8 Jun
2020
8 Jun
'20
11:59 p.m.
Hi all --
I'm working on a PDP-8/A I picked up at VCF West last summer. After a lot
of cleaning and some power supply repair it's showing signs of life (CPU
seems to be at least minimally functional, core memory is going to need
some debugging.)
The transformer in the power supply is humming quite loudly, however, and
I'm curious if this is normal for the 8/A (or is typical for an 8/A of this
vintage). I'm used to the supplies in various other DEC machines not being
exactly silent but this is a rather severe 60Hz buzz that you can clearly
hear over the fans when the machine is in operation.
Thanks,
Josh
9 Jun
9 Jun
1:09 a.m.
Hi Josh,
i have restored last year a PDP-8/a to and had the same experience like you with the
Powersuppy. I felt also wiered about the bang in the transformer in the moment of
switching it on.
Marco
Am 09.06.2020 um 09:02 schrieb Josh Dersch via cctalk
<cctalk at classiccmp.org>:
?Hi all --
I'm working on a PDP-8/A I picked up at VCF West last summer. After a lot
of cleaning and some power supply repair it's showing signs of life (CPU
seems to be at least minimally functional, core memory is going to need
some debugging.)
The transformer in the power supply is humming quite loudly, however, and
I'm curious if this is normal for the 8/A (or is typical for an 8/A of this
vintage). I'm used to the supplies in various other DEC machines not being
exactly silent but this is a rather severe 60Hz buzz that you can clearly
hear over the fans when the machine is in operation.
Thanks,
Josh
4:48 a.m.
Is there a recommended alternative or replacement power supply for the 8/A?
thanks
On Tue, Jun 9, 2020 at 4:20 AM Marco Rauhut via cctalk
<cctalk at classiccmp.org> wrote:
Hi Josh,
i have restored last year a PDP-8/a to and had the same experience like you with the
Powersuppy. I felt also wiered about the bang in the transformer in the moment of
switching it on.
Marco
Am 09.06.2020 um 09:02 schrieb Josh Dersch via
cctalk <cctalk at classiccmp.org>:
?Hi all --
I'm working on a PDP-8/A I picked up at VCF West last summer. After a lot
of cleaning and some power supply repair it's showing signs of life (CPU
seems to be at least minimally functional, core memory is going to need
some debugging.)
The transformer in the power supply is humming quite loudly, however, and
I'm curious if this is normal for the 8/A (or is typical for an 8/A of this
vintage). I'm used to the supplies in various other DEC machines not being
exactly silent but this is a rather severe 60Hz buzz that you can clearly
hear over the fans when the machine is in operation.
Thanks,
Josh
5:41 a.m.
On Tue, 9 Jun 2020, Marco Rauhut wrote:
i have restored last year a PDP-8/a to and had the
same experience like
you with the Powersuppy. I felt also wiered about the bang in the
transformer in the moment of switching it on.
Does it have a ferroresonant power supply? Then the hum and bang is normal
;-)
Christian
7:18 a.m.
On Jun 9, 2020, at 2:59 AM, Josh Dersch via cctalk
<cctalk at classiccmp.org> wrote:
Hi all --
I'm working on a PDP-8/A I picked up at VCF West last summer. After a lot
of cleaning and some power supply repair it's showing signs of life (CPU
seems to be at least minimally functional, core memory is going to need
some debugging.)
The transformer in the power supply is humming quite loudly, however, and
I'm curious if this is normal for the 8/A (or is typical for an 8/A of this
vintage). I'm used to the supplies in various other DEC machines not being
exactly silent but this is a rather severe 60Hz buzz that you can clearly
hear over the fans when the machine is in operation.
Assuming these are traditional transformers with laminated sheet iron cores, check if the
rivets or screws holding the laminations together are loose. If yes, tighten or replace
the fastener.
paul
7:27 a.m.
Related question: Is there any reason, other than historical accuracy and
cost, to not replace the power supplies in vintage computers with modern
switching power supplies?
