Replaycing an old transistor

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I'm an owner of an old Technics SE-A3 and the powertransistors are broken. The devicetype is
OD503A-Q but these parts are discontinued by
Technics. I have no technical paper about these
devices, my measurement's in the no damaged devices
has shown me hfe are 50-60. Has anyone an idea to
replace the output-transistors which another type.
The Technics/Panasonic technican's told me that there
is no chance, but i don't believe it.
The O/P Stage is running at +75 /-75 Volts and is completely
DC coupled in complementair design. Two drivers drive 4
bipolars at the NPN line and two drive the PNP line.
The transistor's are double type's rated at
+/- 150V and +/- 10/15 Amp. the heatsink is big enough
to mount any other types on it TO-220 - T 200 or so.
I love this amp, he must live on....
could anybody help me ?
best regards Bocky
The amp with the big VU-meters, a classic!
there shouldn't be a problem to get the amp back in business. You said that there are no specs available for the transistors so what does this info pertain to??

-The transistor's are double type's rated at
+/- 150V and +/- 10/15 Amp.-

What do you mean by "double type's"? Maybe PNP-NPN combined into one???

MT-200 (sanken type stuff 2sc2922-2sa1216 etc) will need a lot of room, toshiba has a slightly smaller alternative.

The "R" gain range device should work for you. You should try to match the transistors before you installing them since they are going to be paralleled. Lars Clausen of will sell you some matched 2SA1216-2SC2922.
Also, some adjustment may be required to get the DC offset to behave.
Do not forget to check that there are no other broken devices before the power trans. Usually bad things happen for a reason.
If have more detailed questions fell free to ask.

[Edited by grataku on 07-10-2001 at 07:16 PM]
Thanks grataku

What do you mean by "double type's"? Maybe PNP-NPN combined into one???

Yes, that's matches exactly what i mean. Excuse me.
What's is the better one, Sanken or Toshiba
that's the question.

The "R" gain range device ...

I couldn't find any dealer in germany
which allows me to order devices in special ranks.
what happens when i order only the typ without ranks ?
Do they deliver me with a lot of different kinds ?
Do you have an distributor adress to order with ranks ?

Lars Clausen of will sell you some matched 2SA1216-2SC2922...

I'm unable to find this offer, can you give me the exact
link to that offer ?

Also, some adjustment may be required to get the DC offset to behave...

To what value must i adjust ?

best wishes Bocky
do what hifi told you, send an email to LCaudio, Lars will answer, he's a very nice guy. Are you sure you can fit all those MT-200 devices? They are rather large.

As far as the adjustment go is very much going to be an "experimental" kind of deal, do you have the schematics and instrumentation?
Yes, i'm measured out my heatsinks and, yes, it is enough
room for the sanken types.
I have found a dealer ho can deliver me with those types
at range y ( hfe 55-110 ), i have ordered doubled, than
i can match the devices, i hope that's enough or isn't ?
Tell me, what's the best match method ?
Method a la Nelson Pass or is it quiet right to measure only
the hfe ?
I have yet the problem to find a mica insulator, what's better mica, ceramic or kapton ? It must be thin !
But my biggest problem is that i couldn't find the reason
for my broken O/P !!!
Could you give me any tips ? The driver stage should be o.k.
i've checked out any device but couldn't find any failure
could it be, that an driver transistor is half damaged ?
Yes i have an schematic of this amp and an oscilloscope too.
I think it would take a long time to repair them...
greetings Bocky
Great! I would be interested in the Sanken transitor myself, would care to tell me were are you buying them from and how much per trans.? They are 2sa1216 and 2sc2922, right?
The hfe matching is not critical, the internal resistance is what counts! Elliot Sound Products website should also have a "how to" section on matching. Remember to match the NPNs with NPNs NOT the NPNs with PNPs.
Good luck and let me know how is goes.
The trans. cost's 13.45 US$ each, a little expensive maybe.
I'm got them from my hifi dealer near me.
I've matched the trans. with the method of Nelson Pass,
and it looks good, the measurement are 3.97 / 3.98 / 3.99 /
4.03 Volts for the npn and the pnp are a little more different, i think it's o.k. I've mounted them on my heatsink and rewired the pcb connection it looks good.
But i can't finnished, i've no micaplate it's realy hard
to get - nobody knows about MT-200 cases i don't know any dealer to get them. Have no idea. Could anybody help me ?
best regards Bocky
Silicon Impregnated Sheet

