[Heath] [Heathkit:Yahoo] IP-28 Issues

[Garey Barrell]

Hi Gary -

I think your friend is missing part of the circuit operation. The 1 ohm pot never sees more than ~0.36 W, and the 100 
ohm more than ~0.0036 W.

Q6 'senses' the voltage across R13 and R14 in series, and when it reaches ~ 0.6V, Q6 reduces the Base drive to Q4 
dropping the voltage output to maintain that 0.6 VDC.  If he is blowing pots, I would suspect Q6 and its associated 
circuitry is failing to limit that voltage.

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>

Gary H. Harmon, Jr. wrote:
>
>
> A engineering friend of mine locally has been working hard to resolve issues with an IP-28 power supply.  His findings 
> follow:
>
> "I have come to the conclusion that the circuit design on the IP-28 has some serious issues with the power rating of 
> the current limit pot.  All power must flow through this series device and power dissipation of the pot would be 
> around 30W at full voltage.  The ten turn pot is only rated at 2W.  I honestly do not know what the Heath engineers 
> were thinking when they designed this one!  I would suspect that other components in the load current path are also 
> under rated; I know my unit took out three transistors in addition to the pot and another series resistor."
>
> Any thoughts on this problem?
>
>