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Old 30th November 2011, 07:11 PM   #1
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Default Bandpass filters in audio circuit of communications receiver?

Hi. I recently acquired a Realistic DX-160 Communications Receiver, made around 1975. The audio circuit does not work -- the chip is bad. I am going to order a new replacement chip. My question is concerning what I think are bandpass filters surrounding the chip. Can anybody help me determine the nature of these and why they are there? See the image of part of the schematic containing the audio circuit. If needed I can provide a datasheet of the chip. The schematic contains the values of the parts.
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Old 30th November 2011, 07:24 PM   #2
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OK I'll take a shot. There seems to be more than nessisary parts around that chip. Because this is a communications reveiver perhaps they are doing some audio emphasis -demphasis depending on the type of station. They may even switching the tone automatically. I would be intrested in seeing the datasheet.
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Old 30th November 2011, 08:16 PM   #3
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A communications receiver might well have some sort of filtering to make the most of poor signals. The datasheet would be of help in figuring it out. Google draws a blank on it, BTW.

Are those uPC20C chips still available? Better grab one while you can.
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Old 1st December 2011, 12:24 AM   #4
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Quote:
Originally Posted by ingenieus View Post
A communications receiver might well have some sort of filtering to make the most of poor signals. The datasheet would be of help in figuring it out. Google draws a blank on it, BTW.

Are those uPC20C chips still available? Better grab one while you can.
No, the uPC20C is obsolete. Mouser has a replacement -- the NTE1075A. I will attach the datasheet for that one.
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Old 1st December 2011, 12:53 AM   #5
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Hi,

HAM radios AFAIK are designed to maximise speech intelligibility with very
poor signals with loads of interference. Often the interference is in ranges
you don't need for intelligibility but much higher in level than what you wan't.

It gets in the way, so chuck it away and only use the ranges you need.

Not the sort of radio you'd want to listen to music on, basically, but it allows
signals to be understood you would not be able to understand unfiltered.

Some I understand have some very clever DSP adaptive filtering.

rgds, sreten.
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Last edited by sreten; 1st December 2011 at 12:56 AM.
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Old 1st December 2011, 03:19 AM   #6
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Thank you, sreten. That helps me understand why all the filters are there.
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Old 1st December 2011, 06:21 AM   #7
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I've listened to a few DX160s over the years before synthesised tuners were adopted. I wouldn't say the audio was particularly filtered as in a serious comms. receiver, as they made a fair AM receiver too. It is usual in budget receivers to carry out bandpass filtering primarily by the IF bandwidth. Post or audio filtering for various impulse or wideband noise, is by intermediate adjustable/switchable filters.

Unlike modern, one chip audio amps, most early ones needed a bucketload of external caps to make them even function. The likelihood of RF leaked into the audio and mussing up the amp's operation might be a good reason for placing filters there.
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Old 1st December 2011, 03:35 PM   #8
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While trawling the interweb, I found the following bit of info that might be of some use.

Quote:
Click the image to open in full size.

Howard provided some information on improving the audio response of the DX-160. Here are his recommendations:
  • C54 - 22 mfd 50V (If any instability is observed such as excessive time before hearing any sound when powering up, change this capacitor to 10mfd 50V)
  • C56 - 220mfd 16V
  • C58 - 10 mfd 35V or greater
  • C63 - 1000 mfd 16V or greater
  • C64 - Remove this capacitor as this will limit top end response
Howard says that while he doesn't get the bass response he does from a DX-302, he was able to get rid of the nasal sound and adds a lot of punch that it didn't have before. Thanks Howard!

W4JBM's Realistic DX-160 Page
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Old 1st December 2011, 04:08 PM   #9
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@ ingenieus -- Thanks for the link, I saw that already. Unfortunately it applies to the newer ones that use a different audio circuit. The chip and everything is different. But thanks.
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Old 1st December 2011, 06:16 PM   #10
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For one thing, the output coupling cap (C66) certainly is on the meagre side for speaker operation. That one should be like 470..1000 then. C63 seems to be a bootstrap cap, enlarging this by about the same factor seems worth a shot. For everything else, the NTE datasheet is not detailed enough.

In general, I wouldn't trust average quality mid-1970s electrolytics too much any more. If the unit should still be using carbon comp resistors, these should be checked for upwards drift, too.
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