Author Topic: Project Board power supply (Read 12252 times)

Radio Tech · « **on:** March 17, 2013, 04:02:40 pm »

Project Board Dual verible power supply.
Has anyone ever heard of this old supply? I have searched the web over and came back with no results. I had this supply for over 20 years. And for the last 5 years it has been sitting in the storage. Was about to homebrew a bench supply and remembered this one. So pulled it from storage and it still works. Not great but I think it is worthy of a rebuild. Specially since I like old stuff.

It has independent or tracking for both supplies. The 5K voltage adjust pots are in need of replacement. If you turn the right hand pot all the way down it goes to full 20 volt output!. That would sure fix your little project up..

But the first thing is a bottle of Simple Green, man this thing is dirty. But should be a fun little rebuild.

qno · « **Reply #1 on:** March 17, 2013, 07:47:19 pm »

It looks like a hobby project.

Variable power supply is not a brand name.

Radio Tech · « **Reply #2 on:** March 17, 2013, 07:52:19 pm »

Quote from: qno on March 17, 2013, 07:47:19 pm

It looks like a hobby project.

Variable power supply is not a brand name.

You may be right. Although it does have "Project Board" printed on the front panel and the main board is silk screen, I really doen see it being home made.
But probably a kit that someone put together many years ago.

kcav8or · « **Reply #3 on:** January 28, 2015, 05:43:49 pm »

I know this is a very old thread, but it just so happens that I have this very power supply, and I'd also like to refurbish it. I built it from a kit I bought from Radio Shack around 1980. I do have one problem that I hope the original poster can help with. Years ago, something happened that burned out the three transistors on the right side of the board. At the time, teenage me removed the transistors and didn't write down the numbers of the components, and I can't find the original circuit diagram. But if you still have that supply, I'd be very grateful if you could tell me the numbers of those transistors. I don't know whether they are identical to the working left side, or complementary.

Radio Tech · « **Reply #4 on:** January 28, 2015, 08:34:19 pm »

Quote from: kcav8or on January 28, 2015, 05:43:49 pm

I know this is a very old thread, but it just so happens that I have this very power supply, and I'd also like to refurbish it. I built it from a kit I bought from Radio Shack around 1980. I do have one problem that I hope the original poster can help with. Years ago, something happened that burned out the three transistors on the right side of the board. At the time, teenage me removed the transistors and didn't write down the numbers of the components, and I can't find the original circuit diagram. But if you still have that supply, I'd be very grateful if you could tell me the numbers of those transistors. I don't know whether they are identical to the working left side, or complementary.

Well you are in luck!
Give me a day or two. I will post up some pics for you. I also found the origianl manual for this thing along with schematic. ( Which I filed away somewhere ).
This supply works fairly well.

kcav8or · « **Reply #5 on:** January 28, 2015, 09:31:47 pm »

Excellent! I'm guessing that the right side would have PNP equivalents of the left side, so where the 2N2222's are it would be 2N2907, and the 276-2020 power transistor's brother would be the 276-2027. Now I get to see if my guess is right without a puff of smoke.

Radio Tech · « **Reply #6 on:** January 28, 2015, 09:38:54 pm »

These things has old parts, still looking for my manual and part list.
This is what I see so far:
Q1 2026
Q2 A509
Q3 MPS???
Q4 D813
Q5 2N2222
Q6 2907

Things are a bit crusty. Somewhere over the years an explosion occurred in the supply.

kcav8or · « **Reply #7 on:** January 28, 2015, 10:04:31 pm »

Thanks! Looks like Q1, Q2 and Q6 are what I need to replace. Q2 and Q6 probably match, since their counterparts on the other side do. And of course I need to check all the resistor values on that side in case I made a mistake that resulted in them failing in the first place.

Radio Tech · « **Reply #8 on:** January 28, 2015, 10:15:56 pm »

Hey it happens, all in learning. And without failures there is nothing to learn.

Made a mistake above on Q8. Since the Q8 silkscreen was beside Q1 I wrote the wrong designator. Q8 is on the heat sink along with Q7.

kcav8or · « **Reply #9 on:** January 29, 2015, 02:50:10 am »

Somebody in another forum found the manual here, but looks like it's sold out.

http://cavlon.com/zcstore/index.php?main_page=product_info&products_id=13

kcav8or · « **Reply #10 on:** January 29, 2015, 05:25:15 am »

I'm not finding the 276-2026 (Q1) anywhere. What's a modern substitute PNP transistor?

bdivi · « **Reply #11 on:** January 29, 2015, 07:20:35 am »

http://www.theprojectasylum.com/electronicsprojects/radioshacktransistors/radioshacktransistors74.html

PNP TIP30, Power amplifier & high-speed switching, free air 2W, heat sinked to 30W TO-66
replaces: 2N4918, 2SA489, 2SB435, ECG153, HEP246, 276-2026

kcav8or · « **Reply #12 on:** January 30, 2015, 04:35:17 am »

Did you ever find the schematic for the power supply? Also, can you tell me the proper values for R41 and R17 (pictured below)? I'd like to replace the kludge I put in originally with the proper resistor, but I need to make sure I get the right values.

kcav8or · « **Reply #13 on:** January 30, 2015, 04:52:58 am »

Here's the detail of the two resistors I need values for.

