Wednesday, June 13, 2018

Rebuilding LAB Series LS800 Amplifier with a PR-800 Amplifier

I had rebuilt this LS800 amplifier with a L-15 amplifier but I decided to upgrade to PR-800 amplifiers for much more power.  This is a picture of the old amplifier dissembled.

This is a picture the old power amplifier section.  It was really cobbled.
This is how the PR-800 amplifier boards arrived.  Note that the power output transistors will have to be soldered on.
Pr-800 as it arrives
Here I have added the output power transistors.  I also moved the driver transistors to the top side of the board for mounting on the heat sinks that I have.  Their leads barely reach the tops of their holes.
PR-800 with power transistors
This is what it looks like mounted on the heat sink.  I went back and added washers under the heads of the 3/4 inch 4-40 screws.  Note that little plastic washers must be installed in the two To-220 transistors (Q12 and Q15).  Make sure to check for shorts to the heatsink with a voltmeter!
PR-800 Mounted on heatsink
WARNING!!  Something was really wrong, it tripped the circuit breaker and shorted the output transistors!!!  I eventually found the problem.  The 2SD669 (Center transistor Q13) has to be soldered UPSIDE DOWN on the top board!
Upside Down Transistor on PR800
Here is the schematic of the output section.  Note that the 2SC5200 (Q16, Q17, Q18, Q19) transistors are connected to the + power source and the 2SA1943's (Q20, Q21, Q22, Q23) are connected to the negative power source.  Also not that the two boards are the opposite of each other.  The 2SA1943's will always be next to the power filter capacitors.
The schematic was derived from this schematic found on the internet.  The protection circuits do not match what I received.

Here is a labeled picture

I have both channels working fine now, but only one at a time.
PR-800's mounted on heat sinks
As you can see I cut off one side of the heatsink.  I also switched to silicone insulators because they are less messy.  One channel is missing two power transistors because they were fried when I installed the 2SD669 right side up.
This project is finally done!  The cooling fan was mounted to the front cover to make working on the amps easier.  The heatsinks were mounted to the bottom of the cabinet.  The LM3915 LED VU meters have the bottom LED always on and the top two red ones are in parallel thus giving 12 LED's per channel.


Bob Davis said...

The 12 VAC was just for the speaker protection safety circuit. My transformer had a 12 volt winding so I do not know the current requirements.

Erlend Sæterdal said...

What about bias. Did you manage to get some bias from it? I can not. I am turning potentiometer anticlocwise very much and still no bias. It was delivered totally clockwise.