Hello,
but now, most of the voltages are in the right "ballpark", aren´t they?
If it comes to replacing the two diff-transistors, what type is preferred? Can malfunction in that "corner" of the design, result in too large a output offset?
Best regards//lasse
but now, most of the voltages are in the right "ballpark", aren´t they?
If it comes to replacing the two diff-transistors, what type is preferred? Can malfunction in that "corner" of the design, result in too large a output offset?
Best regards//lasse
You need to drive the amp to see if there are other issues.
The repair manual suggests 2N5087 for the diff pair, I would only use a major
brand such as OnSemi or Fairchild.
MPSA18 should be fine but both parts are discontinued.
A dual would be better since they are matched but most are surface mount.
The repair manual suggests 2N5087 for the diff pair, I would only use a major
brand such as OnSemi or Fairchild.
MPSA18 should be fine but both parts are discontinued.
A dual would be better since they are matched but most are surface mount.
2SC2240BL is an obvious choice but getting hard to find, don't buy on ebay.
KSC1845 is an excellent choice
Here is a thread discussing alternatives:
2sc2240 Equivalent
KSC1845 is an excellent choice
Here is a thread discussing alternatives:
2sc2240 Equivalent
Hi,
Yes, I will run the test. Will be interesting to see what come out of that. I Will probably record the outcome on photos taken with my cellphone.
Thanks for the advice on replacement transistors, IF it come to that.
Today, I will take a short rest from sound-related things, bakning christmas bread with my son. ( a Swedish speciality, I think, with saffron, cinnamon and marzipan in them)
Santa is coming soon!
Regards// Lasse
Some measurements...
Hi,
now for some measurements, taken with the obvious limitations of my equipment but to a part, perhaps this is compensated with the corresponding measurement for the "good channel"
I could come up to 72 volts P-P but there is the limit of the input voltage from my computer.
I worried about the DC-offset, displayed (-4,3V), but this is the same for both channels so this is probably a defect of my measuring devices.
Well, does this indicate that my amp is healthy up to a level?
There is still the strange voltage on one off the diff-transistors and the high offset voltage on the output.
regards//lasse
Hi,
now for some measurements, taken with the obvious limitations of my equipment but to a part, perhaps this is compensated with the corresponding measurement for the "good channel"
I could come up to 72 volts P-P but there is the limit of the input voltage from my computer.
I worried about the DC-offset, displayed (-4,3V), but this is the same for both channels so this is probably a defect of my measuring devices.
Well, does this indicate that my amp is healthy up to a level?
There is still the strange voltage on one off the diff-transistors and the high offset voltage on the output.
regards//lasse
Attachments
Possibly healthy, do you have another MM to measure the offset with? That kind of offset I would not plug into speakers, make sure it is measurement error first.
Good job BTW, 10 points for going out and "having a go", and another 10 points for not blowing anything up in the process, really good work.
Hi, thanks for the kind words....
This is my High-school amp so I have a very personal relationship with it. The 70ies made me into the person I am today and the music was a great part of it all.
Yes (see post 58) I have measured a output offset of abt 300 mV. I would never have come this far without the help and support from Pete (PB2) along the way.
best regards//lasse
Yeah it looks like PB2 is knowledgeable and inherently able to transfer that too, good work both of you.
I have one of these old Citation 12's too, which is showing it's age and I will be restoring it in due course.
Just to throw a curve ball, have you seen this: http://www.firstwatt.com/pdf/art_mos_citation.pdf
Yeah it looks like PB2 is knowledgeable and inherently able to transfer that too, good work both of you.
I have one of these old Citation 12's too, which is showing it's age and I will be restoring it in due course.
Just to throw a curve ball, have you seen this: http://www.firstwatt.com/pdf/art_mos_citation.pdf
Yes, I have read that article and Also a few words from people, who have made the modifications of the design. It seems like most are happy with the sound.
I find joy in bringing old stuff back to life, but if the resulting SQ is worse than the alternative, then......
Regards// Lasse
Great work!
Total Ie for both diff pair (36.5 - .69)/8200 = 4.37 mA
first diff estimated Ic = (35 - 34.2)/390 = 2.0 mA
The diff pair is splitting the Ie current reasonably well, this simple design never
has good balance.
Since the offset is negative, it suggests that either one or both of the diff pair are
bad, that the new transistor beta is too high, or that a driver or output transistor
is leaky.
