Rockford Fosgate Punch 100x2

[Perry Babin]

arrownac.com often has very good prices on the 3205s. Use octopart to find prices from multiple distributors.

 

[nikb47]

Cool! I was just looking for best price right now. Thanks for the tip on octopart.

 

[Perry Babin]

You would have found it when you read the semiconductor datasheets page.

 

[nikb47]

I haven't got that far yet Perry! haha but I have seen you mention it several times here on the forums and I never remembered it when I was looking for parts. I have seen 1moreamp mention it several times too. It is really an awesome tool when you're looking for parts and the best deals.

 

[nikb47]

Ok so these gate resistors on this amp are 1/8 watt correct?

 

[Perry Babin]

They are 0805 size. Depending on the materials that make up the resistor, a resistor of any given physical size may have a higher or lower power rating. Virtually any 0805 resistor of the right value will work perfectly fine for this application.

 

[nikb47]

Never mind I got it 0805s

 

[nikb47]

Lol you beat me to the punch

 

[nikb47]

Ok Im back on this one. I replaced all transistors now I'm getting rail voltage to the outputs...

Right output 32.7dcv
Left output -32.7dcv

 

[Perry Babin]

Confirm that you have both positive and negative rail voltage on the 3rd legs of the output transistors.

Also confirm that you have positive and negative regulated voltage on the power supply pins of the op-amps.

Did you measure directly across the speaker terminals or did you have the black probe on the main ground?

If you were not using the main ground for the reference, use it and measure the DC voltage on the bridging speaker terminals for both channels.

 

[nikb47]

Well I don't know whats going on with this thing. Q225 and q115 were shorted between emitter and collecter. Swapped them out and all is good now. Still getting -60mv from right channel and -10.4mv from the left so I guess I'll just have to live with it. I noticed that the biggest caps have leaked, would that have anything to do with the dc offset?

 

[nikb47]

Yes Perry that was Directly across the terminals. Now with the bridging terminals I get

59.9mv right negative
-10.4mv left positive

 

[Perry Babin]

Electrolyte is conductive but it would likely have to be in the area of the differential amplifier transistors to cause any DC offset problems.

60mv is a bit more than ideal but it's not likely to cause any problems.

 

[nikb47]

Ok thanks Perry. Is there a good way to go about correcting the 60mv or is it just not going to be worth it?

 

[Perry Babin]

Is it worth the effort?

It could take several hours to find or could possibly be the first thing you do to find it.

If it was on my bench, I wouldn't chase it down.

 

[nikb47]

Ok sounds good. I just buttoned it up anyway.

I still plan to change the caps before I use it but that will be quick and simple.

 

[nikb47]

Alright I had to open this amp back up. I noticed it is kind of playing music inside the amp. Is this normal? Also, I'm wanting to learn more about why I am getting the 60mv dc offset. If you have the time could you guide me in figuring this thing out?

So far what I have done is:
1. Change the power supply transistor's to IRF3205s and 47ohm gate resistor's.
2. Replaced the IRF540 output transistor's
3. Replaced the IRF9540 output transistor's
4. Replaced the electrolytic capacitor's except for the 10uf 16v smd aluminum can style capacitor's.

 

[seankane]

The 10uf 16v are the most important ones to change. They leak with age.

 

[nikb47]

Thanks Sean. Yeah I learned that from working on the punch150hd. I have a couple other amps coming in for repair so I'm going to wait on ordering them until I get a bigger order put together.

 

[Perry Babin]

Sometimes audio can be heard coming from the transformer. That's not generally a problem because it's not nearly as loud as the audio coming from the speakers. It's generally due to the voltage regulation for the rail voltage. If that's the case, sometimes moving the windings on the transformer slightly can stop it.