--
Anders Nelson
On Tue, Jun 9, 2020 at 10:19 AM Paul Koning via cctalk <
cctalk at classiccmp.org> wrote:
On Jun 9, 2020, at 2:59 AM, Josh Dersch via
cctalk <
cctalk at classiccmp.org> wrote:
Hi all --
I'm working on a PDP-8/A I picked up at VCF West last summer. After a
lot
of cleaning and some power supply repair it's
showing signs of life (CPU
seems to be at least minimally functional, core memory is going to need
some debugging.)
The transformer in the power supply is humming quite loudly, however, and
I'm curious if this is normal for the 8/A (or is typical for an 8/A of
this
vintage). I'm used to the supplies in
various other DEC machines not
being
exactly silent but this is a rather severe 60Hz
buzz that you can clearly
hear over the fans when the machine is in operation.
Assuming these are traditional transformers with laminated sheet iron
cores, check if the rivets or screws holding the laminations together are
loose. If yes, tighten or replace the fastener.
paul
7:34 a.m.
On 2020-06-09 10:27 a.m., Anders Nelson via cctalk wrote:
Related question: Is there any reason, other than
historical accuracy and
cost, to not replace the power supplies in vintage computers with modern
switching power supplies?
3) If it ain't broke, don't fix it
--
Anders Nelson
On Tue, Jun 9, 2020 at 10:19 AM Paul Koning via cctalk <
cctalk at classiccmp.org> wrote:
>
>
>> On Jun 9, 2020, at 2:59 AM, Josh Dersch via cctalk <
> cctalk at classiccmp.org> wrote:
>>
>> Hi all --
>>
>> I'm working on a PDP-8/A I picked up at VCF West last summer. After a
> lot
>> of cleaning and some power supply repair it's showing signs of life (CPU
>> seems to be at least minimally functional, core memory is going to need
>> some debugging.)
>>
>> The transformer in the power supply is humming quite loudly, however, and
>> I'm curious if this is normal for the 8/A (or is typical for an 8/A of
> this
>> vintage). I'm used to the supplies in various other DEC machines not
> being
>> exactly silent but this is a rather severe 60Hz buzz that you can clearly
>> hear over the fans when the machine is in operation.
>
> Assuming these are traditional transformers with laminated sheet iron
> cores, check if the rivets or screws holding the laminations together are
> loose. If yes, tighten or replace the fastener.
>
> paul
>
>
9:04 a.m.
On 6/9/20 10:34 AM, Toby Thain via cctalk wrote:
On 2020-06-09 10:27 a.m., Anders Nelson via cctalk
wrote:
I have a VXT-2000. It is broke. If I could find out what the voltage
and current requirements were and their positions on the power connector
I would replace it in a heartbeat.
bill
Related question: Is there any reason, other than
historical accuracy and
cost, to not replace the power supplies in vintage computers with modern
switching power supplies?
3) If it ain't broke, don't fix it
11:51 a.m.
On Tue, Jun 9, 2020 at 9:04 AM Bill Gunshannon via cctalk
<cctalk at classiccmp.org> wrote:
I have a VXT-2000. It is broke. If I could find out what the voltage
and current requirements were and their positions on the power connector
I would replace it in a heartbeat.
For the VXT-2000 with an H7109-B the rated voltage and current values
are printed right on the power supply label:
+5.1V, 7.81A
+12.1V, 0.62A
-12.1V, 0.46A
-9V, 0.2A
Fixed width character pinout diagram:
+=================+
-9V | Yellow | Orange | +12.1V
+--------+--------+
??? | White | Black | Gnd
+--------+--------+
+5.1V | Red | Blue | -12.1V
+--------+--------+
+5.1V | Red | Black | Gnd
+--------+--------+
Gnd | Black | Black | Gnd
+=================+
The mystery is the White wire. The power supply label only lists 4
output voltages. The White wire appears to be routed to the Ethernet
daughter board. The measured voltage appears that it might be floating
slightly negative, somewhere around -1.5V when the Ethernet daughter
board is installed and around -5V when it is removed. Maybe it is a
high impedance earth ground connection? It appears to be connected to
the shield of the Ethernet BNC, which measures around 1M-Ohm to the
chassis ground when the power supply is disconnected from the main
board, and around 0.75M-Ohm when the power supply is connected.
