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Old 21st July 2013, 09:41 PM   #1081
bcmbob is offline bcmbob  United States
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Thanks, I use that when the new parts arrive.
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Old 25th July 2013, 09:24 PM   #1082
lhquam is offline lhquam  United States
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If an electrolytic cap is used for C3, is the polarity on the board from the DIYAudio Store correct? The silkscreened polarity shows positive on the D output from the board, but when driving a BA-2 complementary output board the D (input) is at about -4V for zero volt output. Thus I would expect the electrolytic to need the opposite polarity.
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Old 26th July 2013, 06:53 AM   #1083
generg is offline generg  Germany
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I posted the same question and suggestion three years ago, nobody answered, but I tell you, you are right!
The polarity is only correct for the SE outputstage.

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Old 26th July 2013, 05:15 PM   #1084
bcmbob is offline bcmbob  United States
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Default Electrician Needed !

Still struggling. Here is what's going on:

1. +/- 24.5 VDC readings are correct at all pads on all three PCBs.
2. On the front end board - reading across the Gnd and D(Drive) -Red/Grn - wires produces 4.5 VDC.
3. Attaching Gnd/D to terminals (A & B) on top bias board produces 4.5 VDC at both the terminals and at R214 (C).
4. Attaching Gnd/D to terminals (1 & 2) on the bottom board produces 0.0 VDC at the terminals and R214 (3).
5. On the bottom bias board both the transistor (Q204) and the small cap (C209) were replaced with new pieces.
6. Doing a swap, both large caps (C207 -1000 uF) allow the correct 4.5 VDC reading on the top bias board (operating channel) at both the terminals and R214.

All that's left is the zener diode (or possibly a resistor) on the bottom bias board. So does that sound like what a blown diode would do? And of course that's the part I didn't re-order. In my stash I have some 12V-1W and 13V-1.3W zeners but no 9.1V-1W. Can either of the two I have be used along with trimmer adjustments? The only other zener availble at my local Radio Shack is 5.1V-1W.
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Old 26th July 2013, 05:19 PM   #1085
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Unplug the Fe and concentrate on the bias and output boards. The FE has nothing to do with bias boards and output in terms of DC conditions
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Old 26th July 2013, 05:26 PM   #1086
bcmbob is offline bcmbob  United States
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OK, is it at all valid and/or productive to make a single connection to one bias board, or is it necessary to attach an output board for proper readings?

I was using this suggestion for the readings reported above
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Last edited by bcmbob; 26th July 2013 at 05:28 PM.
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Old 26th July 2013, 05:29 PM   #1087
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You need to attach the bias boaards to the output boards. that sensing circuit establishes Dc bias for lower fets. I didnt realize at first you were using Ba1 output with Ba3 FE

Measure across Zener, attaching one probe to ground and other to top(cathode) of zener on top bias section. This should read 9v or whatever value xener you have i place. Now measure across pot output, by attaching probe to gronud and D+. For the bias you need, I would expect somewhere in the 4V range, depending on the fets used. You should be able to adjust this value.
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Last edited by buzzforb; 26th July 2013 at 05:32 PM.
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Old 26th July 2013, 05:30 PM   #1088
bcmbob is offline bcmbob  United States
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Will do......
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Old 26th July 2013, 05:59 PM   #1089
bcmbob is offline bcmbob  United States
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Readings with original zeners in place. Trimmer has no effect on voltage at D+ on non-working bias board.
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Old 26th July 2013, 06:06 PM   #1090
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The zener on the board on the rigth is bad if you are getting 0V. I assume you can adjust the voltage at d+ on the left board? Next, measure from neg rail to d- on both boards. You should get about 5v as the article suggest. If this is true, you may only have to replace the zener on the one board. After doing so, you should be able to adjust it. If all else checks out, then you can start final biasing. The bottom fets bias is detemined appx by the Vbe of the bjt divided by the Rs value, whcih is .65/1r=.6A
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