2600 voltage regulator output 4.06V enough?

[reinheitsrobot]

I have a Sears Heavy Sixer that does power but produces dark snow.  I've traced power from the power port (9v) through the switch to the voltage regulator.  Input to the voltage regulator is just over 9v, which is enough.  However, the output at the voltage regulator is consistent at 4.06V and I wonder if this is enough to power the rest of the system.  I do have an extra, and can swap it out but wanted the group's thoughts before doing that.  After that I will measure power to the (A201 - hex buffer??) per the Field Service Manual.  I have not swapped motherboards yet to rule out upper vs lower board...I guess I should do that too.

 

Any thoughts appreciated.  Thanks.

[+Stephen Moss]

I think everything in the 2600 is CMOS so it should just tollerate it, but it is borderline.

A good voltage regualtor should not be 20% off the rated voltage, instead it should be between 4.8 to 5.2V, thus I would be inclined to replace it, preferable with an LM7805CT, or equivalent.

 

Personally, I always like to use regualtors from National Semiconductor (now part of Texas Instruments) as there output voltages were really tight to that specified, whereas some other manufactures they could be down at 4.5V out without any load on them. 

[cwilkson]

On 12/29/2022 at 3:48 AM, Stephen Moss said:

I think everything in the 2600 is CMOS so it should just tollerate it, but it is borderline.

That's not correct.  The 2600 chipset is an NMOS system.  All 3 chips are specified for VCC = 5.0V +/-5% (= 4.75~5.25 V).

With the exception of the CD4050 (if that exists on your motherboard) which is CMOS and will work correctly down to 3V.

 

TTL also requires 5V +/- 5%.  And many cartridges use TTL chips in them.

 

On 12/29/2022 at 3:48 AM, Stephen Moss said:

A good voltage regualtor should not be 20% off the rated voltage, instead it should be between 4.8 to 5.2V, thus I would be inclined to replace it, preferable with an LM7805CT, or equivalent.

 

Personally, I always like to use regualtors from National Semiconductor (now part of Texas Instruments) as there output voltages were really tight to that specified, whereas some other manufactures they could be down at 4.5V out without any load on them. 

 

The above is good advice.  4.06 V is unacceptable.  You have a few likely causes:

• A bad wallwart (transformer).
• A bad filter capacitor.
• A bad regulator.
• A short on the 5V line.  This is less likely.

How you fix it depends on what gear you have available.

• Try a different known good transformer.  It should be 9VDC.  And rated 500 mA to 1 amp.
  • If that fixes the problem your wall wart is bad.  Get a new transformer.
  • If that doesn't fix it....
• Use a bench supply set to 9 VDC.  1 amp is more than enough.  You can clip the supply directly onto the filter capacitor leads.
  • If that fixes it your filter capacitor is likely bad.  Replace it.

If you don't have a bench supply available try the following.  Measure the 7805's output with your meter set on AC volts.  The output should be very close to 0 VAC.

If your meter has a "minimum voltage" function, that can help.  Use it and verify that the input voltage never goes below 8V.

If you have an oscilloscope that's even better.  It can immediately tell you which component is bad.  Let us know!

 

If you have a spare 2200uF capacitor you can connect it directly to the existing one's leads as a test.  If the 7800's output voltage improves a lot then the original capacitor is likely bad.

If you have rock solid input voltage that never dips below 7.5-8V then replace the regulator with a new one from a reputable source.

 

Regulators are cheap and easy to buy from legit distributors so don't risk counterfeits (not ebay or Alibaba or what ever).  So are filter caps.

Always try Digikey or Mouser to start.  Or you can go to Arcade Parts and Repair.  Peter sells good stuff.  Here are some direct links:

https://www.arcadepartsandrepair.com/store/pinball-kits-parts/pinball-parts/7805-voltage-regulator-5v/

https://www.arcadepartsandrepair.com/store/components/capacitors/axial-capacitors/2200uf-25v-axial-capacitor-105c/

 

EDIT:  Fixed a lot of typos.

Edited January 10, 2023 by cwilkson