Orange AD200 Bass MK III not working

[J M Fahey]

it sounds like it's going through an heavy gating effect - only if the strings are played hard the sound briefly comes through, but without sustain

This usually means a leaky coupling capacitor blocking the next stage downstream so it effectively becomes a crude noise gate or an open plate or cathode resistor or shorted cathode capacitor, same thing, but all those throw DC voltages completely out of whack, you checked it does not happen.

So less common but next in line is audio path open somewhere so what you actually hear is audio feedthrough, not through the proper path but because of stray coupling, stray capacitance, imperfect ground,etc.
All those sound ugly.

We have to find where sound stops.

You can test it going forward or backwards.

NOTE: please name tubes as in the schematic, such as RO1 - RO2 - etc. ; "V" something here means rail voltages

Forward, the proper way: inject 100mV 440Hz audio at the input, and measure it along the way comparing what's expected to what you actually find; you need a reasonable Multimeter which can read down to 200mV AC or at worst 2VAC; the $10 ones which only have 2 AC scales: 200 and 500/700VAC don't cut it.

Mind you, best would be a scope, but I guess you don't have one.

OK, injecting 100mV on the passive input and setting multimeter to 20VAC scale we expect:
~5VAC on the node (junction of) C2/R6
~2V AC on the node R6/R7/P1
0 to 2VAC, depending on volume pot position at Ro2 grid

now setting it to 200VAC scale, first test it rejects DC by touching RO2 plate, it should "blink" for an instant and then settle to 0 , this test with volume set to 0 ; then rise volume slowly, you should be able to reach healthy 90-100VAC here, which means the stage clips.

Now with all tone controls on 10, and volume high enough to reach 60-90V on Ro2 plate, and Master on 0, you should get 5 to 10VAC on top of Master volume.

Note 2: use your regular speakers as test loads:
* no danger of DC even if output stage is dead
* you'll blow nothing since amp is not passing audio anyway.
*The instant you hear loud clean sound on them you just turn volume down.
Won't suffer more than actual playing onstage

Now measure on both sides of C14 and start rising master volume, you should be able to get almost same ~5VAC you had on top of master volume.

Now set scale to 200VAC again and measure on Ro7 and Ro8 plates, should be able to get at least 60V RMS there.

Now on Ro9 and Ro10 cathodes, same thing.

Now (carefully, avoid test point slipping) on node C16-R43-R45
Same on node C17-R44-R46

Really, these last tests are suggested to mention all possibilities, if you actually get to find those voltages there, sound should be *deafening* by now ... or you forgot to plug tubes in their sockets or their filaments are cold or plate and/or screen voltages are missing or the OT is open or you forgot to plug the cabinet (or speaker out wiring is open somewhere) but these latter problems are not *amp* problems and you already checked DC voltages.

Good hunting.

[beatnik]

thanks for your reply J M ! that is very informative. I am gonna print it out and add to my library. Will keep for future use !

in the end the problem was something really stupid, a broken wire on the master pot ground connection

the potentiometers are mounted on a pcb that sits on top of them, so the broken wire couldn't be caught by eye !

i am glad this turned out to be a very cheap repair this was my first tube amp debug and i am happy i had the chance to practice the various troubleshooting techniques

thanks a lot to everybody who offered help, I have learned something from you

cheers !!

[Rockhard]

Hi guys, Ive had an AD200 go down and was busy using Beatnik's schematics for reference to repair it a few days ago. The OP's links have since gone dead, does anyone else perhaps have them?

Thanks in advance
Pieter-Jan

[beatnik]

There you go

Good luck with the repair

[Rockhard]

What a legend, thanks beatnik!

Mainly needed the PSU diagram.