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Old 18th May 2009, 03:47 PM   #741
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Quote:
Originally posted by udailey
I am not entirely sure what you mean but I would put each RCA signal through an input cap if you measure any DC from your CDP. A poorly constructed preamp can give some DC if its active and a poorly constructed passive pre can make noise but I dont think its going to generate any DC. If you have no ouput caps on your CDP then your amp should have them. A CDP can give a few volts of it without a cap in the way of DC on each channel. This would destroy your speakers quickly. So measure then make your choice. Dont try to figure it all out first. Measure then decide.
Please email me the parts you would like again and I will order from Mouser for you today.
Uriah

Please ignore my last posting regarding DC offset....I might be tangle up with some AC/DC confusion here.....Anyway thanks for the advice...!!!

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Old 19th May 2009, 01:11 AM   #742
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While trying to diagnose my broken amp, I took voltage readings at lots of points and compared them to the working amp. Interesting that the working amp has 16.5 volts at C14, not 24 as noted on the schematic. Has anyone else ever measured the voltage there? There is 24 volts at the junction of D2 and D3, but 16.5 volts after R14.

On the broken amp, I am getting 30 volts all over the place: BOTH ends of D2 and D3, both ends of R3 (should be zero, I believe), at C14, etc. Voltages on the transistors are all weird, too. The only normal readings are at ZD1, ZD2, R1, R4, C3, C6, C8 and C11, so I know the power supply is okay. I checked all diodes and they seem okay, and all 3 transistors seem to be okay. I used the diode checker on my DVM but didn't pull any off the board. Something is putting 30 volts in lots of places where it shouldn't be. Any ideas? Does this sound like a short somewhere? Bad LM3886 or 318? The amp is out of the case (using a temporary heatsink that touches only the chip), so it's got nothing to do with mounting or connections.

C'mon, guys, I need some help.

Peace,
Tom E
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Old 19th May 2009, 01:17 AM   #743
troystg is offline troystg  United States
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One of the 2 chips is shorted, a solder bridge, or maybe even a bad cap.

With the unit off and the caps fully discharged check the main caps for resistance to grnd.
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Old 19th May 2009, 02:07 AM   #744
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Troy,

Thanks very much for responding.

Main caps check okay--zero ohms to ground on one pin, lots on the other (reading keeps increasing).

By solder bridge, you mean a short to ground? Where I have C21 mounted from under the board, there is a very narrow insulated (open) area around the top of the pad and the surrounding ground plane. If I bridged that space with too much solder on the top of the pad and it touched the ground plane, could that be the problem?

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Tom E
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Old 19th May 2009, 12:48 PM   #745
bluegti is offline bluegti  United States
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Quote:
Originally posted by madisonears
While trying to diagnose my broken amp, I took voltage readings at lots of points and compared them to the working amp. Interesting that the working amp has 16.5 volts at C14, not 24 as noted on the schematic. Has anyone else ever measured the voltage there? There is 24 volts at the junction of D2 and D3, but 16.5 volts after R14.


Here are some voltages I took earlier:

Quote:
Originally posted by bluegti


Well I swapped boards and compared the voltages. They are pretty much the same (recommended voltages in parenthesis), so whatever is happening is happening on both boards:

-C3 -36V (-32.5)
+C8 +36V (+32.5)
+C6 +11.75V (+12)
-C11 -11.75V (-12)
-C1 -36V (-32.5)
+C2 36V (+32.5)
+C14 +16.75 (+24)

They seem to be inline with what you are getting - of course, my amps aren't working yet. If you read the thread above, people seem to think its the speaker protection kicking in because I don't have proper heat sinks.

I'd like it if ClaveFremen would post his voltages because I'm using the same caps as he is.
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Old 19th May 2009, 01:46 PM   #746
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Went over my broken amp with a magnifying glass, checking every solder joint. No bridges, nothing compromised. This amp was assembled at the same time as the working one, every component matched for value, location, and orientation. I am confident this is not an assembly issue.

Peace,
Tom E
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Old 19th May 2009, 02:37 PM   #747
troystg is offline troystg  United States
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Sorry dealing with some issues right now so replies will be sporadic.

If main caps are good and you are confident that there are NO solder bridges between pins or across traces then it is pretty safe to assume that your chip is bad.

But at the same time, the uninsulated chips are notorious for
'bleeding" voltage through the heat sink.
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Old 31st May 2009, 02:44 AM   #748
bluegti is offline bluegti  United States
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Hurray! I got my amps working tonight. I'm surprised that I didn't see the issue before and I'm surprised that nobody else caught the problem either. Its sort of obvious now that I know. Click on the picture to see if you can see the problem.

Click the image to open in full size.

Look at the top-most leg of the LM3886 - Not even in the hole. Amazing how everything works when you have all the connections soldered.

The issue with the screeching sound seems to have been the heat sinks. Thanks to Troy for helping me out with those. I just got done playing the amps at a fairly loud volume for 45 minutes and they are barely above room temperature. My first impression is these sound great.

I'm using them with a Lightspeed Attenuator that I just built. There is some buzzing that I hear when the volume is turned up. I think its the Lightspeed, but I need to swap in another pre-amp to be certain.

Assuming the buzz isn't in the amps, I want to swap out the C13 caps for something better that the Solen's I have in there now, secure the boards in the enclosures then close 'em up. I bought a couple of flavors of Obbligatos to try - 2.2 Copper Case Film caps, 2.0 Film Oil caps (big black cans), and 2.2 Tinfoil/Film caps. Just realized the tinfoil caps are rated at 250V, will that be an issue? Maybe I'll have to get some of those posts that ClaveFremen uses so I can compare. What do people think would be best?
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Old 31st May 2009, 07:46 AM   #749
soongsc is offline soongsc  Taiwan
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check interconnecting wire shielding.
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Old 31st May 2009, 01:18 PM   #750
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Quote:
Originally posted by bluegti
Hurray! I got my amps working tonight.
....
I bought a couple of flavors of Obbligatos to try - 2.2 Copper Case Film caps, 2.0 Film Oil caps (big black cans), and 2.2 Tinfoil/Film caps. Just realized the tinfoil caps are rated at 250V, will that be an issue? Maybe I'll have to get some of those posts that ClaveFremen uses so I can compare. What do people think would be best?
Well done!

250 V rating is not a problem (apart dimensions ), probably it sounds better than the 160V counterpart.

The Obbligatos sounds very good but IMHO Mundorf Zns are better, more transparent.

In the meanwhile, since my personal components selection is definitive, I've removed sockets and soldered all.
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