Hey Fellows,

I'm fixing up an old Heathkit IP-27 power supply for a friend because it isn't working properly. At first It wasn't working at all, but when I took out the fuse and put it back in, it started working again (but not to its full potential). So It was a little dirty and needed some cleaning. Apparently, when you have a load you shouldn't get any voltage drop and the power supply will compensate for the loss voltage. So the voltage stays the same no matter the load. Unfortunately, this isn't happening and when I put a 100Ω resistor around 5V or so the voltage dropped significantly. My first guess is replacing the electrolytic caps since those don't last very long; but I might be wrong. How can I fix this? Here is the schematic because I couldn't find the whole manual online:

http://www.nostalgickitscentral.com/heath/schematics/heathkit_schema_ip27.gif

I'd appreciate the help!

 

You are a minor for several reasons of safety,you should not get advise on
what could be a deadly purpose. I think the guys should remind themselfes,let students select their reading material.Maybe an age requirement should be worded for proper use of site. refer to how old are you.

 

Loosewire,

Might I remind you that I am not an incompetant person who doesn't know anything about electronics. I have had several years of experience in this sort of thing and I know what I'm doing. In fact, I've even started building my own power supply! I appreciate your concern, but I have this under my belt. I just wanted suggestions on how to fix this power supply, end of story. Don't worry, I'll keep one hand in my pocket!

 

If you have an oscope, check ripple internally. A DVM does a lousy job with ripple (voice of experience), where a oscope will tell the tale.

Without a schematic it is impossible to suggest probe points. It looks like a simple design.

Don't assume it is the caps without checking them. It could easily be something else.

Replace the fuse with a new one. Wires can develop high resistance over time, so can fuses (and they are designed to be on the edge of failure all the time).

 

Hello,

Check the rotary switches.
Take special care of S2 the current limiting switch.

Greetings,
Bertus

 

They make contact cleaner that might help with that (rotary switch contacts). Comes in a spray can with a micro straw dispenser.

 

Thanks for the help, I'll look into it.

 

Okay folks, I cleaned it up and now the meter doesn't work! Anyway, I emailed my friend and he said he thinks the heart of the problem is in the voltage regulators. Maybe some transistors need replacing? How the heck am I going to find them? I could put them on my DMM and measure the hFE off of them. Let me know what you think!

 

Troubleshooting transistors is pretty easy, if you approach it logically. Look for the 0.7V BE drop, if it isn't there the transistor should not be conducting. If the transistor is conducting and the BE drop isn't there then doublecheck for other paths, then pull the transistor. Mostly look for alternatives before commiting to a probable problem. This power supply is rather simple overall, so it shouldn't be that much trouble fixing it.

Look at the AAC chapter on meters. Meters tend to be somewhere between a voltmeter and ampmeter. What defines their use in the end is where the resistor is put. If it is in series then it is an ampmeter, if in parallel it is a voltmeter.

 

Troubleshooting transistors is pretty easy, if you approach it logically. Look for the 0.7V BE drop, if it isn't there the transistor should not be conducting. If the transistor is conducting and the BE drop isn't there then doublecheck for other paths, then pull the transistor. Mostly look for alternatives before commiting to a probable problem. This power supply is rather simple overall, so it shouldn't be that much trouble fixing it.

Look at the AAC chapter on meters. Meters tend to be somewhere between a voltmeter and ampmeter. What defines their use in the end is where the resistor is put. If it is in series then it is an ampmeter, if in parallel it is a voltmeter.

Thanks for the help Bill, I'll check those transistors pronto!

 

Ok, I found out that one of the transistors, in the first picture and the right bottom corner, seems to be bad. The one above that had a 0.4V drop and the one below measured 12V. Does anyone know where I can find a substitute for the R256A transistor? I doubt I could ever find one... Also, If anyone could find the specs for that transistor I'd appreciate it!

 

Figure out the power specs (wattage and current), make sure the voltage is adequate, then try whatever you can get your hands on that meets the minimum specs. This design isn't too sophisticated, which is to say, almost anything that can handle the load will work.

 

Hello,

Here is a PDF with the data for the 2N2553 (the transistor in the schematic).

Greetings,
Bertus

 

Changing the electrolyte caps for an old unit like this is always a god idea. Have you also gone through the test-points marked in the schematics. As an example the voltage at the base Q2 should be 5.9 volt (in respect to ground)

 

Yikes! Germanium!

There is a chance that a silicon transistor will work, but that schematic will need some careful review first. I didn't realize it was that old.

 

Yeah, it's pretty old. Thanks for the transistor suggestion Bertus! I'll look into that.

 

Hello,

I also found the data for the 2N2869.

Greetings,
Bertus

 

Thanks! I forgot, what should be Vbe for a gernamium transistor?

 

hmm around 0.2 to 0.3 volt if I do not remember wrong. And it could well be I am wrong. If I was you I would have consentrated on the test-points. That will help you to narrow down the error. It is lots of test point marked in your schematics

 

Hello,

The Ube is indeed much lower as with silicium.
Here is an example of the Ube vs. Ib characteristics for Ge and Si.

This image comes from this page:
http://www9.dw-world.de/rtc/infotheque/semiconamps/semiconductor_amps1.html

The image can be found in chapter 1.2.2

Greetings,
Bertus