Restoration of a PDP11/60

[klapperp]

The H7850 power supplies for our "MOS-Memory" and the "Battery Backup Unit" now also work as they should.

The test setup:


Without load at the output:


With load at the output:


The parts do have a terribly poor efficiency, but that was probably normal when these systems were built.

To be continued ...

 

[pbirkel]

The H7850 power supplies for our "MOS-Memory" and the "Battery Backup Unit" now also work as they should.

It's certainly convenient to have a desktop transformer to provide a proper AC feed to the power supply under test. What AC-input voltage are you using? Also, nice load module -- what manufacturer/model is that?

 

[klapperp]

It's certainly convenient to have a desktop transformer

The transformer is out of an old UPS and delivers 31V and the load comes from the well known chinese source

When you search for "adjustable load tester" you find lots of similar products - the one which @Konstantin uses is already a little bit older.

 

[Konstantin]

It's certainly convenient to have a desktop transformer to provide a proper AC feed to the power supply under test. What AC-input voltage are you using? Also, nice load module -- what manufacturer/model is that?

Hello,
I am using a 1KVA 2x24Vin to 220ish Vout UPS Transformer, reverse connecting the 220V side of it to my 1KW Current Limited Isolation Transformer & Variac. I Adjusted the Variac so that I get ca. 30Vac out of the Transformer. I tested down to 25Vac.

The Electronic Load Module is a 45 Euro Shipped cheap but very good 150Wmax, 60Vmax 10Amax from Ali-express, as Peter mentioned is some years old, but one of the very few lowest cost ones that does add very little noise. I use it for fast low power setups.
Unfortunately only short time later Fake ones appeared on Ali and Ebay that look almost exactly like the ones I have, but are useless....

Cheers,
Konstantin

 

[pbirkel]

The Electronic Load Module is a 45 Euro Shipped cheap but very good 150Wmax, 60Vmax 10Amax from Ali-express, as Peter mentioned is some years old, but one of the very few lowest cost ones that does add very little noise. I use it for fast low power setups.
Unfortunately only short time later Fake ones appeared on Ali and Ebay that look almost exactly like the ones I have, but are useless....

How do you distinguish "fake" from "real"? For example, which are these?

https://www.amazon.com/Vastarry-Electronic-Capacity-Monitoring-Constant/dp/B0B56SP7Y9
https://www.amazon.com/Adjustable-Battery-Capacity-Constant-Electronic/dp/B07V9W8SBD
https://www.amazon.com/Constant-Current-Electronic-Capacity-Electricity/dp/B0DM5GQF61

 

[klapperp]

The power supplies are now fully tested together with the transformers and wiring harnesses!

Base Power Supply BA11-P:


Expander Power Supply BA11-P:


All voltages are correct at the connectors, where the backplanes will then be connected!

Now it's time to re-install everything:


To be continued ...

 

[Konstantin]

Hello,
I am using a 1KVA 2x24Vin to 220ish Vout UPS Transformer, reverse connecting the 220V side of it to my 1KW Current Limited Isolation Transformer & Variac. I Adjusted the Variac so that I get ca. 30Vac out of the Transformer. I tested down to 25Vac.

The Electronic Load Module is a 45 Euro Shipped cheap but very good 150Wmax, 60Vmax 10Amax from Ali-express, as Peter mentioned is some years old, but one of the very few lowest cost ones that does add very little noise. I use it for fast low power setups.
Unfortunately only short time later Fake ones appeared on Ali and Ebay that look almost exactly like the ones I have, but are useless....

Cheers,
Konstantin

How do you distinguish "fake" from "real"? For example, which are these?

https://www.amazon.com/Vastarry-Electronic-Capacity-Monitoring-Constant/dp/B0B56SP7Y9
https://www.amazon.com/Adjustable-Battery-Capacity-Constant-Electronic/dp/B07V9W8SBD
https://www.amazon.com/Constant-Current-Electronic-Capacity-Electricity/dp/B0DM5GQF61

Hello,
with "real" and "fake" I was referring only to my Model, probably bcs of the good performance and build, were sold in larger quantities and therefore it made sense for them to fake them....(the fakes where bad, nothing near to the Original).
Unfortunately my older Model is not sold anymore, the current ones are more fancy, like the ones in the Links you posted, about to those I cannot say anything bcs I didn't used them. Attached a Picture of mine with the Display removed.
Actually I see only one in Ebay.com that from the Pictures, looks like the Original older Model.

Cheers,
Konstantin

 

[Konstantin]

@pbirkel :
At least this one : https://www.amazon.com/Adjustable-Battery-Capacity-Constant-Electronic/dp/B07V9W8SBD
does not score well at this review: https://www.sevarg.net/2018/06/24/the-atorch-purple-fan-mosfet-destroyer/
Here a Review of the model I have: https://www.sevarg.net/2018/06/10/a-tale-of-two-30-load-banks-60v-150w/

Cheers,
Konstantin

 

[klapperp]

The two BA11-P card cages were equipped with the tested cable harnesses:

The left one for CPU and MOS memory:


The right one for the expansion modules:


Thus, these parts are also ready for reinstallation:


To be continued ...