3:10 p.m.
On 6/9/20 2:51 PM, Glen Slick via cctalk wrote:
On Tue, Jun 9, 2020 at 9:04 AM Bill Gunshannon via
cctalk
<cctalk at classiccmp.org> wrote:
Thank you for that. Mine has no labels or anything. It is PC Board
with Digital, Side 1, Side 2 and a number I don't recognize on it.
I guess the -9V is the only odd one but you could easily get that
I have a VXT-2000. It is broke. If I could find out what the voltage
and current requirements were and their positions on the power connector
I would replace it in a heartbeat.
For the VXT-2000 with an H7109-B the rated voltage and current values
are printed right on the power supply label:
+5.1V, 7.81A
+12.1V, 0.62A
-12.1V, 0.46A
-9V, 0.2A
Fixed width character pinout diagram:
+=================+
-9V | Yellow | Orange | +12.1V
+--------+--------+
??? | White | Black | Gnd
+--------+--------+
+5.1V | Red | Blue | -12.1V
+--------+--------+
+5.1V | Red | Black | Gnd
+--------+--------+
Gnd | Black | Black | Gnd
+=================+
The mystery is the White wire. The power supply label only lists 4
output voltages. The White wire appears to be routed to the Ethernet
daughter board. The measured voltage appears that it might be floating
slightly negative, somewhere around -1.5V when the Ethernet daughter
board is installed and around -5V when it is removed. Maybe it is a
high impedance earth ground connection? It appears to be connected to
the shield of the Ethernet BNC, which measures around 1M-Ohm to the
chassis ground when the power supply is disconnected from the main
board, and around 0.75M-Ohm when the power supply is connected.
from a -12V line. I may get to fix it yet.
bill
4:04 p.m.
On Tue, 9 Jun 2020, Bill Gunshannon via cctalk wrote:
For the unlabeled white wire this must be -9V return, such as say with
the H7109-AA PSU used with the VAXstation 4000 VLC, the H7819-AA used with
VAXstation 4000 60/90/etc. and storage expansions or the H7821-00 PSU used
with several systems, where the -9V supply circuit is isolated and used, I
believe, for serial line drivers/receivers. It might be worth checking.
Maciej
For the
VXT-2000 with an H7109-B the rated voltage and current values
are printed right on the power supply label:
+5.1V, 7.81A
+12.1V, 0.62A
-12.1V, 0.46A
-9V, 0.2A
Fixed width character pinout diagram:
+=================+
-9V | Yellow | Orange | +12.1V
+--------+--------+
??? | White | Black | Gnd
+--------+--------+
+5.1V | Red | Blue | -12.1V
+--------+--------+
+5.1V | Red | Black | Gnd
+--------+--------+
Gnd | Black | Black | Gnd
+=================+
The mystery is the White wire. The power supply label only lists 4
output voltages. The White wire appears to be routed to the Ethernet
daughter board. The measured voltage appears that it might be floating
slightly negative, somewhere around -1.5V when the Ethernet daughter
board is installed and around -5V when it is removed. Maybe it is a
high impedance earth ground connection? It appears to be connected to
the shield of the Ethernet BNC, which measures around 1M-Ohm to the
chassis ground when the power supply is disconnected from the main
board, and around 0.75M-Ohm when the power supply is connected.
Thank you for that. Mine has no labels or anything. It is PC Board
with Digital, Side 1, Side 2 and a number I don't recognize on it.
I guess the -9V is the only odd one but you could easily get that
from a -12V line. I may get to fix it yet.
4:13 p.m.
