SA-7800 NSA driver transistors

[VintageMad89]

Hello folks, recently picked up another 7800 for a good price in 100 % original condition- totally unmolested and in full working order

As with all my gear I plan to do a complete recap as well as replace all the small signal transistors as well as the two drivers for the NSA circuit.
The problem is- mouser no longer stock the 2SA1930 used to replace 2SA913 They do however, have limited quantities of the 2SC5171 (to replace 2SC1913) but I was wondering what the current complimentary replacement for these are.

Apologies if this has already been covered but I could not find any info in any relevent threads.

At the moment I have about 25mV offset in the left channel and -50mV in thr right. Bad caps perhaps?

Thanks in advance!

 

[zebulon1]

Thanks Karl, I came across 2SA1930 / 2SC5171 being suggested as a good replacement for the originals, however I am very happy with 2SA1837 // 2SC4793 in that position now.

Mouser stocks the 2SA1930, but no longer stocks it's complement the 2SC5171.

http://www.bdent.com/nsearch.html?#...ze=15&storeid=yhst-864421581610834172249761/1

http://www.bdent.com/nsearch.html?#...ze=15&storeid=yhst-864421581610834172249761/1

http://www.bdent.com/nsearch.html?#...ze=15&storeid=yhst-864421581610834172249761/1

http://www.bdent.com/nsearch.html?#...ze=15&storeid=yhst-864421581610834172249761/1

I didn't check quanity.

 

[Hyperion]

Thanks Zeb.

 

[VintageMad89]

Thanks guys, but it looks like those are also now obsolete

 

[Hyperion]

Thanks guys, but it looks like those are also now obsolete

Oh yes darn it, I just tried to put one in the 'cart'

 

[markthefixer]

Those drivers if working should not be preemptively replaced.

Those parts of the parts lists were intended to indicate REPAIR parts and not pre emptive replacements.
Granted they tended to blow up when the NSA circuits started eating their output transistors...

The speed of those drivers is critical for low distortion in a NSA circuit. At LEAST 100 MHz.


errata: darn it, I should have paid more attention the last few years and stocked up on them.

 

[VintageMad89]

Those drivers if working should not be preemptively replaced.

Those parts of the parts lists were intended to indicate REPAIR parts and not pre emptive replacements.
Granted they tended to blow up when the NSA circuits started eating their output transistors...

The speed of those drivers is critical for low distortion in a NSA circuit. At LEAST 100 MHz.


errata: darn it, I should have paid more attention the last few years and stocked up on them.

Thanks Mark, I've since pulled and tested them and all four seem to be working fine. I may just leave them be if critical as you say.

You successfully talked me through a repair on my other SA-7800 about 5 years ago now, however this one was too good a deal to pass up. Plus, two can't do any harm?
I've already began work changing out transistors on the amp board and moved the two 2.7k 2W resistors on the phono preamp to the top of the board as per your previous recommendation.

As for the unusual offset figures, I'm hoping a full recap should help clear that up... looks like I'll need to make some minor board modifications for the four filter caps- the only ones I can find in those values (50v 8200uF) have a narrower lead spacing than the originals... I guess I'll need to do some thread digging!

 

[VintageMad89]

Bump

Just out of curiosity, would the nsa drivers affect the dc offset if going bad? Or any other pair of transistors? (bear in mind I have still to recap the unit)

Cheers

 

[markthefixer]

DC offset at the output is servo'd by the feedback loop to zero,
or actually the feedback loop's interpretation of zero - which is where the dual transistors on the differential input introduce their error.
Everything in between those two points is adjusted out automatically.

The driver transistors are in that loop, but if they do not respond fast enough, errors occur.

 

[VintageMad89]

DC offset at the output is servo'd by the feedback loop to zero,
or actually the feedback loop's interpretation of zero - which is where the dual transistors on the differential input introduce their error.
Everything in between those two points is adjusted out automatically.

The driver transistors are in that loop, but if they do not respond fast enough, errors occur.

Thanks Mark, that gives me a much better understanding of the circuit operation.

The reason I asked was, I have about 33mV left channel and -50mV right channel.
Both bias adjustments have been recalibrated to little affect... Any ideas?

Cheers

 

[markthefixer]

The SA-7800 does NOT have dc offset adjust.
Idle current (bias) does not affect (much) DC offset.

Three choices:
1. Plug and chug a pair ksa992's to find a pair that gives you lower dc offset in Q1 than the 2sa979
2. Look at the idle adjusts for similar amps higher up the $ model chain, and mod them in.(ksc1845, 62k resistor and 250k multi-turn trimpot 652-3296P-1-254LF)
3. Live with it

 

[VintageMad89]

The SA-7800 does NOT have dc offset adjust.
Idle current (bias) does not affect (much) DC offset.


Three choices:
1. Plug and chug a pair ksa992's to find a pair that gives you lower dc offset in Q1 than the 2sa979
2. Look at the idle adjusts for similar amps higher up the $ model chain, and mod them in.
3. Live with it

Thanks, I think the no.2 mod might be a little out of my capabilities, but not impossible!

I do have another unit with said 2SA979s that I could try swapping out... As far as the KSA992s go, I have them in stock (reel from mouser) BUT- would I be joining two leads together to make the "5 pin arrangement"? Would they need to be gain matched? Which two legs would be joined/connected?

Cheers