Working on a 2250B power supply, have a question

[33713]

Just an update, I retested with DMM all the transistors again and will put back in before the weekend probably. While I was looking around the boards i noticed an emitter resistor that looked odd to me. This is on the bad board. 1 of the 4. It is measuring .1 out of circuit. Wondering if this is a clue? I poked around and measured a bunch of stuff from left side to right side. Just to see if anything jumped out, but it didn't. I was wondering though, some of these diodes on the bad board are reading in both directions. Not sure if that is normal?

So I think I will rebuild the bad board, take those measurements you mentioned in post 78.

After that, I really think I want to know if the other channel will work and I could disconnect both and probe the whole bottom of the board with easy access and see where this bad one starts to show the problems, but I am not sure if that would even help.

Open to any ideas. Thanks @dankik

 

[dankik]

It is measuring .1 out of circuit.

With that low of a resistance, you either need a good meter or you need to make two measurements.

1. First connect the two test leads together and measure the resistance. If it's not 0, note what it is.
2. Next, insert your resister to measure and note the value (in your case it was 0.1 Ohm).
3. You will need to subtract the first measurement from the 2nd to get the true resistance because 0 is not 0.
4. If it still measures 0.1, then get a new resistor

You never answered the question that I had on post #78 and #80.
You can measure continuity on the board with power off checking the resistance between the two points.

 

[33713]

I am back, was starting to put some capacitors in tonight, and noticed some more damage I think! See the heat that the 150 ohm R736 has put on the R719 Trimmer, and the disintegrated R713?

 

[dankik]

Good find. That component where R713 is looks more like a diode than a resistor. Can you tell if it is a diode?
At a minimum, those two components need to be replaced. Maybe the VR also, but we can test it with an Ohm meter.

• On the schematic, R713 is shown as a 1k Ohm resistor.
• Check the other P700 board to see what is installed there.
• The burnt resistor, R736 should be a 470 Ohm resistor.
• R737 on the schematic is shown as a 0 Ohm resistor (wire) but you have something installed there. Check the other board to see if they are the same.

 

[33713]

Good find. That component where R713 is looks more like a diode than a resistor. Can you tell if it is a diode?
At a minimum, those two components need to be replaced. Maybe the VR also, but we can test it with an Ohm meter.

• On the schematic, R713 is shown as a 1k Ohm resistor.

R713 WAS a resistor. the other board confirms that value if I remember right. The body of it is just gone and I have the other leg removed, see picture for the other side below and more info

• The burnt resistor, R736 should be a 470 Ohm resistor.

Looking to see if I have a 470 around, if not I will reorder and replace on both.... I just measured both of these and they both measure 150 ohms and both have been hot. See pictures? Both the parts list and redrawn schematic list it as 470, maybe I need to test out of circuit?

• R737 on the schematic is shown as a 0 Ohm resistor (wire) but you have something installed there. Check the other board to see if they are the same.

It is just a jumper wire with a ceramic cover on it.

RE Emmiters:
OK, so I did try to calibrate the ohmmeter setting, but it is just not accurate enough than to know that all for test .1ohms. I would like to replace them, but do not have any on hand it appears, and it also appears to be the best way to be sure they are within spec. I do have some .33, and I think emitter resistors usually range up to about .5 ohms. Not sure if a sub like that would be ok if I did it to both sides and properly adjusted bias.

In the 2 attached pictures, I finally see a previous repair I believe.

I think that on the "good" board these have been changed

C702 non matching cap
R713, it has no standoffs like the rest of the board
H701 again, no lead protection like the other side. This makes me want to check out all diodes, especially this one.
An area of solder with non cleaned up flux.

BTW, this project has been moved out of my office area and back into the garage where its over 100 degrees everyday. Hence some of the delays in reporting.

That's it for tonight. Thx!

 

[33713]

Back to work on this thing finally. I am replacing

All Diodes and Varistors
All transistors
The Smoked 1K 1/2 watt resistor
All electrolytics

I also found out that out of circuit that suspect diode at H701 tests as faulty in my component tester.

 

[rBuckner]

On those emitter resistors you really need to use the correct parts. Different designs use different values. By adjusting the bias to the same VOLTAGE across them, it is now running at a different CURRENT than it should. Dangerous!!

 

[33713]

On those emitter resistors you really need to use the correct parts. Different designs use different values. By adjusting the bias to the same VOLTAGE across them, it is now running at a different CURRENT than it should. Dangerous!!

Ok, you mean where I mentioned using a different value?

i ended up putting the originals back in and here is why. I hope my logic works here. I do not have a tester accurate enough to measure such a low value. So I had a couple .22 ohms around and measured them in my component tester. They measure .66 which I assume is erroneous. But my original .2’s both measure .64 in the same tester, so I think they are still good and comparable to brand new .22 ohm ceramics.

Does this make sense? Thx

 

[rBuckner]

Ok, you mean where I mentioned using a different value?

i ended up putting the originals back in and here is why. I hope my logic works here. I do not have a tester accurate enough to measure such a low value. So I had a couple .22 ohms around and measured them in my component tester. They measure .66 which I assume is erroneous. But my original .2’s both measure .64 in the same tester, so I think they are still good and comparable to brand new .22 ohm ceramics.

Does this make sense? Thx

Yes, you mentioned a different value. This is what I was referring to.

I agree that your originals are okay. These guys usually are either good or open circuit, but usually no burn marks from this. I've seen them crack apart though!

 

[33713]

Wow, good night tonight. Finished methodically populating the board that was bad. with all new caps, diodes, and transistors, along with the replacement 1K and replacement smoked zeener. I decided to try the original output transistors for dim bulb test, although I do wonder if I should just use the new ones I bought and fired it up. I was nervous, but after a couple seconds I heard the relay click on I became hopeful. Checked for DC at the speaker terminals, have some, but very low. Hooked up speakers and a source, nothing. Then I tried FM. Static. Nice. Ahh, well I hooked up to speaker B for test, but it was not engaged!

As soon as I pressed it AUX came alive. So, looks like its time to continue this restoration as I have AUX/FM/AM all working at the moment.

THANKS EVERYBODY!! I AM PUMPED.

 

[rBuckner]

Awesome! Make sure to set the bias next, especially since transistors were replaced.

 

[33713]

Awesome! Make sure to set the bias next, especially since transistors were replaced.

Will do, started the other amp board this am. Going good so far. Question though. My parts kit has a 3.3uf bipolar replacement that goes in the C701 position, if I recall correctly. I already know it can be bipolar, but I was wondering how important it was to use a small film in this postion? I do not seem to have any around at the moment, but can obtain some. Thanks!