Power supply problem

[mud duck]

I know, I know.....these thing are supposed to be so simple

Its a Pyramid ps 46KX.

I was deep in a conversation a few days ago, when all the sudden I hear a loud humm. A few seconds later (before I could get to the switch), It shut down. No smoke, but it popped the fuse.

I replaced the fuse with a smaller rating one for a test, and it popped it fast too. I then removed the cover and started poking around, the pass transistor heat sinks were really hot..... Ill get to them in a minute.

I unwired the transformer and replaced the fuse and it held. Tansformer didnt blow... A GOOD THING !

Next step was to the rectifiers, 2 of them at 35 amps per unit. Sure as day, both had shorts. Off to the parts store to get some more (maybe some nice 50 amp devices). No luck, 35's only.....so I buy 4 of them.

Instead of modifying for the two extra rectifiers, I decide its a good idea to just replace the two blown ones and see if it fires up first. No luck...

I then head back to the parts store to pick up a pack of 723 regulators.... next part in line that could have failed. Replaced it and no luck.

Getting in deeper now, I unhook all six pass transistors from the circuit
( one driver, five mains). And replace the 723 with another from my pack of four. Unfortunatly This time I installed the chip backwards....sigh.

After replacing the third 723 of my pack of four, I fire it up. Same as above...Nothing. So, I break out the data sheet for the 723 and schematic for the power supply and start digging in. I am getting no signal out of the 723 from pin 10 (wich is the voltage out to drive my first 2N3772). Collector and emitter show full voltage, but I get 0 on the base...... I pull the variable pot out of circuit and test it too....measures good. I checked voltages to the regulator board....good. Im getting 5-7 volts in certain areas of the circuit, and I have read the this is the minimum a 723 can regulate down to within most common perameters.

Im now looking at the protection circuit..... does it clamp down the output of pin 10 to put it in protect mode ? Im wondering, If this circuit took a shot when the bridge rectifiers went south.

Heres a link for the schematic, the area im talking about is at the bottom of the schematic...
http://www.pyramidcaraudio.com/manuals/PS46KX.pdf


Is there something im forgetting to check ? Im pretty sure I tested the pass transisors right, but even with them disconected from the board the problem remains.

Any help will be GREATLY appreciated !

 

[W5LZ]

1. Bridge rectifiers will always show a short in one direction.
2. Follow directions in back of manual for checking.
- 'Doc

 

[Captain Kilowatt]

SCR1 may be shorted. It is supposed to crowbar and short out the supply when too much current is drawn and that causes the fuse to pop. It may be bad or it may be latched on by way of the lead going to the junction of R11 and R12 in which case Q1 may be either shorted or latched on.

 

[bob85]

im not familiar with pyramid psu's but often a loud hum followed by a blown fuse indicates a shorted bridge rectifier, that in itself very rarely damages other components,
if you have full rectified voltage on collector and emitter you usually have a collector emitter short on at least one pass transistor or the driver,
if the 723 is seeing high voltage on its sense pin it likely wont be putting anything on the base of the drive transistor,
you need at least one pass transistor and driver for that circuit to regulate,

the schematic shows a crowbar thyristor for protection which will short the output if the output voltage is above the crowbar setting,
a shorted thyristor or tripped crowbar circuit could take out the rectifiers or pass transistors if its left dumping full current for too long,
since you dont know what caused the rectifer short i would also check that the 5w current sharing resistors measure ok,
check the output terminals dont measure short with the psu turned off

good luck