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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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4th Nov 2019, 9:49 pm | #1 |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
Posts: 2,333
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CR 240 Signal Tracing Confusion
I can monitor an external source through headphones in good quality and volume. On recording to cassette the result is thin and quiet. Playback of pre-recorded cassettes is good.
I'm trying to work out where the fault might lie, but am having trouble understanding part of the manual. See attached paragraph. By 'bypass the erase head connection' do they mean short out the head by connecting the black to the yellow? Does this generate the 400mV required, as there is no input specified? |
5th Nov 2019, 2:59 am | #2 |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
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Re: CR 240 Signal Tracing Confusion
I've got myself stuck again. I've tried to trace a signal from the auxiliary input to the tape heads. I may have found something around the recording amplifier of board 300. The diagram is attached.
The inputs from the previous Dolby section arrive on pins 9 and 13 of the IC. The line to pin 318 is +9.5V, and that to T313 +6.2V. The points MP5 and MP6 are where I should get an output that corresponds to the manual, with the tape heads next in line off to the left. At present I can get a noisy sine wave on either of these points, if I ground the other input. So I can get a reading on MP5 if pin 13 is grounded, and MP6 if 9 is grounded. This also gives me a better signal at the relevant input and output. Still a very noisy wave though. If both inputs are connected, there is nothing registering on MP5 or 6. Is this something to do with the networks connected to pins 8 and 14 - some kind of dodgy feedback? Based on the noise on the wave, the feedback's not doing a great job if that's what it is. Is the IC bust or should I go about changing components in the network? It is a large collection of blue tantalum bead capacitors, though those on the power lines have been changed to al-el. |
5th Nov 2019, 11:08 am | #3 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: CR 240 Signal Tracing Confusion
Did you disable the bias osc as described in the instruction snippet you first posted?
Is the ALC switched off? Remember that the signal levels given in the manual will be in RMS values unless otherwise stated, a 'scope displays peak to peak values. Lawrence. |
5th Nov 2019, 2:50 pm | #4 |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
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Re: CR 240 Signal Tracing Confusion
I did, but it made no difference. I think that side of things is working, as on the 'scope I get ca. 17V pk-pk of very clean sine wave on each channel of the head in stop-record mode with a cassette inserted.
The ALC board is out, as is the meter board. I have 500 Dolby and 200 Recording Preamplifier in. 300 is out connected with jumpers, with a sine signal at about 200mV RMS being fed in to ZUSATZ EINGANG (Aux. Input). This signal can be monitored clearly and loudly through the speaker or headphones. As mper the manual, I have used the front level knobs to set 200mV on MP3 & MP4, which is the volume control. The block diagram suggests that without ALC, the signal should head through the Dolby board at pins 515 and 501, then reaching the RC 4136 IC at pins 321 and 323. The block does not mention the DC supply tapped from the +9.5V rail that also attaches to these signal pins, but from reading about op-amps I suppose that's for biasing. Last edited by Uncle Bulgaria; 5th Nov 2019 at 2:59 pm. |
5th Nov 2019, 4:11 pm | #5 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: CR 240 Signal Tracing Confusion
Quote:
Lawrence. |
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5th Nov 2019, 7:10 pm | #6 |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
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Re: CR 240 Signal Tracing Confusion
I'm just trying to work out why the recording is so different from the monitoring signal. It seems to be getting lost or at least made very noisy and quiet around that IC and before MP5 & 6. I will read up on what a bias trap is, as I don't understand this part of the circuit at all!
