7 Feb
2025
7 Feb
'25
8:42 a.m.
I'm repairing/restoring a later model MITS 8 inch floppy drive. I can not
seem to find schematics for it. It is
the same as Bill Degnan has here:
https://vintagecomputer.net/MITS/88-DCDD/
You can see in his photos (and mine match) that the _plus_ 24 volt DC supply
on the board uses a 7924 (note the
9. negative) voltage regulator, with a small heat sink, directly on the
circuit board. Along with a couple of 7805s,
there is a .Motorola 2n6045 screwed to the large black heat sink. All three
of the devices drop into sockets on the
circuit board to allow the heat sink to be easily installed/removed. On my
unit, the socket for the 2n6045 was burnt
to a crisp. I have replace the socket, the 2n6045, the 7924 and the
electrolytic caps. When I test the +24 volt rail
with a dummy load, it measures +41 volts.
I don't understand this circuit. I know MITS was notorious for its power
supplies. Does anyone have the schematics
for this version of the 8 inch floppy? Any suggestions?
Thanks,
Bill S.
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7 Feb
7 Feb
9:18 a.m.
On Fri, Feb 7, 2025 at 5:08 PM William Sudbrink via cctalk
<cctalk(a)classiccmp.org> wrote:
I'm repairing/restoring a later model MITS 8 inch floppy drive. I can not
seem to find schematics for it. It is
Is it simple enough to trace out the schematic?
the same as Bill Degnan has here:
https://vintagecomputer.net/MITS/88-DCDD/
You can see in his photos (and mine match) that the _plus_ 24 volt DC supply
on the board uses a 7924 (note the
9. negative) voltage regulator, with a small heat sink, directly on the
circuit board. Along with a couple of 7805s,
Remember 'There is no such thing as ground' You can take any point in
a circuit as your reference.
The so-called positive 3-terminal regulators have the negative
terminal common between input and output. The negative regulators have
the positive input and output terminals commoned.
So if the input to a 7924 comes from a 'floating' supply (neither side
of the input supply connected to anything else) it would be fine to
connect the output of the regulator to the connection which is
commonly regarded as ground, at which point the common terminal of the
regulator (and the +ve terminal of the floating supply that feeds it)
is +24V.
-tony
9:40 a.m.
On Fri, Feb 7, 2025 at 5:08 PM William Sudbrink via cctalk
<cctalk(a)classiccmp.org> wrote:
I'm repairing/restoring a later model MITS 8 inch floppy drive. I can not
seem to find schematics for it. It is
the same as Bill Degnan has here:
https://vintagecomputer.net/MITS/88-DCDD/
You can see in his photos (and mine match) that the _plus_ 24 volt DC supply
on the board uses a 7924 (note the
9. negative) voltage regulator, with a small heat sink, directly on the
circuit board. Along with a couple of 7805s,
there is a .Motorola 2n6045 screwed to the large black heat sink. All three
of the devices drop into sockets on the
circuit board to allow the heat sink to be easily installed/removed. On my
unit, the socket for the 2n6045 was burnt
to a crisp. I have replace the socket, the 2n6045, the 7924 and the
electrolytic caps. When I test the +24 volt rail
with a dummy load, it measures +41 volts.
Page 137 of the MITS manual on that site gives the schematic of a
power supply using a 7824 and a PNP darlington power transitor (to
inclrease the availabl output current). It runs off a floating supply
from the transormer T1.
My guess is that using the 7924 in a similar circuit allows the
circuit to be turned upside-down and use a more common NPN darlington
transistor.
Is that what you have?
Is the common pin of the regultor firmly connected to the +24 output
rail? If not the output wlll go sky-hgh.
-tony
1:12 p.m.
Hi Tony,
Thanks for the reply. This is what it looks like to me (I hope this comes out):
AC--- + ---------------------------------------------------FD400 +24
| |
| | ----------------
|+ left| |right |
BR2 C4 2200uf 50V 7924 |
|- |center |
| | |
| B |
AC--- - ----------------------E 2n6045 C--------------FD400 common
2n6045 E/B/C are according to the silkscreen. The actual physical pins are: B-left,
C-center, E-right.
Left, right, center designations are with the tab up for both devices. As far as I can
tell, there are no
other components (capacitors, resistors, diodes) involved in the circuit. I'm
measuring a smooth +41
volts at the FD400 connector with some 24volt automotive bulbs as load.
In case the above is illegible, Circuit is: AC to bridge rectifier. BR+ goes straight to
FD400 +24. BR- goes
to 2n6045 right pin. BR +/- connected with a 2200uf 50V electrolytic cap. 7924 left pin
goes to BR+.
7924 center pin goes to 2n6045 left pin. 7924 right pin goes to FD400 common. 2n6045
center pin goes to
FD400 common.
Maybe I'm not loading it enough? I'm very hesitant to reattach the FD400 until I
understand this. By the way,
plus and minus five volts are fine.
Bill
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2:44 p.m.
Looking at this circuit, it occurred to me that 41 volts is about what you would expect to
see if the 2n6045 was just shorted, E to C (center to right). I checked with the diode
tester in my meter and it looked bad. This 2n6045 was out of a box of random unused
transistors that I've had around forever. I looked through the box again, and through
a couple of other boxes but no luck for a 2n6045 or equivalent. I did have a new pack of
2n6043s. The only difference I can see is that the 45 is rated for 100V and the 43 is
only 60. Since the circuit seems to be running at 41, I decided to try a 43. Now I have
a nice clean 24 volts. I wondering whether it will go bad under heavy load, but I think
I'm willing to very quickly test the FD400 in it. Will report results.
-----Original Message-----
From: William Sudbrink [mailto:wh.sudbrink@verizon.net]
Sent: Friday, February 07, 2025 4:12 PM
To: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Cc: 'Tony Duell' <ard.p850ug1(a)gmail.com>
Subject: RE: [cctalk] Re: Use (abuse?) of a 7924 in a +24 PS...
Hi Tony,
Thanks for the reply. This is what it looks like to me (I hope this comes out):
AC--- + ---------------------------------------------------FD400 +24
| |
| | ----------------
|+ left| |right |
BR2 C4 2200uf 50V 7924 |
|- |center |
| | |
| B |
AC--- - ----------------------E 2n6045 C--------------FD400 common
2n6045 E/B/C are according to the silkscreen. The actual physical pins are: B-left,
C-center, E-right.
Left, right, center designations are with the tab up for both devices. As far as I can
tell, there are no other components (capacitors, resistors, diodes) involved in the
circuit. I'm measuring a smooth +41 volts at the FD400 connector with some 24volt
automotive bulbs as load.
In case the above is illegible, Circuit is: AC to bridge rectifier. BR+ goes straight to
FD400 +24. BR- goes to 2n6045 right pin. BR +/- connected with a 2200uf 50V electrolytic
cap. 7924 left pin goes to BR+.
7924 center pin goes to 2n6045 left pin. 7924 right pin goes to FD400 common. 2n6045
center pin goes to
FD400 common.
Maybe I'm not loading it enough? I'm very hesitant to reattach the FD400 until I
understand this. By the way, plus and minus five volts are fine.
Bill
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