CD104 channel failure - UK Vintage Radio Repair and Restoration Discussion Forum

Uncle Bulgaria · 29th Nov 2020, 4:17 pm

My CD104 was playing happily until the left channel failed.

Historically I have replaced the griplets and the machine's been working.

I thought I could trace this through, but I'm stumped. I can't see the digital signals on my oscilloscope, and don't know what I'm looking for beyond the audio stage.

The PSU is all right.

The voltages on the left channel NE5532 are peculiar, and I can't see why this is. I have 10V on pin 1 where there should be 3.4V. Removing the reed switch has no effect. There is +12 and -12 on pins 8 and 4 instead of the +/-11V specified, but other DC voltages are hovering perhaps a volt high so I'm less concerned about this than the extra 6.6V.

Removing the NE5532 and swapping it with the working channel has no effect on the excess voltage.

As there is not 10V on pin 1 with the NE5532 removed, it must have some effect but I can't see how 10V is getting to that point when the voltages on pins 6&5 are where they should be at 3.4V.

TonyDuell · 29th Nov 2020, 4:59 pm

Go back to the outputs of the DACs. What voltages do you see there on both the good and faulty channel? Looks like one of them is on pin 22 in that schematic clip you posted.

Uncle Bulgaria · 29th Nov 2020, 5:03 pm

Without the NE5532s in place, or the reed switches, I get -0.65VDC on both pins 22. When a CD's playing I got incomprehensible fuzz with an oscilloscope on both, but that seemed to be happy enough to be visible as audio in the good channel by the time it got to pin 1 of the NE5532.

TonyDuell · 29th Nov 2020, 5:15 pm

Ah.. Isn't it a current-output DAC with the first half of the NE5532 acting as a current-to-voltage converter?

I don't know how easy it is to do this, but can you lift that 'resistor' labelled OE that connects to the output of the DAC on each channel and connect them to the 'wrong' DAC outputs. Does the fault stay with the analouge filtering circuitry round the NE5523 or with a particular DAC?

Uncle Bulgaria · 29th Nov 2020, 5:25 pm

Quote:

Originally Posted by TonyDuell

Ah.. Isn't it a current-output DAC with the first half of the NE5532 acting as a current-to-voltage converter?

It could well be - I have no idea so you have the advantage of me there! That would explain where the mysterious voltage is coming from. Should I monitor the DC voltage across pins 2 and 1, as that is where excess current would manifest as voltage, isn't it?

Quote:

Originally Posted by TonyDuell

I don't know how easy it is to do this, but can you lift that 'resistor' labelled OE that connects to the output of the DAC on each channel and connect them to the 'wrong' DAC outputs. Does the fault stay with the analouge filtering circuitry round the NE5523 or with a particular DAC?

That resistor is a little green SMD part, so I can try but I might ping it across the room. Do you mean replace the NE5532s, then jumper the DAC output (now isolated from the failed channel downstream of resistor 3937) to the working channel's NE5532 to see if it's the DAC output?

TonyDuell · 29th Nov 2020, 5:42 pm

The 2 inputs of the current-to-voltage converter op-amp should be at the same voltage. Since pin 3 is grounds, pin 2 should also be at 0V. The idea of the circuit is that the dac sucks current into its output pin. It can't come from the input (pin 2) of the op-amp as thats high impedance. It has to come from the feedback circuit, meaning the output of the op-amp has to go to a non-zero voltage so that the feedback circuit can carry current.

Ok, I suspect that 'resistor' I mentioned is actually a 0-ohm jumper. A shorting link if you like. I'd want to open-circuit it though, somehow. With it out totally, what voltage do you see at the output (pin 1) of the op-amp? It should be 0V as no current can be taken from the input of the circuit with that 'resistor' open of course.

Do not connect 2 such circuits to the same DAC. Rather try connecting them to the 'wrong' DAC -- swap the channels over at that point. You could try swapping the 2 DAC chips over, they're DIL packages I think so not too hard to remove. But watch out for the tiny SMD capacitors hung off some of the pins.

DMcMahon · 29th Nov 2020, 5:56 pm

0E maybe a jumper ?

Edit -sent before seeing Tony's post.

Uncle Bulgaria · 29th Nov 2020, 8:54 pm

Thanks Tony, you didn't mislead me I just failed to clarify that I didn't mean I was going to connect both DACs to one circuit!

