Frankenstein laser pick-up, aka. how to adjust laser power? - diyAudio
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Old 11th February 2014, 10:09 AM   #1
dzseki is offline dzseki  Hungary
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Default Frankenstein laser pick-up, aka. how to adjust laser power?

I was in need of a philips VAM1202/21 laser unit, but that is quite rare and expensive to come by, so decided to transplant the little PCB to a VAM1202/12 which has (supposedly) the same laser unit but the /12 has on board laser driver circuit, /21 does not have this, and the break out cable is also different.
So the transplant was successfull, but the laser current trimmer was set for the old laser, after the transplantation the new pickup was not much better, in terms of reading discs. I added a little more current to the laser and after that it read the cds well, but now I'm unsure if I'm overstress the laser diode. I found the service manual for the philips CDM12.1 which is a close relative to the VAM1202/12, the manual says 4-6V and 60-90mA supply for the laser, but after installation I only could measure 2V on the laser, and the current was around 160mA ,that's quite far from the "ideal" but mine is a VAM the manual is for the CDM. How can I determine the optimal laser current?
By the way the CD player is an USHER CD100, and has a TDA1300T as laser driver/amplifier.
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Old 11th February 2014, 10:47 AM   #2
Mooly is offline Mooly  United Kingdom
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The only correct way is with a laser power meter. No other method exists.

That said, an alternative is to monitor the RF output on a scope and adjust for around 1.5 volts pk/pk as a maximum, but this method is not the same as measuring optical power output.
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Old 11th February 2014, 02:35 PM   #3
dzseki is offline dzseki  Hungary
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Quote:
Originally Posted by Mooly View Post
That said, an alternative is to monitor the RF output on a scope and adjust for around 1.5 volts pk/pk as a maximum, but this method is not the same as measuring optical power output.
And exactly where should I measure this?
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Old 11th February 2014, 02:47 PM   #4
Mooly is offline Mooly  United Kingdom
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Pin 10 of the TDA1300 but that will be impossible to access so just find the RF on the main PCB... one of the traces on the flexiprint from he pickup.
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Old 12th February 2014, 09:09 AM   #5
dzseki is offline dzseki  Hungary
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So measured pin 10 of the TDA1300, It has a DC level about 0,5V and the pattern is also 0,5Vpp on the DC level, this isn't sounds good either...
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Old 12th February 2014, 09:43 AM   #6
Mooly is offline Mooly  United Kingdom
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The amplitude is far to low which can only be the pickup/laser/laser power. Is your scope up to the job ?

The RF should look like this (see pictures),
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Old 12th February 2014, 10:43 AM   #7
dzseki is offline dzseki  Hungary
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The scope is a Meguro 25MHz type, it should be enough for the task, I've taken measurements with the scope probe in both 1x and 10x mode to see if it is not limiting the bandwidth, but the result was the same.
Although with the rewiring I checked my work at least twice, and am pretty sure that everything is wired to the right place, but for a short moment let's imagine I swapped some of the diodes during the wiring, could that cause problem like this?
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Old 12th February 2014, 11:05 AM   #8
Mooly is offline Mooly  United Kingdom
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A 25MHz scope is perfect. Any wiring error would give a non functioning pickup, with an error on just the photo diode array probably giving some odd looking RF.

Impossible to diagnose anything like that at distance I'm afraid.
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Old 12th February 2014, 11:39 AM   #9
dzseki is offline dzseki  Hungary
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I really don't think I made a wiring mistake, and the pick-up actually works if I set the trimmer on the laser PCB a bit higher than it was left with the previous laser, the measurements were taken with these higher settings. I just don't want to see that the new laser dies in weeks...
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Old 12th February 2014, 12:32 PM   #10
Mooly is offline Mooly  United Kingdom
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If it works then there is no wiring mistake, that's a definite. So you have low RF output... it can only be either a pickup problem (a problem with the optics), or low current in the laser. The way laser diodes work mean that a relatively small increase in current should give a large increase in optical output. Its not linear, once a threshold current is reached (perhaps 40 or 50ma) and lasing starts then small increase should see the output (the RF output too) shoot up.
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