On Tue, Jun 9, 2020 at 3:10 PM Bill Gunshannon via cctalk
<cctalk at classiccmp.org> wrote:
On 6/9/20 2:51 PM, Glen Slick via cctalk wrote:
(I was going to change the subject line for this reply to reflect the
VXT-2000 discussion but maybe it is already too late to do that
without making message threading even worse?)
Does your VXT-2000 look significantly different than the images on
this page? Is the power supply in your VXT-2000 a bare board that is
not fully enclosed?
https://terminals-wiki.org/wiki/index.php/DEC_VXT2000
The power supply in my VXT-2000 matches the fully enclosed H7109-B
power supply as shown in this image:
https://terminals-wiki.org/wiki/images/b/bb/DEC_VXT2000_111543249570-2.jpg
On Tue, Jun 9, 2020 at 9:04 AM Bill Gunshannon
via cctalk
<cctalk at classiccmp.org> wrote:
Thank you for that. Mine has no labels or anything. It is PC Board
with Digital, Side 1, Side 2 and a number I don't recognize on it.
I guess the -9V is the only odd one but you could easily get that
from a -12V line. I may get to fix it yet.
bill
I have a VXT-2000. It is broke. If I could find out what the voltage
and current requirements were and their positions on the power connector
I would replace it in a heartbeat.
For the VXT-2000 with an H7109-B the rated voltage and current values
are printed right on the power supply label:
+5.1V, 7.81A
+12.1V, 0.62A
-12.1V, 0.46A
-9V, 0.2A
Fixed width character pinout diagram:
+=================+
-9V | Yellow | Orange | +12.1V
+--------+--------+
??? | White | Black | Gnd
+--------+--------+
+5.1V | Red | Blue | -12.1V
+--------+--------+
+5.1V | Red | Black | Gnd
+--------+--------+
Gnd | Black | Black | Gnd
+=================+
The mystery is the White wire. The power supply label only lists 4
output voltages. The White wire appears to be routed to the Ethernet
daughter board. The measured voltage appears that it might be floating
slightly negative, somewhere around -1.5V when the Ethernet daughter
board is installed and around -5V when it is removed. Maybe it is a
high impedance earth ground connection? It appears to be connected to
the shield of the Ethernet BNC, which measures around 1M-Ohm to the
chassis ground when the power supply is disconnected from the main
board, and around 0.75M-Ohm when the power supply is connected.
12 Jun
12 Jun
7:02 a.m.
On 6/9/20 7:13 PM, Glen Slick via cctalk wrote:
On Tue, Jun 9, 2020 at 3:10 PM Bill Gunshannon via
cctalk
<cctalk at classiccmp.org> wrote:
Sorry for the delay. Since the lockdown started Verizon DSL has
been a total disaster and I frequently go for days without any
connectivity.
As for the subject under discussion. You are right mine did have
a metal cover over it with the voltages, (but not the pinouts!)
It's been so long since I tore it apart to fix that I had forgotten
and had to dig the box out that has everything but the PSU Circuit
Board in it to refresh my memory. Things having changed so much
since then I am going to take another shot at fixing the original
PSU before replacing it with something non-DEC. But, based on the
voltages used, building a replacement from scratch would be
relatively trivial.
bill
On 6/9/20 2:51 PM, Glen Slick via cctalk wrote:
(I was going to change the subject line for this reply to reflect the
VXT-2000 discussion but maybe it is already too late to do that
without making message threading even worse?)
Does your VXT-2000 look significantly different than the images on
this page? Is the power supply in your VXT-2000 a bare board that is
not fully enclosed?
https://terminals-wiki.org/wiki/index.php/DEC_VXT2000
The power supply in my VXT-2000 matches the fully enclosed H7109-B
power supply as shown in this image:
https://terminals-wiki.org/wiki/images/b/bb/DEC_VXT2000_111543249570-2.jpg
On Tue, Jun 9, 2020 at 9:04 AM Bill Gunshannon
via cctalk
<cctalk at classiccmp.org> wrote:
Thank you for that. Mine has no labels or anything. It is PC Board
with Digital, Side 1, Side 2 and a number I don't recognize on it.
