Citronic PPX 900 story.....

[Si Cosis]

Hi to all,

I found this site via google while looking for PPX 900 schematics, and very happy I did, looks like a busy forum so I'll be having a good root around later.

I searched the forums and there are a few PPX related posts, so I thought I'd post my experience and I also have a couple of questions.

I was given the above amp years ago, with the right channel dead, but left channel working fine, which was fine for my purposes at the time, until the left channel also went down. I took it to work and fixed the left channel in my dinner hour. Fault was down to R21, which is a 3K9 2W wire wound resistor, which drops the voltage down from the -80V rail to -15V for the -ve supply to the 2 op amp IC's. I replaced with 2 x 2K2 resistors in series (just what was found in stores at the time) and all has been fine since then (about 5 years ago). I couldn't remove the board from the unit though, it is an old amp and the screws holding the board down just spin and never come out of the board, I didn't fancy drilling out the screws at the time, and the resistor had blown o/c, so I just jumped the new resistors over the top of the old one.

I did a street party on Friday with the amp, and it worked fine, stayed cool and ran all day with no problems. I've been asked to do a couple more parties now, so I thought it might be a good idea to see if I can resurrect the right channel. I pulled out the scope, injected a tone and traced it through, got to the first op amp, and again it wasn't working, no -15V supply, so checked R41, and sure enough, the same resistor had blown on both boards. I thought it would be worth mentioning just in case any else is ever trying to troubleshoot the same board, as the same fault on both boards would suggest to me that it is a common failure, probably due to having the input overdriven at a guess. I've removed the board this time to replace the component properly, but in the process I have broken the volume control pot, I won't bore you with the details, lol. The schems show it as a 10K logarithmic pot, and it is, but on inspection, it is actually 2 x 10K pots in parallel, now that leads me to think why would they do this? Only thing I can think of is increased power handling, so anyone have any idea's on what value (current handling) I could use as a replacement? I don't care about aesthetics, so it doesn't have to look right, just work.

Also, anyone got any tips on how to remove the board from the case without destroying the volume control pot?

Thanks....

[Si Cosis]

All fixed now, would still like to know the correct method to remove the board without taking the volume pot apart if anyone knows........

[skirat01]

Removing boards without damage to pots is simply a matter or removing the back plate. 4 screws and 2 kex/alan keys and this panel comes off and you can simply slide the board back out - can be fiddly but no damage to posts.
Cheers for the other info.
Derek

[skirat01]

I have just fixed my PPX900 - it would seem there are at least 2 different versions of the circuit board around although they are obviously very similar the component numbers are different. If you look on google you will find a pdf with both sets of drawings in - if you want them.

I checked the component listed the post by Si Cosis and mine were all fine, but when I followed signal though I found it wasn't getting passed the 'switch on mute' circuit. Turns out it was exactly the same component doing a different function that had failed 3k9 2W wire wound resistor although mine was numbered R8.

So for anyone with similar problems I would check these first, on mine they are numbers R7 & R8. For the less experienced they are the larger resistors, the body of which is about 1 inch long and just over1/4 inch diameter. The coloring seems to have faded away from the original orange white brown due to the heat but when you put a multi-meter accross them they should read 3.9k ohms (or just under). If they don't read at all or are nearer 60k ohms then they are faulty.

The easiest way to fix is get a replacement (or combination or resistors made up to near 3.9k) and snip off the resistor as close to the body as possible and use the wire ends to solder the new one(s) in. It will save you about an hour stripping the case to get the boards out and putting it all together again.

Good luck