Removing jumper 3937 from the L channel and supplying signal from the right channel (good) DAC to the L NE5532 shows no change - still 10.1V on pin 1 of the left NE5532. Supplying the R channel NE5532 with the output from the L (bad) DAC gets a normal audio output trace, and normal voltages on the pins of the R NE5532.

The NE5532s are not the issue, so it suggests something around the circuit surrounding the L NE5532 is.

ronbryan · 29th Nov 2020, 9:01 pm

R3574 1k8 feedback resistor open so the amp is operating open loop and latching high?

Ron

Uncle Bulgaria · 29th Nov 2020, 9:11 pm

Thoughtful, Ron - but removing 3574 (which measures good OOC) we still have over 10V on pin 1, and pins 5, 6 & 7, and -5.5V on pin 2. This is with no DAC connected to the NE5532. Pin 4 is still -11.3V, and pin 8 is +11.4V.

ms660 · 29th Nov 2020, 9:19 pm

O/C ground?

Lawrence.

ronbryan · 29th Nov 2020, 9:49 pm

With regard to posts #9 and 10, removing R3574 and then testing the voltage on pin 1 is not appropriate, as the amplifier will still be operating open loop with high gain, as the DC feedback is missing. It would be better to tack a resistor between pin 1 and pin 2 on the IC legs (to eliminate any open tracks) and then test the voltage on pin 1.

With regard to Lawrence's post, is pin 3 at ground potential?

Ron

Uncle Bulgaria · 29th Nov 2020, 10:24 pm

Thanks for the clarification, Ron. I didn't realise that. 2 and 1 are still connected via the rest of the network, if that helps. The reed switch is working, so is closed when the power is on. Perhaps 0.8M is too great for the purpose.

Yes, pin 3 has ground continuity.

Uncle Bulgaria · 29th Nov 2020, 10:29 pm

Status of the L NE5532 with break at R3937, no R3574. VDC wrt Ground.

Pin 1: +10.7
Pin 2: -5.5
Pin 3: 0.2
Pin 4: -11.3
Pin 5, 6, 7: +9.9
Pin 8: +11.5

I'm going to try another 1.8k resistor in place of R3574 in case something happened to it when voltage was applied, even though it measured fine OOC.

ronbryan · 29th Nov 2020, 11:05 pm

You must have a DC connection between pins 1 and 2 i.e. a resistor representing R3574 for the amplifier to work as intended. The rest of the feedback network around pins 1 and 2 is AC coupled (i.e. DC is blocked).

I was suggesting that a 1k8 resistor should be tacked directly between pins 1 and 2 of the NE5532 IC for test purposes, to ensure a DC connection, in case a pc track had cracked.

Ron

Uncle Bulgaria · 29th Nov 2020, 11:16 pm

I got that Ron, I just thought I'd update what was what.

With a 1.8k between pins 1 & 2, the results above are exactly the same, except pin 2 is now +9.9V.

Uncle Bulgaria · 29th Nov 2020, 11:49 pm

I just can't see how there can be any current flow as there's no input. I suppose I'll have to remove all the components in the filter network and test them individually.

TonyDuell · 30th Nov 2020, 6:23 am

OK, so the DACs are good, the problem is just round that op-amp chip.

Quick test if you've not already done it. With the NE5532 removed (and the unit turned off) what DC resistance do yo measure between pins 1 and 2 of the NE5532 socket? And do check that pin 3 is solidly grounded.

Have you checked the NE5532 supply voltages _at the pins of the IC_ ?

Can you remove the resistor (I guess SMD, alas that connects the output of the first half of the NE5532 to the second stage? Just in case the odd voltage is coming in that way.

Uncle Bulgaria · 30th Nov 2020, 2:32 pm

I'm sorry, I failed to specify - those voltages above are all measured on the pins (the component side faces downwards when the board's in the normal orientation, so it's quite accessible). Following the tracks on the PCB around the NE5532s, the voltage is constant at all the interconnected places, and there's continuity back to the DACs so I don't think a cracked print is to blame.

Don't worry about the SMD - I've got a little pile of the things now. As Richmall Crompton wrote 'one may as well drink of tragedy to the very dregs when one is about it'.

I did remove R3576 (the link between pin 1 and pin 5) last night before I despaired. There is no link back to the DAC, and no link forward to the next half of the IC. All other components remain in place.

The first half is then 10.6V pin 1; 9.9V pin 2; 0.02V pin 3. Second half: -11/+11 pin 4/pin 8; -2.9 pin 5; -9.9 pins 6 & 7.

