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tom66 21st March 2013 09:39 PM

Repair of Onkyo TX-SR506 (One transistor blown)
I got this Onkyo TX-SR506 from eBay cheap, as faulty. It looks like a fairly decent amplifier, so I want to fix it.

It will power up into standby with a red LED. Pressing the power button makes a relay click. VFD lights up and works, then a few seconds later, the relay goes out, VFD goes out and the power LED flashes continuously until the power button is pressed again.

Upon disassembling the unit, I found one main output transistor on the SL channel to be failed. This is a 2SA1941. This transistor is obviously burnt, with a hole in it and associated burn mark shot up the heatsink. All of the other channels look good, though I'll probably replace the complementary 2SC5198 too, as I believe that may have sustained damage despite showing no signs of failure.

My question is, what part to use for these? I can buy them on eBay, but they're almost certainly fakes. I can't find them from Farnell, either. I -did- find some other parts, such as 2SA1294 and 2SC3263 from Farnell (Sanken/Allegro parts); would these work well? This amp is "only" rated to 0.08% distortion, and I'm not particularly concerned about the distortion being higher, my concern is simply will it work (and not sound awful?)

In addition, do I need to replace the driver transistors 2SA1930 and 2SC5171 and the 0.33 ohm resistors? The 0.33 ohm resistors are a combined, matched device -- do I need to find an equivalent if they're failed? I'll borrow a good Fluke from work tomorrow and test these as well as the test points in the service manual.

Any advice appreciated

JonSnell Electronic 21st March 2013 10:02 PM

The emiter ressistors are 0R22 and fire proof.
The output pair need at least Hfe 50 or more.
The driver transistors are protected but worth checking as are the fire proof base/emmiter ressistors for the drivers.

tom66 21st March 2013 10:29 PM

You're right. 0.22 ohm. I'll check them tomorrow.
The transistors I mention as possible substitutes have a gain of 50 so that's good.
I presume I'll need to adjust the IDLING ADJ after replacing them.

jaycee 22nd March 2013 12:18 AM

Perhaps NJW0281/0302 from Onsemi, or 2STA1962/2STC5242 from ST ?

Bibliophile 22nd March 2013 12:25 AM

Here are some sources of 2SA1941:

2SA1941 Stock and Price by Distributor |

Tube_Dude 22nd March 2013 06:29 AM


Originally Posted by tom66 (Post 3421567)
I presume I'll need to adjust the IDLING ADJ after replacing them.

Yes! Surely.

tom66 22nd March 2013 08:27 PM

OK, I've just had a lesson in WHY disabling protection circuits is a bad idea.

The amp wasn't staying on for long enough to check the bias voltages so I cut one lead of R6173 -- DC fault detect. I'm not connecting speakers, so I figured it's fine.

As expected the output rose to +52V or so... and a few seconds later, two capacitors blew up. LOL. :eek: They were 50V rated across the bases of the main output transistors and across the drive transistors; and experienced around 100V due to the shorted base-collector/ base-emitter junctions. So they very quickly failed. But at least I know where the fault is: looks like I'm replacing both the driver transistors and the main transistors. I tested the 0.22 ohm; it's fine, along with the other parts including the base resistors.

The capacitors were crappy TK Audio capacitors; I think I'll use Panasonic FC for replacements.

I think I'm going to use the ONSemi transistors suggested by jaycee;
NJW0281G - ON SEMICONDUCTOR - TRANSISTOR, NPN, 250V, 15A, TO3P | Farnell United Kingdom
NJW0302G - ON SEMICONDUCTOR - TRANSISTOR, PNP, 250V, 15A, TO3P | Farnell United Kingdom

because they're inexpensive and look decent spec; I can buy them from Farnell (free shipping/no min order to my workplace.)

The next problem is finding replacements for the 2SC5171 and 2SA1930 driver transistors -- any suggestions? These are TO220, insulated case non-heatsinked devices.

tom66 22nd March 2013 08:46 PM

1 Attachment(s)
Schematic of the channel (surround left), for reference.
I think each channel is exactly the same, at least, I can't see a difference.

tom66 23rd March 2013 12:16 PM

Ok, I think I'm going to replace all of the transistors in the amplifier -- it's only going to cost a few pounds more, so it's worth it for my personal assurance. I'm getting the main output transistors from Farnell, and the rest from Kessler Electronics.

Q6053: 2SA1294 -> NJW0281G
Q6063: 2SC5198 -> NJW0302G
Q6033: 2SC5171 -> 2SC5171
Q6043: 2SA1930 -> 2SA1930
Q6003: 2SC1740 -> 2SC2240
Q6013: 2SC1740 -> 2SC2240
Q6073: 2SC2240 -> 2SC2240
Q5003: 2SC2240 -> 2SC2240
Q5013: 2SC2240 -> 2SC2240
Q5043: 2SC2229 -> 2SC2705
Q5033: 2SA949 -> 2SA1145
Q5053: 2SC1815 -> 2SC1815

I've made some substitutions (bold), because the particular component wasn't available. Mainly based on the advice I found on the net and careful reading of datasheets, but I'm not 100% on these. Can anyone confirm that my substitutions are OK?

In addition, I need to replace:

C6043: 47u/50V TK Audio -> 47u/50V Panasonic FC
C5053: 47u/50V TK Audio -> 47u/50V Panasonic FC
R6103: 0R22 matched -> 0R22 x 2 (1%) 3W
R6173: 47k 1/8W -> 47k 1/4W (chopped one leg off disabling DC detect)
R6073: 82R 1/8W -> 82R 1/4W (measured 300 ohms in circuit -- probably bad)

tom66 26th March 2013 08:47 PM

Damn the amplifier board is hard to get out! For reference the back AV plate can be loosened by 5 screws and then the plastic retainers can be broken. The front button/VFD connector needs to be removed too -- careful with the tape so you don't break the cable.

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