 

[MiaM]

View attachment 1321880

A tangent:
Interesting that it uses a push-pull setup similar to tube/valve circuits. I thought that that died out around the time when silicon replaced germanium, more or less?
I know that it's easier to make NPN than PNP transistors in silicon (and the opposite is true for germanium), but still?

 

[klapperp]

Interesting that it uses a push-pull setup similar to tube/valve circuits.

I think the design of the power regulators is much older than the 11/60 and at the time when they were designed this kind of "push-pull" setup probably was the usual way for this requirement.
I also heard from somebody who worked at DEC at that time that power supplies were always a "special topic" at DEC ...

 

[MiaM]

I think the design of the power regulators is much older than the 11/60 and at the time when they were designed this kind of "push-pull" setup probably was the usual way for this requirement.
I also heard from somebody who worked at DEC at that time that power supplies were always a "special topic" at DEC ...

That makes sense.
I would guess that switch mode power supplies are probably more or less universally a special case, perhaps?

 

[Konstantin]

A tangent:
Interesting that it uses a push-pull setup similar to tube/valve circuits. I thought that that died out around the time when silicon replaced germanium, more or less?
I know that it's easier to make NPN than PNP transistors in silicon (and the opposite is true for germanium), but still?

Hello,

Push Pull Designs are still widely used in Car Amp PSU's and in general in mid to High Power Low input Voltage PSU's.
(Push Pull as LV input DC-DC and Half-bridge in mains powered PSU's), another use of PushPull stages is in Power RF Amplifiers.
This PSU design with its only 14W is very inefficient, consisting of a Buck Converter as the PreRegulator, followed by a Push Pull stage which generates the +15 and -12 and the +5V. The +12V are generated using a Linear Regulator from the +15V.

Perhaps they didn't had a better way to generate the negative Rail....

Cheers,
Konstantin

 

[klapperp]

And already the first parts are moving back into place:

First, the two BA11-P card cages along with their wiring harnesses:


And then the two transformers along with the regulators:


To be continued ...

 

[MiaM]

Hello,

Push Pull Designs are still widely used in Car Amp PSU's and in general in mid to High Power Low input Voltage PSU's.
(Push Pull as LV input DC-DC and Half-bridge in mains powered PSU's), another use of PushPull stages is in Power RF Amplifiers.
This PSU design with its only 14W is very inefficient, consisting of a Buck Converter as the PreRegulator, followed by a Push Pull stage which generates the +15 and -12 and the +5V. The +12V are generated using a Linear Regulator from the +15V.

Perhaps they didn't had a better way to generate the negative Rail....

Cheers,
Konstantin

True, having two NPN (or two identical FET) transistors driving a transformer is the common thing for larger car amps.
But I haven't seen a transformer driving the base (or gate) of the transistors since forever. Like I've seen that in 1960's germanium transistor circuits and possibly some of the cheapest leftover sludge consumer electronics from the 1970's.

 

[antiquekid3]

But I haven't seen a transformer driving the base (or gate) of the transistors since forever.

It's still used for driving IGBTs and MOSFETs, particularly in half bridge and H-bridge applications, just as it was used by DEC (as a half bridge).

https://www.analog.com/en/resources/analog-dialogue/articles/implementing-an-isolated-half-bridge-gate-driver.html

 

[klapperp]

An hour under power and one last adventure with a "Bathtub Capacitor"

After all the components were reinstalled, the power supply initially worked perfectly as it should:


After about an hour of test operation (still without backplanes and cards) there was a short pop, smoke, and one phase was gone!

The problem was quickly identified - because we wanted to give 'the little one' a chance, even though many others have a different opinion than we do:


Conclusion - from now on there is no more chance for old electrolytic capacitors - no matter how good they look

A solution has been provided and the next test will follow immediately...


Surprisingly, the PDP11/60 is loud, but compared to modern systems, it produces a quite pleasant sound!

 

[pbirkel]

After about an hour of test operation (still without backplanes and cards) there was a short pop, smoke, and one phase was gone!

The problem was quickly identified - because we wanted to give 'the little one' a chance, even though many others have a different opinion than we do:
View attachment 1322249View attachment 1322250

Well, yuk!

 

[pbirkel]

Surprisingly, the PDP11/60 is loud, but compared to modern systems, it produces a quite pleasant sound!

With all of the panels installed? It was intended for use in office spaces rather than in a raised-floor "white room" environment so I'd expect that noise control would have been important. DEC took special anti-ESD steps with the front panel interface to even protect against carpet-induced static discharge.

 

[klapperp]

With all of the panels installed?

At the moment we only installed the complete power distribution and PSUs including the empty card cages with their big fans on the bottom.
In total there are 11 fans spinning when you turn then power on, four on each transformer unit, one in the expansion cabinet and a big one on each card cage.
All together produce a pleasant sound, probably because of the size of the fans.