Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Digital Source Digital Players and Recorders: CD , SACD , Tape, Memory Card, etc.

Better laser amp transistor than BC338?
Better laser amp transistor than BC338?
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 17th June 2018, 06:22 PM   #1
hollowman is offline hollowman  United States
diyAudio Member
 
hollowman's Avatar
 
Join Date: Jan 2007
Location: Los Angeles, California
Default 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
Attached Images
File Type: jpg Philips-CD-60_laser_amp.jpg (192.3 KB, 124 views)
__________________
"I got banned from a Hi-Fi forum. And I liked it."
- Luv, Katy "Hollowman" Perry
  Reply With Quote
Old 17th June 2018, 06:33 PM   #2
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Better laser amp transistor than BC338?
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.
  Reply With Quote
Old 17th June 2018, 06:57 PM   #3
hollowman is offline hollowman  United States
diyAudio Member
 
hollowman's Avatar
 
Join Date: Jan 2007
Location: Los Angeles, California
Quote:
Originally Posted by Mooly View Post
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.
__________________
"I got banned from a Hi-Fi forum. And I liked it."
- Luv, Katy "Hollowman" Perry
  Reply With Quote
Old 17th June 2018, 07:39 PM   #4
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Better laser amp transistor than BC338?
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.
Attached Images
File Type: jpg E2.JPG (45.4 KB, 88 views)
File Type: jpg E1.JPG (51.1 KB, 87 views)
  Reply With Quote
Old 17th June 2018, 09:36 PM   #5
hollowman is offline hollowman  United States
diyAudio Member
 
hollowman's Avatar
 
Join Date: Jan 2007
Location: Los Angeles, California
Quote:
Originally Posted by Mooly View Post
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?
Attached Images
File Type: jpg IMG_0891.JPG (140.6 KB, 77 views)
__________________
"I got banned from a Hi-Fi forum. And I liked it."
- Luv, Katy "Hollowman" Perry
  Reply With Quote
Old 17th June 2018, 10:23 PM   #6
hollowman is offline hollowman  United States
diyAudio Member
 
hollowman's Avatar
 
Join Date: Jan 2007
Location: Los Angeles, California
Another o'scope capture -- my probe on 10x. Hopefully, a bit clearer.
Attached Images
File Type: jpg IMG_0894.JPG (86.9 KB, 134 views)
__________________
"I got banned from a Hi-Fi forum. And I liked it."
- Luv, Katy "Hollowman" Perry
  Reply With Quote
Old 18th June 2018, 07:26 AM   #7
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Better laser amp transistor than BC338?
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...
  Reply With Quote
Old 18th June 2018, 03:51 PM   #8
hollowman is offline hollowman  United States
diyAudio Member
 
hollowman's Avatar
 
Join Date: Jan 2007
Location: Los Angeles, California
Default So the laser or its trans. are "okay"?

Quote:
Originally Posted by Mooly View Post
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.

Click the image to open in full size.
__________________
"I got banned from a Hi-Fi forum. And I liked it."
- Luv, Katy "Hollowman" Perry
  Reply With Quote
Old 18th June 2018, 05:31 PM   #9
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Better laser amp transistor than BC338?
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 .
  Reply With Quote
Old 18th June 2018, 08:49 PM   #10
hollowman is offline hollowman  United States
diyAudio Member
 
hollowman's Avatar
 
Join Date: Jan 2007
Location: Los Angeles, California
Quote:
Originally Posted by Mooly View Post
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 ????
__________________
"I got banned from a Hi-Fi forum. And I liked it."
- Luv, Katy "Hollowman" Perry
  Reply With Quote

Reply


Better laser amp transistor than BC338?Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Replacing CD laser -- laser power and/or focus trim hollowman Digital Source 3 11th June 2018 06:50 AM
Can bias transistor mount on top(body) of output transistor?? Leolabs Solid State 26 28th April 2016 10:34 PM
One driver transistor per output transistor possibilities? rhythmsandy Solid State 25 27th October 2014 11:11 AM
Frankenstein laser pick-up, aka. how to adjust laser power? dzseki Digital Source 11 17th February 2014 01:03 PM
Transistor laser Nixie Parts 2 26th February 2006 03:17 PM


New To Site? Need Help?

All times are GMT. The time now is 04:37 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki