Philips CD104 radial servo expert needed

[mickeyratt]

Dear Gurus

I'd like to repair an old Philips CD104, but no success
It is a nightmare to me, after 2 weeks of trying.
It enters Service Loop "A" perfectly (so laser, spindle, and focus servo is OK).
When i want to enter Loop "B" (the radial servo switched ON), it is fail. With oscilloscope probe on the radial amp out, there is a full +-12V noise. I changed every part of radial amp. (741 opamp, all transistors, and all passive devices), power supply capacitors, fitted board-through wires.
I fitted a socket in IC 6211 (HEF4053) place, so i can easily remove ic6211 from board, in this case there is no contact to the radial amp inputs. In this case, feeded the opamp inputs with voltage, it worked, the swing-arm moved corresponding to the input voltage, but when the input voltage raises (either to positive or negative direction) the output start oscillating about 1 MHz 1Vp-p(max). (the oscillating signal's amplitude is raising when i raise the feed voltage)
I'd like to ask, it is normal, or maybe the radial motor defective?
Unfortunately i don't have swap parts to localise the problem.


Thank you in advance!

[oshifis]

Hi and welcome,

This other thread (w ref to SM and repair doc) might help:

1984 Philips CD104 CD player restoration - UK Vintage Radio Repair and Restoration Discussion Forum

[mickeyratt]

Oshifis:

Thank You, i've already seen that thread.
I think, my cd104 suffer from a not common problem

Thanks!

[amc184]

When you replaced all of those parts did you replace any diodes? Last time I repaired a player like this (a Revox B225 with a bad radial servo) the fault was due to a leaky diode. The other components I would suspect are the small silver film capacitors, they tend to go open circuit. They're not something you'd normally check either, as film capacitors are a generally reliable type of component.

My suggestion is that you go through the radial servo section methodically with an oscilloscope. The service manual details the correct waveform at a whole heap of test points. Start from the end, where the signal is not correct and work backward until you find a signal that is. This is how I fixed the B225.

[mickeyratt]

amc184, Thank You for tips.
You mean: diodes in power supply? No, i don't replaced them, just some of the capacitors. (to low esr types). Examining the power rails with scope seems to be very clean.

You are right: "film capacitors are a generally reliable type of component", there is many in servo board, i'll check them.

It is hard to check the entire radial servo stage, it is a very difficult unit. Unfortunately there is no some "theory of operation" section in service manual, so it is nearly impossible to understand the radial servo operation. (for ex.: "xd", "xk" sections in block diagram).

Thanks again, i'll check the condensators and go through step-by-step as i see in the service manual.

[mickeyratt]

amc184:

There is a little "theory of operation" section in the Revox B225 service manual. So i can examine that!

It is an older philips cd system (seems close to the first CD players from philips), but the main operation concepts are the same.

Thank you again.

[poynton]

You say you replaced some capacitors to low esr types.
What type ?
Some caps are very sensitive to type and value changes .

Andy

.

[amc184]

You mean: diodes in power supply?

No, I mean the signal diodes in the radial servo.

You don't need to understand the whole theory of operation. What you need to do is go through the test points on the radial servo and look at each with an oscilloscope. The manual list what should be seen on each. Find out which points don't match, then try to figure out why.

For me it turned out to be diode 6255, which protects the input of the HEF4053B near the end of the servo. The diode had become leaky and was pulling up that input. I would have never found that one leaky diode without methodically testing each point in the player.

It is hard to check the entire radial servo stage, it is a very difficult unit.

Sure is, but you could spend a lot of time and effort trying easy fixes and never solve the problem.

[mickeyratt]

Dear amc184, and others.
Thanks for your valuable help. Finally managed to fix the cd104.
The biggest help was the Revox B225 service manulal, because it has a lot of measure points with correct values in "stop" and "play" modes.

First i assembled a circuit on a small PCB to replace the original Philips radial servo drive unit (i followed the Revox B225 schematic, the Philips way of driving the radial coil is absolutelly wrong - i think. It is oscillating about 1MHz if the input signal is positive, it seems to me, every philips player which utilizes this cd104 base suffer from this problem, maybe that's why Revox engineers don't keep that schematic too.)
With this new drive cicuit the problem remains the same.
A started to check all of the measure points from end, (as amc184 proposal -thanks) and found a leaky transistor: BC548B (6243), the RC-0 inverter unit. After replace, the radial servo worked perfectly.

There was a second error too, the left channel did not produce any sound, but it was easier to find that one of the output muter reed-tube is failing. (did not short circuit if get magnetic field).

So thank You all, it was a hard project, but I learned a lot from it.

[austriuz]

hello
i am new there, and i have problem with my CD104 too, it luks like mickeyratt sayd "There was a second error too, the left channel did not produce any sound," it produse only noise on left speaker. what i need to do?
p.s. sorry for my english.