this winter look forward to getting it on the air. Good old analog radio with tubes, tune it up, load it up, have fun.
I had not considered that option but you might not have adjusted the Quint power supply to a voltage of +28V. That would cause your transmitter to get a bit weird! We do not pre-set the power supply but all other voltages on the board are preset before we ship them.
Give it a try and thanks!
Aside from over-processing, over driving, or a crummy mic, I’ll bet a round of drinks that the 28.5 VDC supply is providing 24 VDC. That will likely produce a distorted signal. Bias points will be off, ALC won’t respond properly, etc. And yes: my QUINT was set at 24 VDC when I got it. I adjusted it to 28.5 VDC.