PPI Powerclas PC2150 and PC275 need repair ideas

Dr Zeus

Member
2010-10-17 5:12 am
PPI Powerclass PC2150 and PC275 need repair ideas

Let me start by saying that I've got a certain nostalgia for Precision Power, and in the 90s I loved using their PC450 and PC1400 (Big boy). I regret selling them, but alas theres enough used even now to go around.
Unfortunatly time must be getting the best of PPI. Amps I just picked up used both have a similar issue as I have what appears to be a common problem between both... Two amps are experiencing identical noise in the form of Buzzing from at least one or both channels. Whats really funny, is that back ten years ago I had to do a warrantee repair on my PC450 for the same issue as I'm posting today.

Today's amps are both the Charcoal Grey Precision Power Powerclass amps, one is a model PC2150 (150x2 4ohm / 600x1) and the other is a PC275 (75x2 4 ohm / 300x1).

The setup:
I'm powering a single sub with the PC2150. The sub is 4 Ohm and the amp is running 4Ohm bridged (600w). The PC275 is running 4 Ohm door speakers. Amps are running seperate RCA cables from a Pioneer HU.

The problem:
PC275
Left channel is Fine / Perfect sounding quality. This one is the model I go by for the rest of the problems as this channel sounds totally better than any other over both amplifiers. THIS IS THE GOOD CHANNEL

Right channel has a distinguished noise or buzzing sound, most audible when turning off the HP crossover (Playing full range) and playing a sin wave from CD in the 100 - 2000hz range.

PC2150
Both the left AND right channels are playing the same noise or buzzing sound as the other amp! Because this is a SUB amp its really hard to tell from the sub but is apparent at very low volumes listening very close to the sub. For a more acurate test I swapped plugs with the PC275 and ran the door speakers off my PC2150. Indeed both door speakers make an exact buzz like the PC275's Right channel.

In-Car troubleshooting: With the engine off... I've eliminated any RCA cabling issues because I have two seperate cables of different type; both sets do exactly the same things. I've also tried flipping the speaker-out plugs up-side down, and the problem at least on the PC275 follows the bad Right channel. If I unplug the left (good) channel's RCA, the right channel's buzzing gets about 20% less noticable. If I half pull out the right (Bad) channel's RCA - leaving only the inner connector attached - the problem gets another 25% better. If I leave both RCAs half-way pushed on and almost hanging then the problem gets about 75% better at a cost of overall sound quality - I think the PPI's internal noise reduction system starts eating into musical sound at this point.

On-Bench troubleshooting: Neither of these amplifiers had been opened before me; the original warrantee sticker was still on them when I got the amps off Craigslist from seperate owners / listings. I'm fair to good with a soldering iron so I started going to town on re-soldering mostly resistors and some capacitors. I probably resoldered only about 15% of the PC275's PCB, and very very little on the PC2150. It didnt help, and I think it may have actually made the PC275's bad channel a little worse. There are no burned up or noticable problems with the PCBs on these boards. I've vaccumed, brushed, and checked most of the 3w resistors with my Fluke DVM. Unfortunately thats about the extent of my knowledge of these things. I wish I had an osiliscope.

After searching google and this forum all I came up with was a very similar problem from another member with a PPI Art A600 amp: (I cant link because my post count is low) Go0gle seach "ppi a600 noise"

Please help, I hope I didnt buy these in vein. If you know how to fix please post, PM, email, or even give me a call

supertegrs@yahoo.com
Cell 678-778-5533
 
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Do not apply power to these amplifiers without having the bottom installed and the screws tightened down. Do not cross-thread the screws when threading them back into the heatsink.

Does the noise change when you move the RCA cables where they plug into the amp (when fully inserted)?

If you have no RCAs plugged into the amp and touch one meter probe to each of the two input RCA shields on the amp, what is the resistance?
 

Dr Zeus

Member
2010-10-17 5:12 am
Holy nocternal-ness batman. I even got the 'man' to respond and all I had to do was go to bed for a few hours.

Perry,

Measured resistance as you said (Input RCA rim to rim) and both amps come up with 20ohms.

Wiggling the RCAs make no difference so Im sure the connections are good. If I remove half or one of the RCAs then the noise changes.


Thanks for those other link guys. I think I found and read those threads last night but I'll read them again.
 

Dr Zeus

Member
2010-10-17 5:12 am
I read both those links and I dont -think- I have the same problem. The second link talks about shorts and cutting out Fets due to things running really hot. The only go forth I have from that post is looking forward to replacing any and all capacitors.

The first post is more relevant at least for my PC2150. The PC275 does not have the same SIP boards rising out of the PCB so I'm kind of stuck there. I also dont want to try re-soldering those boards unless I'm told to give it a try.

Again, there are no visual signs of component failure anywhere on these PCBs. I did find some cold looking solder joints but I've fixed all that I could of those and the problem persists.

