PPI a404.2 Project Repair Refurbish - diyAudio
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Old 29th October 2013, 01:52 AM   #1
nikb47 is offline nikb47  United States
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Default PPI a404.2 Project Repair Refurbish

I just started on a repair/refurbish on one of my amps. 96 PPI Art series a404.2 It had lots of noise coming from right rear channel. I bought this thing to repair it, it had been opened by the previous owner. Upon inspection 2 resistors are MIA at R185 and R186 right by the rca in's (the 10ohm 5% 2 watt) and you can see on the board where they got hot. I went ahead and pulled R137 and R138 which looked a little tan like they got a little hot, although they did test out good.
Click the image to open in full size.

Click the image to open in full size.

I did a quick continuity check on the transistors and they all seem fine, although there is one that seems to be different than all the other ones. It is located at Q5.

Click the image to open in full size.

Almost all of the caps looked like they were starting to leak. But those were on the list already! Upon removal of capacitors I found the cap at C10 had one leg broken off! I have all caps and resistors on the way from mouser.com and I've already begun pulling caps. I am going to fix these things mentioned and then see if it will come back to life with clean power. Stay posted for updates within the next few days. Also, if anyone has any other suggestions as to what may be the cause of noise at right rear let me know. Oh, and if the experts want to chime in feel free! 1moreamp I'm talking to you! lol Thanks for stopping in!
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Old 29th October 2013, 06:18 AM   #2
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Hello nikb47, Do I know you ? Just have to ask because of your thread request.

Those heat damaged resistors usually fail when outputs fail for the corresponding channels RCAs they connect to. So I would be looking for damage in the channels those RCAs feed if I were in your shop at your bench. They are current limits and they are meant to fail when your amp is passing DC back into the RCA shield back towards your other equipment like your crossovers and Head unit. This can occur for a couple of reasons, one being the amp has been pushed into hard clipping excessively and constantly. The other reason being the outputs blow shorted and pass DC rail supply out to your speaker load and the amps design sends it back towards those RCA shields going back towards you head unit or other peripheral gear you may have in circuit. It the Dc and electrical noise is looking for earth ground so your head units ground looks good to it electrically speaking so it heads that way. PPI floated the secondary of their amps away from 12 volt ground by opto isolation and the power toroid like many other amp companies that saw it as ground loop noise issues and just plain good design practice. SO any excessive DC and ultra-sonic signals generated by clipping that get caught by the zobel filtering through out the amp and power supply generally head back towards the RCA shield and the only possible earth ground source possible, your head unit. Those resistors were designed into the amp to burn open at some point hopefully saving your head units ground circuitry.
So I tend to replace those a lot with brand new even if they read good as the heat build up is a sign of stress and its just one of my ways after all these years of doing these amps. If its been beat up, then replace it. It will head off premature failure down the road, especially since its internal structure is likely weakened by what has happened to it already. Its a resistor that was meant to act like a fuse in this design so if it got hot enough to change it's color, well I think you get the idea.

I didn't see you mention ordering or replacing the silicon heatsink compound. That stuff dries out due to the silicon grease literally migrating away due to heat build up over time. That is why it is so paste like, and caked up. All that is left is the aluminum oxide powder after the silicon grease migrates away. It literally creeps away when ever it gets heated up. I scrub as much as possible off and clean the insulators for reuse or replace the mica insulators with kapton tape or similar. It will be a big mess, and if you do clean it all up you only need to apply a small thin coating under each device, nothing like what you see now. PPI had a design issue with their bottom plates not clamping down consistently and over time. I also suggest you take take time to straighten all those tangs/finger of the bottom plate so they apply as even a pressure as possible before you reassemble. Put the end plates on last so you can inspect how your bottom plate sits as you tighten it down. I have seen them cranked all the way down warping the bottom case and still having terrible clamp pressure. it has tangs on each end and when they have bottomed out any extra torque you apply will simply just be bending the case bottom uselessly. Also you will begin to see the bottom plate curve in the center, this can cause the power toroid to hit the bottom of the case. PPI even added plastic inserts to prevent shorts, but I suggest good mechanical practices and keeping an eye of what your doing so the bottom plate does not contact the toroids and caps lids...

