PCEngineFans.com - The PC Engine and TurboGrafx-16 Community Forum
NEC TG-16/TE/TurboDuo => TG/PCE Repair/Mod Discussion => Topic started by: YoshiKnuckles on April 02, 2016, 12:01:48 PM
-
Hello,
I recently finished recapping a PC Engine Duo and have encountered a crackling sound issue. I've been pointed towards a few of the OP Amps, and other IC's related to the audio circuitry. I only have a basic DMM here, and an old analog scope. I have access to a slew of DSO's and Fluke 87-V's at school though.
What should I check moving forward? I'd like to fix it if possible.
Background: I'm currently going to school for Electronics Engineering Technology, yet I'm still early in my studies.
-
The board might need a dishwasher if old capacitor fluid got on the MB. It's hard to see but I had this happen to me once.
Sent from my iPhone using Tapatalk
-
It did have electrolyte leaked all over. I cleaned it up with some 91% rubbing alcohol before recapping it. Are you saying I should give cleaning the board again a try?
-
Id recommend dishwashing it, if you search an old post I had this exact same issue happen. I tried cleaning too but eventually the dishwash method worked. You want to use something very subtle like either just water only or 7th Generation dishwash soap. The crackling is shorts caused be leaked fluid and it's in places you cant see.
Also Google PCB dishwash for more info.
-
Gentle dishwasher cycle with minimal detergent. No heat dry.
After washing, preheat your oven to its lowest temp. Put the board inside, turn off the oven and leave the board in there overnight. Works like a champ!
-
Yes good tips Beemer!
Sent from my iPhone using Tapatalk
-
I also just put my board in the garage and set it on its side and had a box fan dry it off for a day.
Sent from my iPhone using Tapatalk
-
Box fan works good too!
-
odds are you have an open via by the 10uf caps behind the heatsinks
-
Thanks thesteve! I was going to check for that after I gave the board another good cleaning to rule out any electrolyte causing issues under IC's.
Is there a good way to check for damaged vias?
-
I had 2 different sound issues after my recap, both were due to bad lines, via etc going to the ground pin on 2 different 4558 op amps. Running a new ground on both solved it.
Sent from my SM-G920V using Tapatalk
-
Bust out the ohmeter and just checking out all the traces in the audio circuit
Thanks thesteve! I was going to check for that after I gave the board another good cleaning to rule out any electrolyte causing issues under IC's.
Is there a good way to check for damaged vias?
-
With every complete capacitor change I typically remove 2 op amps, clean under them really well, and then reinstall. The one near the AV jack and the other near the M5205 chip are the ones known for getting old cap goo under them.
-
Alright, I re-cleaned the board and rinsed it off really well, leaving it to hang and air dry overnight.
Today when I got home from my Lab class, I plugged it in to see what I'd get. The result? What was seemingly no audio output. However, after cranking up my PVM to max I was able to barely hear it, and the same outcome with headphones and the headphone jack.
My guesses at this point are either a dead via, or a dead op amp. I'll be taking it to school with me tomorrow to check it out under the microscope in the electronics lab, and see if that gives me any hints. Any other things I should check for?
-
Check to see if the op amp near the AV jack is getting around 3.85 volts on the power input pins. If not then there is a chance there is a bad trace or that the amp is bad. Also, try testing the ground trace as well
-
Checked the Op Amp this morning, 1.4V on the output on both sides, 3.8ish volts on the normal input, 4V on the inverting input.
Power rail shows +7.8 volts, ground rail is showing -0.2 mV for some reason... Hmm.
-
so the amp is working and something is pulling the inverting input out of range
-
Hmm, so what should I check? I think it might be time to crack out my Electronics Troubleshooting textbook and do some reading.
-
yes, look up simple OP-AMP circuits
check your grounds, check your input couplings, check your resistors between out and -in
you need to figure out whats pulling the - inputs up
-
Will do. Won't be covering them in class till next spring I think though. =/ Have to get through all the Circuit Analysis and such first.
Guess it's time to turn to The Art of Electronics and my Electronics Troubleshooting textbook from the 90's and actually learn some stuff.
-
Not sure if this will help point you in a better direction, but the fix my repair guy did for this exact same problem involved cleaning under some of the slightly raised chips in that area, where some acid had leaked underneath. I'm not techy, so I can't say which one's to look for, but the fix did involve a small broken trace underneath a chip, due to prior leaky caps.
-
That's the general conclusion I've come to. I've looked the PCB over under a microscope I have access to at school, and I didn't see anything jumping out at me. Have a feeling the dead trace/via causing my issue is under one of the op amp chips. I think I know exactly which one too. Need to poke around with the meter a bit more and confirm it before sucking off chips though.
I haven't looked at it for a week or so now, will have to give it another go sooner or later.
Going to college for Electronics Engineering Technology is a time killer. =x
-
Clean the audio and PSU section with isopropyl alcohol. Push and pull a headphone jack into the headphone socket with some isopropyl alcohol a few times.
-
Fixed it! :D
Decided to probe around the board a bit more yesterday and found 5 volts on the negative input pin on the back of the board. Sure enough, the via connecting that pin to the ground plane was corroded away!
So I ran a jumper from ground on the next chip over to the op amp, and it's back to normal. =) The damaged via was right by a capacitor that had leaked horrendously before the recap, so it makes sense now. :lol:
The pin is back at ground, and the jumper has 1.96mA flowing through it to ground, so I'm not worried about it at all.
-
Update: Went to tune in the disc drive, managed to get the disc spinning, read, and playing, but no audio from my PVM speaker.
I do however faintly hear audio from the headphone jack. Based on this, I'm assuming I have another amplification-related problem.
I've checked the DAC with my Rigol DS1054Z oscilloscope, the audio is present, I've traced it from there to the 4560 Op AMP on the top of the board, yet I'm having trouble finding the next spot in it's path.
I'm learning a lot of stuff working on this Duo.
Edit: Figured it out! Had a via out that kept the 5V rail from making it to the pin. (I traced it out first) Soldered a jumper wire from the 5V rail to pin 8, and she's working perfectly again. =)
Edit: After checking another thread, I checked a few caps in the audio section of the board and found that C162 has no voltage on the positive side. I think I found part of the problem. =)