Unknown transistor replacement (555a)

[Bert Beukema]

Hi guys,

I have a Sansui 555a on my bench belonging to a friend. It's main problem is loss of volume and a 'weak' and somewhat distorted left channel. I've narrowed it down to the pre-amp section. Now, the machine was ready for a full recap but before I do that I like to get it working if the problem is not capacitor related.

Seeing that distortion + weakness commonly implies a bad transistor I decided to pull the four on the board. However, it seems someone has been there before me as none of the transistors are 2sc871r. In fact, none of the transistors on the boards are of this type (four more). Now, shouldn't be a problem if the replacement is a valid one.

And here we do have a problem. You see the guy who was in there before me decided to put a dot of yellow paint on each of these transistors. After a lot of *painstaking* work I could manage to get this reading:

*632A
L71

The star stands for an unknown number or letter, it was directly under the yellow damned blob. I've been trying different combinations but haven't had much luck in finding them. Do these numbers mean anything to anyone with more experience in this Sansui 555a?

Otherwise I think i'll just replace all of them with SC1815.

Thanx!

Regards,

Bert

 

[petehall347]

maybe this http://english.electronica-pt.com/components-cross-reference?ref=2SC632A
should work ok .. but maybe faulty anyway bc549 may work ok .check specs first ..
1815 are ok too . i have hundreds of them .

 

[ConradH]

Wild guess, it might be a 2SC632, which would be labeled C632. I'm working on a 2000A that uses those instead of the 458s shown on the schematic. Sansui did sometimes change things, and yellow dots of paint (or other colors) often indicate a selection process for gain or other parameters. Fairly high Hfe, similar to 2N5088 or KSC1845. Unless it's something else! FWIW, the KSC1845 seems to substitute for almost everything, having high Hfe, high CE voltage and very low noise.

 

[The Fuxtor]

There are lists for good replacements! Just search "au-555a recap list"
Complete cap and transistor replacements.
I used one and all worked good , just make sure to double check the NPN/PNP subs as the list I used mixed up a set .....

 

[slimecity]

As Conrad says above - you wont always find the parts that are listed in the schema actually inside the amp - I've found all sorts of different types inside various sansui amps. You can usually go off what is listed in the schema as the part that was actually used (if its different) will generally be a sub with very comparable values.

 

[Bert Beukema]

Thanx guys!

I think that these are indeed a form of 2SC632 (Sanyo would be my guess). Actually it's not that strange for builders to swap around / change components but the higher rated Sansui's i've had on the bench were generally a good reflection of the service manual. Now pick a 101 or a 505 and the fun begins, they almost seem like 'spare parts' builds with many different components. SC632 isn't known for it's high failure rates or other problems, then again it's also not a 'too go to' transistor. I'll pick them off the board and test them, the one i've got here is already suspect.

Regards,

Bert

 

[Hyperion]

Just use 'AK search' for the type number alone - '2SC632' (and uncheck all the search options) - not that many hits on that particular one, but enough to give you the answer.

 

[stereofun]

Hi Bert - rather than pull those 2sc632, test them, and possible put them back I would recommend to just replace them with ksc1845fta. I had those in my Eight phono board that was quite noisy until I replaced with 1845's.

 

[Bert Beukema]

Hi Bert - rather than pull those 2sc632, test them, and possible put them back I would recommend to just replace them with ksc1845fta. I had those in my Eight phono board that was quite noisy until I replaced with 1845's

Hmmm, my experience is that in circuit testing for these transistors almost always generates results that are incorrect. Which is why I generally pull them from the board to diagnose them in a 'clean' setting. If you have another method i'd be glad to hear it!

It seems that these are indeed the sanyo ones, i've got a Sony Ta-88 open (psu problems) which uses the same 2CS632 transistors but they are a bit different, including gold plating on the legs.

 

[stereofun]

That's what I meant: " pull them " = off circuit.

But even that I wouldn't bother. I'd just replace them period. Especially with the 1845 being an excellent low noise replacement and likely an upgrade.

