[R-390] R-390 restoration

[Barry Hauser]

Les wrote:

>I lean toward the use of an external 300 VDC lab supply rather than trying
to modify the internal regulator. Keep in mind no matter what you do
internally, you still have to get rid of the IR drop heat. Instead, present
the regulated bus with the right voltage from an external unit that can be
disconnected to restore to the OEM design.


That sounds like a good idea -- with the reg. supply set to 180 vdc.  If you
need the P/S for other work, just won't be able to listen to your non-A at
the same time.

However, the heat dissipated by the SS VR circuit is much lower than what is
given off by the 6082 pair and Dr. J. offers some heatsinking suggestions in
the HSN article which get the heat out of the chassis altogether -- e.g., by
mounting the heatsink on the outside of the chassis using existing holes
alongside the audio deck area.  Warning on that -- the transistor that needs
the heat sink will probably have a "hot" case or tab, so needs a mica
insulator and heat sink grease -- plus an insulated cover over the
transistor itself so as not to have a very oversized, external and dangerous
B+ "test point".  If you don't like the idea of an outboard heat sink, one
can be positioned inside and a small fan can be used to draw the heat out
through a nearby "porthole".  BTW -- this is a good idea when running the
6082's.  The heat buildup is aggravated by the fact that they are upside
down and the heat otherwise rises to cook the deck and warm up the whole
chassis on that side.

However #2:  The '391 I have (that sparked the article) has no heat sink.  A
tab mount style transistor is just "floating" on the perfboard.  It looks
like the guy originally had it mounted to the shield that encloses the 6082
area, as there is a small hole and a residue of heat sink grease there.
Maybe he had trouble isolating the tab, I don't know.  But it's the coolest
spot in the rig and the mod was probably done over 15 years ago, judging
from some of the components used.  I should point out that this was done
irreversibly and the audio was solid stated out using one of those handy
dandy ECC modules.  There is only one tube remaining on that audio deck.
The rest of the radio is stock "tubular" and the solid state VR is supplying
B+ to all of that.  Yet, no heatsinking and not much heat.  So ... I dunno.

Barry