Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chocopower on March 01, 2012, 10:54:03 am

Title: Orange Overdrive weird noise problem
Post by: chocopower on March 01, 2012, 10:54:03 am

Hi there.

I have an Orange Overdrive from 90´s with high frecuency noise (continuous) and interference type noises (intermittent).
Measuring voltages, i found that both plates in V1 are in the 75vdc range. (In some 80´s Orange schematic claims for 170vdc in both plates)
Power suply before plate resistors seems normal (330vdc)
I checked plate resistors, cathode resistor and cap, input grid resistance, coupling cap and of course, tubes.
With master all way down noise disappear.
Noise disappear too, with bias trimmer at minimun.
100ohm resistor in heater circuit seems "burned". I put a new one.

Power and OT trafos had been changed and rebobined respectively in past.
Moving grid wires in preamp tubes noise get better (or worse... :POd: ).

Any other idea???
thanks in advance!

Title: Re: Orange Overdrive weird noise problem
Post by: rzenc on March 02, 2012, 10:37:37 am

Hi Choco!

Try chopsticking components/solder joints. Maybe a solder joint went bad/crackled.
Do you have a schematic?

Best Regards
R.

Title: Re: Orange Overdrive weird noise problem
Post by: chocopower on March 02, 2012, 04:53:18 pm

Hi! a long time since last time!

that schematic is the closer one i found

http://www.orangefieldguide.com/OFG_SCHEM/ODschem_A.gif (http://www.orangefieldguide.com/OFG_SCHEM/ODschem_A.gif)

Title: Re: Orange Overdrive weird noise problem
Post by: rzenc on March 03, 2012, 07:49:27 am

Divide and conquer man!

I would take P.I. out and check for noises. Crank MV all the way up. Is there noise yet? If so, maybe master pot has gone south/caps from concertina maybe leaking DC and making pot scratch?

If there is no noise anymore, it must be before P.I. tube. Then I would go check solder joints. If it's PCB I would not hesitate to reflow all of them.

Check E-caps too...maybe, V1 e-cap drifted and is out of spec?


P.S:I built this amp - expect por master volume - very nice amp...but I like the AC concertina better, IMHO.

Best Regards
R.

Title: Re: Orange Overdrive weird noise problem
Post by: chocopower on March 03, 2012, 09:12:42 am

No noise with Master at cero. So the problem should be befora.
With Volumen pot at cero, noise is still present.
My idea is that the problem is in the last filter stage and round...

thanks man!

Title: Re: Orange Overdrive weird noise problem
Post by: rzenc on March 03, 2012, 11:39:19 am

Quote from: chocopower on March 03, 2012, 09:12:42 am

No noise with Master at cero...With Volumen pot at cero, noise is still present.

Ok, I believe you have isolated the problem.
Try this: disconnect FAC switch. Disconnect V2 grid wire. You have removed all componets between V1b and V2a. Jumper V1b -after the coupling cap (68nF) to V2a grid. Add grid reference (1M) to e-cap that supplies V2a.
Is there noise?

Title: Re: Orange Overdrive weird noise problem
Post by: chocopower on March 03, 2012, 04:12:57 pm

Hi!  thanks. Along next week i´ll chek that.

thanks a lot for your help!