I picked up a Kunkin KP184 this last year, and it does nicely for testing those brick regulators and the power control boards in isolation (a bit more convenient than the old light bulb or power resistor approach.) Some of the large electrolytics in those bricks will likely have dried out. I disassemble and ?reform" these with a bench supply and current limiting resistor on any of these bricks I have not yet personally serviced (Yes, I know the reforming thing is controversial for some. I think its safe to say it at least doesn?t hurt anything :-) Check the big bridge rectifiers carefully; I?ve seen a few of those cracked or toasted. It?s also been my experience that beyond the bricks that are dead or fail at the start, some smaller fraction will also let go after the first few hours of continuous operation. The don?t tend to fail with fireworks ? you?ll generally get a slowly increasing whiff of magic smoke so generally plenty of time to shut things down. The operation indicator bulbs on these seem almost always dead. You can sub CM7381 for the +5V regulators, and OL-6003BP for the -15V. Last, if you have H742 supplies on your 11/45, be sure to check the small muffin fan above the power control board. It is quite common for parts of the internal wiring harness to rub up against these and stall them out, eventually leading to a fried controller board. Good luck! Good winter time project :-) ?FritzM.

Thanks for the tips. The transformer that drives the bricks is a real beast. Did you find an alternative more convenient way to power them on the bench? I don't see the Kunkin unit for sale except for a couple of items on ebay from China. I am really reluctant to buy anything at all from China. I couldn't find a manufacturer site for them, and I was wondering if there are alternatives since they don't seem to be particularly available. I did find this which is a bit more expensive (I know Rigol is Chinese, but it seems to be decent quality kit) https://www.rigol-uk.co.uk/product/rigol-dl3021-programmable-dc-electronic-… Thanks Rob

I have a variac rated at 2.5A. I am not quite sure whether that would be sufficient to test the bricks individually. Otherwise, I guess one of these would do the trick https://cpc.farnell.com/block/steu250-48/transformer-250va-230-400v-2-x/dp/… ?

So, to supply the bricks on the bench, would a variac rated at 2.5A be OK? I am not sure I know how much current the bricks will draw at 20VAC, and at what voltage the 2.5A rating is given. Otherwise, would this do the trick? https://cpc.farnell.com/block/steu250-48/transformer-250va-230-400v-2-x/dp/… Thanks Rob

"As a general rule, a variable transformer (Variac) can provide full rated current at any output voltage. So a 2.5A unit can provide 2.5 A at 1V, 10V, 120V, etc. With a 20V output, that is 50 VA (Watts, sort of)." Up to the current rating of the variac.? When you draw more current than the transformer can deliver then the voltage will sag. On 12/8/2021 4:22 PM, Will Cooke via cctalk wrote:

Seems to me that the actual coil resistance will limit the max-current at lower voltages. 20 A through wire sized for 1 A seems ... unlikely? -----Original Message----- From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Mike Katz via cctalk Sent: Wednesday, December 8, 2021 5:58 PM To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>; wrcooke at wrcooke.net Subject: Re: PDP-11/70 Boards More accurately up to it's rated wattage "Power = Voltage * Current" after all. If you have a 100W max variac you can draw 20A @ 5V (approx) but only 1A at 100V. On 12/8/2021 4:44 PM, Mike Katz via cctalk wrote:

That would depend on the type and size of wire (solid core, stranded, strands of stranded, etc.). On 12/8/2021 5:06 PM, pbirkel at gmail.com wrote:

My previous response was a bit misleading and not completely accurate. Sorry. I was trying to rush out the door. Variacs are rated for max current. In the stated case, 2.5 amps. They can provide that current at any output voltage. So, the maximum wattage (actually volts-amps, which is a bit different) will be 2.5 amps * highest output voltage. (Some are possibly rated at for that current when the output voltage does not exceed the input. They can usually go higher than the input by 20% or so) They can provide that same current at any voltage UP TO that max output voltage. In theory, they could provide higher current at lower output voltages. The magnetic flux is proportional to current times turns (of wire on the core.) A lower output voltage implies fewer turns. BUT, the rest of the transformer is not designed for that higher current. So it is a bad idea to try to pull more than the rated current. However, pulling that rated current at any output voltage up to the input voltage is fine. You can get a pretty good idea how much power a variac (or any transformer) can handle by its weight and size. The 3 amp variable transformer I have on my bench weighs about 25 pounds. It is a 120V input so that is about 360 volt-amps. One that can handle 600 volt-amps will weight around twice as much (the relationship isn't linear, though.) Why do I keep saying volt-amps instead of Watts? Watts are actual power. Volt-amps are apparent power. If there are capacitors or inductors in the circuit the current and voltage will be out of phase. That phase change means the current and voltage won't peak at the same time and therefore the actual amount of power used will be less than it "appears" by multiplying volts * amps. But the core and winding of the tranformer will still have to handle the apparent power, even though the actual power used is less. The actual power used is the apparent power times the cosine of the angular difference in phase. I'm not familiar with the DEC power supplies, but it is almost certain they have capacitor input power supplies. That means a phase change. That means, then, that your input supply will need to provide more apparent power than actual power used. A rough approximation is to double the input from what the supply outputs, assuming reasonable efficient supplies. So if one of those DEC supplies provides, say, 5 volts at 10 amps, that is 50 watts output. I would start with a variac (or whatever) that can supply 100 volt-amps at the input voltage. So, for 20V input it would need 5 amps. Will

Hello Tony, Are you saying that I can test them with no load? I thought they would need some load?

Yes, just about any programmable DC load in this class will suffice. It?s just convenient to be able to punch in current or power limits, dial them up and down, and have everything including fans, thermal management, etc. in one compact package on the bench. At the time I was looking, the Kunkin, while yes, from China, had a lot hands-on reviews on YouTube. So I just went ahead with that. Worked out okay for me, but one kind of annoying thing is the banana jacks on the five-way binding posts don?t seem quite standard (!) ? I will probably change out the posts on mine.