With the NE5532 removed (I'm going to have to put a socket in at this rate!) I get open circuit between pins 1 & 2 (R3574 1.8k was on the pins). It measures 1.8k exactly. <1R to ground at pin 3.

TonyDuell · 30th Nov 2020, 5:30 pm

Yes, fit a turned-pin socket and re-fit that 1.8k resistor.

Now with the DAC disconnected and the link to the second stage disconnected, what voltages do you get at pin 1?

29th Nov 2020, 4:17 pm	#1
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,332	CD104 channel failure My CD104 was playing happily until the left channel failed. Historically I have replaced the griplets and the machine's been working. I thought I could trace this through, but I'm stumped. I can't see the digital signals on my oscilloscope, and don't know what I'm looking for beyond the audio stage. The PSU is all right. The voltages on the left channel NE5532 are peculiar, and I can't see why this is. I have 10V on pin 1 where there should be 3.4V. Removing the reed switch has no effect. There is +12 and -12 on pins 8 and 4 instead of the +/-11V specified, but other DC voltages are hovering perhaps a volt high so I'm less concerned about this than the extra 6.6V. Removing the NE5532 and swapping it with the working channel has no effect on the excess voltage. As there is not 10V on pin 1 with the NE5532 removed, it must have some effect but I can't see how 10V is getting to that point when the voltages on pins 6&5 are where they should be at 3.4V. Attached Thumbnails Last edited by Uncle Bulgaria; 29th Nov 2020 at 4:29 pm. Reason: Circuit diagram

29th Nov 2020, 4:59 pm	#2
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: CD104 channel failure Go back to the outputs of the DACs. What voltages do you see there on both the good and faulty channel? Looks like one of them is on pin 22 in that schematic clip you posted.

29th Nov 2020, 5:03 pm	#3
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,332	Re: CD104 channel failure Without the NE5532s in place, or the reed switches, I get -0.65VDC on both pins 22. When a CD's playing I got incomprehensible fuzz with an oscilloscope on both, but that seemed to be happy enough to be visible as audio in the good channel by the time it got to pin 1 of the NE5532.

29th Nov 2020, 5:15 pm	#4
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: CD104 channel failure Ah.. Isn't it a current-output DAC with the first half of the NE5532 acting as a current-to-voltage converter? I don't know how easy it is to do this, but can you lift that 'resistor' labelled OE that connects to the output of the DAC on each channel and connect them to the 'wrong' DAC outputs. Does the fault stay with the analouge filtering circuitry round the NE5523 or with a particular DAC?

29th Nov 2020, 5:42 pm	#6
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: CD104 channel failure The 2 inputs of the current-to-voltage converter op-amp should be at the same voltage. Since pin 3 is grounds, pin 2 should also be at 0V. The idea of the circuit is that the dac sucks current into its output pin. It can't come from the input (pin 2) of the op-amp as thats high impedance. It has to come from the feedback circuit, meaning the output of the op-amp has to go to a non-zero voltage so that the feedback circuit can carry current. Ok, I suspect that 'resistor' I mentioned is actually a 0-ohm jumper. A shorting link if you like. I'd want to open-circuit it though, somehow. With it out totally, what voltage do you see at the output (pin 1) of the op-amp? It should be 0V as no current can be taken from the input of the circuit with that 'resistor' open of course. Do not connect 2 such circuits to the same DAC. Rather try connecting them to the 'wrong' DAC -- swap the channels over at that point. You could try swapping the 2 DAC chips over, they're DIL packages I think so not too hard to remove. But watch out for the tiny SMD capacitors hung off some of the pins.

29th Nov 2020, 5:56 pm	#7
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: CD104 channel failure 0E maybe a jumper ? Edit -sent before seeing Tony's post.

29th Nov 2020, 8:54 pm	#8
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,332	Re: CD104 channel failure Thanks Tony, you didn't mislead me I just failed to clarify that I didn't mean I was going to connect both DACs to one circuit! Removing jumper 3937 from the L channel and supplying signal from the right channel (good) DAC to the L NE5532 shows no change - still 10.1V on pin 1 of the left NE5532. Supplying the R channel NE5532 with the output from the L (bad) DAC gets a normal audio output trace, and normal voltages on the pins of the R NE5532. The NE5532s are not the issue, so it suggests something around the circuit surrounding the L NE5532 is.

