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Distorted Receive on NorthStar 990v

Mr.715

Mr.715

Member
Nov 30, 2019
25
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Stevens Point, WI
Hello looking for some guidance on this one. The radio has this issue in all modes. It does not matter how far down the signal generator goes the distortion is always there. If you cut back the RF gain it will clear up and return to normal. If the radio is not receiving a signal and you work the volume control it does not show this issue with just the static noise floor that appears normal. Issues only pertain when it is receiving a signal. I've never seen this problem on this type of chassis which it uses a 360014B board.

Many thanks for any suggestions on the approach here.

View attachment 1000005577.mp4
 
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Edit... The volume knob (does not) run through the echo/vc/robot board. I bet that board is the source of your issue.

Sorry, I crossed the lines on the schematic. It does sound like the robot though.
 
Last edited:
The volume and mic gain share a ground at pins 44 and 45. Maybe if that ground was missing, rx audio could feed into the mic gain and into the board. Lowering rx gain would stop it, but does lowering the Mike gain change it?
 
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I will report to you back on this Friday when I'm at the shop. Yes RF gain when cut back will make the distortion go away. I did not however check to see if the Mic gain would have an effect. I will let you know when I'm back in.
 
sp5it said:
What is next step if after recap is no difference?
Isn't it better to diagnose before blindly replace radio components?
Mike
Click to expand...
I was just thinking that the chances would be good that a failed capacitor is the cause. Guessing that the radio is approaching 40 years old, the capacitors need to be changed anyway.
 
sp5it said:
radios older thatn 40 years, and never had to replace
Click to expand...
As Indiana Jones said: "It's not just the years, it's the miles, too". Heat exposure and operating hours affect service life. A new-in-box radio that's 40 years old may not have one bad electrolytic. A radio that was powered 24/7 for 20 years will be more likely to have failed caps in it. A shorted or open capacitor is not the only failure mode. Reduced capacitance, internal series resistance can compromise performance without putting the radio off the air like a shorted part. Quality level varies, too. Old SBE radios seem to have a higher percentage of bad electrolytics than Uniden made radios of the same age.

Oh, about the DX99. I would check C39, a 47uf cap. Schematic says 10 Volt rating. Parts with that voltage rating seem to be the canaries in the coal mine. They tend to fail first from age. Simply taking a new cap and pressing the leads against the foil pads for C39 will put your "test" cap in parallel with C39. If it has failed open, you'll hear an immediate improvement in receiver audio quality.

73
 
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nomadradio said:
As Indiana Jones said: "It's not just the years, it's the miles, too". Heat exposure and operating hours affect service life. A new-in-box radio that's 40 years old may not have one bad electrolytic. A radio that was powered 24/7 for 20 years will be more likely to have failed caps in it. A shorted or open capacitor is not the only failure mode. Reduced capacitance, internal series resistance can compromise performance without putting the radio off the air like a shorted part. Quality level varies, too. Old SBE radios seem to have a higher percentage of bad electrolytics than Uniden made radios of the same age.

Oh, about the DX99. I would check C39, a 47uf cap. Schematic says 10 Volt rating. Parts with that voltage rating seem to be the canaries in the coal mine. They tend to fail first from age. Simply taking a new cap and pressing the leads against the foil pads for C39 will put your "test" cap in parallel with C39. If it has failed open, you'll hear an immediate improvement in receiver audio quality.

73
Click to expand...
I'm in tomorrow I will check this cap as well thanks gentlemen for helping me on here.
 
Ok, worth a shot. Cold solder joints and loose grounds are common with these boards. Id start looking at the caps around the audio IC and rx audio chain. Some caps pass audio further down the chain, some shunt unwanted audio frequencies to ground, and both shape the audio to its destination. Time to scope the path and replace all 10v capacitors.
 
nomadradio said:
As Indiana Jones said: "It's not just the years, it's the miles, too". Heat exposure and operating hours affect service life. A new-in-box radio that's 40 years old may not have one bad electrolytic. A radio that was powered 24/7 for 20 years will be more likely to have failed caps in it. A shorted or open capacitor is not the only failure mode. Reduced capacitance, internal series resistance can compromise performance without putting the radio off the air like a shorted part. Quality level varies, too. Old SBE radios seem to have a higher percentage of bad electrolytics than Uniden made radios of the same age.

Oh, about the DX99. I would check C39, a 47uf cap. Schematic says 10 Volt rating. Parts with that voltage rating seem to be the canaries in the coal mine. They tend to fail first from age. Simply taking a new cap and pressing the leads against the foil pads for C39 will put your "test" cap in parallel with C39. If it has failed open, you'll hear an immediate improvement in receiver audio quality.

73
Click to expand...
Did you mean C37? C39 is a chicklet style cap. 01uF 50v C37 is in the same general area 47uF rated at 10v?
 
Mr.715 said:
Did you mean C37? C39 is a chicklet style cap. 01uF 50v C37 is in the same general area 47uF rated at 10v?
Click to expand...
Chicklet mylar capacitors can also fail, although not as often. I wouldn't overlook them. If you see any brown glue gooped over any components, remove it. Sony bond glue is corrosive and conductive, and causes many odd symptoms and failures.
 
Cable Guy said:
Chicklet mylar capacitors can also fail, although not as often. I wouldn't overlook them. If you see any brown glue gooped over any components, remove it. Sony bond glue is corrosive and conductive, and causes many odd symptoms and failures.
Click to expand...
I removed C37 and C39 both tested good on the cap wizard. Replaced anyway. No difference made I think you're correct. I may just do a dive and cap the entire radio. But I would also like to take some voltage tests for the AGC IC as well I think.
 

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