I guess the -9V is the only odd one but you could easily get that
from a -12V line. I may get to fix it yet.
bill
I have a VXT-2000. It is broke. If I could find out what the voltage
and current requirements were and their positions on the power connector
I would replace it in a heartbeat.
For the VXT-2000 with an H7109-B the rated voltage and current values
are printed right on the power supply label:
+5.1V, 7.81A
+12.1V, 0.62A
-12.1V, 0.46A
-9V, 0.2A
Fixed width character pinout diagram:
+=================+
-9V | Yellow | Orange | +12.1V
+--------+--------+
??? | White | Black | Gnd
+--------+--------+
+5.1V | Red | Blue | -12.1V
+--------+--------+
+5.1V | Red | Black | Gnd
+--------+--------+
Gnd | Black | Black | Gnd
+=================+
The mystery is the White wire. The power supply label only lists 4
output voltages. The White wire appears to be routed to the Ethernet
daughter board. The measured voltage appears that it might be floating
slightly negative, somewhere around -1.5V when the Ethernet daughter
board is installed and around -5V when it is removed. Maybe it is a
high impedance earth ground connection? It appears to be connected to
the shield of the Ethernet BNC, which measures around 1M-Ohm to the
chassis ground when the power supply is disconnected from the main
board, and around 0.75M-Ohm when the power supply is connected.
9 Jun
9 Jun
4:57 p.m.
New subject: Was: PDP-8/A transformer hum Is: VXT-2000 stuff.
On 6/9/2020 11:51 AM, Glen Slick via cctalk wrote:
On Tue, Jun 9, 2020 at 9:04 AM Bill Gunshannon via
cctalk
<cctalk at classiccmp.org> wrote:
For the VXT-2000 with an H7109-B the rated voltage and current values
are printed right on the power supply label:
+5.1V, 7.81A
+12.1V, 0.62A
-12.1V, 0.46A
-9V, 0.2A
Fixed width character pinout diagram:
+=================+
-9V | Yellow | Orange | +12.1V
+--------+--------+
??? | White | Black | Gnd
+--------+--------+
+5.1V | Red | Blue | -12.1V
+--------+--------+
+5.1V | Red | Black | Gnd
+--------+--------+
Gnd | Black | Black | Gnd
+=================+
The mystery is the White wire. The power supply label only lists 4
output voltages. The White wire appears to be routed to the Ethernet
daughter board. The measured voltage appears that it might be floating
slightly negative, somewhere around -1.5V when the Ethernet daughter
board is installed and around -5V when it is removed. Maybe it is a
high impedance earth ground connection? It appears to be connected to
the shield of the Ethernet BNC, which measures around 1M-Ohm to the
chassis ground when the power supply is disconnected from the main
board, and around 0.75M-Ohm when the power supply is connected.
As long as we are talking VXT-2000, I have this on file, figure it would
be useful for future googling of the list if
this were here as well.? May be a repeat of info from an earlier post,
or maybe elsewhere.
I didn't save the source of the info, but looks like someone named Matt
Millman did it.
The setup is as follows: HP Envy laptop running MOP boot daemon,
connected to Lantronix LTX-C twisted pair
ethernet to AUI converter (not the same as a twisted pair MAU). This is
then connected to a cabletron coax
MAU with a 10BASE5 vampire tap. The coax runs over to another MAU, which
is connected via an AUI cable to
my VXT2000. I didn't think of it, but it would have been nice to have a
real DEC MAU on the VXT2000, I've
got several DEC h4000's I could have used.
https://www.youtube.com/watch?v=A5T2GlAN2N4
http://tech.mattmillman.com/projects/10base5/
7:35 a.m.
