Yet another butchered 250

[goldswimmerb]

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!

From what I have read, Q503 being too low in HFE could cause the amp to loose ~some of~ it's "unity" gain configuration, which may or may not matter critically.
I know a guy who stuck a KSA992 in his for that location in a 250M and had no issues at all, thing has been running for 6 months now without issues.

Looks like MPSA65 would work as long as the 5pf cap is added in.

 

[tonebells]

The MPSA65 works in mine. The amp sounds good. I just can't get more than 7mv bias. I will need to take another look at this one soon. I have a spare set of V2 boards. Don't ask what I paid for them. I have been thinking of updating them to B.

Tony

 

[ocdecio]

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!

My measurement of a SPS1952 I pulled was hFE = 246 so it seems like a high hFE is not really critical here. Now MPSA65 is a different animal and may need a 5pf Miller capacitor.

Now what I am scratching my head is with Q801 (SPS1950) that is inside the epoxy block in the models with the germanium diode. That guy measured hFE of more than 500 on a good board. MJE180 is around 120 and my theory is that it would only work with the newer models with a MPS6519 transistor (no diode).

 

[ocdecio]

The MPSA65 works in mine. The amp sounds good. I just can't get more than 7mv bias. I will need to take another look at this one soon. I have a spare set of V2 boards. Don't ask what I paid for them. I have been thinking of updating them to B.

Tony

I won't. This is a hobby for me, not a business

 

[ocdecio]

From what I have read, Q503 being too low in HFE could cause the amp to loose ~some of~ it's "unity" gain configuration, which may or may not matter critically.
I know a guy who stuck a KSA992 in his for that location in a 250M and had no issues at all, thing has been running for 6 months now without issues.

Looks like MPSA65 would work as long as the 5pf cap is added in.

Other than a darlington MPSA65, BC560C could work, it has hFE of around 500. Can't find it in the US though.

 

[ocdecio]

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!

BC557C has hFE above 600, just measured a couple.

 

[rickl]

Other than a darlington MPSA65, BC560C could work, it has hFE of around 500. Can't find it in the US though.

i checked my thread and I'm using a BC560C for Q503.

When I fixed my 250, i pulled and checked every transistor and replaced any bad ones. Most of them were bad. What gets difficult is some of the modern replacements used are now obsolete.

I think even the transistors in the protection were bad.

Threads like this help others find these older transistors with available replacements

 

[ocdecio]

@rickl that would make sense. BC560C is an old an reliable low noise / high gain transistor. I was able to find mostly all transistors I needed, but it takes some digging and willingness sometimes to buy some crazy minimum quantities. For example MPSA65 can still be found (it is the Q503 darlington used in some newer 240's) but I ended up with 400 of them - at 5 cents each but still.

The only ones that cannot be sourced are the SJ and SPS. These are lost in the agreements between Motorola and Marantz.

 

[Bvers1]

Here is some of the 250 information from AK on transistor replacements that I've gathered so far. The green's are the ones I bought so far. It's been a struggle to find everything. I didn't realize the pace of obsolescence. I still waiting for KSA1200's to become available. This hobby makes you work, hunt and think. Spend money too. There may be some mistakes so feel free to let me know.

 

[ocdecio]

@Bvers1 good list. I would just point out something I read a lot around here - try avoiding NTE as much as you can. Also - MPSA65 is a darlington with upwards of 100k hFE. I will be curious to see how well your 250 does with the MJE's - I couldn't make them work for my 240. BC's are great but their pinnage is reversed.

I agree - as a hobby this is great. It gives me a lot of appreciation for those that do this for a living, not easy by a mile. I probably spent more in parts and equipment than if I had bought what I have already restored. But then what's the fun in that?

 

[rickl]

@rickl BC560C is an old an reliable low noise / high gain transistor.

I think I had these in my spare parts bin for an old project

 

[goldswimmerb]

I'm debating whether it's worth swapping the drivers outright, or just pulling, testing, and if they're good, continuing to use them, since this one does have late enough drivers to be color coded.

 

[ocdecio]

I'm debating whether it's worth swapping the drivers outright, or just pulling, testing, and if they're good, continuing to use them, since this one does have late enough drivers to be color coded.

That's possible, but I would check their hFE and see if they fall inside the ranges the SM asks for:



If they are too far off you may need to swap them anyways.

 

[goldswimmerb]

That's possible, but I would check their hFE and see if they fall inside the ranges the SM asks for:

View attachment 2195879

If they are too far off you may need to swap them anyways.

That's what I figured, as for the whole pulling and testing.

 

[goldswimmerb]

Yeah, I wound up having to swap them... For now I was trying MJE parts since it's what I have and it worked on my 250M.
I also have 2N5416/3440 for predrivers, MJs as drivers, 2N3906/3904 for Q505/506
2N5961for the diff pair, BC547A for Q504, my Q503 that's in the unit is an MPS4250 for some reason.
DC offset, with new pot installed was showing .39v at minimum...

I was checking for a bit, and the bias voltages did not make sense, had -.8 on one and -1.8 on the other... Then after a bit the side with -.8 just dropped out...

Anyway, I don't think MJE 180 can replace that block style Transistor, at least not on its own...

Gonna have to search around.

 

[goldswimmerb]

Alright, so, the negative half of the bias is working, but the positive half is where the issues are. I'm not quite sure why, I decided to try moving the one bias block I do have over and no change at all, which leads me to the conclusion that the MJ is not the cause of there being no bias.

 

[goldswimmerb]

Another, interesting thing I noticed, is with my current parts in the DC offset pot affects my bias voltages.... Strange...

 

[goldswimmerb]

So just an update, I have about .56 to 6v on my positive predriver (this is the bias voltage) and -.7 to -.9v on my negative predriver, what's interesting is:
Both do adjust with the bias pot.
BUT the DC offset pot affects the bias voltages a good bit, maxed out DC offset seems to be about 66-80mv, and I can only adjust it up, as it rises the positive predriver collector voltage drops out, while the negative side approaches -2v. Really strange.

Gonna take a break while I wait for Transistors to arrive...

2N5961 does not seem to work for the diff pair as the gain is too low.

 

[ocdecio]

2N5961 does not seem to work for the diff pair as the gain is too low.

You can use BC550C with hFE of around 530 (twist the legs like in the photo) or go for 2N5089 which has hFE upwards of 700.

 

[goldswimmerb]

You can use BC550C with hFE of around 530 (twist the legs like in the photo) or go for 2N5089 which has hFE upwards of 700.

I'll probably try the 5089s. Still have to figure out why my bias is being dependant on the DC offset pot... Have checked my resistors and they're all testing okay.