Re: A problem with a cd player

If you have access to an oscilloscope, the HF "eye" pattern can be observed, and may be noisy. This would account for the deteriorated performance first on CD-R's. As previously noted, this is often the result of dust and smoke contamination. The finest particles get inside the laser, and settle on the turning mirror as a fine film. This diffuses the reflected laser light coming back off the disc before it can really be read. It is sometimes worthwhile to adjust the Focus Offset for maximum amplitude as viewed on the scope. If it makes a difference of about 20% or more, this may solve your problem, at least for the short term.

Mark Z.

Mark Z.

get further into a disc, its rotational speed falls to maintain

Hello

I have an old Telequipment scope who goes up to 1 mhz, it should do the job.

I've look on the main pcb and there is 4 trimpot; T.BAL , F.BAL , FEOF , TEOF

Wich one are for adjusting the Focus Offset ?

Thank a lot

Gaetan

FEOF should be the one, but a 1 meg scope won't really get it. You can adjust for maximum envelope size, which probably is good enough, but you can't look at the diamond shaped center area of the expanded "eye" pattern as viewed on a 30mHz or higher scope. Sometimes a x10 probe is required, but in my experience it is rare for a direct probe to load down this measurement.

You can "Google CD eye pattern" and see what I'm talking about.

Mark Z.

All that Mark says, but before you start, CAREFULLY mark the EXACT positions of the pots before attempting adjustment. If you do adjust more than one of them (the temptation *will* be there - trust me !!) and you do not have the proper bandwidth 'scope, and the experience to know exactly what you are adjusting and why, it is staggeringly easy to get those pots to a point where nothing will be read at all, often with just a few degrees mispositioning of a couple of them. Re-finding the correct places to get it going again, can then be a most frustrating excercise ...

Arfa

Hello

I will borrow a Sencore SC61 scope to a friend who do not use it, maby I will buy it anyway since it is doing nothing in his basement.

Btw, what would be T.BAL , F.BAL, TEOF ?

Thank for your help

Bye

Gaetan

T.B(AL) is tracking balance. Sometimes also known as E-F BAL (named for the designations of the two photodiodes, "E" and "F" in the pickup, which are responsible for generating the tracking control signal). It is required in order to remove unwanted positional offsets of the lens which would otherwise be caused by imbalances in the tracking servo photodiodes in the pickup, and the following opamp in the tracking servo processor.

F.B(AL) is focus balance. Does the same for the focus servo circuitry as TBAL does for the tracking.

TE(OF) is tracking error offset. Allows the electrical zero point of the tracking servo to be set to match the lens suspension mechanical zero. It is needed to offset mechanical errors in the construction of the laser, and electrical servo errors which may creep in as a result of imperfect opamps etc in the servo amplifier.

FE(OF) would be the same thing for the focus servo, and is needed to correct for such things as an error in the turntable height, and similar servo electrical errors.

You may find a pot called PLL on some players. The setting of this is

It is rare to find any of these adjustments present, in modern players. The servo processor ICs are these days capable of performing an analysis of the signal, and introducing and storing appropriate correctional offsets.

Other pots which may also be encountered are T.G(AIN) and F.G(AIN). These set the operational gains of the tracking and focus servo loops. They are often to still be found in modern players. Adjusting these up slightly, can often improve a player with marginal performance, but if set too high, particularly in the case of the focus servo, will result in the lens making a mechanical 'hissing' noise, particularly on a marked or scuffed disc.

Finally, there is always a pot on the laser which sets its output power. With a few notable exceptions such as Pioneer, this pot is sealed and MUST NOT be adjusted. Even a small overdrive of the laser diode, can result in its partial, or even complete destruction.

Arfa