Philips CD104 with intermittent audio fault (griplets fixed) - advice appreciated!

[evendine]

Found a CD104 I'd tucked away in a cupboard many years ago and have been trying to get it running for the last few days, with limited success.

Initially, it couldn't find the TOC, but with the help of Mike Leach's servicing guide and the many posts on this forum and others, have made some progress...

Checked all of the PSU rails with a disk playing - everything is within a few hundred millivolts.

Fixed the 'griplet' connections between the double-sided boards by cleaning out the original solder (drilling through where needed) and soldering a through wire on each side, making sure of a good connection to the ground plane. There are 2 on the servo board and 11 on the decoder board. This allowed TOC to be found, but no audio, despite tracks being accessible via the controls, the disk spinning normally and the display doing what you'd expect

Greased the draw mechanism and cleaned and tested the 'draw closed switch'. A slight change - occasional audio for about 15 seconds - it's fine initially, but becomes more and more distorted and then stops.

Checked the mute signals (there are two), which seem correct. Tested the 'anti-bump' reed relays with a magnet - sound fine, but bypassed both channels with wire bridges to be sure. No change.

Re-soldered every connection on the decoder board and changed all of the 22uF coupling / decoupling caps. No result...

Running through the decoder board (checking against the service manual voltages) it seems the 16.8v expected on pin 10 of the DAC chips is high (17.5 and 18.3v). Does anyone know if that would be enough to cause a problem?

Throughout the process, once in a while, the 15 seconds of audio popped up randomly (in different location son the disk), which suggests the transport and chipset is basically functional, but something is interrupting either the data stream or audio signal somewhere in the circuit...

Thinking about changing the power supply caps and perhaps the 18v regulator, but if anyone has come across this problem and can suggest a solution, your advice would be much appreciated!

 

[evendine]

Thinking the wiring loom / connectors may be at fault. The ones between the servo and decoder boards have a hard time when removing and replacing the pcbs...

 

[Willi Studer]

If during playback the time display works correctly and smoothly, then the whole digital train is healthy and the problem lies in the decoder or analogue stage.
Does the sound intermittently appear on both channels?
I would backtrace the audio signal by playing a CD with a 1kHz test tone and look for it starting from outputs and all the way back to the analogue output of DACs.
Did you recap the player?

 

[evendine]

Yep - have been focusing on the analogue board. The only issue on the servo side seems to be the progr/review button, which yields an error code, but otherwise fine.

Yes - both channels. Leaving the unit on for an hour or two results in maybe two or three 15 second 'output episodes'... initially sounds respectable descending into increasing distortion, and then silence.

Have a feeling it's a psu load issue, or maybe a failure of the wiring loom - probably due to being messed about with while trying to fix the other bugs.

Have done basic voltage tracing on the audio stage as you know, but although i measured a healthy 'eye' on the scope, it then itself became intermittent unfortunately, so relying on DVMs at the mo...

Ordered a set of electrolytics last night, so will be interesting to see if that helps. Strikes me that as both channels are effected equally, must be something in common, most likely the PSU. Earthed the chassis today with that in mind, but no result...

Thanks for the input

 

[evendine]

Recapped + re-soldered the PSU board and swapped out the -18v regulator, but no change. Still getting intermittent audio...

There are a couple of caps left to change on the decoder/servo boards and I'll continuity test the interconnection leads next.

Pity my scope packed up when it did - looks like it's going to be needed...

 

[evendine]

Got the scope going again and followed the decoder data lines through as recommended in Mike Leach's guide. Appears that there is data all the way to the pin 1 on the DAC chips, but no analogue output on pin 22. All of the voltages as specified in Philips service manual are correct on both chips. The fact that both channels work intermittently suggests the DAC chips are basically functional. Hmm...

What could be effecting both chips, if not the rails? The DAC clock on pin 4 looks fine - 14 pulse groups followed by gaps. Could this be an intermittent fault within the DAC itself? Seems unlikely as both are behaving the same way...

 

[evendine]

Changed the remaining caps and spotted an anomaly - there's a bipolar cap on the servo board (#2214). Looks very similar to the many light blue 22uF electrolytics in the system, so worth being aware of - could do damage and be very hard to find when fault finding...

Also noticed an out of spec cap in the PSU (#2465) which was upstream from the -18v regulator: 220uF rather than 470uF. That supply rail to the DAC had drifted up by 2v to almost -20v (due to excessive ripple?), so was replaced and now sets pin 11 to 18v +-5 millivolts.

