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Replace NAD 3220PE (~3020E) power transistors?

J

JohanH

pfm Member
Hello all,

I'm in the process of repairing a NAD 3220PE which is a 3020E with the added "power envelope" power supply.
For now I know the drivers on 1 channel are shot and the power transistors measure (surprisingly) ok.
I'll be replacing the drivers with:
BD237 BCE 100V 2A 25W 3MHz hfe 40-
BD238 BCE 100V 2A 25W 3MHz hfe 40-
or
2SA1837 BCE 230V 1A 20W 70MHz hfe 100-320
2SC4793 BCE 230V 1A 20W 100MHz hfe 100-320

Originals seem kinda difficult to come by and above are cheap enough to buy more than needed and match them.

My question is about the power transistors: because they are 30 years old I intend to replace them.
In other venues people recommend replacing the original types with modern ones and add emitter resistors and basestoppers for added reliability.
I have pairs of MJ15024/25 which would seem ok for the job but the originals are (of course) also readily available.

Which would be the preferred course of action?
Thanks in advance,
Johan
 
JohanH said:
Hello all,

I'm in the process of repairing a NAD 3220PE which is a 3020E with the added "power envelope" power supply.
For now I know the drivers on 1 channel are shot and the power transistors measure (surprisingly) ok.
I'll be replacing the drivers with:
BD237 BCE 100V 2A 25W 3MHz hfe 40-
BD238 BCE 100V 2A 25W 3MHz hfe 40-
or
2SA1837 BCE 230V 1A 20W 70MHz hfe 100-320
2SC4793 BCE 230V 1A 20W 100MHz hfe 100-320

Originals seem kinda difficult to come by and above are cheap enough to buy more than needed and match them.

My question is about the power transistors: because they are 30 years old I intend to replace them.
In other venues people recommend replacing the original types with modern ones and add emitter resistors and basestoppers for added reliability.
I have pairs of MJ15024/25 which would seem ok for the job but the originals are (of course) also readily available.

Which would be the preferred course of action?
Thanks in advance,
Johan
Click to expand...

They should be ok. Try and get drivers of similar spec to originals. I wouldn't add base stoppers.
 
Thanks.
To be clear: your advise is to replace the original power transistors with th MJ pair and add emitter resistors? I feel just as comfortable to buy original 3055/2955 and use those.

Original driver specs are:
2SB649A-C BCE 180V 1,5A 20W 140MHz hfe 60-200
2SD669A-C BCE 180V 1,5A 20W 140MHz hfe 60-200

I kinda lean towards BD replacement because of 2A spec. The 3220 has +/- 40V voltage.

Cheers,
Johan
 
JohanH said:
Thanks.
To be clear: your advise is to replace the original power transistors with th MJ pair and add emitter resistors? I feel just as comfortable to buy original 3055/2955 and use those.

Original driver specs are:
2SB649A-C BCE 180V 1,5A 20W 140MHz hfe 60-200
2SD669A-C BCE 180V 1,5A 20W 140MHz hfe 60-200

I kinda lean towards BD replacement because of 2A spec. The 3220 has +/- 40V voltage.

Cheers,
Johan
Click to expand...

The Toshiba drivers should be better. It's very unusual to find a BJT amp with no emitter resistors! Are you sure about that? If non are fitted then I'd leave it that way as it's possible that other measures have been taken to help with bias stability.
 
Some say the lack of emitter resistors is what makes these amps special. Also 3055 pair inherently doesn't seem to need them (not my words)

Now trying to add schematic: not working, bear with me
 
NAD%203220PE%20amp1.jpg


Here's the schematic.
The 1 ohm resistor is shorted; only used to set bias
 
A single output pair must be easier to maintain bias than multiple pairs. Do the output transistors warm up without a signal?
 
Ok, thanks for the input.
I'll replace output transistor with original pair and drivers with Toshibas.

@demotivated: Don't know if the output transistors warm up without a signal. Probably not much as bias is to be set at 28mA.

Cheers,
Johan
 
If bias setting VBE mu;ltiplier (Q409) is attached to output heatsink, should get adequate bias stability, due to circuit design. Basically, multiplier is set to give 4 times VBe, and tracks output transistor pair, so varies at 4 times the rate of change of single VBe with temperature.

If drivers stay cool, their bias hardly changes, so as stage warms up, bias requirements change more slowly than multiplier output, giving stability; output will become underbiased when hot, which is safe.

Neat, safe, low cost approach, with minor sacrifice of sound qualtity under hard running.
 
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