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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 12th Feb 2020, 7:57 pm   #61
Atmosferit
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Default Re: Diagnosing temperature related noise?

Quote:
Does not appear to make much sense why no real signal other side of C22 ?

When you say the signal also disappears on the other channel, I assume this must be the good channel at C21, when you loose the good channel at C21 on the scope, do you also loose the audio output
Yes, totally correct.
As soon as I put my probe on the other side of the capacitor, the output from the line out vanishes as well.

And yes, you are correct, I'm playing my 3khz test tape as a reference signal.

Quote:
Are the DC voltage levels good at the pin 5 inputs, in all conditions, including when you connect scope
I've not tried that, I guess measuring voltage is going to be my next step

Thanks.
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Old 13th Feb 2020, 7:08 pm   #62
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Default Re: Diagnosing temperature related noise?

So, with the digital multimeter, I measured 12.45v on the pin 16 (v+) of the dolby chip, 6.02v on the pin 5 (with and without the oscillocope being connected).
Obviously that also gives the same 6.02v on one side of C22 (since it's connected to the pin 5), and like 0.1mv on the other pin of the capacitor (but on both channels, so I guess it's normal).

Somebody suggested that I rent a thermograph imager to compare the temperature on both sides of the pcb when noise starts to appear.

Does that makes sense or would that be a waste of time and money?
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Old 13th Feb 2020, 9:26 pm   #63
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Default Re: Diagnosing temperature related noise?

Technically it sounds good, but I do not know how effective it might be with a small PCB, especially in terms of cost verses potential likelihood of detecting anything that would help the diagnosis.

I have used them myself a little bit, but on high voltage power circuit breaker panels where they can be very useful.

I do not want to condemn it as such because it is a difficult problem, however on balance I personally consider it might be a waste of time, will be interesting to see what others think.

Last edited by DMcMahon; 13th Feb 2020 at 9:27 pm. Reason: Typo
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Old 23rd Feb 2020, 6:22 pm   #64
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Default Re: Diagnosing temperature related noise?

Well, I did try a bit more, poking here and there, and as far as I see the problem happens in the dolby/filter bloc, but I did not manage to understand how the whole thing works.

It looks like the audio data always goes through the Dolby chip, and that depending of the switch positions the data gets filtered or not, but I was not able to understand exactly what did what.
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Old 24th Feb 2020, 10:25 pm   #65
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Default Re: Diagnosing temperature related noise?

Yes I think that is fairly common with Dolby recorder circuits, the audio signal goes into and comes out of the Dolby circuitry even when Dolby operation is switched off, but obviously when Dolby is switched on, the Dolby circuitry additionally does the Dolby processing.
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Old 25th Feb 2020, 9:05 am   #66
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Default Re: Diagnosing temperature related noise?

Ok, makes sense.

Is there even a remote possibility that the Dolby IC could be the cause of the problem?

I've known ICs that totally worked or totally did not work, or that had some combinations of gates that worked/did not work, but I've never seen seen integrated circuit to fail after they get warm.
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Old 25th Feb 2020, 1:47 pm   #67
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Default Re: Diagnosing temperature related noise?

In a word, yes. If the packaging has failed to the point of letting in moisture, this may cause erratic operation. Substitution may be the only sure test.
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Old 25th Feb 2020, 2:24 pm   #68
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Default Re: Diagnosing temperature related noise?

I guess I could try something "simple": Unsolder both Dolby chips, install sockets, swap left and right, see if the fault appears on the other channel.
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Old 25th Feb 2020, 5:10 pm   #69
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Default Re: Diagnosing temperature related noise?

Might be less disruptive to buy another chip off an auction site - there appear to be a few to be had.
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Old 25th Feb 2020, 5:12 pm   #70
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Default Re: Diagnosing temperature related noise?

If you are good at unsoldering and soldering in the IC sockets then yes this is a good idea. However if any of the associated artwork has plated through holes/tracks on both sides of the board then depending soldering skills then could be somewhat risky, i.e. damage to artwork tracks and worst case could end up making the left channel bad also.

I personally would go for it, have you tried any sort of gentle cooling down of the suspect Dolby chip ?
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Old 25th Feb 2020, 5:17 pm   #71
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Default Re: Diagnosing temperature related noise?

Swapping the chips over involves a finite risk of damaging the good channel - I vote for replacement. If the fault is proved, you'll need a new chip anyway.
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Old 25th Feb 2020, 5:24 pm   #72
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Default Re: Diagnosing temperature related noise?

I would probably not try desoldering if it was a double sided pcb, but this thing only has tracks on the solder side, that's why the board is such a mess of cables all over the place

I was about to try cooling until I read that cooling could actually damage other components, but yeah at this point I guess I could just try that first.

Regarding ebay, are we relatively certain that these chips are actually working ones and not cheap cloned that don't work the same or have different tolerances or simply don't work at all?
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Old 25th Feb 2020, 7:04 pm   #73
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Default Re: Diagnosing temperature related noise?

Impossible question to answer. Buying any component from any selling site has a risk, that worst case the part supplied is a cheap clone of poor quality or old unknown operationally condition.

I think in most cases the components sold are good, I would certainly enquire as to their origin. i.e are they OEM parts such as Signetics/Philips/Sharp (to name just 3 for this device). Would steer clear of unbranded parts.

Getting components from a component distributor such as Farnell, RS (to name just 2) may give more confidence that they are original/non cloned parts, Note - I have not checked to see who might sell this particular chip.
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Old 26th Feb 2020, 9:11 am   #74
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Default Re: Diagnosing temperature related noise?

Does the various versions of the chip (as you said Signetics, Philips, and Sharp) have the exact same specs? If the remaining chip is say a Signetics version, and I get a Philips on the other side, would that have possibly some impact in term of volume, noise/signal ratio, etc... or are they pretty much identical?
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Old 26th Feb 2020, 10:42 am   #75
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Default Re: Diagnosing temperature related noise?

Yes any official version (not cloned copy) will have the same spec/performance.

Signetics as a company are long gone and were brought out by Philips long ago.

Signetics devices can normally be identified by the large S on the chip.

Because the Signetics company are long gone, any Signetics devices purchased would be either used or NOS (New Old Stock). I find NOS a bit misleading, i.e. "New" could be anything to > 40 years, maybe OUS (Old Unused Stock) would be more accurate, but of course OUS does not sound so good for selling.

I have no idea if the NE-645B is still manufactured new by anybody.
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Old 26th Feb 2020, 10:46 am   #76
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Default Re: Diagnosing temperature related noise?

I forgot to add that I see that the Signetics data sheet (link previously provided by Lawrence) has some useful or at least interesting technical information.
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Old 26th Feb 2020, 8:39 pm   #77
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Default Re: Diagnosing temperature related noise?

I just ordered one of each (Signetics and Philips), these things are surprisingly cheap and in quite large quantity.

Guess now I just need to wait
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Old 18th Mar 2020, 10:00 pm   #78
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Default Re: Diagnosing temperature related noise?

I just received a couple of Philips chips, guess I'll have to do some desoldering, socketing and testing this weekend!
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Old 19th Mar 2020, 8:59 pm   #79
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Default Re: Diagnosing temperature related noise?

I removed the old chip and put the Philips in place (on a socket)
https://twitter.com/DefenceForceOrg/...16328442507264

Currently listening to some music, so far, so good.
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Old 19th Mar 2020, 9:14 pm   #80
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Fingers and toes crossed
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