Frankenstein laser pick-up, aka. how to adjust laser power?

[dzseki]

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.

 

[Mooly]

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.

 

[dzseki]

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?

 

[Mooly]

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.

 

[dzseki]

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...

 

[Mooly]

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),
http://www.diyaudio.com/forums/digital-source/226288-sony-cdp790-kss240-restoration-project.html

 

[dzseki]

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?

 

[Mooly]

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.

 

[dzseki]

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...

 

[Mooly]

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.

 

[Extreme_Boky]

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...

You could try to find the RF test point, it should be labeled on the PCB. Check the amplitude is within 1.5V pp + and - 0.3V.

Some CD's have high reflectivity, producing around 1.8V pp, which is perfectly fine. You will find CD's that have low reflectivity and those may give you around 1.3V pp or lower. This is fine as well as long there's no tracking issues.

By the way, try to adjust the trim-pot on the "previous laser" slightly higher. You'll probably find that it will work okay for another 10 years.... The trim-pot setting on all laser pick-ups should be re-adjusted after a while (5 years or so) as part of the corrective maintenance (as I call it) because the laser output deteriorates with time. This is normal.

I would not be worried at all about the new laser setting. If all your CD's play okay without drop-outs and tracking issues -> you are ok to go.

Nick

 

[dzseki]

Thanks guys!

It turned out my DMM was bad at current measurement mode, so the laser current is in the lower end of the specified range, and the eye-patter is still only around 0,5V, but it reads the CDs well, and don't have to be afraid to kill the laser.