Il giorno lunedì 15 aprile 2002 13:56:13 UTC+2, Jan Wemmel ha scritto:
Thank you very much to everybody who wrote a reply, it is good to see that
there is so much knowledge out there.

Even that I have been trained as a Radio and Television Technician in
Germany (admittedly 10years ago) and that I have a degree in IT and
Telecommunication, I have never seen a diode with varistor characteristics.
Unfortunately, the source for the part that Paul mentioned does not stock
the MA27A1, only the MA27A which is totally different. Panasonic in the US
offer the part via their website (http://www.pasc.panasonic.com) but this
service is not offered in Europe (UK), however the shop that Paul
recommended will get hold of the part from Panasonic. Do not try to get hold
of part from Panasonic directly in Europe a service manual would arrive in
6! weeks and would cost you £20+ VAT and postage (approx. $40). I got a
manual within 2!days from here http://www.divdev.fsnet.co.uk/man.htm. I can
only recommend the service!!!

Anyway, Goran was absolutely right, the Diode (varistor) controls the basis
voltage on a transistor (Q312) that controls the bias voltage of the amp.
Currently the transistor opens up completely and the transistor have the
full negative voltage (-53V) on their basis (not very good, luckily the
protection is VERY quick, I would be very upset to have seen my B&W speakers
go up in flames). I assume the diode has move the basis voltage of Q312 a
little causing the transistor to open up completely. However, I think there
is probably an additional problem because when I measure the resistance
against the chassis the right and left channel a completely different. Still
a lot of fun to be had... But I think the repair is still worth it as the
amp, even compared to todays amps still has some very good characteristics
and do you know of an end user amp that has two power supplies, one for each
channel (transformer, rectifier AND caps)?

Again thank you very much for your help!!

Jan



Hi, sorry for resuming a 13 years old thread.
I have a problem similar to Jan's. I did some research and found a possible replacement candidate fo the diode: Panasonic MA2B027 (datasheet here http://www.datasheetarchive.com/dlma...SA-471901.pdf).


It seems there are several different versions of the diode (variants A/B, WA/WB, TA/TB, QA/QB). I think in my case the WA/WB variants are the correct ones (see also thread http://www.audiokarma.org/forums/sho...99#post8216699)

BUT the part shows as discontinued in Panasonic website: http://www.semicon..panasonic.co.jp/...0&part=MA2B027

Any idea for a replacement?

Thanks in advance

I have a few parts units around, what part exactly do you need? MA2B027 or MA27A1?

Dan

Il giorno martedì 5 maggio 2015 23:32:46 UTC+2, ha scritto:
I have a few parts units around, what part exactly do you need? MA2B027 or MA27A1?

Dan

Hi Dan,
thanks for replying.
I think the right replacement would be MA2B027WA.
How many of them do you have? Would you able to ship me some, let's say 10, of them (to Germany)?
Do you happen to know a suitable replacement which is easy to find nowadays?

I think those "varistors" can't be replaced with normal silcon diodes as correct biasing would require them: I did once a try and I was about to blow up the final stage (again).

Thanks a lot in advance,
-- Marco

On Tue, 5 May 2015 13:49:26 -0700 (PDT), wrote:

Il giorno lunedì 15 aprile 2002 13:56:13 UTC+2, Jan Wemmel ha scritto:
Thank you very much to everybody who wrote a reply, it is good to see that
there is so much knowledge out there.

Even that I have been trained as a Radio and Television Technician in
Germany (admittedly 10years ago) and that I have a degree in IT and
Telecommunication, I have never seen a diode with varistor characteristics.
Unfortunately, the source for the part that Paul mentioned does not stock
the MA27A1, only the MA27A which is totally different. Panasonic in the US
offer the part via their website (http://www.pasc.panasonic.com) but this
service is not offered in Europe (UK), however the shop that Paul
recommended will get hold of the part from Panasonic. Do not try to get hold
of part from Panasonic directly in Europe a service manual would arrive in
6! weeks and would cost you £20+ VAT and postage (approx. $40). I got a
manual within 2!days from here http://www.divdev.fsnet.co.uk/man.htm. I can
only recommend the service!!!