On Jun 9, 2020, at 10:27 AM, Anders Nelson via cctalk
<cctalk at classiccmp.org> wrote:
Related question: Is there any reason, other than historical accuracy and
cost, to not replace the power supplies in vintage computers with modern
switching power supplies?
--
Anders Nelson
RFI? Old computers are not well shielded, and putting a switcher into them might well
create significant radio interference. It may be ok if the power supply is a well
shielded subsystem.
Also watch out for minimum load. Classic power supplies don't mind being unloaded;
switching supplies often need a minimum load current to keep them healthy. It's
probably ok but it is a new consideration.
paul
7:45 a.m.
On 06/09/2020 09:27 AM, Anders Nelson via cctalk wrote:
Related question: Is there any reason, other than
historical accuracy and
cost, to not replace the power supplies in vintage computers with modern
switching power supplies?
The old discrete transistor machines often used fairly odd
voltages, not like 5V and 12V.
Jon
10:03 a.m.
On Tue, Jun 9, 2020 at 10:46 AM Jon Elson via cctalk
<cctalk at classiccmp.org> wrote:
On 06/09/2020 09:27 AM, Anders Nelson via cctalk
wrote:
DEC's transistorized machines used +10V and -15V supply rails for the
logic, and some other large voltage (+28V?) for the core.
Later machines still used odd voltages. Lamp-based PDP-8 front panels
used voltages like +8V, and later core was IIRC +20V.
There are a couple versions of the PDP-8/a PSU, for core memory and
for MOS memory. MOS memory is standard 1970s triple-voltage stuff
(4096s and 4116s) but they pull a lot of +12V compared to later
machines.
It's often easier to just fix the old PSUs than replace them. At
least linear supplies aren't so hard to debug.
-ethan
Related question: Is there any reason, other than
historical accuracy and
cost, to not replace the power supplies in vintage computers with modern
switching power supplies?
The old discrete transistor machines often used fairly odd
voltages, not like 5V and 12V.
11:19 a.m.
Interesting, good to know!
--
Anders Nelson
On Tue, Jun 9, 2020 at 1:04 PM Ethan Dicks via cctalk <cctalk at classiccmp.org>
wrote:
On Tue, Jun 9, 2020 at 10:46 AM Jon Elson via cctalk
<cctalk at classiccmp.org> wrote:
DEC's transistorized machines used +10V and -15V supply rails for the
logic, and some other large voltage (+28V?) for the core.
Later machines still used odd voltages. Lamp-based PDP-8 front panels
used voltages like +8V, and later core was IIRC +20V.
There are a couple versions of the PDP-8/a PSU, for core memory and
for MOS memory. MOS memory is standard 1970s triple-voltage stuff
(4096s and 4116s) but they pull a lot of +12V compared to later
machines.
It's often easier to just fix the old PSUs than replace them. At
least linear supplies aren't so hard to debug.
-ethan
On 06/09/2020 09:27 AM, Anders Nelson via cctalk
wrote:
> Related question: Is there any reason, other than historical accuracy
and
> cost, to not replace the power supplies in
vintage computers with
modern
switching
power supplies?
The old discrete transistor machines often used fairly odd
voltages, not like 5V and 12V. 
11:54 a.m.
Den tis 9 juni 2020 kl 19:04 skrev Ethan Dicks via cctalk <
cctalk at classiccmp.org>:
On Tue, Jun 9, 2020 at 10:46 AM Jon Elson via cctalk
<cctalk at classiccmp.org> wrote:
DEC's transistorized machines used +10V and -15V supply rails for the
logic, and some other large voltage (+28V?) for the core.
Later machines still used odd voltages. Lamp-based PDP-8 front panels
used voltages like +8V, and later core was IIRC +20V.
There are a couple versions of the PDP-8/a PSU, for core memory and
for MOS memory. MOS memory is standard 1970s triple-voltage stuff
(4096s and 4116s) but they pull a lot of +12V compared to later
machines.
It's often easier to just fix the old PSUs than replace them. At
least linear supplies aren't so hard to debug.
