White Dot, No Sound, Proper Voltage at Logic Board Connector

[klydefrog]

Hi All,

New to the forum, and I hate to start off with asking for something, but hopefully I get this resolved and the resolution will contribute to the community.

As the title states, I've got the somewhat classic problem of the white dot, but in my case I have no sound (which also appears common).  I have retrieved the service manual and troubleshooting guide and have also searched the web quite a bit.

I was hoping that the solution in this video would apply to me (clean the J204 connector).  In my case, however, it looks like the proper voltages are reaching the logic board.

I've pulled a couple ICs and reseated, but no luck there.  Following the troubleshooting steps for "No Vector", I have noticed that the voltages at T503 and T401 (RF chokes) on the power board side are not the 9V specified, but instead 25V.  The procedure I'm referring to is:

"Check that there are no shorts between the +9 and -9 power supply to GND.  (T503 (IN) - T401 (IN) )."

Since I don't have a strong EE background, I've been avoiding the power side steps, but it looks like the next step for me is to check the power transformer:

"EP105 (center tap) to EP106 = 8.6 VAC."
"EP105 (center tap) to EP104 - 8.6 VAC."

I had skipped that step both due to FUD and not finding those points in the schematic initially (I now see them as part of T102).  The only mention of 25V I see in the schematic are the caps very near T102, but they are wired to a 9V line it looks like, so I imagine they are just spec'd above the V that should reach them.

With proper voltages at the logic side, am I going down the wrong path here, or does anyone know if this might cause the symptoms?

Thanks in advance.

[klydefrog]

Update:
EP105 - EP106 = 11.1V
EP105 - EP104 = 11.1V

[VectorX]

Jun 12, 2014 9:37:22 GMT -5 klydefrog said:

New to the forum, and I hate to start off with asking for something,

Don't worry about it Welcome aboard, and hopefully you can get this sorted out soon.

[klydefrog]

Thanks. Hope so too. Looks like with my limited electronics background that this is going to be a long term project for me. Just ordered a cap kit. Figured it won't hurt given the age even if it doesn't fix. Can't give up on the ol' gal. She's almost as old as me.

Started tinkering with my Raspberry Pi. Perhaps I can get the specs for the ICs and arrange to test them on the breadboard, but that's a tall order for me. Barely got an LED to flash last night. Or it least it took 10 times as long as I thought (I tend to dive pretty deeply into documentation of things I don't understand thoroughly, which is nearly everything EE related). Turns out I majored in CS when I really wanted to be an EE.

[klydefrog]

Jun 12, 2014 9:37:22 GMT -5 klydefrog said:

I've pulled a couple ICs and reseated, but no luck there.  Following the troubleshooting steps for "No Vector", I have noticed that the voltages at T503 and T401 (RF chokes) on the power board side are not the 9V specified, but instead 25V.  The procedure I'm referring to is:

"Check that there are no shorts between the +9 and -9 power supply to GND.  (T503 (IN) - T401 (IN) )."



So I've been pouring over the schematic and I'm fairly certain that this is my problem.  I see that both the sound amp and the yoke receive this expected 9V input (and are getting 26V on my system), so it stands to reason that this could very well be the entire cause of my symptoms.  The mains to the power board are correct, so the capacitive power supply section looks to be the issue and a bad cap in that section seems consistent with the increased voltage as a reduction in capacitance would in turn reduce reactance and raise the voltage.

I didn't mention it, but yesterday as I was obsessively probing things (unsure why I felt the need given I already found a spot where the voltage was way out of whack) I accidentally shorted something to ground with my probe and the system responded by emitting a beeping/squealing sound, which did not return after unplugging and replugging.  I think what may have happened is the resulting drop in voltage allowed some manner of signal to be sent to the speakers.  Hopefully I didn't harm anything w/ that mistake, but it was over on the HV side, so hopefully whatever new route the current took that brief moment was through things that could take it.  FWIW, I took the initial high V reading at T401 prior to doing this, so I didn't cause that issue I know for certain.  No physical indication of any damage.

Caps on the way.  Guess I hold tight until then.

References:
en.wikipedia.org/wiki/Capacitive_power_supply
en.wikipedia.org/wiki/Electrical_reactance

[klydeFrog-asguest]

Long story short, replaced all caps and added molex connectors for yoke output and A/C input to powerboard. I've broken it worse as now I have no dot on the screen, filament isn't lit.





Think I've introduced a short to ground somewhere. Ugh.