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On Aug 14, 2019, at 6:22 PM, daven9ooq via Groups.Io <daven9ooq@...> wrote:

Hi everyone: This is my first post since the group changed to IO.

I been in need of a good Dual Voltage variable power supply for my bench, I been looking around on line for awhile when I came across a Tektronix PS 280, up to that point all that I was seeing is cheap Chineese PS , I already own lots of Tek TM 500 equipment , So I thought the Tek PS280 would fit right in.

The PS280 is around late 70's early 80's vintage It was advertised as working on ebay, I but I had my duoghts, I felt I could probably fix it , having built many of my own, I looked up the schematic and manual in advance, I found it on W140.com Tekwiki, I found all the semiconductors were still available ,even the Intersil meter chips.

For the money , you couldn't even build one for the hundred bucks + shipping!

Although it has the Tek name, it doesn't look like a Tek products that I'm used to seeing, Was after Tek sold out you can tell it was built with a budget in mind, but is of fairly good quality, the two varabile supplys are 0-30volts @2amps they can be run independently or in series or
parallel, Neither of the variable sides worked properly, there was some low voltage but nowhere near the range it should have and not a smooth adjustment but very choppy. I ordered the electrolytics and replaced all the caps except for the 5 volt supply that was still working havent checked the ripple yet.

Since the board was difficult to remove and I'm working one handed due to a stroke also wanted something I could depend on well into the future.

After the recap things were a lot better voltage wise but still not working right to my surprise.

The boards looked pristine , no one had their fingers in before me I did find a cold solder joint on one of the 470uf electrolytics , the boards were waved soldered.

Upon visual inspection nothing else looked bad ,burnt or smelled bad,
I have not been able to find any obvious bad components .
There has been many revisions and the schematic is somewhat correct on the general circuitry but not the leadgonds for the connectors, there are many interconnects and wire jumpers on the board but the numbers on the connections don't jive, which makes me think I don't have the correct schematic for the revision.

Looking at Tek wiki the one that I have looks like the latest version.
The serial number is TW 50125 the main board is marked
GPC- 848A-1 B

The supply looks like it was farmed out and built under many Brands
But has a legit looking Tek Serial number tag.

This power supply is unusual to me having built many of my own over the years, it is dual tracking and constant voltage and current that uses a bunch of Asian transistor logic and 4 relays, but uses none of the usual regulator designs like LM317 or LM723 but it does have some of the usual regulators but used only for reference, it starts with a 7815 as a reference that goes into LM741 op amps and LM301 then another 741 to a driver transistor then to the usual series pass transistor a 2N3055.

Most of the 7800 series regulators and op amps are bulit proof which makes me think the culprit probably has to be a cold solder joint or a bad zener, or transistor.

I only had one basic course of electronics in college the rest is self taught.
I'm not able to follow a signal through a maze of transistors and op amps, but I know enough to be dangerous!

I would really like to gain a better understanding of this power supply
And get it working of couse. Many thanks! in advance.
Dave N9OOQ