CR 240 Signal Tracing Confusion

[Uncle Bulgaria]

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?

[Uncle Bulgaria]

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.

[ms660]

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.

[Uncle Bulgaria]

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.

[ms660]

Originally Posted by Uncle Bulgaria

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.

If you want to measure the signal voltage at the record head or the other side of the bias trap you should disable the oscillator, and if you want to compare to that shown in the manual you're source voltage and frequency should be that as specified.

Lawrence.

[Uncle Bulgaria]

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.

[ms660]

Originally Posted by Uncle Bulgaria

I will read up on what a bias trap is, as I don't understand this part of the circuit at all!

The bias trap is there to ensure that the bias voltage that's applied to the head in record mode doesn't find its way back in any large quantity back into the record amplifier, if it did it would mess things up because it's amplitude is considerably more than the amplitude of the audio signal.

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.

[Uncle Bulgaria]

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?

[TIMTAPE]

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?

[ms660]

Originally Posted by Uncle Bulgaria

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.

T310 and T312 aren't switches, they're signal amplifiers, the signal follows the same path (input to base, output at collector) irrespective of the Cr setting, with the Cr setting selected the output from their collectors will be higher than it would be with the other tape settings selected, with Cr selected their emitter resistors are bypassed by the AC network that's switched into circuit by T306 and T307, remember that in simple terms that an un-bypassed emitter resistor acts as a -ve feedback element so far as the signal is concerned, -ve feedback reduces gain, less -ve feedback (Cr selected) increases gain.

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?

It shouldn't unless the trap frequency is wrong by a good margin, but looking at the levels in the block diagram you will see that the outputs from the final record amplifiers (points 328 & 316) are quite low compared to the levels earlier on in the amplifier chain, so if you intend 'scoping up to the heads you might find the presence of the bias waveform confusing to your measurements because the bias voltage will be considerably higher than the record signal voltage by a large margin.

Lawrence.

[Uncle Bulgaria]

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.

[Uncle Bulgaria]

Originally Posted by ms660

T310 and T312 aren't switches, they're signal amplifiers, the signal follows the same path (input to base, output at collector) irrespective of the Cr setting, with the Cr setting selected the output from their collectors will be higher than it would be with the other tape settings selected, with Cr selected their emitter resistors are bypassed by the AC network that's switched into circuit by T306 and T307, remember that in simple terms that an un-bypassed emitter resistor acts as a -ve feedback element so far as the signal is concerned, -ve feedback reduces gain, less -ve feedback (Cr selected) increases gain.

Lawrence.

Of course - I meant T306 and T307! T310 and T312 are always in circuit because their bases are fed by the signal, it is only the emitter resistance that changes when T306 and T307 are in circuit. Or is it the emitter capacitance that changes as it's a bypass network, resulting in an increase of AC gain? I deduce that T336 and T338 are regulating the collector current of T310 and T312 as they are connected to the 9.5V rail via the record switch S1 and have a diode bias.

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.

[Uncle Bulgaria]

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.

[ms660]

Originally Posted by Uncle Bulgaria

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.

Try feeding in a signal as per the block diagram, ie: 500mV RMS to the Aux. input and adjust the levels for 400mV RMS at the specified test points....according to your 'scope you're feeding in a signal that's 250mV peak to peak (88mV RMS).....big difference.

Lawrence.

[Uncle Bulgaria]

Originally Posted by ms660

Try feeding in a signal as per the block diagram, ie: 500mV RMS to the Aux. input and adjust the levels for 400mV RMS at the specified test points....according to your 'scope you're feeding in a signal that's 250mV peak to peak (88mV RMS).....big difference.

Lawrence.

I was trying 250mV (I thought peak rather than RMS as it wasn't specified in the manual, and I recalled your advice from the playback amplifier about overloading) based on the injunction in 2.10.2 "Regulation of Recording Current for Channel Balance with Tape". It's also the maximum output I can get from my 'phone which is generating the signal!

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...

[ms660]

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.

[Uncle Bulgaria]

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.

[ms660]

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.

[TIMTAPE]

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.

[Uncle Bulgaria]

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.