Gorgar
Williams, 1979I am having an issue where all my sounds and speech are working but the volume is too low. Here is what I have done so far:
Recapped the ps and sound board.
New headers all around.
I eliminated the main pot as the source.
I eliminated the speaker as the issue.
I replaced the transistor that drives the dac
I am not sure what to test next honestly. I did check 2 pins on the dac per an old thread, but that didn't show an issue. Hoping someone can help.
Quoted from Cheddar:I think if you touch the tab on the amp and you hear a loud hum it means the amp is good
Touch with my finger? Thanks
Ok, so I get nothing when I do that. I think I see the input pin on the schematics, but how much voltage should I see there? Also, does it always how power or just in gameplay? Thanks for your help!!!
Quoted from Cheddar:Yeah and I'd do it right when you turn it on so it isn't hot yet
Quoted from pins4u:Not the tab because usually that is earth, but certainly the pins that solder to the PCB. One of them is the input and it should blurt a healthy noise to the speaker when touched.
It's a signal not a voltage as such applied to the input pin BUT you certainly should make sure the voltage is applied to the correct pin on the audio amplifier IC. Probably about 12 or 15V DC on pin 5 if this is the correct schematic for your machine? Yes, the amplifier is always powered when the machine is switched on.
amp (resized).png
Quoted from pins4u:It's a signal not a voltage as such applied to the input pin BUT you certainly should make sure the voltage is applied to the correct pin on the audio amplifier IC. Probably about 12 or 15V DC on pin 5 if this is the correct schematic for your machine? Yes, the amplifier is always powered when the machine is switched on.
[quoted image]
Thank you! Yes, that looks like the same schematic. I will double check though.
Quoted from Tomass:Ok, so at pin 5 I am getting 10.75v is this close enough?
Should be plenty.
You need to figure out if the low sound problem is BEFORE the amplifier - AFTER the amplifier (IE speaker or wiring/plugs/sockets) or the actual amplifier circuit.
This is a very simple circuit that rarely gives trouble - I would be replacing the electro caps (you said you replaced them BUT with the correct value and installed in the correct direction?), checking the value of all the resistors around the chip and see what you find.
So I have one of these that makes it easy to trace the audio signal once it's been generated. I've used it several times to identify or eliminate the amp as a problem on a sound board. As often as you'd use it it wouldn't recommend paying more than $16 for it.
ebay.com link: Progressive Electronics 200EP Inductive Amplifier by Tempo Untested
But.. after you've double checked the caps it would cost less that $16 to replace the amp anyway
I know the caps are correct value and polarity, but there was one listed as "low leaking". I put a ceramic cap in this spot after trying an electrolytic first. It was originally an electrolytic. I ordered a tda2002 and figured I would swap it out and see.
I also tried a second speaker to test that.
Well, I swapped out the tda2002 and still the same issue. I notice that I am not getting much wattage at all if I read the volts at the speaker. I did check all the resistors around that amp and they read good. I am kinda stumped again.
I know the data signal is good since I get all the sounds. I still get no loud hum if I touch the legs of the amp. All test points show proper voltage.
I didn't look at the wiring of this machine, I assume there are 2 speakers? Have you tested the speakers at all? Measured their coil unplugged? You have sound, but it's low, does the volume control affect the volume at all? The voltages are good, no loud hum even if the volume is cranked up to max and you touch the input pin with a finger?
I see only 2 things that can happen if the amp doesn't make the loud hum, the input pin is pulled down from the volume control so low that it muffles anything coming from the input, but the pot and the amp have been changed to same results.
One of the 2 speakers, or both, are damaged and it is actually pulling down the output because the impedance of the coil has changed. From the schematic posted above, unplugging the volume control would make sure nothing is pulling down the input pin. If you don't hear a loud hum or static touching the pins then with the volume control unplugged, I'd definitely focus on the output stage.. The caps you changed included those? Maybe test the resistor values as well.. But check the speakers and measure their coil with resistance (ohm) on your meter.
Quoted from Cheddar:can you swap the roms between boards? Use the known good board to test any swappable components of the bad board.
I did put the gorgar rom in it. The only other thing I could swap is the 6808. I will do that next and at least eliminate that. I don't think a pia would do that though, would it?
Quoted from pins4u:It's the voltage supply voltage point to the amplifier chip.
There are about 4 parts in this entire circuit. I would just replace them all.
Problem is I already replaced the caps and the amp. All those resistors read in spec. Not sure what else to replace.
Quoted from pins4u:Those blue 1% resistors are stupidly difficult to read the values with the colour bands. Are you 100% certain they are the correct values, verified by a meter?
Yes! It is working now! I did test each resistor, but they are labeled on the paper. Not sure if it was a resistor or one of the new caps were bad, but I replaced them as well.
Thanks for your help, everyone!
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