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Distorted Audio on my TS-940S after installing the upgrade  

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ak6ok
(@ak6ok)
Member Admin

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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

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Posted : 09/10/2018 10:55 pm
ak6ok
(@ak6ok)
Member Admin
There are many possible causes for distorted audio on SSB Transmit with the TS-940.  However, I doubt any of them could be directly related to the PS Upgrade unless the old PS voltages were dramatically out of spec when the radio was serviced.
 
Here are some suggestions
 
 
·         ALC.   It is important to observe the behavior of the ALC while transmitting to prevent distortion.  This is one area where the PS upgrade could have a minor impact.  There are several voltages derived from the primary power supply, most importantly the 21V supply, which we have noted before can be somewhat critical in the TS-940.  Typically, the factory power supply in older TS-940’s had this voltage lower than it should be.  Normally this should not be too critical since the 21V is mostly used as a input bias for downstream voltage regulators that make up for any variance.  

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. 

·         Kenwood microphones.  There are many on-line discussions and modifications regarding the Kenwood MC-60 and MC-80 microphones and their sensitivity to RF, causing audio distortion on transmit. However, this is normally endemic to the microphone and not a problem that typically pops up after years of use. 
·         Driver and Final Bias.  Incorrect bias to the PA Driver and Final transistors can cause distortion in the output signal due to shifting the operating point of the amplifier into a non-linear region.  This is usually easily corrected by performing the bias adjustment procedure. 
·         Carrier alignment.  The transmitter linearity, adjusted through the carrier balance, is also a source of distortion.  In this case, the distortion will be markedly different between LSB and USB.  Normally, correct alignment solves this problem as well, but it is best performed using a two-tone (300 & 2700 Hz) audio test and observing the output waveform on a scope.
 
If the owner would like, I would be happy to put the radio on my bench and discover exactly what is causing the problem.
 
Regards

Dave

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Posted : 09/10/2018 10:56 pm
ak6ok
(@ak6ok)
Member Admin

Dave/Jeff:

 
I wonder if he set his Quint to 28V when he tested the board. And would a failure to do THAT cause low enough voltages to cause distorted audio? When I received my kit, the Quint was at 24V, I think. Kim said his was too.
 
Dave: I too like to run a straight, clean signal and then use a small (400W) linear to make the rig more competitive in pileups. My SWAN 700 CX's produce 400W without a linear or compression, so other OPS comment on the clean sound. But yeah, processors. YUK.
 
John
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Posted : 09/10/2018 10:56 pm
ak6ok
(@ak6ok)
Member Admin

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.

 

Cheers

Dave

 

ReplyQuote
Posted : 09/10/2018 11:02 pm
ak6ok
(@ak6ok)
Member Admin

Hi Kerry,

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!

Jeff AK6OK

 

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Posted : 09/10/2018 11:26 pm
ak6ok
(@ak6ok)
Member Admin

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. 

 
73,
 
Kim N5OP
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Posted : 09/10/2018 11:29 pm
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