Grundig 7500 (Phil CD303, Mar CD73) radial servo malfunction: advice please - diyAudio
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Old 19th March 2009, 12:30 PM   #1
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Default Grundig 7500 (Phil CD303, Mar CD73) radial servo malfunction: advice please

Hi all

I bought a dodgy Grundig CD7500 (= Philips CD303 = Marantz CD73), yesterday based on its looks, its phenomenal weight, and web reputation.

I should eventully figure this fault out, but I'm hoping someone who knows the model will immediately recognise the symptom and say "It's componet X!"

It contains CDM-0 mechanism with brush/commutator motor (version A07) and ceramic TDA1540-D dacs

The player mechanics and syscon logic all seem normal.
Power supply voltages are good and ripple free.
It quickly reads the TOC and displays correct number of tracks.

On play, it spins the disc at correct speed, the focus is OK,
BUT the radial servo is not working correctly.
It continually pushes the radial arm up against the starting position rubber cushion. And it makes chirping sounds as the radial arm hunts and oscillates around its nominal position - always bumping up against the rubber end-stop.

If I move the arm to the middle of the disc, the time display momentarily shows a correct disc time, then the radial arm shunts back to the start position, and continues to bump up against the rubber.

When I move the radial arm manually - against its 'will' - across the disc, the spindle speed drops, exactly as it should, and returns to the usual +/- 500 rpm as soon as the radial arm is left alone - as it goes back to the start point.

I can't figure out the test modes exactly, but I have managed to get it into a test mode where the disc spins, and the focus servo operates, but the radial servo is disabled. In this test mode, focus is all good, spindle speed is correctly maintained for any disc position, but the radial servo is quiet, and doesnt try to track or hunt - BUT it does stay hard up against the buffer rubber.
I know on the 16bit+CDM1, in test mode the radial arm should swing freely. Is that how the CDM-O should be? In which case, there sees to be an erroneous DC offset in the control system (but there is NO adjustment anywhere in the radial servo).

Any hints please?

When I scope the radial error signal - I get a FREQUENCY modulated sinewave; that foxed me, I was expecting an amplitude modulated sine (the 600 Hz wobble).
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Old 20th March 2009, 05:22 PM   #2
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On all boards, but especially the main boards, resolder thoroughly all of the places where the top ground plane foil is soldered through to the bottom side of the board, and replace all <1000ufd Philips &/or Siemens electrolytic caps(recommend Nichicon or Nippon ChemiCon). Also check that the platter height allows about 2mm space between lens & cd, and adjust the bottom bearing(#10 Torx) of the platter motor clockwise if bearing wear has caused loss of clearance. I'll be very surprised if the player doesn't work flawlessly after these steps are taken.
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Old 20th March 2009, 08:07 PM   #3
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Thanks stephensank for input:

Quote:
resolder thoroughly all of the places where the top ground plane foil is soldered through to the bottom side of the board
There are no ground-plains on any boards! Single sided boards, but they are mounted inside metal cages (like TV tuners). The soldering looks very good throughout.

Quote:
and replace all <1000ufd Philips &/or Siemens electrolytic caps(recommend Nichicon or Nippon ChemiCon).
Did you mean bigger than 1000uf, or smaller than 1000uF?
I changed the electrolytics, no fix. Now I'm eyeing the 1% polystyrene ones - I'm suspicious of them since I had 2 go bad in my ReVox, and they are not easy to source locally.

Quote:
Also check that the platter height allows about 2mm space between lens & cd,
Yep done that - got the focus offset signal to absolutely zero, even though it was barely off even before I adjusted.

And PS: If I hold the optic on any part of the disc, I do get audio, although it's verrry skippy - not surprisingly. But it shows the data decoding etc. is all working Ok.
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Old 21st March 2009, 05:45 AM   #4
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Not the same generation of boards as I was thinking. As for caps, you may have realised this from working on Revox gear, but generally speaking, any German made electrolytic cap is always a likely suspect(though not as likely to be bad, in my exp, as a Rubycon, Elna or Panasonic cap), but it seems like the smaller they are, the more likely they will be bad.
Assuming it's not a cap or soldering problem, if there are any discrete transistors or opamps in the radial/tracking servo system, they may be noisy.
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Old 22nd March 2009, 12:53 AM   #5
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My powers of prophesy are surprising even to me! Two of the 6n8 1% caps were short circuit.
I have a fully functional Grundig now! Wir tanzen und trinken!
But I will still change *all* of the caps in due course.

The control system is quite primitive compared to the Sony CDP101 capabilities, but this one has 'the look'.

What I also like is, apart from the microcontroller, there are only general purpose ICs used throughout. No chance of a future dead impossible-to-find chip.
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Old 2nd October 2009, 10:25 PM   #6
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This thread could be also of interest in this case:
cx7933,cx7934,cx7935,saa7030 with tda1540
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Old 3rd October 2009, 01:01 AM   #7
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Since the time I first opened the machine, and got it working, I have studied the Philips & Marantz service data and found the Grundig is somewhat different to the early Philips in the processing of data. Although on the outside the machines look like the same machine, the Grundig uses no Sony chips, and has improved microcontroller firmware (it finds tracks WAY faster that the Philips). Also it has a different PCB layout in the analogue section, resulting in better sound performance than the apparent 'clones', according to web sources.
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Old 16th April 2010, 12:14 PM   #8
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Quote:
Originally Posted by Steerpike View Post
Since the time I first opened the machine, and got it working, I have studied the Philips & Marantz service data
To Steerpike.I have a problem with my Philips cd100, it is like your Grundig with CDM0 inside.It can not read TOC.It spins a disk with a light high tone like a noise.Laser turn on and after 2 sec. disc stops .
Help me, please, what is the problem with?
I checked all power supply outputs-they are OK.
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