Again, thanks for the quick responces here guys. I'm both curious and content on any welcoming ideas.
 

Dr Zeus

Member
2010-10-17 5:12 am
Ive only powered the amp up in the car. If you'd like I can try it on a bench but I'll first need to go buy a few alagator clips and get something to source audio from - like my ipod. In the meantime I've tried the following.

After playing and driving around all day, I found on the PC275 removing both input RCAs made a different noisy issue out of the right (bad) channel. A new noise in the form of a low volume deep ticking of a heartbeat - offbeat like the human heart - came ouf of the right channel speaker. It didnt sound like a thump but more of like an on-off sound like something kept surging on and off.

All excited I ran inside to fire up my computer and get on this forum. While booting I decided to try a few more things on the car before I reported this find, and I'm glad I did this. After the PC275 had been powered off for about ten minutes or less I could not get the heart rythm sound to present itself. This time and while the RCAs were disconnected the PC275 sheds no noise at all. It was silent.

The PC275's problem seems to be getting more pronounced either because it is further failing or I'm just noticing more faults in it. I now find that hooking up only one - either left OR right channel RCA will cause the right channel to play the same buzz-hum. This may tell ya that theres some cross-talk of noise from left to right. While playing a sin wave from CD at 400hz the noise changes but does not go away if I fade the HU from full left and right. The right channel on the PC275 always plays low volume noise no matter what audio material is played.

The buzz-hum is present 100% of the time while playing audio, but while the music is silent I only hear some noise about 10% of the time. Also noticce making a bluetooth phone call in the car causes the most noise; probably from the increased RF. Whether the car is running or not does not change the noise level or output quality.

The PC2150 is a little harder to determine since its hooked back into the sub, but it too only presents noise while the RCAs are attached and playing audio.

If I had my choice; I'd like to get the PC275 working first. its less expensive than the PC2150 and more expendable, plus the problem is much more audible.


I hope this isnt too much info to decipher...
 

Dr Zeus

Member
2010-10-17 5:12 am
Neither Amp clips / distort at all. The louder I go with it, the less I hear the noise-buzz. The noise is not random but constant at all volume levels. If the noise was not superceded by volume I''d have to permanently remove the amp. The noise is always prevalent but is most sticking for the first 1/3 of volume control.

I just bench tested both the PC2150 and the PC275. The PC2150 actually tested out very well, very hard to discern any problems. Lets put the PC2150 aside for the time being and lets just say its OK for now. I had some doubts it was that bad anyways.

The PC275's right channel failed miserably on the bench. Let me see if I can explain a few more scenerios.

-The right speaker always has low volume noise while playing sound and none when no audio is fed to the amp, and again none if both RCAs are disconnected.

-When one speaker is connected to the right channel and both L & R RCAs are in, I can remove the right input RCA and the speaker goes 98% silent. The left RCA doesnt do anything to the right speaker under this circumstance.

-When two speakers are connected (L & R), if I remove the right channel RCA, the right speaker continues to play noisebuzz and very slight audio - Like crosstalk from the L channel. If I disconnect both RCAs with both speakers connected the amp goes 98% silent.
 
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Dr Zeus

Member
2010-10-17 5:12 am
Gain and QBass dont change anything other than what they are designed for. The PC275 has an HP/LP button, a Crossover active button, and a -12db button. Pressing the crossover buttons cause a little bit of static and but its not the same ans the noisebuzz.

Hey Perry, can you give me a call at 678-778-5533 ? I've got the PC275 on my workbench with the rear cover off. Ive also fabricated brackets to sandwitch the transistors to the heatsync which means I can run this amp open on the workbench.
 

Dr Zeus

Member
2010-10-17 5:12 am
I think I may have found a bad cap at C73. Its a blue can-style 220uf rating of 25v. I'm not 100% positive on how to test caps, but with my DVM common to ground and set to meter resistance and the amp power disconnected, I touched the top of every electrolitic cap. All caps reported back with some sort of value in resistance; while C73 comes up open on the DVM. Hmmm
 
Yes. You'll use it to check the signal on the output of each of the op-amps to see if you can find where the noise is entering the signal chain.

The electrolytic capacitors should read a low value resistance when the probes are initially touched to the terminals and then the resistance should climb. When you reverse the probes, it should do the same. This is for capacitors that are not in the board. When in the board, the results may be different.
 

Dr Zeus

Member
2010-10-17 5:12 am
IMG_20101017_234933.jpg

IMG_20101017_234939.jpg

IMG_20101017_235018.jpg
 
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Dr Zeus

Member
2010-10-17 5:12 am
Click on this link. Its a 2 minute video clearly showing the buzznoise.

Also that snapping sound is some sort of feedback coming from my laptop. When I play the same sound from my cellphone the snapping does not occur. The humming does.


Click here for the video!
PPC PC275 :: VID_20101018_000537.mp4 video by unclemeat2010 - Photobucket

Thanks and best regards
 
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