By the way after you get the amp to power up please make sure your power devices are clamped back down to the heatsink especially those lower rail regulator pass elements. between the rectifiers and the outputs, and should be the same device as the outputs but not connected to the audio output stages. They have 25 to 35 volts into them and only 15 volts out so that 20 volt drop across them will cause them to heat up so fast they will tend to fail if they are not clamped to the sink with enough integrity. Perry and I both suggest you make some clamp bars out of aluminum or steel. Perry used simple right angle aluminum and some spare hardware to clamp the devices back down to the sink so you can work on the amp powered on without the bottom plate being installed back on the amp. I used tube steel myself, drilled appropriately and spare hardware to install them. This is a commonly overlooked thing so please pay attention that the amp should not be powered up without these make shift clamps for more then 10 to 15 seconds. That is all the time it takes to cook off those pass elements for the lower rail regulators, and they will short out and pass 25 to 35 volts to your front end op-amps along with toasting up your board where they are located. SO please don't over look this important issue, that these amps were not meant to be powered on without the bottom of the case installed or some suitable replacement in place.

Is that some of what you were looking for??? Hope some of its helpful...
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Old 29th October 2013, 06:36 AM   #3
nikb47 is offline nikb47  United States
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Thanks for stopping by 1moreamp! No, we don't know eachother. I have just read a lot of your posts and a lot of other posts about you knowing what your talking about. And I believe that you may be "The Stig." lol

I am planning on going the Kapton route on the heat sinks but man that stuff is expensive! I just forgot about it until after I placed my order at mouser. Is there a cheaper place to get it? And also just regular silicone paste right?

I have read a lot about the clamps for these old ppi's and plan to build a set for myself because I have several other art series amps that I would like to rebuild later on.

Right now I am just waiting on parts from mouser so I need to clean the board and sink up really well.

Thank you for all of the tips and also for explaining about those resistors!
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Old 29th October 2013, 09:03 AM   #4
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Try this ebay link if it works or just search ebay for kapton if the link does not work...
Kapton | eBay

You can also buy kapton like used in SoundStream amps from jandrelectronix on ebay. its a paper like thermal sheet material. SoundStream used it in all of their American made amps from the old school days REF and RUB series. I have some stock of it myself as I have seen the paper grade insulator develop shorts through the membrane surface due to debris from the screws used to clamp those amps together thru the PC Board. in fact I just replaced a REF 705's entire insulator sheet only to find the short was underneath one of the 15 volt regulators after it was cleaned up.
Kapton tape comes in different dimensions and thickness's 1 mill is fine, 2 mil will also work. Orion used kapton tape many years ago in many of their old school amps I even have to-3 insulators in kapton used in aero-space and jet aircraft. You will see kapton wire as used in satellites also when you search. Thin is fine as long as you don't damage it while loading and clamping the amp back together. Orion used 1 mil stuff in their old transistor only based amps like the GX series, and they had bar clamps with plenty of pressure in their designs. Hard to believe that PPI, Orion and ADS all went on the auction block together and folks don't realize that they were all owned by the same people....
So I did not invent this insulator, the industry did, and its been around for years. These PPI amps do not have enough clamping pressure to use the grease-less Sil-pad designs currently available. Those designs require pressures as high as 50 foot pounds per square inch to operate properly and the bottom plate clamping system will not even come near that level of captive pressure.

Silicon heats sink compound, plain and simple. Do not use those metal based thermal compounds you see for CPUs' they contain copper and silver and they conduct electricity along with heat. CPUs are isolated inside their packages, Power devices used in car amps are not isolated from their heatsink surfaces, so that CPU grade compound could short out all your hard work.

I wish you good luck on your restorations. None of this info I posted is secret, although most of it only applies to PPI amps. I am sure you can find similar info elsewhere on the web from others like us here on the DIY. Please don't leave out the others that contribute here so selflessly. I think you will find them an intelligent and generous bunch to collaborate with. I also suggest you look up Perry Babin's tutorial links under any of his posts. His tutorial is as good as it gets IMHO, and I know you will glean much from his hard work and efforts.