Radio Tech · « **Reply #14 on:** January 30, 2015, 10:06:35 am »

Quote from: kcav8or on January 30, 2015, 04:35:17 am

Did you ever find the schematic for the power supply? Also, can you tell me the proper values for R41 and R17 (pictured below)? I'd like to replace the kludge I put in originally with the proper resistor, but I need to make sure I get the right values.

Honestly have not had time yet. My week end starts at 1pm today if all goes well. I have a bit of cleaning up in the shop so will be looking for that manual. I was not where I thought it was. I have the supply sitting on a bench with the cover off, soon as I get home today I will get those values for you.

Radio Tech · « **Reply #15 on:** January 30, 2015, 09:16:43 pm »

Both are 82 ohm.

kcav8or · « **Reply #16 on:** January 30, 2015, 10:15:10 pm »

Even if you can't find the manual, a couple of good resolution photos of the circuit board from a couple of angles would be a big help. I can almost read the resistors on the photo you posted earlier, but can't quite make them out.

Thanks!

kcav8or · « **Reply #17 on:** January 31, 2015, 04:13:08 am »

Thanks. Looks like our posts crossed.

kcav8or · « **Reply #18 on:** February 09, 2015, 04:57:37 am »

I finally got all the components installed. I started with the voltage pots all the way to zero for both sides. As soon as I moved the right-side pot off zero, the meter immediately pegged, and Q2 went up in a puff of smoke. That suggests to me that I most likely have either a short or a wiring mistake on that side. Would you concur?

ddrl46 · « **Reply #19 on:** February 09, 2015, 09:41:49 am »

I should have the service manual laying around somewhere, I'll have a look and upload it if I can find it.

There we go, scanned and uploaded: https://www.dropbox.com/s/qy34sb6bt6lgy5h/Tandy%20Project%20Board%20Dual%20Power%20Supply.pdf?dl=0

kcav8or · « **Reply #20 on:** February 09, 2015, 08:17:47 pm »

This is fantastic! I can't thank you enough. Now to find the problem(s).

jamesd168 · « **Reply #21 on:** February 11, 2015, 04:57:04 pm »

why would you want to refurbish this? you can get a brand new variable dc power supplies at very good price. less than your time's worth for sure. I got mine at www.mastechpowersupply.com for $169.95. it has two adjustable outputs 0-30V, and one fixed at 5V, and the two variable channels can be operated independent, series or parallel.

Commander · « **Reply #22 on:** September 12, 2017, 11:15:27 pm »

Any chance you could repost the manual? It would be greatly appreciated!

Riccardo Macri · « **Reply #23 on:** November 04, 2017, 11:05:30 am »

Just found this thread. I built this from a Radio Shack kit in 1979, when I started I was still in primary school. Ambitous kit and a very fragile design, an output short (eg: +18 with -18) could blow out the transistors and the opamps (as could ESD from an inverter circuit).
I still have it and it worked as of a few months ago. Its not in regular use.

My manual is literally in pieces and faded. I'd love scans particularly of the board layout and circuit (that is the most faded)! Can't see myself working on it much as I'm now mostly blind and those DPS3005 etc. modules blow it away. I keep it more for nostalgia.

I remember getting fed up with the price of the RS transistors and replacing them with cheaper and more robust ones I had in my junk box, and it survived the decades since. Can't remember which and opening it up will be a fragile operation given its age.

A debugging trick I remember was to power it with 2 9v batteries (non alkaline and probably half flat), each across the 2 main filter capacitors. A lot less than the 30+V the transformers delivered there when plugged in. This let me verify the voltage control and tracking basically worked, in retrospect its amazing it worked at all like this, as a kid I was just happy to get it together and it served me well.

Chris56000 · « **Reply #24 on:** November 07, 2017, 07:23:19 pm »

Hi!

I'd appreciate a copy reposting as well please if anyone still has it - I've just found one in my friend's front room!

Chris Williams


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Author Topic: Project Board power supply (Read 12252 times)

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