SONY TA-N55ES Repair/Restore - Need Help

p4spooky

Active Member
Hi All: Bought this on the bay for cheap since it had the protect light on. Here the issues I found (right channel)

1. Final outputs C-E short (2SA1215/2SC2921)
2. BIAS transistor 2SC3423 C-E short
3. Several fusistors open (R356,R357,R358)

Left channel appeared OK and DC offset was 2.3mV measured at R315

Repair Strategy:

1. Replace final outputs with working pull (bought from bay)
2. Replace 2SC3423 with same ordered from bdent
3. Drivers 2SA985 and 2SC2275 were OK - removed from board and tested OK
4. Replace 2SA1049 (Protect) with same part ordered from bdent
5. Replace bias pots with bourns single turn for both channels
6. Replace input cap on both channels
7. Replace diodes and both resistors on Speaker board (10R resistor on right channel was burnt)
8. Replace both 5A relay with 8A (Part# G2RG-2A4 DC24)
9. Replace all caps on PSU section and preamp section (note this was done AFTER the amp was repaired but still has the DC offset issue)

Completely rebuilt right channel with all new MF resistors/diodes/one input cap and new transistors from above.

Power up:

Powered up using DBT and came out of protect on first shot and no smoke :)
Bias adjusted to 7.14mV (spec is 7mV - pic shows bias while connected to DBT)
Took it out of DBT and readjusted bias to 7.14mV on both channels
Played music and sounds great.

Problem

Right channel has high DC offset (180mV) :eek2:

Measure all voltages on right channel (see attached) and sure enough they are off. Emitter voltage on final output is 180mV instead of close to zero. Detailed measurements attached. Verified all resistor values and found them to be accurate. Went though and touched up all solder joints. Dc offset does not budge.

Need help to figure this out experts! Amp is running OK, and sounds good although I am yet to connect to scope. Want to fix the offset issue first. Attaching some relevant pics
 

Attachments

  • Voltage Measurements.pdf
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  • Right Channel.jpg
    Right Channel.jpg
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  • Right Channel Bias.jpg
    Right Channel Bias.jpg
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  • Preamp - Rebuilt.jpg
    Preamp - Rebuilt.jpg
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  • Speaker Output board - Rebuilt.jpg
    Speaker Output board - Rebuilt.jpg
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The Differential amp for the l and r amplifiers exist in the IC (STK3102). According to your voltage readings, that is where the 200mv+ difference is starting at pin 5 and pin 6 of that IC. I would think that that chip is your issue.
 
Exactly Tom - I noticed it and put a box around it in the attachment. Did now know the differential amp is in the IC. I have a NOS STK 3102. I will swap and see what happens. This IC runs hot as well. Thanks for the tip.

@SV12004A: Yes ordered 2 pairs of A1216/C2922 from bdent. Will put it in once I get the offset fixed.
 
I think the fact that the STK got no heatsink is why so many die... have seen a few ppl on here mount a heatsink to these to ensure longevity
 
Thanks to @Tom B and @SVI2004A the issue is now resolved. As Tom correctly diagnosed it was the STK3102 iii chip. Replaced it and all voltages came back normal on the right channel and DC offset is now down to 5.2mV. I also noticed the sound is awesome now - did not know what I was missing before!

STK3102 is now installed on a heat sink (used a chipset heat sink from a dead motherboard)

Fixed - Right Channel DC Offset.jpg STK3102 Heatsink.jpg Replaced Parts.jpg
 
Curious where you found the replacement stk3104? My source for (real?) obsolete semis has zero stock.
 
Bought it from a seller on the bay (US seller) claiming NOS. Compared the weight of the one that was defective to the replacement and they were the same. I know this is not a definitive test to eliminate knock offs. The replacement has been working well so far.
 
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Hi. I'm also Repairing a ta-n55es and need a little advice as im having trouble finding this resistor or an equivalent. Its R306 and R356 and both are 220ohm 0.5w 5% fusible risistor. All my searches draw a blank as nothing matches the band colours.

20181222_233322.jpg
 
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