HELP! Rescuing Dad's old PL400 S2 (pics)

[lightfall]

I am doing these OTs lightly finger snug, the torque setting is very low as I recall.

 

[laatsch55]

Too loose and they wont transfer heat well. They MUST be more than finger tight.

 

[lightfall]

Too loose and they wont transfer heat well. They MUST be more than finger tight.

Roger that, testing drivers now

 

[lightfall]

Actually this AGX 10 32v fuse appears to be too small for the AGC 10 slot, this might sound blasphemous but should I shim it or something?

 

[laatsch55]

Well, if the AGX won't make contact, leave the AGC in there...

 

[lightfall]

Heh, I never even had an AGC or appropriate fuse. I just used your fuse with a little bundled piece of copper in the cap so that it fit snug. Drivers are good, no beeps from the multimeter so I am going to fire up the DBT.

 

[lightfall]

EUREKA!! Bulb was bright for a split second and then immediately went very dim. The front LEDs lit up far left on both. I powered down promptly. Now to study the bias procedure.

 

[laatsch55]

THAT is a VERY good sign. If that holds true on the first set of 15024's we're halfway home...

 

[lightfall]

THAT is a VERY good sign. If that holds true on the first set of 15024's we're halfway home...

First row of 15024's look good with the MM and DBT! Now trying to figure out bias testing and adjustment.

 

[laatsch55]

OK, power down long enough to blee the PS caps. If in doubt, measur DC volts left in caps, Pos lead on B+ cap terminal and Neg lead on B- cap terminal. Wait till they are below 4 volts, THEN AND ONLY THEN, hook up leads for bias adjustment.

For bias adjustment, pos lead on the base bus bar, neg lead on the emitter common tie point. Volts in DC millivolts.

 

[lightfall]

OK, power down long enough to blee the PS caps. If in doubt, measur DC volts left in caps, Pos lead on B+ cap terminal and Neg lead on B- cap terminal. Wait till they are below 4 volts, THEN AND ONLY THEN, hook up leads for bias adjustment.

For bias adjustment, pos lead on the base bus bar, neg lead on the emitter common tie point. Volts in DC millivolts.

I am thinking I need the clip type leads for this since I am working in a live circuit.

Is there an "explain like I am 5" bias adjustment guide? I am reading the manual and just want to hear all of it in different words to make sure I do not mis-step.

 

[laatsch55]

Yes clip type leads and you are in territory where really bad stuff happens if you slip. And even if power is off the ps caps can carry plenty of current so be sure they are bled off before attaching or removing leads.

 

[laatsch55]

I am thinking I need the clip type leads for this since I am working in a live circuit.

Is there an "explain like I am 5" bias adjustment guide? I am reading the manual and just want to hear all of it in different words to make sure I do not mis-step.

I can't be much plainer than that. Have your meter hooked up and on before powering up. Use your fingers or small PLASTIC screwdriver to adjust the pot. Watch the meter while you do it. Keep it below 300 mv for now.

 

[gadget73]

You want something like these for this job:

search for minigrabber test leads. Many places sell them.

 

[lightfall]

I just bought a pair of mini alligator clip leads from radio shack, who wanted to charge me double the price for them that their own website listed...

Anyway, now I am setting up to test this bias and the caps etc.

You want something like these for this job:

search for minigrabber test leads. Many places sell them.

 

[Gepetto]

I just bought a pair of mini alligator clip leads from radio shack, who wanted to charge me double the price for them that their own website listed...

Anyway, now I am setting up to test this bias and the caps etc.

Rat Shack is fighting for life unfortunately...kids don't experiment with electronics anymore.

 

[lightfall]

OK the MM shows me .357 DC V at the points you indicated (base bus and common tie of emitters). What now good sir.

 

[laatsch55]

Adjust that below 300 ON EACH channel...then you can add a row of outputs and bring it up again, then the next row and set your bias for the last time AFTER you have all rows in and the bulb stays dim, then plug directly into the wall . That's when you adjust bias for the last time. Give it 15 minutes of warm up time before the last bias adjustment....then you're good to go.

You're almost there.......don't take a shortcut now because you're excited. Drain the caps to unhook or rehook your test leads.....

 

[laatsch55]

Your looking for a final bias of 370-380....

 

[lightfall]

Sorry I did NOT adjust yet, that was the reading without any adjustment. I am still looking for a small enough piece of plastic to turn the two pots on the PCB.

I assume I make a small counter clockwise adjustment to each side respectively and when changing sides I wait >5 minutes to attach or detach leads.