Punch 500x Random excessive current draw

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I am repairing a Rockford Punch 500x that came in with a bad channel and a blown power supply. I repaired the bad output section. The power supply used MTP75N05's and 10-ohm resistors. I replaced with 3205's and 47-ohm resistors.

The issue I am having is it has a seemingly completely random excessive current draw issue in the power supply. I can turn it on many times in a row and it will power up and draw ~0.70A. But all of a sudden on powering up it will draw 42A and one side of the power supply transistors get immediatly hot! Shut the amp off and back on and it may power up fine, or it may draw too much current again.

I have tried moving things while it is idling normally, I have flexed the board, nothing seems to make it pull excessive current once it's powered up normally. And vice versa, in the time I can let it run, if it draws too much current, nothing I do will make it start idling normally again unless I power cycle.

Powering through a current limiter allows the amplifier to power up normally every time and produce clean audio (at low levels of course).

No channel has significant DC offset when the high current draw occurs. Also none of the output devices heat up when it happens.
 
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It does this in or out of the heat sink. It's possible it's a bias pot, but I have tried moving them and it doesn't seem to affect it. I also adjusted bias because they had creeped up it seems. Current draw before adjusting was 2.5A. Now it is around 0.70-0.75A

It's just odd that I can power it on and off as slowly or as fast as I'd like and it is hit or miss whether it will power up normally or draw too much current, without touching anything else.
 
Intermittent bias pot?

In one Rockford amp (same era but I don't remember which model), the bias transistor legs were intermittently shorting to the MEHSA strip and causing a similar problem.

Perry
most Rockford amps have parallel resistor on the Bias trimmer to eliminate this "trimmer" problem.
it looks like the problem came from the Power Supply. maybe one of the FET gate resistors not welded good
 

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Perry
most Rockford amps have parallel resistor on the Bias trimmer to eliminate this "trimmer" problem.
it looks like the problem came from the Power Supply. maybe one of the FET gate resistors not welded good

Resistors have good connection. In fact I have changed them trying different values. It seems the lower value I use, the more often it draws excessive current, the higher I go the less often it does it. Could be purely coincidental.

Perry, I will try to source those bias pots
 
I'm sorry. I misread the OP.

IT's not the bias pots if nothing in the output stage is heating.

If one half of the PS FETs are heating up, I'd suspect a bad PNP driver or a bad solder connection on the PNP driver. Also, sometimes, problems on one side can make the other side heat up. So, a problem on the cool side can sometimes be causing the other side to heat.

Again, I'm sorry that I misread the OP.
 
I'm sorry. I misread the OP.

IT's not the bias pots if nothing in the output stage is heating.

If one half of the PS FETs are heating up, I'd suspect a bad PNP driver or a bad solder connection on the PNP driver. Also, sometimes, problems on one side can make the other side heat up. So, a problem on the cool side can sometimes be causing the other side to heat.

Again, I'm sorry that I misread the OP.

PNP drivers were changed on both sides. I will double check for good connection, but fairly sure I have done so. I have no problem re-checking though
 
What if the wiper loses connection?

I've had bias pots that were intermittent. Replacing them solved the problem.

Perry
With all the respect (and i have lot of respect) to you
we are talking about Rockford Fosgate products and the parts they use.
I never found any trimmer they use that have problem like wiper loses connection
even not in the old PIHER Trimmer. i am working with Rockford Corporation from 1994 and i know they are not perfect but... i know how they chose the Parts for the products they made (what made in the USA)
Anyway it can happen what you say "wiper loses connection"
if you remove out completely the Bias trimmer it will be almost 0 because the parallel resistor. if you think i am wrong let me know about it i am not perfect :D
 
I can tell you for a fact that I remember having to replace 2 of the blue/white pots that were causing precisely this problem.

I don't know what causes the failure inside the pot. Did you try lifting the base of the bias pot to simulate that failure to see if the bias would spike?

You also, previously said that the 494 type ICs never have to be replaced. I've had to replace those due to various failure modes.

I don't know why you're making a big deal out of this.

To make absolutely sure that I was not wrong, I pulled a working 225.2 off of the shelf. Confirmed that it was OK then desoldered the base leg of the bias transistor to simulate the open wiper circuit. As soon as the mute delay ended and the differential amp was biased, the amp pegged out the power supply.

If the 1k resistor would do as you suggested, the bias transistor could never allow excess bias.
 
As i told you before maybe my country get different product from Rockford.
I have in my place tons of Original Rockford fosgate parts. the parts stay with me, they don't move from me to repair Rockford products.
To be honest?
i really like this Rockford brand. i also do that job so long time for other brands that i represent in my country Like Audison Hertz Massive audio and more.
 
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