Best guess is that the driver/outputs are okay since there was no upset in the
output stage.
If you are able to remove the diff pair without damaging them you could try a new
pair. I think that the dual was a fragile device since there are a lot of reports of
them failing noisy, high offset, etc. I'd guess that static discharge when plugging in
the input cable might be the reason.
B&D has many hard to get transistors but I don't know if they ship to your country.
Total Ie for both diff pair (36.5 - .69)/8200 = 4.37 mA
first diff estimated Ic = (35 - 34.2)/390 = 2.0 mA
The diff pair is splitting the Ie current reasonably well, this simple design never
has good balance.
Since the offset is negative, it suggests that either one or both of the diff pair are
bad, that the new transistor beta is too high, or that a driver or output transistor
is leaky.
Best guess is that the driver/outputs are okay since there was no upset in the
output stage.
If you are able to remove the diff pair without damaging them you could try a new
pair. I think that the dual was a fragile device since there are a lot of reports of
them failing noisy, high offset, etc. I'd guess that static discharge when plugging in
the input cable might be the reason.
B&D has many hard to get transistors but I don't know if they ship to your country.
Hi,
I think that it would be a good idea to test a new diff-pair but the only replacement variant mentioned by you or anyone else I´ve seen on the net , that is available in Sweden is the KSC1845FTA and not before the middle of February 2018.
Can you think of any other suitable replacement device apart from the ones mentioned? They seem to be discontinued.
By the way, it is a good thin to buy a few dozens and match them, isn´t it?
Do you think that I should wait for new diff-transistors or could I try the amp, given the measurements mentioned before?
Regards//lasse
I think that it would be a good idea to test a new diff-pair but the only replacement variant mentioned by you or anyone else I´ve seen on the net , that is available in Sweden is the KSC1845FTA and not before the middle of February 2018.
Can you think of any other suitable replacement device apart from the ones mentioned? They seem to be discontinued.
By the way, it is a good thin to buy a few dozens and match them, isn´t it?
Do you think that I should wait for new diff-transistors or could I try the amp, given the measurements mentioned before?
Regards//lasse
Hi again,
I see that the 2N5401 has been recommended before as diff-transistors.
They are available in Sweden. Should I try?
Regards//lasse
I see that the 2N5401 has been recommended before as diff-transistors.
They are available in Sweden. Should I try?
Regards//lasse
Look at the hFE at 1 mA here, min of 250 probably 400 typical:
http://www.mouser.com/ds/2/308/2n5087-d-1190230.pdf
2N5401 is not a particularly high gain type, I believe that there is a -16 version
selected for gain is that available. The hFE is min of 50:
https://www.onsemi.com/pub/Collateral/2N5401-D.PDF
I just noticed that you need PNP, I might have called out NPN previously
so you need the compliment if I did that, check of course.
http://www.mouser.com/ds/2/308/2n5087-d-1190230.pdf
2N5401 is not a particularly high gain type, I believe that there is a -16 version
selected for gain is that available. The hFE is min of 50:
https://www.onsemi.com/pub/Collateral/2N5401-D.PDF
I just noticed that you need PNP, I might have called out NPN previously
so you need the compliment if I did that, check of course.
I have many transistors here.
I have 1970s 2N5087s if you want some.
I have to look at what I have for new 2016 ish parts.
Can I send you something?
I have 1970s 2N5087s if you want some.
I have to look at what I have for new 2016 ish parts.
Can I send you something?
Hi Pete,
and thanks for the generous offer but I have already ordered a large (40 st) number of:
Fairchild KSA992
stated as the PNP-version of KSC1845. It was available in Sweden and the total cost was reduced by a factor of ten compared to ordering from the US.
Since this (as most of the things I take on) is a learning process, I intend to match the diff-pair and to do so, I need a large number of units.
They are only like 13 cents each so...
I hope this was a wise choice...
regards//lasse
Excellent, let us know what Hfe grade you get when they come in any
should be fine but just curious.
If they came from the same wafer they should be easy to match.
Many place them face to face on the flat side with heat shrink or tie wrap around the pair.
Obviously, pay close attention to the pin out when installing them.
should be fine but just curious.
If they came from the same wafer they should be easy to match.
Many place them face to face on the flat side with heat shrink or tie wrap around the pair.
Obviously, pay close attention to the pin out when installing them.
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