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PPI pro mos 450 pulses when powering on
PPI pro mos 450 pulses when powering on
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Old 24th November 2020, 11:54 PM   #1
GEM 592 is offline GEM 592  United States
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Default PPI pro mos 450 pulses when powering on

Hello,

I have a PPI pro mos 450 that a friend bought used then sent to me to check.

I connected the amp, and checked all four channels - seemed fine. About 30 seconds after playing, the sound cut out, and the power led started pulsing.

I can connect ground and +12V without any apparent issues, but when I connect the remote to +12V the amp just pulses with the power led flashing.

I can't see any obvious signs of damage, short circuits in power supply devices, etc.

Any ideas are appreciated, thank you.
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Old 24th November 2020, 11:57 PM   #2
Perry Babin is offline Perry Babin  United States
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Did you check the output transistors to see if any were shorted?
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Old 24th November 2020, 11:58 PM   #3
GEM 592 is offline GEM 592  United States
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Will do right now.
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Old 25th November 2020, 12:10 AM   #4
GEM 592 is offline GEM 592  United States
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There is a bank of 5 2N6487 with base to emitter impedance reading very low, 0.2 or 0.5 Ohms. Not seeing this in the other banks of these, typically minimum of about 47 Ohms elsewhere.
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Old 25th November 2020, 12:12 AM   #5
Perry Babin is offline Perry Babin  United States
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Find the lowest one and remove it from the circuit. After that, do the rest read like the others?
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Old 25th November 2020, 12:34 AM   #6
GEM 592 is offline GEM 592  United States
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I wound up having to remove 2 of the 5, the remaining 3 now read base to emitter resistance 47 Ohms, which is similar to corresponding measurements on the other side of the amp.

Out of circuit, base to emitter is reading 0.5 Ohms on both removed devices.
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Old 25th November 2020, 12:37 AM   #7
Perry Babin is offline Perry Babin  United States
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Were you testing with the bottom cover removed?
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Old 25th November 2020, 12:39 AM   #8
GEM 592 is offline GEM 592  United States
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No, I know that acts like a clamp.

But, interesting story, when he received the amp, one screw was missing. That was replaced first, but maybe it caused the problem previously.
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Old 25th November 2020, 12:42 AM   #9
GEM 592 is offline GEM 592  United States
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How much matching would you recommend for the replacements? Obviously, at least that bank of 5 should be date-code matched, should I replace the complimentary 5 2N6490 on that channel too at least?
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Old 25th November 2020, 12:42 AM   #10
Perry Babin is offline Perry Babin  United States
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You'll have to replace all 5 transistors in that group. I'd suggest the other 5 in that channel as well. Each group of 5 needs to have matched date codes.

Carefully check the emitter resistors as well.
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