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Post subject: Deluxe 90 DSP Power Transistors Shorting
Posted: Tue Jul 27, 2010 11:27 am
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I have a Deluxe 90 DSP that i have had laying around for 2 years now. I figure it's time to try and get it running again. The power transistor keeps blowing. There is a short some where on that area of the board. Any ideas? Im a tube amp junky and dont mess with solidstate too much

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Posted: Tue Jul 27, 2010 6:41 pm
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it's just as easy to debug transistor circuit (but without the drama of high voltage in a tube circuit).

pretty clear where the problem is: at the power transistor. the surest way to blow a transistor is to over drive it and over heat it - it will be come fused or burnt and be rendered useless. first, check that there nothing aroung it that's shorting (check ALL wiring, right to the speakers), make sure it is properly heat sunk (this is real important because without heat sinks, heat will build in the transistor until it burns), and check the bias voltages (transistor has to be properly biased to work well).

btw, transistor are like valves, but they are current valves controlled by the current going into the base. the more current going into the base, the more current can be passed from collector to emitter. current is "measured" by observing the voltage across some load (typically a resistor). again simple stuff.

ciao & good luck.


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Post subject: Re: Deluxe 90 DSP Power Transistors Shorting
Posted: Wed Jul 28, 2010 1:14 am
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triodeamplification wrote:
I have a Deluxe 90 DSP.... The power transistor keeps blowing... Any ideas?...


First, make sure to unplug the amp from electricity before doing any testing.

A thorough visual inspection is a good place to start, look for signs of heat damage, or burnt components. Make sure that your speaker isn't shorted, it should be between about 4 ohms to 8 ohms. Check all solder joints with a magnifier for cracking or cold joints, touch up any questionable looking joints after you check the following components. The output transistors are Q22 and Q23, check both for a short (typically). Also check Q20 and Q21, the smaller transistors between the output transistors for a short. You will probably have to unsolder each component to check it while it's out of the circuit. If you check them while still in the circuit, you can get false readings. Then check all of the diodes around the transistor circuit for shorts or opens, D36, D37 (schotkey) and D38, D39, D41, D43, D44, D45 (regular diodes) and D40, D43 (Zener). The diodes will typically be shorted, so you can usually check them in the circuit. If the diodes are open it will usually be visibly damaged. If you get funny readings, then unsolder one side and check them again. Then check the resistors R123, R124, R125, R126, R127, R128, R129, R130 you might find an open resistor, usually visibly damaged, but they can be bad without showing any damage. Again, you may have to unsolder one side of each component to check it accurately.

That should keep you busy for awhile. :lol: If you don't find anything obvious, you may want to take it to a tech. If your output transistors are shorting out, it is likely that there are other components that have gone bad causing the transistors to short.

I was looking at the amp schematic found here:

http://www.fender.com/support/amplifier_schematics.php

:)

.

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Posted: Wed Jul 28, 2010 12:33 pm
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Thankyou for all the info. Oh yeah it will keep me busy. Thats why it's been on the shelve for 2 years now. I am so busy but this info will help me to eliminate a lot of what if's. Thanks and will update soon.

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