Uncle

[Sunnbobb]

I have to call uncle on an issue I am having with a PL400 II. I started with a perfectly working amp (after I recapped, installed new filter caps, rectifier, op amps and large resistors) and decided to install a WA speaker protect. While messing around and before the install, I stupidly crossed two output wires. (Don't ask, it was stupid). Before going on, I put the unit back on the DBT and yup, it remained brightly lit. First I pulled one row of transistors at a time, hoping to find a bad transistor. I did find one, however, the DBT remained lit. After removing all the transistors, It remained lit.
Next, I checked every transistor, resistor, op amp and cap on the board. (I had already recapped the board, installed new resistors and opamps prior to my screwup and everything had worked fine). Still the DBT remained bright. The filter caps and rectifier were brand new as well. I tested them anyway, no issues. I discovered if I removed the wires from one of the filter caps, the DBT would work properly. I then disconnected the board from the back plane, and the dbt still remained lit. Disconnecting the filter caps, I tested the leads on the rectifier. 79 V dc came out of each rectifier lead, 122V AC was showing on the input leads to the rectifier. I disconnected the output wires and still I was stuck with a bright DBT. Finally decided to bring it up on the variac without the dbt. All three fuses blew.
I crawled all over the back plane with a magnifying grass and found nothing amiss. I also disconnected the light boards. still bright DBT.
So My thinking is that something is wrong with the transformer. although I'm a bit puzzled as to why I get a reading of 79V on the leads, and if I disconnect one of the filter capacitors, the DBT goes out and the remaining filter cap charges properly.
Hive mind thoughts PLEASE...

 

[mlucitt]

The DBT identifies high current loads (shorts) in the Primary Power and to a lesser extent, downstream. Crossing output wires shows the amplifier an impedance of 0 Ohms and the amplifier will generate all the available Power across that short circuit - damage ensues. Many times the damage cannot be seen unless the amplifier is powered up. This makes troubleshooting hard.

I would replace the Bridge Rectifier (fairly inexpensive) and try two new Main PS Capacitors. Go slow. I have seen a bad BR in a PL700B, the symptoms were similar to what you have. The transformer is the most robust component in the amplifier.
Good Luck!

 

[Sunnbobb]

The DBT identifies high current loads (shorts) in the Primary Power and to a lesser extent, downstream. Crossing output wires shows the amplifier an impedance of 0 Ohms and the amplifier will generate all the available Power across that short circuit - damage ensues. Many times the damage cannot be seen unless the amplifier is powered up. This makes troubleshooting hard.

I would replace the Bridge Rectifier (fairly inexpensive) and try two new Main PS Capacitors. Go slow. I have seen a bad BR in a PL700B, the symptoms were similar to what you have. The transformer is the most robust component in the amplifier.
Good Luck!

Thanks, I have replaced both the rectifier and the PS caps. Problem still persists. sigh.

 

[Gepetto]

Did you start by removing all wires from both bulk caps, reconnecting ONLY the 2 wires that come from the bridge to the bulk caps, taping off the other ones and then reporting back DBT results. This cluster of parts represents the basic Power Supply.

If you take things one step at a time Richard, we will be able to hopefully pinpoint your problem area. This is a good first step, to see if the basic Power Supply works with no other connections.

This step limits itself to the mains fuse, the line cord, the transformer, the bridge and snubber, and the 2 bulk caps. If this works on the DBT, the problem is external to this. If this does not work on the DBT, you need to look within this cluster of parts.

 

[Sunnbobb]

Thanks Joe, that step was the last thing on my mind before I fell asleep at the bench last night, LOL. I'll do that this morning and report back. I wasn't sure that would show me anything, now I know it will. Appreciate it. I assume I would be leaving the ground bar attached to the caps?

 

[Sunnbobb]

OK, dbt stays lit with above configuration... note I crashed the system before i swapped out new ps caps and rectifier. I tested the new ones just now and they are fine, the dbt remains lit however. New fuses and i checked the cord and temp sensor. Guess that leaves the transformer. Anybody want to sell me one?

 

[Gepetto]

Next, disconnect both leads from the bridge to the bulk caps and tape off. This leaves the transformer and bridge. Report back after DBT testing

 

[Sunnbobb]

Done. The dbt doesn't light, not even flash.

 

[Gepetto]

Done. The dbt doesn't light, not even flash.

Connect one bulk cap at a time and figure out which is bringing things down.

 

[Sunnbobb]

either one keeps the dbt lit. The one closest to the bottom of the unit dims ever so slightly and then lights right back up

 

[Gepetto]

either one keeps the dbt lit. The one closest to the bottom of the unit dims ever so slightly and then lights right back up

Do you have them in backwards? Backwards caps will behave as you describe

 

[Sunnbobb]

Son of a... The leads from the Rectifier were crossed. dbt now goes out. Sheez, does this forum have a detention room? I feel the need to write "I'm an idiot" on the blackboard 500 times. Weird, I'm not sure how that happened, as I swapped the originals out one at a time. Perhaps when I soldered up the rectifier.. Thanks for your time Joe. You are a truly great help.

 

[Gepetto]

Son of a... The leads from the Rectifier were crossed. dbt now goes out. Sheez, does this forum have a detention room? I feel the need to write "I'm an idiot" on the blackboard 500 times. Weird, I'm not sure how that happened, as I swapped the originals out one at a time. Perhaps when I soldered up the rectifier.. Thanks for your time Joe. You are a truly great help.

That's what I am here for Richard...