Better laser amp transistor than BC338?

The attached photo show a portion of schematic from a Philips CD60 vintage CD player (there is another thread with more about repairing this player after some damage).

Note 6502, the BC338 npn transistor powering the laser diode.

It's possible that the +10v rail this trans. is connected to experienced a power spike, as one of the fusible resistors was blown (see other thread) .

The laser power was waaaaaaaaaay off after the fusible resistor was replaced and Laser Output Adj (3520) was tweaked in order to get a CD to boot.

Some discs still don't track well (bouncy eye pattern), or produce mild audible distortion.

It's possible the BC338 was "affected", tho' it tests "okay" (its hFE is a bit on the high side).

Suggest an alternative (maybe even an improvement).
In my kit, I have Toshiba 2SC2240 and BC547.

If you think the original BC338 is okay, say so.

Thx
 

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I've had a very occasional failure of those transistors, usually giving very low or no laser current . Absolutely non critical and the original type is fine. Replacing it should not alter the set laser current because the transistor is within the whole 'optical feedback loop'. The chip will increase or decrease the base current (low or high transistor gain) to maintain the optical output of the laser as determined by the preset resistor.

If the RF 'bouncy' then that is caused by the disc not running true. It will be worse on the tracks near the end of the disc because the run out will be magnified or worse there. Check for debris on the platter as a first easy option. Beyond that and the platter spindle may be out of true.
 
If the RF 'bouncy' then that is caused by the disc not running true. It will be worse on the tracks near the end of the disc because the run out will be magnified or worse there. Check for debris on the platter as a first easy option. Beyond that and the platter spindle may be out of true.
Most discs play w/o bouncy RF.

The ones that do bounce also have audible distortion. It sounds like very faint ("fuzzy") clipping distortion (or radio static).

Some other discs have no bounce, but very faint "fuzzy" audible distortion.

I've heard some CD-R (or RWs) can produce worse eye patterns. Mine all look as good as commercial/pressed CDs. Then again, they are Taiyo Yuden's burned on Benq or LG drives.
 
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If you look at the spinning disc edge on it should have absolutely no discernible runout.

CDR/RW usually do have definitely inferior RF, either/or amplitude being lower (CD-RW) or the relative amplitudes of the parts that make the eye pattern (the individual sines) being wrong... or at least different to a pressed CD.

This shows an eye pattern I photographed myself on a Sony player I was restoring. Yours should look as good in every way and have an amplitude of around 1.2 volts peak to peak.
 

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If you look at the spinning disc edge on it should have absolutely no discernible runout.

CDR/RW usually do have definitely inferior RF, either/or amplitude being lower (CD-RW) or the relative amplitudes of the parts that make the eye pattern (the individual sines) being wrong... or at least different to a pressed CD.

This shows an eye pattern I photographed myself on a Sony player I was restoring. Yours should look as good in every way and have an amplitude of around 1.2 volts peak to peak.
The physical disc spins w/absolutely no wobble on the edge.

But on some discs with some wobbly eye pattern, I hear it as fuzzy distortion -- especially on bass notes.

One thing (perhaps a bit off-topic) is that I can't (now) tweak Focus Offset at all. I was (a few hours ago) able to have a trimpot span of 0 - 200mV. And 145mV seemed to have been a sweet spot. Now, the whole trimpot range is one value: 133mV. I can't get it to move.
Tracking is fine (no dropouts/glitches). But discs that are scratched or have rough surface now manifest themselves audibly as that same fuzzy distortion.

Moving on ...

My old, all original, Tek 465 doesn't image as well as yours. The attached image was one I just captured from the problematic CD60.

I'm only getting 1.0v p-p. How do I increase this to 1.2v you suggested?
 

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That looks pretty good actually.

Laser power is the only adjustment you can make to alter final amplitude. If you are on 0.05V/DIV with a divider probe then that looks about right to me. There is no harm in taking it a fraction higher and seeing if it changes the fault.

Because manufacturers call things by different names (as I mentioned somewhere) you could also confirm that tweaking the offset pot does not alter the clarity of the RF. Assuming it doesn't, then adjust it as per the service manual (assuming they give a set up procedure). If it does then obviously adjust for best clarity of the diamond shape.

But that all looks good...
 
So the laser or its trans. are "okay"?

That looks pretty good actually.
[...]
But that all looks good...
So the laser or its trans are "okay"?

One thing I noted, right after the accident, was that the Laser Offset Adj. trimpot (see diagram) was at "min" (all the way counterclockwise). Very unlikely the moderate shock of accident moved its postion--this was factory setting. I did not adj. this during initial troubleshooting. Hence, that whole, now-long thread you are no doubt familiar with: Philips (Marantz) CD-60 not spinning up

The Serv. Man. outlines a procedure for troubleshooting laser power. First, you place a DMM's (+) lead on test point <11> and (-) lead on ground. And, then, you adj. the Laser Offset trimmer until the DMM reads 1k. (See diagram for <11>. It's near that BC338 trans.)
For me, this required about 1/2 turn (clockwise, of course). After this, you fine-tune the Offset via an o'scope and the eye pattern.

However, just that 1k setting got the disc spinning, TOC read, and playing! That now-long thread roughly collapses with this simple adj.

Recall the 1-ohm safety resistor (at +10v pwr rail) blew about midway during the troubleshooting procedure. The exact same symptomatic problem (disk not booting up) remained after the safety R was replaced.

That laser amp trans -- BC338 (6502) -- is powered, on its collector, by that same +10v rail. And that's why I suspect it.

687357d1529258619-laser-amp-transistor-bc338-philips-cd-60_laser_amp-jpg
 
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The 1k setting for laser power is an initial start point intended to give a workable current. The Philips method for final adjustment of laser power was to monitor the voltage across a resistor to the photodiode array, the idea being that the optimum laser power allowed the photo diodes to conduct enough such that they would drop 50mv across the test resistor.

To be honest I never really liked that method, I always liked to see the RF and see its quality, however the official method should give an RF value of around 1.2 volts peak to peak which is a fairly standard value .
 
The Philips method for final adjustment of laser power was to monitor the voltage across a resistor to the photodiode array, the idea being that the optimum laser power allowed the photo diodes to conduct enough such that they would drop 50mv across the test resistor.

To be honest I never really liked that method, I always liked to see the RF and see its quality, however the official method should give an RF value of around 1.2 volts peak to peak which is a fairly standard value .
Yes, I forgot to note, I did tweak that photodiode test resistor drop (it's cropped off in that schematic portion I posted) to that 50mV value while I looked at the eye.
nota bene: I found that 50mV extremely touchy to set ... uh, microscopic turns of that Laser Pwr trimpot. If that 50mV value is critical, maybe the accident (impact) did mess up laser current ????
 
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On a 'good' player of any make and model I have always found that adjustments have a wide tolerance... meaning that it will still play correctly even if the settings are off considerably.

Once things start getting super critical then there is usually something else going on beside simple mis-alignment. I wouldn't hesitate to both increase and decrease the laser power a little (by monitoring the RF amplitude) and to see if that alters this distortion fault you mention in the other thread.