QBert no-sound / just a hissing noise

primerano

New member
Joined
Jan 24, 2024
Messages
21
Reaction score
1
I rebuilt my power supply and got my qbert working again but the sound board appears to be dead. I just get a hissing sound out of the speaker when I have the sound board installed.

  • Voltages are good now that I have rebuilt the power supply.
  • The sound board has no discoloration. Nothing burnt. It looks really clean
  • The heat sink on the LM379S did not give off any heat after playing for 10 minutes. Considering the size of the heatsink I assumed it would be warm.
Does a dead LM379S result in just a hiss for the output or should I look elsewhere?
 
Another thing to check is to make sure your speaker coil isn't shorted. Disconnect the wires to the speaker, then check the speaker coil resistance with a DMM. Should be somwhere between 4-8 Ohm, I'd think. If less than 2 ohms or so, problably a shorted speaker.
 
Also check the pots. If you adjust R15/16, do you hear any change? Always a good idea to clean volume pots with some DeOxit.
 
@tomservo thanks for the flow chart pointer. It does not say I'm turned on. The volume knob does adjust the level of the hiss.

The test button does not produce a tone.

Chart would imply the amp is bad but next time I am at the machine I'll check the other items you mentioned.

Thank you,
Tony
 
@tomservo I replaced the amp but it didn't fix the issue. I will say at least this amp heats up the heat sink so it must be doing something. But given that I have the same result I'm starting to think the prior amp was fine. I forget if I had tested pin 10.

Speaker ohms is about 4 and I even attached another speaker with the same results.

From here I started testing some capacitors. between pin 10 and audio out is c36 and it is within 5% of 470uf.
  • r21 good
  • r22 good
  • c37 good
  • r18 good
  • c33 -- my meter is reading 54uf vs an expected 0.1uf!
    • I started reading some of the other 0.1uf caps on the board and they were also high but then i realized that they are all going from 5v to ground and have an additive effect.
    • and then i realized c33 is in series with c38 (47UF) and probably another cap I am missing so the 54UF reading is not unexpected.
Anything specific I should check?

I should note that my power supply was bad and I rebuilt it last year. I am not sure what voltages the sound board was exposed to, but nothing looks burnt.
 
what TYPE of capcitor should be in C33? mine blew up and i dont know what it was other than .047uF and 25v. is it ceramic?
 
C33 is a 0.1uf cap. 25v according to p27 of the manual. But I think people often replace it with a 50v version as it is handling 30v. There are almost 20 of them on the board but the others are filtering 5v.
 
craaaaaaap. i read that wrong. i took the values at the bottom not at the top/first line :(
but what is the type/style? ceramic? film? etc
 
@tomservo I replaced the amp but it didn't fix the issue. I will say at least this amp heats up the heat sink so it must be doing something. But given that I have the same result I'm starting to think the prior amp was fine. I forget if I had tested pin 10.

Speaker ohms is about 4 and I even attached another speaker with the same results.

From here I started testing some capacitors. between pin 10 and audio out is c36 and it is within 5% of 470uf.
  • r21 good
  • r22 good
  • c37 good
  • r18 good
  • c33 -- my meter is reading 54uf vs an expected 0.1uf!
    • I started reading some of the other 0.1uf caps on the board and they were also high but then i realized that they are all going from 5v to ground and have an additive effect.
    • and then i realized c33 is in series with c38 (47UF) and probably another cap I am missing so the 54UF reading is not unexpected.
Anything specific I should check?

I should note that my power supply was bad and I rebuilt it last year. I am not sure what voltages the sound board was exposed to, but nothing looks burnt.
No specific advice here, but the same sound board was used in some gottlieb system 80 pinballs and there might be some repair logs on those that should overlap somewhat with q*bert.

Section 5.19.3 of pinwiki https://www.pinwiki.com/wiki/index.php/Gottlieb_System_80#System_80_.2F_80A_Sound_and_Speech_Board
 
Back
Top Bottom