I meant the erase oscillator seems to be working as the signal at the heads without it shorted is the one mentioned. Shorting it wasn't affecting the signals at the IC, but I'll try again as I probably did something wrong. |
5th Nov 2019, 9:31 pm | #7 | |
Dekatron
Join Date: Apr 2011
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Re: CR 240 Signal Tracing Confusion
Quote:
In this machine the bias traps are composed of L2 & C11 and L3 & C12 that's a separate trap for each channel, looking at the schematic you will see that the two components of each trap are connected in parallel, when a circuit consists of an arrangement like that we call it a parallel resonant circuit (aka a tuned circuit) It's resonant frequency is adjusted to be the same frequency as the bias frequency, when that happens the circuit presents itself as a high impedance to the bias frequency, therefore the amount of bias that gets back into the record amplifier will be minimal but the traps impedance presented to the much lower audio frequencies will be low in comparison thus allowing the audio signal from the final record amplifier to pass through to the head. The different types of tape that this machine is designed to use need different bias levels for optimum recording, the bias voltage is derived from the erase oscillator voltage that feeds the erase head, a proportion of that voltage is fed to the heads via the tape type selector circuit and C9 & R9 for one channel and C10 & R21 for the other channel, you will already know that the switch has three settings for the different types of tape (Cr, FeCr and Fe) and you can see how the three different bias levels are obtained by the switch and the three resistors associated with it. You will also see that the switch also in effect grounds the base bias resistors of T3 thus turning T3 on which then conducts and connects the 6.2 volt supply rail to the base bias resistors of T306 and T307 thus turning those two transistors on, that results in a frequency dependant network being connected in effect across the emitter resistors of T310 and T312 (both those transistors are the final record amplifiers) you will notice the increased level of the signal voltage as a result of that action given in the block diagram (50mV for Cr against 30mV for FeCr and Fe) If that's any use. Lawrence. |
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6th Nov 2019, 1:13 am | #8 |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
Posts: 2,333
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Re: CR 240 Signal Tracing Confusion
That's a lot more helpful than the hits I was getting online. Despite my search parameters I got a lot of bias binding for sewing! Much appreciated.
So the concept of the bias trap is a similar idea as radio tuning, but as we know the frequency of the oscillator there's no need for variability except for trimming adjustment, instead of having a variable capacitor. Very clever. So when T310 and T312 are 'on', because of the Chrome setting engaging T2, the resistance in their emitter circuits is lower than R340/341 (180k), meaning the signal is amplified there rather than following the path for the other tape types. How does shorting that erase head affect what I might be measuring on the IC? I see that the bias is applied after the trap, so the circuit on the record amplifier of board 300 should be unaffected. Does this imply there is an issue with the IC, or the components around it? |
6th Nov 2019, 8:01 am | #9 |
Octode
Join Date: Jul 2006
Location: Perth, Western Australia
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Re: CR 240 Signal Tracing Confusion
With the symptom of OK playback but weak recording, the first thing I would do is check the tape head for cleanliness and wear, and check the record bias is correct. Electronic faults do happen of course, and increasingly with time, but more often with record problems it's head and bias level issues IME. Also are any bias and EQ switches set correctly for the test tape?
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6th Nov 2019, 11:47 am | #10 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: CR 240 Signal Tracing Confusion
Quote:
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6th Nov 2019, 12:05 pm | #11 |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
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Re: CR 240 Signal Tracing Confusion
The head looks all right, from my limited experience. The record bias is set after adjusting the record level with a trimmer. I think the problem is electrical first of all because any recording made at the level required is so quiet as to be barely audible through headphones at full volume over the hum and hiss.
I think I need to investigate the components around the IC, as surely only having a signal when one input is grounded is peculiar. T2 is being engaged by the tape bias switch - pin 322 has an emf of ~6.2V when Chrome is selected. However, there is no visible change in output from pins 328 and 316 (only the bias trap lies between this point and the head) with any of the tape bias settings. Certainly no jump with the Chrome setting as the block diagram suggests should happen. |
6th Nov 2019, 12:15 pm | #12 | |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
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Re: CR 240 Signal Tracing Confusion
Quote:
I'm only trying to get the expected signal at MP5 and MP6 at the moment rather than going to the head yet. Something's happening to the signal before it gets to the bias trap - with the board out and the connections jumpered, the signal is getting lost even without the output pins connected so the bias can't be getting back into the board. |
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6th Nov 2019, 5:05 pm | #13 |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
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Re: CR 240 Signal Tracing Confusion
Keeping at it, I've replaced C338 which seems to have dealt with some of the inconsistencies around the IC.
