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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 7th Nov 2019, 3:16 pm   #21
ms660
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Default Re: CR 240 Signal Tracing Confusion

Quote:
Originally Posted by Uncle Bulgaria View Post
With the erase head shorted and 400mV RMS at 528 and 514 (333Hz signal from Aux), I get 6.4V DC at 515 & 501
6.4 volts DC seems high to me given the supply voltage you quoted earlier.

For info...from the blurry schematic....I make the top resistor of the potential divider that supplies the non-inverting inputs DC voltage as 3.9k and the bottom resistor as 4.7k, might be worth measuring the voltage at the actual divider and comparing it with DC voltages on the non-inverting input pins.

Lawrence.
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Old 7th Nov 2019, 4:33 pm   #22
Uncle Bulgaria
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Default Re: CR 240 Signal Tracing Confusion

Things are looking up! I replaced the IC this morning with a new Texas Instruments RC4136N and some improvements are noticeable.

The divider now measures 5.18VDC at the junction (I read the same resistor values), with 5.16VDC at the non-inverting inputs.

With 400mVAC RMS at 528 & 514 we now have 20mVAC RMS at 328 & 316 on Fe and ca. 30mV on Cr.

This is still with the erase head shorted. I can make a recording and get the same mV back at MP3 & 4 as the output. I haven't tried with the bias in circuit yet.

There is relatively significant hiss and crackle in the headphones, which tempts me to look at the coupling capacitors.
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Old 7th Nov 2019, 4:46 pm   #23
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Default Re: CR 240 Signal Tracing Confusion

If you haven't already seen it the article given in the link below shows a circuit (Fig.2) that's basically the same setup as the Op Amp in your machine and gives some explanation which you might find useful.

https://www.analog.com/media/en/anal...y-problems.pdf

Keep at it.

Lawrence.
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Old 7th Nov 2019, 5:56 pm   #24
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Default Re: CR 240 Signal Tracing Confusion

Interesting! Thank you. Mostly over my head, but I expect it will go in with repeated viewings. I'll print that out.

Slightly OT - I was reading the associated text for figure 2 and they mention suitable values for the feedback resistor R2 as being 100k - 27k for supplies 15V-3.3V. Here we have a complex feedback network of resistors and capacitors (is that an M-derived filter? I was reading my 1947 Radio Handbook last night and the filter section had something that looked similar), but the first series resistor is R362 at 1M.

The hiss is present without any of the audio boards in place, and doesn't change with volume (only some hum at maximum) so I'll look for that later in the final output stage on the main board.

If it was working from the factory, and the original paint is still on the trimmers, I feel I should be looking for some more component issues before cranking them up to get the output levels right for the block diagram. Could it be another case of 'beware the blue tants' as I read in an article linked here the other day?

Or is it an evening playing 'sort the bias trap'?

Tune in for another exciting instalment!
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Old 7th Nov 2019, 9:03 pm   #25
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Default Re: CR 240 Signal Tracing Confusion

Adjusting the bias trap inductors as per the manual (attached - I'm using a blank Chrome cassette in case it matters), I can get them both down to about 5mVpp at MP5 & MP6 by 'scope (I assume the requested 'LF voltmeter' is 'low frequency' as opposed to 'RF', but mine doesn't detect any voltage for the 98kHz).

However, that makes the situation at the output pins 328 & 316 noticeably worse. See attached. When they were 1.4Vpp and 3Vpp before adjustment, one's now 6Vpp! These are both on Chrome, one distorted but both pretty strong.

If I adjust them for minimum at the pins, the signal is worse at MP5 & MP6. Do I adjust as they say, or trace the source of the distorted wave and try to get it down at the pins?
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Old 7th Nov 2019, 9:29 pm   #26
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Default Re: CR 240 Signal Tracing Confusion

Did you read what I mentioned earlier about 'scope probe/lead capacitance.....

Lawrence.
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Old 7th Nov 2019, 9:53 pm   #27
Uncle Bulgaria
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Default Re: CR 240 Signal Tracing Confusion

Yes, silly of me. I was hung up on clarifying the metering part after that post. So it's worthless metering the pins. With a DMM that only reads AC to 1kHz, I don't know how to set the trap without the 'scope at MP5/6. The 'electrical alignments' section mentions 'connect an oscillograph to the output of the voltmeter (internal resistance 1M)' which I suppose, dredging up possibilities, is to do with loading or shunting in some way.

Unless they mean to connect it in parallel with whatever meter they would have been using I don't know if that method would apply to today's DMMs...

Regardless, I've just tested recording with the requested 250mV/333Hz in at the auxiliary, with 200mV supposed to be set and measured at MP5/6 on recording and playback...and I get 250mV out! Ten times better than the 20mV I was getting earlier today. A twist of the trimmers and I'll be there. The internal mic. is also working now, and some Elgar has just made it from the radio onto tape with great success!

Happy Thursday everyone!
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