Weak output on 1 channel of PPI PCX-4125

[EriCCirE]

Hey guys,

I have a very weak output on one of the rear channels of my PPI PCX-4125. This does not seem to be a problem when bridging the two rear channels. I opened her up and there looks to be no problem areas on the main board. However, on the daughter board it looks like some components have leaked or burnt.

The problem components are a crossover freq pot, crossover slider switches and a red worm looking component (8 legs, labeled M8-3-103G 216)

3 of my crossover slider switches have the tabs broken off, So I plan to replace all five. They look to have 3 legs and two grounds. They are not labeled, so I am having a tough time trying to locate replacements.

My problem pot has 12 legs and is labeled C503.

Take a look at the pictures. I know I need to clean up the pins. I tried with some electrical component cleaner with no luck. I will be getting some Alcohol and trying again.

Main Board:


Close Main Left:


Close Main Right:


Daughter board top:


Daughter board bottom:


Problem pot:


Problem pot pins:


Problem "red worm":


Problem "red worm" pins:


Problem switch:


Problem switch pins:


Problem switch 2 pins:


Problem switch 3 pins:


Thanks for taking a look guys, and I appreciate all input and help.

[Perry Babin]

The red worm thing is a resistor network. It's likely four 10k ohm isolated resistors.

The switches are DP3T switches (no ground). You can confirm that they're functioning normally by using an ohm meter. In each position, make sure that each pair of contacts reads ~0 ohms for each position. The ones in the attached image are essentially double what you have. The ones in your amp would essentially be what's in the black square.

The pot has 4 sections. Each one is 50k ohms. The 'C' is the taper.

If you're going to have to buy isopropyl alcohol, buy the 91% concentration. Acetone works better but you have to be a bit more careful using it.

Can you post a better photo of the markings on the preamp board's connector?

[EriCCirE]

Hope this can help. What is the dark color on the dirty pins? I will be buying some 91% isopropyl alcohol to clean it all up... Do you just use a q-tip, or is there something better to use?

[Perry Babin]

Are the LFQ, RFQ, RRQ and LRQ the signals to the main board?

Do they all have the same signal level when the corresponding inputs are driven.

[EriCCirE]

I am not sure if they are the signals to the main board. I will do some testing tomorrow. Would it be sufficient to measure with my dmm on ac voltage?

[Perry Babin]

Yes. You're trying to determine if the loss of signal is on the preamp board or on the main board. If all of the signals are the same on that connector, the problem is likely on the main board. If one signal is low on that connector, the problem is likely on the preamp board.

[EriCCirE]

Ok, got her all cleaned up:









And all my switches are operational. This is how they are linked:


Just waiting till everything is dry before I test and take note of the AC voltages. I am thinking test tones would be the best (no variation), any specific frequency I should pick? Also, if I should use something else for the input let me know.

[Perry Babin]

I generally recommend low frequencies between 50 and 100Hz. Set all crossovers to flat or full range.

[EriCCirE]

OK, My DMM sucks but here has what I've gotten so far. (on 200V AC setting of DMM). 80hz test tone. I had to crank my gains all the way up to get any readings:

LRQ -> LFC = .02
LRQ -> LFQ = .02
LRQ -> RFQ = .02
LRQ -> RFC = .02

RRQ -> LCF = .00
RRQ -> LFQ = .00
RRQ -> RFQ = .00
RRQ -> RFC = .00

The only time I could get a reading using the RRQ was going to the LRQ and that was .02

[EriCCirE]

After cleaning everything up I did some more testing with an actual speaker. It seems that the rear left channel is too loud and the rear right channel is too quiet.