Repairing A PPI PC450

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Jump to Latest
E

elvi

Member
Joined 2010
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
 
P

Perry Babin

Member
Joined 2003
bcae1.com
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.
 
E

elvi

Member
Joined 2010
Perry Babin said:
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 :)
Click to expand...

Great support; thank you.
 
P

Perry Babin

Member
Joined 2003
bcae1.com
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.
 
E

elvi

Member
Joined 2010
Perry Babin said:
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.
Click to expand...


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 ..
 
E

elvi

Member
Joined 2010
Perry Babin said:
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?
Click to expand...

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.
 
E

elvi

Member
Joined 2010
Perry Babin said:
I'd replace the drivers since you have to order
The large resistors are 0.22 ohms. You need to confirm that they're within tolerance.=QUOTE]
Good advice as usual.. I pulled the failed 91's, there fully shorted. R27, R28, R31 and R32 all read 98 Ohm, all within tolerance. BUT R2 and R3 were open. They appear to be 3 watt based on their dimension. I don't have those in my parts bin so a trip to the shops is in order. As soon as the power \ IO plugs show up I will reassemble and test the 3 remaining channels.
Click to expand...
 
E

elvi

Member
Joined 2010
Fixed

Perry; Thank you for your expertise; it is appreciated. The Amp has been repaired. Final failed components; both 91's, there fully shorted. R2 and R3 were open. I replaced all drivers on your suggestion for good measure. It seems to be fine on the bench. I have a jl Audio 300/4. Do I follow their instructions for setting levels? The HU is a Pioneer FH- P800BT. I have no documentation for the PPI Amp. Thanx again for your support!
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Top Bottom