29th Nov 2020, 9:01 pm	#9
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,959	Re: CD104 channel failure R3574 1k8 feedback resistor open so the amp is operating open loop and latching high? Ron

29th Nov 2020, 9:11 pm	#10
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,332	Re: CD104 channel failure Thoughtful, Ron - but removing 3574 (which measures good OOC) we still have over 10V on pin 1, and pins 5, 6 & 7, and -5.5V on pin 2. This is with no DAC connected to the NE5532. Pin 4 is still -11.3V, and pin 8 is +11.4V.

29th Nov 2020, 9:19 pm	#11
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: CD104 channel failure O/C ground? Lawrence.

29th Nov 2020, 9:49 pm	#12
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,959	Re: CD104 channel failure With regard to posts #9 and 10, removing R3574 and then testing the voltage on pin 1 is not appropriate, as the amplifier will still be operating open loop with high gain, as the DC feedback is missing. It would be better to tack a resistor between pin 1 and pin 2 on the IC legs (to eliminate any open tracks) and then test the voltage on pin 1. With regard to Lawrence's post, is pin 3 at ground potential? Ron

29th Nov 2020, 10:24 pm	#13
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,332	Re: CD104 channel failure Thanks for the clarification, Ron. I didn't realise that. 2 and 1 are still connected via the rest of the network, if that helps. The reed switch is working, so is closed when the power is on. Perhaps 0.8M is too great for the purpose. Yes, pin 3 has ground continuity.

29th Nov 2020, 10:29 pm	#14
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,332	Re: CD104 channel failure Status of the L NE5532 with break at R3937, no R3574. VDC wrt Ground. Pin 1: +10.7 Pin 2: -5.5 Pin 3: 0.2 Pin 4: -11.3 Pin 5, 6, 7: +9.9 Pin 8: +11.5 I'm going to try another 1.8k resistor in place of R3574 in case something happened to it when voltage was applied, even though it measured fine OOC.

29th Nov 2020, 11:05 pm	#15
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,959	Re: CD104 channel failure You must have a DC connection between pins 1 and 2 i.e. a resistor representing R3574 for the amplifier to work as intended. The rest of the feedback network around pins 1 and 2 is AC coupled (i.e. DC is blocked). I was suggesting that a 1k8 resistor should be tacked directly between pins 1 and 2 of the NE5532 IC for test purposes, to ensure a DC connection, in case a pc track had cracked. Ron

29th Nov 2020, 11:16 pm	#16
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,332	Re: CD104 channel failure I got that Ron, I just thought I'd update what was what. With a 1.8k between pins 1 & 2, the results above are exactly the same, except pin 2 is now +9.9V.

29th Nov 2020, 11:49 pm	#17
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,332	Re: CD104 channel failure I just can't see how there can be any current flow as there's no input. I suppose I'll have to remove all the components in the filter network and test them individually.

30th Nov 2020, 6:23 am	#18
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: CD104 channel failure OK, so the DACs are good, the problem is just round that op-amp chip. Quick test if you've not already done it. With the NE5532 removed (and the unit turned off) what DC resistance do yo measure between pins 1 and 2 of the NE5532 socket? And do check that pin 3 is solidly grounded. Have you checked the NE5532 supply voltages _at the pins of the IC_ ? Can you remove the resistor (I guess SMD, alas that connects the output of the first half of the NE5532 to the second stage? Just in case the odd voltage is coming in that way.

30th Nov 2020, 2:32 pm	#19
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,332	Re: CD104 channel failure I'm sorry, I failed to specify - those voltages above are all measured on the pins (the component side faces downwards when the board's in the normal orientation, so it's quite accessible). Following the tracks on the PCB around the NE5532s, the voltage is constant at all the interconnected places, and there's continuity back to the DACs so I don't think a cracked print is to blame. Don't worry about the SMD - I've got a little pile of the things now. As Richmall Crompton wrote 'one may as well drink of tragedy to the very dregs when one is about it'. I did remove R3576 (the link between pin 1 and pin 5) last night before I despaired. There is no link back to the DAC, and no link forward to the next half of the IC. All other components remain in place. The first half is then 10.6V pin 1; 9.9V pin 2; 0.02V pin 3. Second half: -11/+11 pin 4/pin 8; -2.9 pin 5; -9.9 pins 6 & 7. With the NE5532 removed (I'm going to have to put a socket in at this rate!) I get open circuit between pins 1 & 2 (R3574 1.8k was on the pins). It measures 1.8k exactly. <1R to ground at pin 3.

30th Nov 2020, 5:30 pm	#20
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: CD104 channel failure Yes, fit a turned-pin socket and re-fit that 1.8k resistor. Now with the DAC disconnected and the link to the second stage disconnected, what voltages do you get at pin 1?