MAX 115 300 ii no sound from speaker after an instant of power on

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bvbrain
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MAX 115 300 ii no sound from speaker after an instant of power on

Post by bvbrain » Fri Nov 12, 2021 9:02 am

I searched and found a schematic for the 2012 versions and mine is manufacture date January of 2014. Headphones and tuner work fine.
Preamp voltages all seem to check out ok, items marked for +15 are just a bit over 15 and items marked -15 are measuring 14.91. I checked all of the points mentioned in the schematics. Perhaps I do not have a good schematic.

Are there some useful voltages to check on the power amp board?

Woofer Speaker measures around 5 ohm.

No visible burns or damaged components.

I did have one loose cap, C32 had one leg broken just inside the can (negative). When I turned it on it sparked there. Replaced both C31 and C32.

Suggested troubleshooting?

thanks in advance. Brian.

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Enzo
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Re: MAX 115 300 ii no sound from speaker after an instant of power on

Post by Enzo » Fri Nov 12, 2021 4:45 pm

Use your FX loop jacks or preamp out and power amp in jacks. Apply a signal to the return to test the power amp. Get sound? Play into the regular input and run a cord from the FX send to some other amp. Get sound there?

Lug a spare cord from send to return, sound come back?

"No sound" will not be a problem with a voltage being off a little. No sound will result from missing voltages. Of course it may not be a voltage issue at all. Is the speaker utterly silent? Or is ther a little background hum or hiss?

Post the schematic.

If the power amp board is separate, what number board is it? Look on the solder side.

bvbrain
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Re: MAX 115 300 ii no sound from speaker after an instant of power on

Post by bvbrain » Fri Nov 12, 2021 5:23 pm

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power amp schematic pic with voltage readings in power section. High + is 39.5 volts and High - is 50.9 volts. I am suspecting the voltage regulating diodes D40 may be open.
20211112_163849.jpg
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photo of the board where these are located.
MAX_115 PowerAmp_2012.pdf
(155.92 KiB) Downloaded 153 times
Max 112_115 Preamp_2012.pdf
(174.79 KiB) Downloaded 139 times

bvbrain
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Re: MAX 115 300 ii no sound from speaker after an instant of power on

Post by bvbrain » Fri Nov 12, 2021 8:14 pm

20211112_163824.jpg
20211112_163824.jpg (2.98 MiB) Viewed 3311 times
20211112_163849.jpg
20211112_163849.jpg (3.84 MiB) Viewed 3311 times
MAX_115 PowerAmp_2012.pdf
(155.92 KiB) Downloaded 134 times
Max 112_115 Preamp_2012.pdf
(174.79 KiB) Downloaded 130 times
I was able to check more voltages on the power amp board. I found the +High was at 40.9 v and -High was at 50.9 v.
I am suspecting the voltage reg diode D40 may be open. It measures -50.9 on the input side then 0.

I am no electrical engineer so I may be reading this all wrong and the main digital amp may be causing the issue but it did not appear damaged or over heated. But the difference in voltage between +High and -High are very suspect.

Ideas?

bvbrain
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Re: MAX 115 300 ii no sound from speaker after an instant of power on

Post by bvbrain » Sun Nov 14, 2021 2:46 pm

I put a 22v and 18v zener in place of the 20v d40 d41 to get a bit further. They were 10% vs the original 5%. It shows the correct drop over the first zener. There is no voltage drop over d5. In circuit test of d5 shows 0 in diode check mode both directions. D4 has readings in both directions. What is more likely, d5 or q6 tip107 shorted?

I am working with the 2012 schematic as mine is a 2014 amp.

bvbrain
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Re: MAX 115 300 ii no sound from speaker after an instant of power on

Post by bvbrain » Sun Nov 14, 2021 6:23 pm

I checked d41 out of circuit and it is shorted. 0 ohms both directions.

Beach
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Re: MAX 115 300 ii no sound from speaker after an instant of power on

Post by Beach » Thu Nov 25, 2021 11:03 am

Obviously both D40 and D41 are dead. Probably Q6 as well (short).

The power amp chip will have some circuitry to measure the difference in supply voltage and will shut down rather soon after switching on. I did not investigate the power amp chip in detail, but if that is the case, your amp will turn on for a moment and then goes into shut-down mode.

bvbrain
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Re: MAX 115 300 ii no sound from speaker after an instant of power on

Post by bvbrain » Thu Nov 25, 2021 4:21 pm

So far I replaced d 40 and 41. 40 was good but I had new ones anyway. Q6 also. But putting it together I did not get the isolation washer in place so I blew the t4a fuse. Tough sucker to find. Waiting on a pack of those from a MO zon. Put in a didderent tip107 and metered to be sure bass was not shorting to ground any more. Hopefully this is it.

bvbrain
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Re: MAX 115 300 ii no sound from speaker after an instant of power on

Post by bvbrain » Fri Nov 26, 2021 8:23 pm

Success. Fuse replaced and it works again. Sprayed pots with deoxit after starting and they were scratchy. Good to go now. Thanks for the steering.

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