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Old 14th February 2010, 05:09 AM   #1
elvi is offline elvi  United States
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Default Repairing A PPI PC450

Howdy.

Been lurking for months, time for a post.

Picked up a dead PPI PC450. I will be using it to drive 2 8's and 2 full range and am looking for SQ not boom.

I actually have yet to power it up for 2 reasons.

1. Pretty obvious to me it had problems, it was in pieces when I got it
2. The previous owner had sold the input \ output plugs
I do not have a schematic, and would like one if anyone has access. (Please)

So; logically I started testing for shorted drivers. Found a couple.

Q4 2N6491
Q5 2N6491


I assume I also need to replace the 2 2N6488 at Q7 and Q8?
While I am in there, shall I replace all 16 drivers? (91's and 88's?)
Any other things I need to look at for this amp?
Typical problems \ issues?
Replace all of the PSU caps? (1000uf and 470uf)

Mouser and Digikey show the 91 on backorder.
2N6491
2N6488
No big worries, its only a couple of weeks.

Thanks in advance for your support; it is appreciated!

Elvi
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Old 14th February 2010, 03:40 PM   #2
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The amplifier will power up and produce audio with those two transistors out of the circuit, if there are no other missing parts or any other problems.

This amp uses the bottom cover to clamp the transistors to the sink so you need to replace it and tighten the screws before applying power. When you power it up, do so through a fuse rated for no more than 15 amps.

Do NOT cross-thread the screws for the bottom cover. Start them and thread them in as far as possibly by hand.

The 6488s and 6491s are the output transistors, not the drivers. The drivers (in this channel) are Q14-Q17. Those should be replaced so order them (MPSA06s and MPSA56s) when you order the outputs. Check the 100 ohm resistors connected to the driver transistors to confirm that they're within tolerance.

You only need to replace the outputs in the defective channel. The outputs in the other channels don't need to be replaced if they're not defective.
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Old 14th February 2010, 07:38 PM   #3
elvi is offline elvi  United States
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Quote:
Originally Posted by Perry Babin View Post
The amplifier will power up and produce audio with those two transistors out of the circuit, if there are no other missing parts or any other problems.


Thank you. I can simply pull the shorted parts out and test? Will order a set of I\O plugs and give it a spin.

This amp uses the bottom cover to clamp the transistors to the sink so you need to replace it and tighten the screws before applying power. When you power it up, do so through a fuse rated for no more than 15 amps.

Understood. I will also use a current limiting resistor, input gain set almost zero and 8 ohm speakers on all 4 channels until it's all sorted out.

Do NOT cross-thread the screws for the bottom cover. Start them and thread them in as far as possibly by hand.

Yes; I saw the other 4 or 5 threads in which you made this observation. I am tempted to drill tap and take the size up to number 6 and\ or add a machined aluminium heat spreader. (Nothing I own is "stock"; but is upgraded for serviceability)

The 6488s and 6491s are the output transistors, not the drivers.

Sorry; my bad. I call all transistors that drive a signal "Drivers"

The drivers (in this channel) are Q14-Q17.

I guess I need a schematic; OK, I see them 2 for the 91's and 2 for the 88's.

Those should be replaced so order them (MPSA06s and MPSA56s) when you order the outputs.

Do I replace all 4 (I assume so)? By the marking these are Motorola. I see them in Fairchild and ON; do I order them in ON?

Check the 100 ohm resistors connected to the driver transistors to confirm that they're within tolerance.

OK, looks like 1/8 watt, 100 ohm 5%, R27, R28; R31, R32; R33, R32; R37, R38. Do I need to lift the legs on anything to check them properly? On the non shorted transistors, they show 130 Ohm. On the 2 shorted 91's they read .062. I assume I am reading back through something else (No schematic)
Do I need to worry about the large R2-R5 as well? (2.2 at 5%)


You only need to replace the outputs in the defective channel. The outputs in the other channels don't need to be replaced if they're not defective.
OK, I will order 2 91's and 2 88's.
BTW; what channel is this please? (And I do need to locate a a schematic
Great support; thank you.
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Old 14th February 2010, 09:37 PM   #4
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I'd replace the drivers since you have to order parts anyway. You should replace all 4.

Pull the known defective components. Power the amp up with the 2 shorted outputs out of the circuit. If it's works properly, then you can pull all 4 drivers and recheck the resistors.

The large resistors are 0.22 ohms. You need to confirm that they're within tolerance.

If you're going to do anything with the screw holes in the sink, use Helicoil (or similar) threaded inserts.
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Old 15th February 2010, 12:34 AM   #5
elvi is offline elvi  United States
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Quote:
Originally Posted by Perry Babin View Post
I'd replace the drivers since you have to order parts anyway. You should replace all 4.
Parts on order; but some are backordered.

Any issues with these replacements?

863-2N6491G
863-2N6488G
512-MPSA06RA
512-MPSA56D26Z
71-PTF56100R00FZEK

Thanx for your assistance; it is appreciated!
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Old 15th February 2010, 01:15 AM   #6
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The resistors in your board are likely 1/4w, not 1/8w. Those resistors are also far more expensive than they need to be. Standard 5% tolerance resistors are good enough.

Otherwise, everything appears to be OK.
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Old 15th February 2010, 03:28 AM   #7
elvi is offline elvi  United States
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Quote:
Originally Posted by Perry Babin View Post
The resistors in your board are likely 1/4w, not 1/8w. Those resistors are also far more expensive than they need to be. Standard 5% tolerance resistors are good enough.

Otherwise, everything appears to be OK.

Understood. My background is technical; but not at the component level.
Thank you.

I have 1/4 watt resistors in my parts bin ; so I am good to go.

Do you have access to a schematic? Can you share? I hate to fly blind ..
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Old 15th February 2010, 04:14 AM   #8
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I don't have a schematic diagram for this amp. Unless there is a complex problem with the protection circuit, you don't really need a schematic diagram.

Did you power it up to see if there were any other problems?

Last edited by Perry Babin; 15th February 2010 at 04:17 AM.
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Old 15th February 2010, 05:17 PM   #9
elvi is offline elvi  United States
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Quote:
Originally Posted by Perry Babin View Post
I don't have a schematic diagram for this amp. Unless there is a complex problem with the protection circuit, you don't really need a schematic diagram.

Did you power it up to see if there were any other problems?
I have the power \ IO plugs on order. They should be here at the end of the week. As soon as they arrive I will bench test it with the failed 2N6491G's removed. I hate to set it up with a hand full of alligator clips. One slip and I get to debug something else

BTW; Digikey and Mouser are back-ordered on the 2N6491G in "ON" semiconductor. MCM shows to have a generic in stock but their descriptions are poor.
Any additional vendors you like to use? I am in the lower 48..

And once again; thanks for your assistance; it is appreciated.
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Old 15th February 2010, 07:06 PM   #10
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2n6491 - Octopart
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