Odd receiver fault: experienced tech advice needed

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TassieJay

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I'm in need of some advice on the repair of a receiver fault where it appears to be a little deaf, in an odd way.

The receiver in question has a mute threshold of 0.25uV (or -125dBm) where a pretty noisy signal results, and 20dB quieting is achieved at 0.35uV (or -122dBm). That's only about 3dB variation between a very noisy just-able-to-open-the-mute signal and one that is essentially noise free.

Compared to a 'good' receiver in the same model of radio, a mute threshold of 0.11uV (-132dBm) and 20dB quieting at 0.28uV (-124dBm) is measured; that's an 8dB spread between a noisy just-able-to-open-the-mute signal to a noise free one, which is much more like normal.

So in the problem radio, the 20dBq sensitivity is OK, perhaps just a little bit down, but below that the signal drops off extremely rapidly. I've replaced the front end preamp active device and the first mixer active device, neither made much more than 1dB of improvement. The 1st & 2nd Local Oscillators look & sound pretty clean on the test bench.

Would I be correct in assuming symptoms like this is typical of an IF fault? Or could it still be something in the front end eg: faulty bypass caps etc?
 

jackj

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Your faulty receiver appears to be pretty close to normal. Without knowing the receiver's specs, it is impossible to know for sure but I don't think I'd spend much time looking for about 0.1µv. I doubt if you could tell the difference in the real world. As for trouble-shooting at a distance, good luck but I don't think I'll try it.
 

TassieJay

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Close to normal - at signal levels around 0.35uV or greater, yes.
But below that, where a weak but still quite usable signal exists: most definitely no!

The key concern is that there's only 3dB difference between noise free and nothing. No gentle fade, just... nothing. That makes a weak signal very 'choppy' to listen to, and it's not the mute that's the cause. And because I'm out on the country roads a lot for my job, this choppy action is very annoying. As a ham, I'm passionate about wringing the best performance out of my system as possible, and I *know* this equipment can do better.

Tait (T700) list the 20dBq performance as "better than -113dBm" (or 0.5uV) - which is just the manufacturer being conservative to cover themselves. But with another two other radios of the same model not exhibiting this 'cliff' in receiver performance, I know something isn't right, and want to put it right. Call me a perfectionist, but I'm not going to adopt the 'it meets manufacturer spec' stance when I know it can do better.

I must have been half asleep when doing the uV to dBm conversions in my original post, it should have read:
Faulty RX: mute threshold 0.25uV / -119dBm and then 20dBq at 0.35uV / -116dBm. (still only 3dB between literally no signal and full quieting)
Normal RX: mute threshold: 0.11uV / -126dBm and then 20dBq at 0.28uV / -118dBm.

Having been in the same situation myself, I know how hard it can be to diagnose by distance. All I'm really looking for is to see if other experienced techs have come across similar symptoms in general (in any receiver, not just specifically the T700), and wether or not my suspicions that the problem lies at the 2nd IF, close to the discriminator, in their experience, is likely to be correct or not.
 

WA0CBW

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Have you tried comparing a working radio with the non-working one? Maybe you can follow the signal through both radios and see where the signal degrades when injecting the same signal level in both radios.
BB
 

n0nhp

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Long shot, don't even know if the radio uses them but have you checked the PIN steering diodes? I have seen them do some pretty strange things at low signal levels.
Bruce
 

prcguy

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What you are describing is limiter action, and when done right will give you the best compromise between too wide of a range between noisy and no reception and what you describe as full quieting to nothing with a very small change in level.

Not sure if there are specs for the limiter or AGC in your radio but you might look for an alignment spec and compare both radios to see which one is closer to reality.
prcguy
 
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