ADA amp

[Valvehead]

Ive got a pretty rare ADA Viper combo at home that all of a sudden developed a big problem.

The "dirty" channel has super low output , but the clean channel has full output . This amp has worked fine for years(its my friends) , and this just happened.

Anyone worked on one of these? I know its pretty rare. Damn thing looks like the space shuttle inside !

I have schematics for it if anyone wants to take a gander . I can email them or i think they are up at Adadepot.com

[Don Symes]

Please send the schemo to das_float at yahoo dot com.
The ZIPs at ADAdepot won't unzip for me (Stuffit 10.0/OS-X).

I'm going to make a blind guess and suggest you look at the channel switching (duh). If it's LDRs or JFETs, I expect that, either the part at the dirty's output isn't turning ON, or the shunt switch that ground the signal somewhere within (or immediately before) the dirty circuit isn't turning all the way OFF.

LDRs _can_ burn out if they're run with too high an LED current, but that seems unlikely.

[Don Symes]

Got the schemo - thanks!
No power amp pages, but we may not need them.

As far as I can tell, the clean channel is called GREEN and dirty is RED.

First, does the problem move from RED to GREEN if you swap tubes V1 and V2? Yes? Bum tube or loose socket pin are the first suspects, then the wiring between the RED and GREEN boards - bad solder connections and broken wires are pretty common.

No? Q9 and the circuit that drives it need a look.

Q9 is a 2SK364 N-channel depletion mode JFET. It needs a voltage of -1.5V from Gate to Source to turn OFF (otherwise it's ON). Check that Vgs voltage in both modes. If the voltages are good (one negative, one either GND or positive), then it may be the part. If the voltages aren't right, you need to follow the signal GRNCHCONT (tube & tone sheet, section A6) back to its source - CR9/R21 on the schematic-input sheet, section B5 - to GRNCHSEL on that sheet and on the channel-logic-...decoder sheet, section C1.

If the voltages all look OK, you need to go through the channel switch functions - footswitch and front-panel.

Hope this helps!

[Valvehead]

wow---thanks Don !!

some of these amps are beyond me............


i will check everything out you mentioned--hopefully tomorrow !

i did put new 12ax7 tubes in it by the way.

forgot to mention--the channel switch buttons on the panel work and so does the footswitch

[Valvehead]

Q9
green mode
0v on outside leads, 0.49v on center lead

Red mode
0v on outside leads 13.8v on center lead


CR9
green mode
0.5v and 0v

red mode
13.7v

cr9 is grounded good and tests good on diode tester


R21
green mode
0.5v and 13.4v

red mode
13.8v both sides

[Don Symes]

So no negative voltages anywhere in that chain in either mode?

Do you have a -15V supply (U13 pin 11)?

In GREEN mode, what are the voltages at U13 pins 2 (input), 3 (reference) and 1 (output)?

In RED mode, pins 1 and 2 should change, pin 3 should not.

In one of these modes, pin 1 should go negative.

If it's practical to remove U13 or lift pin 1, does that get you the RED mode?

If that last move works, U13 needs to be replaced. I suggest getting a good socket (http://www.mill-max.com/images/products/pdf/011.PDF -- digikey has them for $1.31 http://www.digikey.com/scripts/DkSea...580941&Site=US ) and usign a ty-wrap to keep the new IC in place.

Otherwise, I expect you'll be chasing down a problem with getting teh -15 rail to where it's supposed to go.

Hope this helps!

[Valvehead]

tetsing............

So no negative voltages anywhere in that chain in either mode?
sorry . i meant negative 13v on those


Do you have a -15V supply (U13 pin 11)?

Yes

In GREEN mode, what are the voltages at U13 pins 2 (input), 3 (reference) and 1 (output)?

pin1 +13.4v , pin2 4mv , pin 3 +7.0v

In RED mode, pins 1 and 2 should change, pin 3 should not.

pin1 goes to -13.9v , pin 2 goes up to +10.6v , pin 3 does not change stays 7.0v

In one of these modes, pin 1 should go negative.

yes , in red mode it goes neg.

If it's practical to remove U13 or lift pin 1, does that get you the RED mode?
i dont see any way of doing that, its a fairly big chip! I suppose i can try to clip the leg..

If that last move works, U13 needs to be replaced. I suggest getting a good socket (http://www.mill-max.com/images/products/pdf/011.PDF -- digikey has them for $1.31 http://www.digikey.com/scripts/DkSea...580941&Site=US ) and usign a ty-wrap to keep the new IC in place.

Otherwise, I expect you'll be chasing down a problem with getting teh -15 rail to where it's supposed to go.

Hope this helps!

[Valvehead]

Good news !!-------------

I was looking at the solder joints on U13 and the chip next to it. A few of them looked dodgy, so i re-flowed them. BINGO .

I now have a full functioning amp ..........so far........Im letting it idle and playing through it , i will post back in a while .

[Valvehead]

ok--------


amp seems great now..working fine.


Don, i will buy you a steak dinner if you come through the Iowa region......
not kidding...


thank you




man...I owe Enzo about 27 steak dinners . waiting for him to claim one.

[Don Symes]

amp seems great now..working fine.

Don, i will buy you a steak dinner if you come through the Iowa region.

Thanks, but I just like trying to help.

I think that's why most of us are here.

[Enzo]

When it gets up to 30, I'll bring the wife. She loves a good leg of cow dinner.