However, the problem remains. Voltages are matching on each channel around the final transistor output. Here's what I get on the 'scope. The signal at about 250mV as it enters the machine. When it is entering board 300 after leaving the Dolby section, it's strangely overlaid and jittery, but still about 30mV and recognisable. At the output of the recording section on the way to the bias trap it's descended into noise. Perhaps just discernible down at the 2mV/div. I'll "keep on trackin'"! Oh, this is all with the erase head shorted. Last edited by Uncle Bulgaria; 6th Nov 2019 at 5:15 pm. |
6th Nov 2019, 5:24 pm | #14 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: CR 240 Signal Tracing Confusion
Quote:
Lawrence. Last edited by ms660; 6th Nov 2019 at 5:29 pm. |
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6th Nov 2019, 5:35 pm | #15 | |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
Posts: 2,333
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Re: CR 240 Signal Tracing Confusion
Quote:
I'll try out of my PC, though I fear it will be far too noisy. A good idea though, as you're right - it could just be swamped. However, it does come out clearly in the speakers at this level, and I didn't have to adjust the level control anywhere near maximum to attain the 200mV peak on the MP3/4. Although this could also be intended to be an RMS... |
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6th Nov 2019, 7:23 pm | #16 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: CR 240 Signal Tracing Confusion
For a fair assessment you need to adjust the levels so there is 400mV RMS at points 514 and 528, in 'scope money that's approx. 1.13 volts peak to peak.
Lawrence. |
6th Nov 2019, 8:34 pm | #17 |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
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Re: CR 240 Signal Tracing Confusion
With the erase head in circuit and no input signal, it looks like the bias trap is not doing its job.
Attached is what I get at different tape-type positions at the L & R outputs of board 300. If a wavelength is about 5µs then I make it a frequency of 100kHz. The clipping's obvious at the chrome setting... Does this seem like the trap is incorrectly set, and worth fiddling with the trimmers, or does it seem so out of whack that a broken component is likely to be to blame? I'm assuming that there should be no signal behind the trap towards the amplifier if it's working correctly. With the erase head shorted and 400mV RMS at 528 and 514 (333Hz signal from Aux), I get 6.4V DC at 515 & 501, and no measurable AC with AVO or DMM. |
6th Nov 2019, 9:03 pm | #18 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: CR 240 Signal Tracing Confusion
I thought you were signal tracing using a 'scope not an AVO or DMM.
Regarding the bias trap you should 'scope at the points specified in the manual to see if it's doing its job, not at the trap itself otherwise the capacitance of your 'scope probe/lead will alter the frequency it's tuned to. No photo yet. Lawrence. |
6th Nov 2019, 9:58 pm | #19 |
Octode
Join Date: Jul 2006
Location: Perth, Western Australia
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Re: CR 240 Signal Tracing Confusion
I'm finding the copy of the schematic hard to read.
With very low audio around that area, and common to both channels, what immediately springs to mind is a "record mute" circuit switched in when it shouldnt be. |
6th Nov 2019, 10:09 pm | #20 |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
Posts: 2,333
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Re: CR 240 Signal Tracing Confusion
ms660 - I had the message open for too long (typing and measuring!) and the attachments didn't hold. Here they are. For new readers, 316 and 328 are the output channel pins from board 300, which head to the bias trap then the head.
I have been using my 'scope, but thought my DMM with 'TRUE RMS' would be good for setting the required levels. On metering the junction of the Dolby and Record boards, I thought to use them to get a mV level rather than try to calculate it from the 'scope. TIMTAPE - the diagrams are awful, but that's all I've got! It's a PDF download and a bad scan. I've found a magnifying glass handy, and some of the pins I've had to work out and annotate by following tracks. I've tried to get a high resolution by maximising the forum's image upload limits, and what I download here is much the same as what I have printed out and on screen. I have just ordered a genuine printed service manual and diagram from a Uher service chap in Germany, so I hope to have some better scans soon! The unit is in stop and record, so monitoring is enabled. If I just record from that position (400mV at 528 and 514), the recording is thin, and very, very quiet. A DMM reading of the AC at the volume control on playback, that should be equal to the monitored signal on record, is perhaps 20mV. I have had some improvement with swapping out the RC4136DB on the amplifier board with one of the two on the Dolby board (one per channel). I have a new one on order in case it's the culprit. Based on my experience with the playback amplifier, that one of the tantalum bead capacitors was the cause of the fading audio, I'm tempted to replace all of them on the recording board too with new electrolytics. I understand these should be extremely reliable if not decoupling a power rail, but that doesn't seem to be the case in this unit so far. |