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Yet another butchered 250

Part of the service bulletin asks for replacing the bias trimpot from 1k to 250ohm 2W and add a 56Ohm resistor. That can be a tall order.
 
ocdecio said:
What transistor and diode are you using? I measured the original (working) transistor inside the epoxi and its hFE was very high - more than 500. The MJE180 seems only to work with Q521 ( MPS6519 ) versions (no diode).
Click to expand...

Most are original save for the differential pair, outputs and one I don't recall of the top of my head.

Tony
 
tonebells said:
Q503 SPS1952 (MPSA65) was the other.

Tony
Click to expand...
Were you able to confirm that SPS1952 is indeed a darlington? That's what I have in my 1200 and I was planning to use MPSA65 as a replacement if needed.

Also - does your version have a 5pf Miller compensation capacitor on it?
 
Well I decided to answer that myself. NOT a darlington.

upload_2021-4-8_20-54-48.png
upload_2021-4-8_20-55-11.png

Therefore no need for the 5pf capacitor (I think)
 
ocdecio said:
Were you able to confirm that SPS1952 is indeed a darlington? That's what I have in my 1200 and I was planning to use MPSA65 as a replacement if needed.

Also - does your version have a 5pf Miller compensation capacitor on it?
Click to expand...

You will need to excuse my ignorance. I am not sure if I know where the 5pƒ would be..

Tony
 
tonebells said:
You will need to excuse my ignorance. I am not sure if I know where the 5pƒ would be..

Tony
Click to expand...
We are all learning (well I am doing my best :)

upload_2021-4-8_21-0-29.png
Here's a description on a 240 Addendum:

upload_2021-4-8_21-2-3.png

I am assuming these amps share most of the power amp circuit.
 
@tonebells it is hard to keep up with the various versions. There seems to be different layouts for the 1200 non B and the 1200B is further different (more like the newer 240s) :

upload_2021-4-8_21-16-36.png

Here we have a MSPA65 and the 5pf cap.
 
So a V2 is an original V1 that went through the modifications listed in the Addendum for Model 1200, NOT a 1200B.

In any case - I am unsure if in this case SPS1952 can be replaced by the MPSA65 Darlington.
 
ocdecio said:
So a V2 is an original V1 that went through the modifications listed in the Addendum for Model 1200, NOT a 1200B.

In any case - I am unsure if in this case SPS1952 can be replaced by the MPSA65 Darlington.
Click to expand...

I know SPS1951 can be safely replaced with BC547A, looks like for SPS1952 people have used BC557 with success.
 
I've got three partially butchered 250's I'll be working on soon. Got a parts list and bought all the transistors that were available. Many went obsolete before I could send the order in. Q503 went obsolete fast and someone in AK suggested a BC557BG. Its hFE is only 400. The BC550B has a better HFE. My boards have the SPS1952.
Please let me know if BC550B works? I have a current BOM with recent available parts if you need it. I just ordered the Germanium Diodes from Small Bear and some colored #20 wire. Good Luck!
 

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Here was my parts reference for the 250M I do a little while back
 

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This image is of the 250Ms Circuit but can be pretty easily modified to apply to any revision if you know what you're doing.
 

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