Started my build, have issues

[dave5701]

I have PL400. Need some brain help here. Bought enough TO-3's to change out one side of amp, got cap upgrade, got speaker protect. All from WO and Wattsabundant. First step was to check TO-3s to stop fuse from blowing. Went through every one, used Fluke 77 in diode mode to check em. Found two shorted. I replaced all TO-3's on one channel and used leftovers to replace two bad ones. Fuse still blows. Could be more TO-3s went bad in the repair process? Can I pull all to-3's out and see if fuse blows. If fuse does not blow, can I put in a couple at a time to see which one causes problem? Another approach?? I would like to get the old gal powered up without fuse issues then tackle the caps then speaker. Want some stability before causing any issues I might make.

 

[Gepetto]

Yes follow the bring up procedure...starting at step 11. One row at a time...

 

[AngrySailor]

Are you sure your power supply caps aren’t causing the fuse to blow? Dim bulb tester bright?

 

[dave5701]

Thanks to all you guys for the help. I built a DBT and had to come up with my own mods for that. I did not have a 100w bulb (Wally World is 100% LED friendly so Dollar General helped me out) so I bought cheap lamp bulb replacement sockets and had an old school "Y" for two bulbs in one socket and used 3 40w appliance bulbs in parallel. Finally got all TO-3's in WITHOUT blowing fuses. Did the cap change out, that was much easier than I thought AND it was obvious the difference they made proven by the DBT. The original caps would power up in 2-3 seconds (came on bright then dimmed down to nothing), but new caps took 4-5 seconds to dim out. Next step is speaker protection and I have all the stuff for that. This is truly an interesting project.

 

[WOPL Sniffer]

3 40w appliance bulbs in parallel????


Interesting, Hmmmmm

 

[J!m]

With dimmer and strobe circuit.

 

[dave5701]

yeah. 40 x 3 = 120w thats my 100w or actually 120w bulb. worked for me.

 

[AngrySailor]

Not blowing fuses without the dim bulb now? What’s your bias and offset readings? This is all Stock PL right?

 

[dave5701]

Yes stock PL. not sure on bias but I think by offset you mean what DC voltage do I see at output? DC at output is .003VDC on each channel.

 

[AngrySailor]

Search the site here for the pl-400 manual and service manual. Not sure I can phone post it. Also read Joe’s documents/instructions (they’re the eff’n PL bible...). Bias is measured across the B-E on the outputs. 350mV is about what you’re after. Too much bias will turn on the +\- rails at the same time and could be the cause of your “short”. If it runs hot idle good bet there’s a bias issue...

 

[dave5701]

Hmmm. Ok. I will look into that. I have some service manuals and other stuff I found and my memory is crap but I think I remember something like that somewhere.