XL-280 circuit card trouble-shooting - diyAudio
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Old 9th April 2005, 12:21 AM   #1
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Default XL-280 circuit card trouble-shooting

A certain mod shop recently sold leftovers so I got 3 circuit cards for the XL-280. They were advertised as "pulls" but from working amplifiers. Well, none of the amplifiers these cards came from were "working."

All 3 cards exhibit output gate voltages up to around 14-15 volts, until the 15v zener MOSFET gate protection diodes kick in. Running up the variac above more than 20 VDC or so to these cards for testing makes no difference. Here are the voltages present with rail voltages of +/- 20 VDC:

Card 1 has a negative 14VDC gate voltage on the negative side of the circuit
Card 2 has a positive 14 VDC gate voltage on the same negative side of the circuit
Card 3 has a negative 14 VDC gate voltage on both sides of the circuit.

I assume a problem with the input diff circuit or is there a better place to search? Many amps get damaged with misuse of the bridging circuit. If so, where does this damage typcially show up? In transistors or resistors?

Well, I know you can only generalize from my data, but anything will help me get started sleuthing.

I have all the transistors and MOSFETs to fix this amp but have no experience with FETs. This circuit uses the 2SK147/2SJ72 devices. Are they available? But, they would have to be matched. Right? My meter measures hFe but will it help match FETs?

Thanks for your help.
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Old 10th April 2005, 01:33 AM   #2
mlloyd1 is offline mlloyd1  United States
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Hmmmm ...

I hope for your sake the input JFETs are ok. They will be very difficult and expensive to replace, especially needing matched sets.

The service manual is available at the hafler site and I think (not sure) that voltage measurements are included. That would help troubleshooting.

Good luck. The XL-280 is a pretty decent amp as far as I'm concerned.

mlloyd1
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Old 10th April 2005, 03:11 AM   #3
fab is offline fab  Canada
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See http://www.borbelyaudio.com/audiophile_components.asp for matched jfet parts but check price too first!!!

Since you do not know what happened with these boards I would check all transistors anyway. You can check the junctions using a meter in diode verification mode (or resistor mode -if you do not have diode mode- to check for short circuit in bjt or jfet. And this is done with no power applied.
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Old 10th April 2005, 03:13 AM   #4
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I fear you are confirming what I was afraid of, that the JFETs would be difficult to find and even more difficult to get in matched pairs.

Can I remove them from the circuit and check them with my meter? I can do this for regular transistors, including measuring hFe. Can I do the same for the JFETs?

I have the manual downloaded from Hafler.com and it does have a drawing of the back side of the circuit card with voltage numbers at various locations. But, the resolution on the drawing is so poor that it will take some real sleuthing and comparisons to get a definite drawing of which exact point to test. Not simple to get a good print from a PDF file. It works best when I capture the screen to the clipboard, go into PC-Paint to get a BMP file, them put the BMP file into my photo rendering software to tweak brightness, sharpen the image, etc., to finally get something readable I can expand to size in WordPerfet and then print. Oh my!!

Time to check Erno borbely's web site it seems.
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Old 10th April 2005, 03:18 AM   #5
fab is offline fab  Canada
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Quote:
Originally posted by Dick West
...Can I remove them from the circuit and check them with my meter? I can do this for regular transistors, including measuring hFe. Can I do the same for the JFETs?...

You do not need to remove them from the circuit. Simply put your meter probes on the transistor pins while no power supply applied. Yes jfet can be checked this way too. They still have np and pn junctions as bjt.
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Old 10th April 2005, 03:58 AM   #6
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OK, I think I understand. Lots of detailed work to do. Tedious.

I assume I can do most of this with the cards disconnected and lying "naked" on the bench? That is the best way for me to get meter probes in place quickly and easily. Let's hope that from 3 cards I can eventually get two that work. . .

Thanks for the help.

Dick
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