Hello,

I have a vintage electronic chess board from the 1980's. Its 9VAC input. There is a quiet buzzing sound from the internal speaker. Its not too loud but I want to confirm that its nothing to worry about.

I checked all the grounds, electrolytic capacitors, etc. Is there something else I should be looking for? Also, the regulator gets pretty hot. I understand that its going to release excess voltage in the form of heat but other similar boards (different manufacturer) do not generate this amount of heat.

I was also wondering what the diode on the ground pin of the 7805 is for? And why the thin plastic in between the heat sink and chip? Shouldn't there be some thermal grease? This is a somewhat rare unit and difficult to replace. I want to do everything I can to insure its operating properly. Thanks!

 

How have you checked the electrolytics?

Change them anyway especially the large blue one in the top corner, it may help with the buzzing and doesn't cost a lot.

The diode in the ground pin of the regulator will raise its output voltage to 5.6V or so (5V + a diode drop).

The only reason I can see for using a mica washer between the heatsink and the regulator is for isolation and the possibility of the heatsink touching some other component or track. If you are sure the the heatsink isn't touching any other part of the circuit you could replace it with a little heatsink compound to reduce the temperature.

 

You can grease both sides of the mica and put the hot part back where you found it.
Sometimes you can glue just about anything to a TO-220 package to help it radiate heat.
It depends on whether you have room.

 

HI, Yes, I replaced ALL the electrolytic including that large blue one after these photos were taken. I'll take a look at the heat sink as you suggested. I was suspect of the pair of voltage regulator diodes as seen to the left of the large capacitor but there are really no markings on them to determine specs. The speaker buzz is sort of a pulsating buzz sound. This unit was factory upgraded many years ago with an improved program and faster speed, hence all the modifications such as the large chips in the undersized sockets.

 

It could be picking up noise from your CPU. With all that messy wiring it would not be surprising.

Hum from bad filters is pretty distinctive.

Here are some samples

I like the coon skin cap

Around 1:20

 

Another from wiki

http://en.wikipedia.org/wiki/Mains_hum

 

Golly!
"Electrical network frequency (ENF) analysis is a forensic technique for validating audio recordings by comparing frequency changes in background mains hum in the recording with long-term high-precision historical records of mains frequency changes from a database. In effect the mains hum signal is treated as a time-dependent digital watermark that can be used to find when the recording was created, and to help to detect any edits in the sound recording."

 

HI, Thanks for this, yes it must definitely be the 60Hz hum. I guess its something I will have to live with. As for the heat I did remove the mica, cleaned all surfaces and re-greased it as per #12.

It could be picking up noise from your CPU. With all that messy wiring it would not be surprising.

Hum from bad filters is pretty distinctive.

 

When you 'pull the plug', literally remove power by unplugging does the hum linger for a very short time or stop exactly (no part of a second) when the power is removed? This may tell you something.

 

HI, The sound immediately ceases when unplugged

When you 'pull the plug', literally remove power by unplugging does the hum linger for a very short time or stop exactly (no part of a second) when the power is removed? This may tell you something.

 

"immediately ceases when unplugged"
That is consistent with power supply hum and maybe cpu noise.
It depends how long the circuit is alive after the power is cut off.
If the display does not linger at all this test is not so useful.

If you can run the device from a bench supply or battery that should eliminate ac hum as the culprit.