[Heath] The Heathkit IP-27

[Kenneth G. Gordon]

On 6 Jun 2012 at 12:15, MICHAEL TALLENT wrote:

> I have an IP-27 that I built in 1972 and It does not work.  I would
> first suspect the capacitors and then the PNP devices.  I am getting
> close to starting to fix mine.

When you do, please keep us informed.

> I don't see any reason to switch to NPN devices as silicon PNP devices
> work just fine and it would make the switch much easier.

Well, any PNP device seems to be much more expensive than NPN devices 
with similar or equivalent characteristics. I think PNP devices have been 
being slowly phased out over the past 10 years or more.

>  Silicon has
> a higher base-emitter voltage drop

On the other hand, Ge PNP devices have, generally, a much higher Veb 
than Si devices: 20 V in the case of the 2N2553 vs 2 v in the case of its Si 
equivalent. At least the Si equivalents I have found so far do.

> so that might cause some problems
> in the Q1 and Q2 circuits

Yes. Exactly

> and also the higher frequency response of
> the newer silicon devices may cause some oscillations that have to be
> snubbed.

Yes. The 2N2553 has an Ft of 200 KHz. Most Si devices have an Ft up in 
the MHz range. This is likely to be a significant problem.

> Just looked through my parts drawers and I have most of the original
> transistors except for Q1.

Q-1 is somewhat weird: it is a DTG-600, and there appears to be no direct 
sub for that. Its Ft was quite high for the time, something like 500 KHz. An 
NTE-179 is SUPPOSED to be a sub, but I wouldn't trust that.

Also, the DTG-600 has a Pd of 106 watts.

Well, it looks as though I have more research to do.

Thanks for your input.

vy 73,

Ken W7EKB