Neg Rail on JLH Amp Output - diyAudio
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Old 12th November 2011, 03:57 AM   #1
john65b is online now john65b  United States
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Default Neg Rail on JLH Amp Output

I have been checking the components on this amp for a couple days now and cannot figure why I have -20 volts on the outputs. I have the JLH Amp from Geoff's design from Skycoral

JLH-2005 – NOTES

Only different thing is I am using the FJL4315 (2SC5200 equiv) for output devices.

I have rails at +/-20V. The two pots (50 ohm and 2K) for Iq bias and DC offset adjustments do nothing. Not current through teh emitter resistors.

I bought these boards from someone here a long while ago and I think I remember them saying there is a fixed trace some where...

Tried 2SA970 / BC559 for Q5 Q6, Q7, and have 2SA970 for Q4 (in correct spot). Q3 is 2SC3421, and Q8 is 2SA1358

Any one?
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Last edited by john65b; 12th November 2011 at 03:59 AM.
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Old 12th November 2011, 08:23 AM   #2
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Either the positive rail fuse is blown or you have connected a transistor as_s about face. Geoff's designs are good.
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Old 12th November 2011, 12:02 PM   #3
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The positive rail is intact and operational. I see both rails throughout the board.

Only thing I can think of is the Q3 is 2SC3421, and Q8 is 2SA1358 possibly being fakes, as they were cheap and from ebay (China - no offense), as I had problems sourcing them.
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Old 12th November 2011, 01:35 PM   #4
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Just ordered BD139/BD140 from Mouser for the replacement of Q3 and Q8. We will see if that does anything...
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Old 12th November 2011, 03:32 PM   #5
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I also found two pairs of 2SB649/2SD669 in my parts bin...seems same pinout, but have not looked at the specs
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Old 12th November 2011, 03:35 PM   #6
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John, it is always a good idea to say what led up to the problem. For instance poking around with a metal object, etc. Then the guys here does not have to think that there is a design problem to analyze and can often help you to quickly trace the fault if they know it was working moments prior to the problem..
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Old 12th November 2011, 03:59 PM   #7
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OK, sorry. The amp has not fired up (bias wise) since putting it together.

Only thing I can think of is I put in the 2SA970 (ECB) into the Q4, Q5, Q6, Q7 (CBE). Q4 has the second ECB for the 2SA970, so I moved it there. I removed the 970s in Q5, Q6, Q7 and replaced with new BC559 (with correct CBE pinout)

No smoke, hiss, pops or anything leading me to damaged part... I realized the mistake and made the corrections.

Still has -20v on outputs.
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Old 12th November 2011, 06:51 PM   #8
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2SC5200/FJL4315 BCE pinout...
Your board pic shows EBC

If emitter junction reverse avalanched
possible that HFE has been degraded.

Last edited by kenpeter; 12th November 2011 at 07:04 PM.
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Old 12th November 2011, 08:41 PM   #9
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Yes, I had the FJL4315 wired correctly - Board showed different pinout of EBC - I managed to catch that at the beginning.

OK, so the 2SA1358 was toast - shorted on all pins. Still checking the C3421

EDIT - the SC3421 is also toast.
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Old 13th November 2011, 01:47 AM   #10
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Bingo. Replaced the 1358 and 3421 and all is good. Sounds nice so far.

I have one channel working, and sounds quite clean. Pulled the neg rail fuse and put the amp meter across the tabs and read 2.75 amps, so about 32 or so watts per channel at 4 ohms. The 4 pound heatsink gets nice and toasty hot - I used the Nelson Pass way to check temperature - if I can hold my hand on the sink for 5 seconds, its about 140 degrees.

Incidentally, have the one channel powered by two basic HP laptop power supplies (+20VDC @ 3.2 Amps each for a dual +/-20VDC). When using one of these power supplies on my laptop, I see it pulls 230 watts from the wall, so I biased up the amp up to read 450 watts from the wall to match the load from my laptop. I know a laptop load is somewhat variable, but this works great. These power supplies are clean and quiet, but no one ever believes me...

One more thing, I never put an input cap on any of my amps - Krell KSA-50, Aleph 5, F5, (all my pres have one tho), so why have one here?
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Last edited by john65b; 13th November 2011 at 01:56 AM.
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