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Precision Power ProMos450
Precision Power ProMos450
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Old 17th April 2019, 11:04 PM   #1
Bills amps is offline Bills amps  United States
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Default Precision Power ProMos450

Looking for some schematics for this amp. Has issue with the rear channels. draws excessive current. The power supply fets (IRFZ34) get hot and one bank of the BTJs (2N6487)gets hot. But i cant find the reason. Nothing seems to be out of order. I have removed each transistor and checked for defects and reinstalled. Real head scratcher here. Thank you in advance.
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Old 17th April 2019, 11:07 PM   #2
Bills amps is offline Bills amps  United States
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Sorry needed to add these....
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Old 17th April 2019, 11:15 PM   #3
Perry Babin is offline Perry Babin  United States
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The biasing may have drifted on the channel that's getting hot if all of the outputs in that channel are heating (roughly) evenly.

Measure the voltage across all of the emitter resistors in that channel and compare the readings to the voltage across the emitter resistors in the other channels.
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Old 18th April 2019, 01:27 AM   #4
Bills amps is offline Bills amps  United States
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Using the center tap of the transformer for negative the emitter voltage was 2.1v . And on the good side it was 0.5v. AM I correct in this measurement?
If so where do I make the adjustment?
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Old 18th April 2019, 05:11 AM   #5
Perry Babin is offline Perry Babin  United States
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You measure the DC voltage directly across each emitter resistor.

Since there is no bias pot, biasing is likely done via transistors and thermistors. One or more may be defective. There may be a way to replace fixed resistors with the bias pots (see locations for what may be bias pots).

How much does heating/cooling the thermistors between make the idle current change?
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Old 18th April 2019, 12:15 PM   #6
Bills amps is offline Bills amps  United States
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I measured the thermisters at room temp. all were the same. in circuit were 2.7k ohm. I applied power and heat to the closest one to the bank and o change to the over current. The thermister did go down in value though. It draws over 10 amps at idle with no speakers attached. Attached is the area of issue.
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Old 18th April 2019, 02:54 PM   #7
Perry Babin is offline Perry Babin  United States
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Did you heat the thermistor on the channel that is drawing too much current?
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Old 18th April 2019, 11:55 PM   #8
Bills amps is offline Bills amps  United States
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yes that is correct. heated the one that was drawing the high current
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Old 19th April 2019, 12:57 AM   #9
Perry Babin is offline Perry Babin  United States
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Does shorting across the thermistor make a difference?
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Old 19th April 2019, 01:07 AM   #10
Bills amps is offline Bills amps  United States
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i put a 10 ohm resistor across the thermister and the voltage came down to 1.8v. But the amp still draws 10+ amps. i have a current regulated power supply for testing. max current is 10 amps. I also pulled C6, C26, Q64 and replaced the 4 output transistors with new ones. The caps all checked good and Q64 checked good as well. Nothing has changed.
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