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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, 06:43 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 07:00 PM.
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Old 27th January 2018, 10:51 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, 03:00 AM   #23
Cheeto333 is offline Cheeto333
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Bump?
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Old 31st January 2018, 09:25 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, 02:12 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, 03:20 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, 04:32 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, 12:09 PM   #28
thoglette is offline thoglette  Australia
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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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Old 19th February 2018, 10:21 PM   #29
Cheeto333 is offline Cheeto333
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Applying ~120VAC to the primary of the power transformer (with secondary disconnected) yields ~41 VAC across 3-4 and 4-5 of the secondary, and ~81 VAC across 3-5 of the secondary. Are these values expected? If so, what should I check next? The schematic shows +/- 60 VDC from the rectifier (U2), does that correspond to +/- 80VAC input?

It's possible the glow in the bulb I saw previously was normal (and I chickened out). Should I expect glow in the bulb at input voltages as low as 4AC?
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Old 20th February 2018, 12:34 AM   #30
thoglette is offline thoglette  Australia
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Quote:
Originally Posted by Cheeto333 View Post
Applying ~120VAC to the primary of the power transformer (with secondary disconnected) yields ~41 VAC across 3-4 and 4-5 of the secondary, and ~81 VAC across 3-5 of the secondary. Are these values expected? If so, what should I check next? The schematic shows +/- 60 VDC from the rectifier (U2), does that correspond to +/- 80VAC input?
Ok, so 80VAC center tapped (that is, +/- 40VAC) will give roughly +/- 60 VDC.

That is, the center tap needs to be tied to earth and the two outside taps sent to a pair of diodes, one for each of the rails.

If you're rectifying 80VAC you're going to get +/- 120VDC. Which you don't want.

It should look like this
Click the image to open in full size.
(from All about circuits)

If your bulb is glowing brightly you're pulling mains current. It's got to be going somewhere. It is possible, given the age of the amp, that one of the filter capacitors has started leaking badly. Off the top of my head I can't recall what the formula for acceptable leakage is (Anyone? Ferris?) but you put a resistor in series with the cap and measure the voltage ratio (12VDC will do for starters). Be aware that the cap will appear as a dead short on start up so your resistor needs have enough wattage for this.

Conrad Hoffman (who's sometimes hereabouts) wrote a short article on this topic

Last edited by thoglette; 20th February 2018 at 12:46 AM. Reason: Add picture, talk about caps
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