[sebhc] H89 Power-up problems.

Lee Hart leeahart at earthlink.net
Sun Jul 9 10:52:44 CDT 2006

Mark Garlanger wrote:
> I was able to locate the right schematic and check the power going
> into the TLB board (strange that it comes from the CPU board and not
> the power supply).  From your earlier posting:
>> These symptoms both point to problems on the TLB. I'd check the supply
>> voltages. The TLB needs +8v, +16v, and -16v to work. It has onboard
>> regulators to convert these to +5v, -5v, +12v, and -12v.
> The values on my DMM are +7.3V +17.6V and -18.1V...
> Most likely the +7.3V is the source of my problems. Based on the
> schematics, it appears that the keyboard encoder uses Vcc2 (one of the
> 5V supplies).

Something's wrong with the 8v supply; 7.3v is definitely too low.

A common problem was for connector P101 at the extreme rear of the
little power supply board under the fan to burn up the pins for the
yellow wires. The standard fix, used by Heath themselves, is to cut the
two yellow wires, and solder them directly to the AC pins of the bridge
rectifier on the metal heatsink under the fan.

Another possibility is that your bridge rectifier is bad, and that only
2 of the 4 diodes in it still work. This makes it a halfwave rectifier
instead of fullwave, with a resulting drop in in voltage and current

> But then I checked the wall voltage: 117V!!! what it should be 120V?

No, that's not the problem. The H89 will run just fine on even 100 vac.
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in    --    Leonard Cohen
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
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