Sony STR-3800L - Transistor replacements on output board

[david winter]

Hi,

I need to replace the transistors on a Sony STR-3800L output board as both outputs are shorted.
The last four transistors in the list differ with the service manual. References in bold were originally installed in this amp, which may be an early specimen (serial at rear is in the 5007xx range, chassis serial is in the 4007xx range).

Q701-702 (differential): 2SC1364 -> KSC1815 ?
Q703: 2SA896 -> KSA1381 ?
Q704: 2SC1811 -> KSC2383 ?
Q705: 2SC1809, 2SC1475 -> ?
Q706: 2SA777, 2SA773 -> KSA1013 ?
Q707: 2SD531, 2SD476A -> BD203 ?
Q708: 2SB521, 2SB566A -> BD204 ?

Any advise appreciated. Thanks.

 

[PE9ZZ]

Just curious: how did you manage to destroy all 16 transistors in the power amp of this nice receiver? Surely not all of them? As for advice: what's in your inventory is leading. If the replacements you mention are available, use them. Check the parameters in their respective datasheets first though. Package, V(ceo), P(c,max) and I(c,max) must be equal or better. For instance, BD203 is beefier than the 2SD531 so it can be used.

 

[david winter]

I didn't destroy them. The amp came with one shorted output transistor. I replaced the two outputs by BD203/BD204. After a minute or so the bias became very unstable and both outputs were found shorted. I prefer changing 8 transistors rather than leaving an unstable one which may cause some damage soon or later.
Check what quick tests reveal:
2SC1364: Hfe=682 (ok but close to max spec)
2SC1364: Hfe=549
2SA773: Hfe=27 (NOS: 200tp)
2SC1475: Hfe=31 (NOS: 100-650)
2SC1811: Hfe=97 (NOS: 150tp)
2SA896: Hfe=97 (NOS: 150tp)
Bias diode: Vf=2.628V@5mA

 

[PE9ZZ]

So you upgraded from one shorted output transistor to two. Did you check the bias before you plonked them in? Did you use DBT? And what is "tp"? I never test h(FE). If it works it works. Bias usually becomes unstable due to bad trimpot. Replace it. The bias diodes used here (D701/51) are known to fail. You can opt to redo the bias by building a V(be) multiplier with a BD137 or somesuch and remove RT701 and VT751 (sic) altogether.

 

[david winter]

I obviously replaced the bias trimpot before the output transistors failed. "tp" means "typically". I checked the bias diodes with my DCA75 Pro.
I finally replaced Q703 by KSA1013, Q705 by KSC2383, and the outputs by TIP41C/TIP42C. Bias now adjusts fine, amp works fine.