Last edited by 1moreamp; 29th October 2013 at 09:06 AM.
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Old 30th October 2013, 05:56 PM   #5
nikb47 is offline nikb47  United States
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Ok, so since the one power supply fet seems to be different, I have decided I'm going to go ahead and replace all the transistors and rectifiers on this thing. So the power supply mosfets are the irfz43n. Should I replace them with original or is there a good replacement? The rectifiers are byw29f-200. Again any replacements? And then the 2n6490 and 2n6487. Thinking of going with the 6491 and 6488. Also would op amp upgrades be worth it and what should i look at? Lm837n is what its got. Thanks!
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Old 30th October 2013, 08:47 PM   #6
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Quote:
Originally Posted by nikb47 View Post
Ok, so since the one power supply fet seems to be different, I have decided I'm going to go ahead and replace all the transistors and rectifiers on this thing. So the power supply mosfets are the irfz43n. Should I replace them with original or is there a good replacement? The rectifiers are byw29f-200. Again any replacements? And then the 2n6490 and 2n6487. Thinking of going with the 6491 and 6488. Also would op amp upgrades be worth it and what should i look at? Lm837n is what its got. Thanks!

MY burning question is, are any of those components damaged in any way ? I ask because the "upgrade" devices your asking about will not alter the amps SQ or power capabilities.
Your amp has been abused by previous owners this is evident by the over-heated current limit resistors your replacing. This abuse typically manifests itself as unusually high DC offset on the speaker terminals. Have you checked the DC offset on the speaker terminals yet?
This is adjustable to a given point by a offset adjustment that exists for each channel. Bias is given at ~100 micro-volts and if you raise it by adjusting it, the max you will likley be able to get is in the one milli-volt range before the amps thermal tracking begins to compensate for the rise in heat this will generate.

The things you want to change in your post may increase the amps durability somewhat but will not do much else. The upgraded outputs will only result in them being capable of higher rail voltages and the amps regulated power supply will not allow you to alter that upward without some intense redesign and rework.

My best suggestion is that you get the parts you have ordered and place them and then test the amp to see what is left to repair or realign.

Changing out the op-amps may result in unpredictable behaviors of the front end. The current op-amp LM837 is a fair op-amp, but its output is 600 ohm capable and its output will pass thru those rather large 4.7 ufd mylar caps which gives PPI their SQ that most owners are loyal too. So after all of that I have serious doubts that any real tangible gains can be had by going the route you asked about.
Improving a solid electronic design goes a lot deeper than just placing upgraded power factor devices in place. The sum of the whole was designed to be what it is, and without a complete redesign of the power supply your not going to gain any more power from this amp. And for what it was designed to do, I think your going to find that the heatsink surface area will not allow too much alteration. Try to remember that these amps also featured the ability to add liquid cooling to their heat sinks on the end plates. And if you really want to run one ohm with these I suggest you look into that option as my bench testing at 2 ohms stereo per channel caused them to run very hot to the point of thermal shutdown most likely due to the voltage drop across the outputs and the smooth surfaces the Art series used in the sink design. Heat sinks rely on large surface areas to emit heat to ambient atmosphere, so the more surface area the sink has the more heat load it can handle to a given point of saturation. Again this is all designed into the amp to begin with, and when you go about making too many changes you will run into these engineering barriers along the way... Just being frank and honest about this before you get too involved financially into this sort of venture....
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Old 31st October 2013, 05:59 AM   #7
nikb47 is offline nikb47  United States
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Ok thanks again 1moreamp. Just like youre saying about getting it put back together is what my original plan was. Then i started browsing forums and reading about "upgrades" and such and I guess I caught the bug... I thank you for setting me straight again. I am also posting this up over at DIYMA and someone suggested upgrades thats why i started looking around.
Most of my parts should be here tomorrow so I'll be getting started soon!
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Old 1st November 2013, 03:21 AM   #8
nikb47 is offline nikb47  United States
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Got a little done today. Put a handfull of caps in. Kapton is applied. I will finish up first thing in the morning. I just need to get some silicone thermal paste and I should have everything! I also built my clamp bar for testing. Oh and cleaned some more of the thermal paste after the pic. I had to do that stuff in stages its a major pain.

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Old 1st November 2013, 06:52 AM   #9
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LOL welcome to our world of clean up lol.....

So what brand name caps did you get?
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Old 1st November 2013, 06:54 AM   #10
nikb47 is offline nikb47  United States
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Those are the panasonic fm. Is that a good choice?
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