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Replacing Burnt Components on Gallien Krueger RB400 Bass Amp
Replacing Burnt Components on Gallien Krueger RB400 Bass Amp
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Old 15th January 2018, 07:43 PM   #21
GeorgK is offline GeorgK  Austria
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Quote:
Originally Posted by Kay Pirinha View Post
If you have set your DMM to diode test mode, your measurements indicate taht all these transistors are faulty. In this case I'd check the remaining ones also.
I also did not get if you listed just the ones that tested faulty or just the ones you took out?
But as you have decided to change them all, which is certainly the best idea, it won't matter.

Last edited by GeorgK; 15th January 2018 at 08:00 PM.
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Old 27th January 2018, 11:51 PM   #22
Cheeto333 is offline Cheeto333
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So I replaced all transistors, found 2x that were faulty (including one of the outputs). I also checked the resistors in that area and replaced R20 (it was open). Any advice on next steps before slowly bringing up the voltage with a variac and bulb limiter?
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Old 31st January 2018, 04:00 AM   #23
Cheeto333 is offline Cheeto333
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Bump?
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Old 31st January 2018, 10:25 AM   #24
thoglette is offline thoglette  Australia
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I'll bite. (PRR , Gno and George must be busy elsewhere).

First, if you can power up everything EXCEPT the output stage, do so and check that's all good.

Second, fast blow fuses in your power rails to the output devices is a good idea.

Then check that your DC voltages (and temperatures) are good before trying to apply any signal or load
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Old 1st February 2018, 03:12 AM   #25
Cheeto333 is offline Cheeto333
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How would I power up everything except the output stage? A bunch of power supplies? If so, I unfortunately don't have access to any...

Where are these fast blow fuses located? Are they something I'd have to install myself?
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Old 1st February 2018, 04:20 AM   #26
thoglette is offline thoglette  Australia
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I usually disconnect the collectors or leave the power devices out until I've tested the rest of the circuit. Sometimes you can't easily do that and/or it messes with the FB loop too much.

Fuses just go in series with the power connections. Make them much smaller than needed for full power while you check that nothing is awry. Really only need to support the standing bias current plus a bit more to charge up some capacitors.
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Old 11th February 2018, 05:32 AM   #27
Cheeto333 is offline Cheeto333
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So I reinstalled the power amp board, disconnected the preamp board, and began powering up (variac >> bulb limiter >> amp, no speaker connected). The bulb began to glow around 4 VAC (measured on the power transformer primary), so I quickly powered down. I checked the PT resistances, they measured ~1.5 ohms across the primary (1-2), .5 ohms between 3 and 4, .5 ohms between 4 and 5, and .9 ohms between 3 and 5. There was no continuity between the primary and the secondary windings.

What do you all recommend as my next step? Should I disconnect the PT and check its voltage at full 120VAC input?
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Old 14th February 2018, 01:09 PM   #28
thoglette is offline thoglette  Australia
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Quote:
Originally Posted by Cheeto333 View Post
Should I disconnect the PT and check its voltage at full 120VAC input?
Yup, with the bulb in series.

Looks like you need to start from the input and find out what's drawing so much current.
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