Checking the tda1540 DAC chip spec, it turns out that the chip is +very+ sensitive to supply voltage - the operating range is limited to between -16.5 to -18v. The absolute maximum is -20v. Looks as though the chips are operating at their limits, which might explain the intermittency? Since the reading initially was just under -20v, the chips may have survived, with luck.

I'll stick a resistor between the -18v supply and pin 11 and see what happens...

 

[evendine]

Still no improvement - using a diode to drop the -18v rail by 0.7v lowered the vREF on pin 10 below the operating range limit of 16.5v and even intermittent audio stopped. Reverted to standard and fortunately, the new regulator seems to have burned in and be stable at -17.8v, with -17v on pin 10, which seems reasonable...

Spotted a voltage anomaly on the 7010 demodulator chip laser data input op-amp feed on pins 7 and 8. There's a 1uF cap associated with it which is in the post and has yet to be changed, so that's next on the list, along with the electrolytics on the servo board.

 

[evendine]

Changed all servo electrolytics (other than the 3 tantalums) and the remaining 1uF on the decoder board. The latter seems to have fixed the voltage anomaly on pins 7 & 8 of the 7010 (now both 1.84v), but still no audio out. 'Eye' pattern on pin 7 is reasonably stable and about 1.25v p to p, which is in range. The MCES clock on pin 4 looks to be in good shape.

Since the TDA1540's provide current output on pin 22, to be sure the 5532's aren't the problem (i.e. not sinking current from the DAC), removed one and connected a 100R across pins 2 (signal in) & 3 (GND) to perform I/V. No sign of an audio signal, so must assume either the DACs are blown, or I'm missing something...

Time for new DACs or a replacement decoder board, it seems. Looks like the -18v regulator drift took them both out

Can anyone advise on a good source for TD1540's?

 

[Willi Studer]

Are you the first owner of this player? Did it work properly the last time you used it? It would be unlikely that both DACs blow at the same time, unless because of the excessive voltage which, anyway, you did not measure. Besides, if there is intermittent sound, they must be OK and, as the phenomenon happens on both DACs, then it should be something that is common for them. How about the SAA? Do change the tantalums, those are old generation ones.

 

[evendine]

I've owned it maybe 20 years Willi, but as i recall, it was bought as part of a system with several other units and didn't work originally.

Agree that it does seem unlikely that both DACs are blown unless the regulator fault caused it, but since the -18v rail was sitting very close to -20v initially, my DVM hasn't been calibrated recently, and the regulator was damaged, it seems possible that the actual voltage was above the -20v absolute maximum rating at some point. Is it possible that internal damage was caused to both DACs which appears as intermittent audio? Honestly unsure if that is a failure mode for the TDA1540...

The digital signal pathway described in Mike Leach's service manual seems ok, suggesting that the SAA chips are functioning.
(see: fig 4 - http://www.tvcaudio.com/tvc/Philips/documentation/servicing%20the%20Philips%20CD104%20CD%20Player.pdf)

Perhaps a more detailed look at the SAA chipset is needed?

I'll replace the tantalums on the servo board as you suggest. Guess they are ok currently though, as TOC is working reliably.

 

[evendine]

As they are a common factor, took another look at the key SAA clocks; pin 8 on the 7000 - 4.23 MHz and pin 10 on the 7010 - 4.35MHz and all seems well. Also checked the uProcessor clock on pin 16, which is 6MHz as specified.

By chance had the scope connected to pin 22 of the DAC while intermittent audio was present today (across the temp I/V resistor) - waveform looked normal.

I've ordered another tested decoder board to prove or disprove that it's causing the issue. Should be helpful in tracking down the fault through comparison, in any case...

 

[Willi Studer]

Does any of the ICs run excessively hot?

 

[evendine]

No - they all seem fine - nothing unusual...

 

[evendine]

Fitted the replacement decoder board today and the player works, finally!

Can hear some distortion in the audio when the source has a lot of bass, which suggests that the (original) caps on the new board need replacing. Will transplant the new set from the old board step by step. Most likely the 22uF pair in the signal path are the issue, I guess...

 

[evendine]

A quick update - all chip decoupling capacitors on the new decoder board were replaced with OS CONs, the coupling capacitors in the audio output path with Panasonic FC's and other sundry electrolytics with Nichicon FW's.

A problem in the op-amp section then emerged where the output would saturate randomly. Swapped the pre-eq + kill relays and all has been well to date (a few weeks later).

I'd recommend changing the decoder electrolytics and swapping the relays as a starting point for any renovation. Tested the removed caps and most were way out of spec...