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Post subject: BDRI repair problem (resistors blew)
Posted: Fri Dec 27, 2013 12:17 pm
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So, it finally happened. My BDRI blew a resistor in the tube mount area. The specific resistor is R62 if you're looking at the service manual. Amp was purchased used in 2009 and I have used it for gigging and recording non stop since. I think what happened was that a power tube shorted and it arc'd over the PCB burning out resistor 62 and leaving the traces burned.

I have since replaced R62 and also R61 for good measure. I replaced them with 3W 470 ohm FP resistors.

I had to jump the tracers on PCB and solder the resistor to the V4-A pin 4 (power tube) and PW4B-2 (jumper wire).

Resistor 61 was okay to be soldered back into the PCB.

I turned it on, Biased the power tubes (different set than what was in it when it blew the resistor).

When I flipped it off standby it plays, but you can barely hear it when cranked all the way up to 12. On both channels. And it sounds very thin like when one of the power tubes is dying.

I'm unsure where the issue could be.

Diode 5, was also blackened on the PCB and I'm not sure if it needs replacing.

Any help would be much apreciated!


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Post subject: Re: BDRI repair problem (resistors blew)
Posted: Fri Dec 27, 2013 5:12 pm
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Hi JohnnyHarpo,

Possibly still a bad connection for the screen grid resistors (R61 and R62), a connection common to both, not allowing the output tubes to conduct, or D5 could be shorted, definitely check D5 and D6. Did you actually have current through the output tubes, you mentioned biasing?

I think that you are correct in the assessment of the tube shorting, taking out the SGR.

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Post subject: Re: BDRI repair problem (resistors blew)
Posted: Fri Dec 27, 2013 5:49 pm
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I'm not sure, how would I check the current?


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Post subject: Re: BDRI repair problem (resistors blew)
Posted: Fri Dec 27, 2013 6:01 pm
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How did you "bias" the output tubes?

Using the test point when setting the bias, you are measuring the current through both output tubes via the one ohm resistor.

Using bias probes, you can measure the current through each output tube individually.

Measuring the negative voltage at the grid of the output tubes is really useless as far as bias current is concerned.

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Post subject: Re: BDRI repair problem (resistors blew)
Posted: Fri Dec 27, 2013 6:25 pm
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I used the test point to bias them.


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Post subject: Re: BDRI repair problem (resistors blew)
Posted: Fri Dec 27, 2013 6:56 pm
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What was your reading? If you have approx 60 mV or more, then that indicates enough current through the tubes to confirm that the SGRs and circuit are good.

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Post subject: Re: BDRI repair problem (resistors blew)
Posted: Fri Dec 27, 2013 8:40 pm
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Okay sooooooooooooo! Good news. I pulled all the tubes and the issue was that one of the 12ax7s wasn't completely pushed in...........and one of the pins was smashed down :shock:

Now its ROCKING again!


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Post subject: Re: BDRI repair problem (resistors blew)
Posted: Fri Dec 27, 2013 9:11 pm
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Awesome! 8)

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