While I'm not personally a great fan of silicon impregnated thermal insulators; I've been led to believe they do not perform as well as traditional mica, but that could be BS. It is however available as a sheet so you can cut it out to suit whatever size you like, it's a lot less messy to use, and has the dandy property of not being carcinogenic.

Not sure where you are, but check radio spares.


OD503A transistors

reference the post on these transistors for the output stage of the Technics SE-A3.
Did anyone come to a suitable conclusion as to a source for the original transistor or a direct replacement which didn't upset the amp's balance?
Any info most welcome.
SE-A3 output BJT's.


Was passed your 'working' OD-503AQ dual complimentary BJT module to check out for suggested replacement of failed others.

I have found the following:-
hfe approx. 90 @Ic=1A
Ft approx. 80MHz (based on measured BW of 880kHz and hfe)

The voltage rating you gave (150V) may not give much room for safety.
I have read earlier in this thread the supply rails in SE-A3 are +/-75V. I would suggest that the BJT's need to be rated to at least 110% of rail to rail voltage. Also if the supply is not regulated against mains voltage variation it would be prudent to check or work out what he worst case high DC supply rail voltage is at high mains input. (Say you have +/-75VDC rails for 230Vrms mains input then I would allow for a +15% on the mains which would bring the DC rails up to +/-86.2VDC, rail to rail voltage is then 172.5VDC and allowing another 10% safety margin brings us to about 190V voltage rating for the BJT's.)

The current rating of 10A is probably quite adequate as there are, according to you, 4 comp. pairs of BJT's per channel. As Amp is 200W class A the standing current in o/p BJT's is probably between 7A and 14A depending on the rated load and other stuff. This current split 4 ways is much less than 10A so there seems plenty of over head in that department.

The Sanken parts that are mentioned earlier in the thread (2SA1216 & 2SC2922) would have right hfe if 'P' suffix are acquired but look a little slow at Ft=40MHz. Still an Ft within 50% to 200% of original part stands a good chance of working fine. Voltage and current rating look OK also. All this assumes that the part you gave me is representative of it's younger self, some parameters may have degraded in it's c20 year life, plus I might have the measurements wrong!

I have asked Mark to get schematics from you as these could help me be more certain about the advice. It would be nice if those guys from start of thread could let us know if they had any success.

Happy listening,
Hi Soldersplash,
you should have now a fairly comprehensive copy of the circuit diagram for the amp, i hope this helps.
I spoke to the Technics technician in Stamford and i think the only other info on the transistors he had was the frequency being 50MHZ which would be about right as the amps age is from an era of slower devices.
Give me a call on 07767 205205 or text your number and i will call you.
cheers Toby
I have an original instruction manual with circuit diagram available, please give me a ring on 07767 205205 I can always tell you what value the component is or fax/scan the diagram. From memory it is quite a large diagram (4 or 6 sheets of A4) but it is very comprehensive.

I'm presuming the capasitor that failed is one of the two main PSU ones, to be honnest they want replacing anyway if they are the original ones as they start to leak and generally stop working when they get old. I'm surprised it went bang though, that normally only happens with reverse polarity applied to them.
Hallo Bocky,
I have the same problem with an SE-A3 from Technics. Today i repaired a SERTEC S500.
In this ampl. the transistors MJ15024 and MJ15025 are used.They are 16A/250V. Maybe that can help you.
What is the solution you found for the SE-A3? Thanks.
You also can repaet me in german.
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