Anyway, Goran was absolutely right, the Diode (varistor) controls the basis
voltage on a transistor (Q312) that controls the bias voltage of the amp.
Currently the transistor opens up completely and the transistor have the
full negative voltage (-53V) on their basis (not very good, luckily the
protection is VERY quick, I would be very upset to have seen my B&W speakers
go up in flames). I assume the diode has move the basis voltage of Q312 a
little causing the transistor to open up completely. However, I think there
is probably an additional problem because when I measure the resistance
against the chassis the right and left channel a completely different. Still
a lot of fun to be had... But I think the repair is still worth it as the
amp, even compared to todays amps still has some very good characteristics
and do you know of an end user amp that has two power supplies, one for each
channel (transformer, rectifier AND caps)?

Again thank you very much for your help!!

Jan



Hi, sorry for resuming a 13 years old thread.
I have a problem similar to Jan's. I did some research and found a possible replacement candidate fo the diode: Panasonic MA2B027 (datasheet here http://www.datasheetarchive.com/dlma...SA-471901.pdf).


It seems there are several different versions of the diode (variants A/B, WA/WB, TA/TB, QA/QB). I think in my case the WA/WB variants are the correct ones (see also thread http://www.audiokarma.org/forums/sho...99#post8216699)

BUT the part shows as discontinued in Panasonic website: http://www.semicon.panasonic.co.jp/e...0&part=MA2B027

Any idea for a replacement?

Thanks in advance


Here is the modern sub. from Best Buy's part division. 45 days to
restock but you can order it now.
https://www.partstore.com/Part/Matsu...A27B1/New.aspx

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Il giorno mercoledì 6 maggio 2015 23:50:15 UTC+2, Chuck ha scritto:
On Tue, 5 May 2015 13:49:26 -0700 (PDT), ... wrote:

Il giorno lunedÄ› 15 aprile 2002 13:56:13 UTC+2, Jan Wemmel ha scritto:
Thank you very much to everybody who wrote a reply, it is good to see that
there is so much knowledge out there.

Even that I have been trained as a Radio and Television Technician in
Germany (admittedly 10years ago) and that I have a degree in IT and
Telecommunication, I have never seen a diode with varistor characteristics.
Unfortunately, the source for the part that Paul mentioned does not stock
the MA27A1, only the MA27A which is totally different. Panasonic in the US
offer the part via their website (http://www.pasc.panasonic.com) but this
service is not offered in Europe (UK), however the shop that Paul
recommended will get hold of the part from Panasonic. Do not try to get hold
of part from Panasonic directly in Europe a service manual would arrive in
6! weeks and would cost you �20+ VAT and postage (approx. $40). I got a
manual within 2!days from here http://www.divdev.fsnet.co.uk/man.htm. I can
only recommend the service!!!

Anyway, Goran was absolutely right, the Diode (varistor) controls the basis
voltage on a transistor (Q312) that controls the bias voltage of the amp.
Currently the transistor opens up completely and the transistor have the
full negative voltage (-53V) on their basis (not very good, luckily the
protection is VERY quick, I would be very upset to have seen my B&W speakers
go up in flames). I assume the diode has move the basis voltage of Q312 a
little causing the transistor to open up completely. However, I think there
is probably an additional problem because when I measure the resistance
against the chassis the right and left channel a completely different. Still
a lot of fun to be had... But I think the repair is still worth it as the
amp, even compared to todays amps still has some very good characteristics
and do you know of an end user amp that has two power supplies, one for each
channel (transformer, rectifier AND caps)?

Again thank you very much for your help!!

Jan



Hi, sorry for resuming a 13 years old thread.
I have a problem similar to Jan's. I did some research and found a possible replacement candidate fo the diode: Panasonic MA2B027 (datasheet here http://www.datasheetarchive.com/dlma...SA-471901.pdf).