The MOS PDP-8/a PSU provide +5V, +15 and -15V. So no 12 V there.
The Core PDP-8/a has -5V and +20V for the core memory.
To be able to test and debug Omnibus boards in my lab (aka garage) I
converted a MOS 8/a box into switched PSUs. Managed to squeeze five single
output PSus in the box to be able to run both 8/e core memory (which
require quite a lot -15V) and also newer 8/a core memory in it.
It is so much less heavy to move around on the lab bench.
https://i.imgur.com/3DW9ukm.jpg
/Mattis
On 06/09/2020 09:27 AM, Anders Nelson via cctalk
wrote:
> Related question: Is there any reason, other than historical accuracy
and
> cost, to not replace the power supplies in
vintage computers with
modern
switching
power supplies?
The old discrete transistor machines often used fairly odd
voltages, not like 5V and 12V. 
-ethan
7:44 a.m.
On 06/09/2020 01:59 AM, Josh Dersch via cctalk wrote:
Hi all --
I'm working on a PDP-8/A I picked up at VCF West last summer. After a lot
of cleaning and some power supply repair it's showing signs of life (CPU
seems to be at least minimally functional, core memory is going to need
some debugging.)
The transformer in the power supply is humming quite loudly, however, and
I'm curious if this is normal for the 8/A (or is typical for an 8/A of this
vintage). I'm used to the supplies in various other DEC machines not being
exactly silent but this is a rather severe 60Hz buzz that you can clearly
hear over the fans when the machine is in operation.
The old PDP machines used ferroresonant transformers to
provide better voltage regulation.
These do hum a bit. The cause is that they run part of the
iron into the saturation region, and there is a lot of
leakage magnetic flux. It may not be the actual transformer
that is buzzing, but any piece of steel nearby. You might
put your hand on various pieces in the area and see if you
can find one that has developed a loose bolt or whatever.
Jon
2:10 p.m.
On Tue, Jun 9, 2020 at 7:44 AM Jon Elson <elson at pico-systems.com> wrote:
On 06/09/2020 01:59 AM, Josh Dersch via cctalk wrote:
Thanks Jon (and everyone else). Yes, this is a ferroresonant transformer
(w/matching motor-run cap), I'd just never heard one in a DEC machine that
was so very loud. I'll see if there's anything that needs to be screwed
down a bit more.
- Josh
Hi all --
I'm working on a PDP-8/A I picked up at VCF West last summer. After a
lot
of cleaning and some power supply repair it's
showing signs of life (CPU
seems to be at least minimally functional, core memory is going to need
some debugging.)
The transformer in the power supply is humming quite loudly, however, and
I'm curious if this is normal for the 8/A (or is typical for an 8/A of
this
vintage). I'm used to the supplies in
various other DEC machines not
being
exactly silent but this is a rather severe 60Hz
buzz that you can clearly
hear over the fans when the machine is in operation.
The old PDP machines used ferroresonant transformers to
provide better voltage regulation.
These do hum a bit. The cause is that they run part of the
iron into the saturation region, and there is a lot of
leakage magnetic flux. It may not be the actual transformer
that is buzzing, but any piece of steel nearby. You might
put your hand on various pieces in the area and see if you
can find one that has developed a loose bolt or whatever.
Jon
2:36 p.m.
On 6/9/20 2:10 PM, Josh Dersch via cctalk wrote:
On Tue, Jun 9, 2020 at 7:44 AM Jon Elson <elson at
pico-systems.com> wrote:
Thanks Jon (and everyone else). Yes, this is a
ferroresonant transformer
(w/matching motor-run cap), I'd just never heard one in a DEC machine that
was so very loud. I'll see if there's anything that needs to be screwed
down a bit more.
Well, humming isn't always bad--I'd like my transformers to be able to
hum "Coro a bocca chiusa" from Madame Butterfly... :)
Ferroresonant supplies do work, but make ugliness on the AC line.
Cheers,
Chuck
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