PCE Duo - No Sound *solved*

[HuMan]

System: PC Engine Duo (black)
Symptom: No sound, before and after cap kit

When I first got this Duo, it was sold as working (HuCards and CDs), working video, non-working audio on both HuCards and CDs. I opened it and found a partial cap kit applied, somewhat sloppily. There wasn't much capacitor damage, just one ripped trace on one of the big leaded caps behind the 7805s.

I applied a near-full cap kit (can't easily reach the two 100uF 16v leaded caps above the BA6290A). I used a cap kit and instructions from Console5. The video quality improved back to what PC Engines are known for, HuCards and CDs still work, still no audio on either of them.

So in a way, nothing changed after the cap kit, so the audio issue is being caused by something else... Or via/trace corrosion which I hope not. When I have some more time for soldering I'll check the op amps and volume IC. Anything else I should check?

[YoshiKnuckles]

Have you tried cranking your set's volume up to max, or plugging in headphones with them at max to see if you get anything at all out of it?

Basically you want to find out of the system is producing audio from it's chipset or not.

 If it is producing audio than you have an amplification issue, and should start checking all of the Op Amps and the M51131L.

You want to get the card sound working, then try to get the CD sound working.

For the CD section, start with the DAC to see if it is outputting anything. Not exactly easy to do though as it is on the bottom of the board.

Hopefully that helps a bit.

[HuMan]

No sound from either the AV port or the headphone jack. Completely silent.

I'll check all the Op Amps and the M51131L. For checking traces, does anyone know the general layout of the sound? Where does it start and go through on its way to the AV port and headphone jack?

[YoshiKnuckles]

For the card sound, you want to check the Hu6280. It's the CPU of the console, and the sound starts here. The pins are rather small though so be careful not to short them out.

https://console5.com/wiki/Hu6280

Pins 17 and 18 of the 6280 output the sound. It's easiest to check this with an oscilloscope, as a multimeter isn't going to show you if the signal is there or not. Also check the 5V audio rail on pin 19 and the audio GND on pin 20. All you really need to do is see if it is producing a waveform or not.

[Keith Courage]

most likely your problem is the op amp closest to the AV Jack. Not that the amp is bad though. you should remove the amp and clean underneath it really well. I like to use rubbing alcohol and a toothbrush. then reinstall the amp.

I would say almost 85% of every Duo that I change the capacitors on needs this done now.

most of the voltages from the amp should read about 3.8 volts. the power input should be 7 volts.

another issue with this area is even after cleaning sometimes the ground trace is no good. So you might need to run a new wire or capacitor lead to get a good ground.

if all the traces to the amp seem fine then it's probably a bad op amp and you need to replace it.

[HuMan]

I could certainly buy some 4558 op amps but my only source is china and that would take a month. I only have time for this Duo this week... Hopefully it's just a bad trace.

Where is this op amp exactly? Is it near where the DC jack board is? Or is it near the 100uF, 4.7uF smd caps and headphone jack?

[YoshiKnuckles]

There is an op amp right by the A/V jack. I believe that's the one he is talking about. It's next to a couple of caps and some inductors.

China? Please. They stock 4558 op amps on Mouser and Digikey.

Within a minute of searching:
http://www.mouser.com/Semiconductors/Amplifier-ICs/_/N-4frvm?Keyword=4558&FS=True

Like $0.50 each + shipping. Not sure of the exact one to get though.

[Keith Courage]

get a reading of the voltages on the pins for the amp right by the AV Jack and post them here. 

[HuMan]

IC506 or IC507? They both look like op amps to me and they are "near the AV jack".

I tested pin 8 on both of them, they are 3.11v. Ground seems to be working on both as well. If I could find a place to attach the alligator clips on my logic probe, I could try using that too.

Also, I don't live in the US, shipping isn't cheap from either mouser or digikey.

EDIT: I put a fresh battery in my volt meter. The 4 op amps and the volume control chip are 3.3v. I've poked around and in general, this area of the board is of a low voltage. The AC adapter voltage is 9.4v, 7805 regulators input is 8.1v, output is 5.03v, it goes down lower as the power passes through various passive components. The op amps and volume chip only get 3.3v. What is going on here?

[YoshiKnuckles]

I'd assume he means IC506.

Pin8 should be around 7-8V, not 3V That's the +V input for the Op Amp. That would be part of the reason you have no sound I'd guess. You'll have to figure out why you're not getting 7V on those.

Likely a dead passive, transistor, missing ground or dead via is my guess. I'd start with tracing the +rail back from one of the op amps to see where it is created. Notes and drawing your own schematic might help.

[HuMan]

7 - 8v? Yeah there's a significant drop going on here. The voltage from the 7805s is 5v, so I guess there are passive components that amp the voltage up to 8v between the 7805s and the op amps/volume IC. I will investigate that area and determine where the voltage drops are occurring.

Would anyone happen to know what kind of transistors are used on a PC Engine Duo? The tiny ones with no markings on them. I get the feeling there may be one or two dead ones. Last night I checked one near the 7805s and op amps and I think its output was 4v, that doesn't sound right. The PC Engine Duo doesn't use 3.3v for anything, right? Is it just 5v and up to 8v?

[Keith Courage]

all of the pins on the op amp should be about 3.5 volts except for the ground pin obviously and then the Power Pin which should be 7 volts.

the amp that I have been talking about is the one to the right of the AV Jack.

you should also double check that the amps on the bottom of the board have these readings as well.

if the amps are not getting these voltages then the problem is most likely old capacitor goop underneath one or more of the amps causing shorts or that there are bad traces to one or more of the capacitors that you replaced.

[HuMan]

All 4 op amps and the volume IC read about 3.3v. One of the op amps under the board looks like its been reflowed in the past. Other than that, all 5 of those chips look okay.

When I was replacing all the capacitors, there was one big trace between a couple leaded caps that was gone, I jumpered that trace while putting the new caps in. I didn't see any other damage.

Any ideas which of the big caps behind the 7805s are used for the op amps?

[Keith Courage]

Okay, lets try to narrow this down then. What's the voltage on each one of these caps circled in red? https://postimg.org/image/c41vw27ah/

Also, can you by chance hear some audio if you crank up your TV really high?

[HuMan]

Cap near volume wheel - 1.3v
Caps to left of AV jack op amp - 1.3v
Caps to right - 1.89v (3rd one from top is 1.5v)
Cap near super cap - 0.05v

The Duo outputs dead silence at normal level. If I put the speakers to max, there's just humming. I actually use a 3 piece stereo system rather than the TV speakers.

The 22uF 16v cap near the super cap is probably the culprit. Do you know what it's connected to?

EDIT: I noticed that 22uF 16v cap sits right above the one 4558 op amp under the board. I think the negative side of the cap goes through to the underside next to the 4558 (on its right in the picture). That via seems to be corroded, no continuity through it. I take it this is what you meant by op amps being ravaged by caps? I'll try removing that op amp, cleaning underneath and resoldering it.