It seems there are several different versions of the diode (variants A/B, WA/WB, TA/TB, QA/QB). I think in my case the WA/WB variants are the correct ones (see also thread http://www.audiokarma.org/forums/sho...99#post8216699)

BUT the part shows as discontinued in Panasonic website: http://www.semicon.panasonic.co.jp/e...0&part=MA2B027

Any idea for a replacement?

Thanks in advance


Here is the modern sub. from Best Buy's part division. 45 days to
restock but you can order it now.
https://www.partstore.com/Part/Matsu...A27B1/New.aspx

---
This email has been checked for viruses by Avast antivirus software.
http://www.avast.com

Hi Chuck,
thanks for your reply.
Is there a place where I can find the datasheet for this part you mentioned? I just want to double check it suits as a replacement and my inability to find the datasheets makes me a bit reluctant to just place an order and see what arrives.

Thanks in advance,
Marco

On Fri, 8 May 2015 12:39:15 -0700 (PDT), wrote:

MA27B1

Don't know of one, but this is the substitute that Panasonic
recommends.

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Il giorno martedì 12 maggio 2015 18:58:39 UTC+2, Chuck ha scritto:
On Fri, 8 May 2015 12:39:15 -0700 (PDT), wrote:

MA27B1

Don't know of one, but this is the substitute that Panasonic
recommends.

---
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http://www.avast.com

I ordered some MA2C029WA from ic-chips.com. We will see if it suits....
Quite pricey the expedition costs to Europe... not sure if it's worth but I'm sentimentally bound the my Technics. Probably a bad thing

I'l try with these you recommend if those I ordered turn to be a not suitable / bad replacement.

Best,
Marco

Il giorno giovedì 21 maggio 2015 22:14:15 UTC+2, ha scritto:
Il giorno martedì 12 maggio 2015 18:58:39 UTC+2, Chuck ha scritto:
On Fri, 8 May 2015 12:39:15 -0700 (PDT), wrote:

MA27B1

Don't know of one, but this is the substitute that Panasonic
recommends.

---
This email has been checked for viruses by Avast antivirus software.
http://www.avast.com

I ordered some MA2C029WA from ic-chips.com. We will see if it suits....
Quite pricey the expedition costs to Europe... not sure if it's worth but I'm sentimentally bound the my Technics. Probably a bad thing

I'l try with these you recommend if those I ordered turn to be a not suitable / bad replacement.

Best,
Marco

I now gained quite some more experience on the subject. I can confirm MA2C029WA are a suitable replacement for the MA27-A2. I tested that for years in "production" on my own Technics SU-V3 amp. Of course I can't guarantee about extreme conditions but it seems to me that the key function of "stabilization" is performed by the negative slope of the VF-Ta curve which would guarantee that, in the configuration in which the diodes are used, the lowering of VF with temperature increase will change the bias point of the circuit in a way that prevents thermal runaway.
The MA2C029 series offers a very wide range of forward voltages and it basically serves the case of "double diodes". I need to do more research if I had to replace the MA26 itself but I think the search for a replacement may be much easier for that one.

See this thread also: https://audiokarma.org/forums/index....ma26-2.621043/

Also some other hard to find diodes in these amps are the OA90 diodes. I was able to find some NOS on ebay at very reasonable prices and I stocked them up. See below for the datasheet.

Some technics amplifiers, for example the SU-8088, use the dreaded SVDSTV4HG.
Since that is attached to the heatsink with its very long leads (needs to be thermally coupled to the final transistors) it happens that sometimes the leads break because of mechanical stress and the bias circuit doesn't like that basically causing some cascade breakdown of the final stage.
See this dicussion
https://audiokarma.org/forums/index....-story.498025/


and also the datasheets:
https://www.alldatasheet.com/datashe...C/MA2B027.html
http://datasheet.octopart.com/MA26-P...eet-108588.pdf
https://www.alldatasheet.com/datashe...C/MA2C029.html
https://pdf1.alldatasheet.com/datash...ETC1/OA90.html (on the third page!)