Help with mkiii restoration

I recently acquired a pair of mkiii mono blocks, with the intent to upgrade the boards.  The amps worked well on receiving them, but lacked a bit in fidelity.  I chose to upgrade to the tubes4hifi dual 12AU7 board.  I decided to do one of them first.  I'm very capable with a soldering iron, but have limited understanding in circuitry.  I opened up the chassis and noticed everything was pretty much old and nasty looking.  I decided to replace everything, only re-using the chassis and transformers.

I removed everything from the chassis, cleaned it, acquired all new parts and assembled everything with the new boards.  The tubes light up, biasing works and music plays.  However, when the amplifier gets to a decent volume, it starts to break up and has distortion.  It's bad enough that I don't want to harm my speakers with attempting any higher volume.

I triple checked every connection, checked every resistor with an ohm meter to make sure everything was in it's right place, and switched everything to a new set of tubes from the other mkiii that works fine.  I then retrimmed and tinned all of the wiring and resoldered everything.  The amp acts exactly the same.  I tried 3 different speakers with varying levels of sensitivity, and same results.  I tried 3 different amps I have, and they all work perfectly.  I also tried 2 different preamps, same result.  A friend put it on a scope with a load, and it shows oscillation occurring after it warms up and gets driven a bit.

Any help with where I might go from here is greatly appreciated.

NewDIYGuy wrote:I recently acquired a pair of mkiii mono blocks, with the intent to upgrade the boards.  The amps worked well on receiving them, but lacked a bit in fidelity.  I chose to upgrade to the tubes4hifi dual 12AU7 board.  I decided to do one of them first.  I'm very capable with a soldering iron, but have limited understanding in circuitry.  I opened up the chassis and noticed everything was pretty much old and nasty looking.  I decided to replace everything, only re-using the chassis and transformers.

I removed everything from the chassis, cleaned it, acquired all new parts and assembled everything with the new boards.  The tubes light up, biasing works and music plays.  However, when the amplifier gets to a decent volume, it starts to break up and has distortion.  It's bad enough that I don't want to harm my speakers with attempting any higher volume.

I triple checked every connection, checked every resistor with an ohm meter to make sure everything was in it's right place, and switched everything to a new set of tubes from the other mkiii that works fine.  I then retrimmed and tinned all of the wiring and resoldered everything.  The amp acts exactly the same.  I tried 3 different speakers with varying levels of sensitivity, and same results.  I tried 3 different amps I have, and they all work perfectly.  I also tried 2 different preamps, same result.  A friend put it on a scope with a load, and it shows oscillation occurring after it warms up and gets driven a bit.

Any help with where I might go from here is greatly appreciated.

I would start checking DC voltages. Of course there is a problem as this in
not at Dynaco MkIII, does the tubes4hifi board has notes of DC voltages ?

What preamps are you using?

Are you sure all of your voltages are within range?

Do you have other tubes to use?

Did you try driving the amps to the same level when you first got them, before the restore? If you did and they didn't distort then it's probably something you did or a component you installed, could be a bad new resistor or capacitor.

What is the oscillation?
Can you post a scope photo?

Do you have the manual?
If not, it is available at HiFiEngine

check the B+ voltage (about 500vdc) and check the voltage going to the PCB (on the other side of the large 6.8K resistor on the quad cap) should be around 380vdc.
If those are OK, check the voltages on the PCB especially plate voltage to each driver tube (see schematic that came with documentation)

tubes4hifi wrote:check the B+ voltage

Be careful:

1. use a clip lead for 0v [gnd]
   
2. insulate most of the +ve lead tip with heat shrink, tubing or electrical tape
   
3. stand on an insulated surface
   
4. keep one hand in your pocket
   

Ideally, testing should be done on a GCFI protected outlet