Fisher BA-6000, help finding a transistor sub?

[Nick_the_'Nole]

So I'm finally getting around to working on a Fisher BA-6000 power amp that I picked up a couple years ago. Normally I wouldn't bother with 'Stupido Standard' gear, but this thing actually seems to be pretty decent quality... must be a fairly early example, from before the company completely devolved into BPC.

Anyhow, the situation is that one of the STK-0100II outputs is blown. That itself is a simple enough fix, and I have a replacement on the way. However, apparently when it went out, it also took some of the driver circuitry with it, including a couple of 2SB718 transistors. Those are a bit more of a problem... they don't seem to be available anymore, except from sketchy sellers for inflated prices, and I'm not sure how much leeway there is on a substitute (especially since the complementary 2SD758s are also involved).

Here's the general run-down on the parts:
2SB718: TO202 200V 50mA 1.25W 60-320hfe 70mhz

And I've attached the schematic for the amp board, with the transistors in question highlighted. Could one of you transistor gurus please help me figure out what my options are? :scratch2:

 

[FootFungus]

KSA1381E should do the trick.

http://www.mouser.com/Search/Refine.aspx?Keyword=ksa1381

I'm guessing Q11 and Q15 are probably 2SB718s as well? You should probably replace those as well while you are at it.

Be careful on the orientation when replacing.

 

[Nick_the_'Nole]

Yep, Q11-Q18 are all 2SB718s... there's four per channel.

So a 150mhz part is OK to use in place of a 70mhz one here? I know that such a big jump can sometimes be a no-no, primarily on output transistors... but I take it there's nothing in this particular case that would cause it to matter?

The other question is, what about the 2SD758s? If I'm changing out the 2SB718s for KSA1381s, shouldn't I change the 2SD758s out for KSC3503s as well?

 

[gadget73]

cant answer the frequency question, though I suspect it does not matter. I would change the 2sd758's to whatever the complimentary is to the KSA1381E though, and just so everything is matched I would change them on both channels.

 

[Nick_the_'Nole]

That's kinda what I was figuring... and the KSC3503 I mentioned is the complement to the KSA1381.

Unfortunately Mouser only carries the 'D' (60-120 hFE) grade of the KSC3503 and the 'E' (100-200 hFE) grade of the KSC1381, so if matching them is important I'll have to find a different source for one or both types.

 

[srinath]

How did this pan out

Hi,
What did you do and how did it work out.
I am working on a ba6000.

Thanks.
srinath.

 

[srinath]

Hello,
Has anyone got anything further on this amp.
I am working on one and want to eliminate any issues.
Thanks.
Srinath.

 

[srinath]

I am finding out on the interwebz that the complimentary to the 2sd718 is a 2sb688 ?

Why does the amp have 2sd758 ?

Thanks.
Srinath.

 

[srinath]

I am getting a staticky and weaker output from the right channel, the same one that blew a chip on the previous time.
Cool.
Srinath.

 

[Nick_the_'Nole]

I just now stumbled back across this thread and saw the further questions. While it may not do srinath much good at this point, I know how frustrating it can be to find a discussion about a subject without any sort of closure or solution, so I'll offer a much-belated follow-up in case the information proves useful to anyone.

I did succeed in getting the amp working, and it is quite a nice piece of kit. However, getting it going proved to be more of a chore than it initially seemed. First of all, the KSC1381E/KSC3503D combo worked perfectly for the driver transistors. I matched the HFe of the two grades of transistors as closely as I could... I had enough of them to choose from that I got them fairly close. There were also a couple of 2SC1570s upstream that were dead, which got replaced with 2SC1815s... or maybe it was 2SC2240s? I have tons of both on hand and can't remember which I used, but it doesn't really matter-- there's a bunch of different small signal transistors that would work just as well in the application. The BZ250 zeners got replaced with 1N4749s, and any damaged resistors got replaced with equivalent value 1% metal films. Even though only one channel was bad, I replaced parts on both so there wouldn't be any mismatch. I also went through and replaced all of the electrolytic caps except the main filters (which were still ok), along with a couple film caps that looked a bit sketchy from being near the burned area. (Did I mention that the middle of the board had a dime-sized divot in it from where it had been on fire? Yeah, this was not a small project.)

The real biggie I encountered on this was that most of the solder joints on the main board were bad. I am not kidding when I say that over 75% of them were cracked. I don't know if it was caused by some environmental factor over my particular unit's lifetime, or if it was just poor manufacturing, but I ended up going through and redoing every single solder joint in the damn thing.

The final part of the process was replacing the odd-looking 0.47ohm cement resistors in front of the STK packs. I discovered that one of them was bad after finding distorted and weak audio during my first post-surgery test. I ultimately settled on using standard 5W axial lead resistors, even though the originals were 7W. The originals were NLA, and there wasn't sufficient space to fit 10W parts. The down-rated resistors have not proved to be a problem even after 4 years of use.