Now if suitable replacements weren't available, then that a different story.

 

[SoundOfSound]

Here is some info on it.

http://www.datasheetarchive.com/dlmain/57623ddef5475a2bf188e4b2f41a4c308d8c8c/M/2SC632

 

[Bert Beukema]

'd just replace them period.

Agreed big time. I only reinsert pulled transistors if I am certain they are working correctly and are difficult / expensive to replace. These ones don't fit the profile as they are just bulk transistors. In fact, when I encounter certain types I pull and replace them whatever the situation ;-).

 

[cb1975]

Wild guess, it might be a 2SC632, which would be labeled C632. I'm working on a 2000A that uses those instead of the 458s shown on the schematic. Sansui did sometimes change things, and yellow dots of paint (or other colors) often indicate a selection process for gain or other parameters. Fairly high Hfe, similar to 2N5088 or KSC1845. Unless it's something else! FWIW, the KSC1845 seems to substitute for almost everything, having high Hfe, high CE voltage, and very low noise.[/QUOTE}
Hi there, I'm finding the exact problem with a 555a. A yellow blob of paint and all.
I'm having trouble finding correct data, but it looks like both are NPN, but the pinout is backward? I have no way to test so was hoping someone had done this and could report back.
To be clear, I'm attempting the change out the preamp (for now) for what's in there currently which seems to be 623A T 7 1 with KSA1845? As you pointed out before the schematic for 555a states, these should be 2sc871, but they are not?
Thanks
Chris

 

[Bert Beukema]

You should use either 2SC1845 or KSC1845, but remember that the KSC has a lower gain. Also watch out for FTA versions of the KSC1845 as these have a much lower gain. Personally I used the 2SC1845 and that solved the problem immediately. Before you pull anything, be sure to take a multimeter and check voltages across the board at the transistors. It's an easy way to check if they are operating within spec; if they are then the problem is somewhere else. Usually a resistor or diode in the chain. Pulling transistors to test them is not easy, they get damaged quickly which might give you a 'false' sense of problem discovery. Good luck!

 

[The Fuxtor]

I think I used MJE350/MJE340's..
Just pay attention to placement! Works fine
Edit: This was on the amp board.... I think you are dealing with the preamp board? Been a bit since i had mine opened up!

 

[Bert Beukema]

I've started the rework this week, mostly because the left channel was very weak and i'd like to keep my vintage machines in proper order. It's just been very busy the past months with other projects. Done so far:

- Reworked the phono / equalizer board (using 2SC1815)
- Reworked the pre-amplifier board (using 2SC1845)
- Replaced all capacitors here with Silmic II
- Replaced the main power capacitors with Vishay caps

Voltages on some of the transistors are off somewhat from the schema, but they are consistently off. All resistors were checked and fine, diodes on the power supply were replaced with modern ones and so were the resistors there. All voltages coming off from the main power supply are as specified. Some voltages being off can have a thousand reasons but seeing that they are consistent it should not be a real problem. Voltages on the output transistors are good and in spec, I don't suspect an issue there. The bias transistors also appear to be OK.

Strangely though, this rework did not change the issue. I had previously tested the machine by separating the pre-out and feeding it into another amp. This reproduced the issue, which led me to conclude that the primary issue was in the pre-amplifier.

I've checked the whole chain for the volume -> balance -> filters but this all checks out OK. Sound is coming from both speakers, but when I use the balance control and turn it to the left channel, it is silent. If I turn it perhaps 10% up, there is sound on both channels. If I turn the knob completely to the right, there is also sound on both channels. None of this actually makes any sense to me at the moment.

 

[skippy124]

How are you connecting the speakers - side by side, or one on top of the other?

 

[The Fuxtor]

I was testing my first unit and was getting confused until I realized the A channel speaker terminals are stacked on the left side and B channel on the right..... Normally they would be one channel on the top, the other on the bottom.....

 

[Bert Beukema]

No, there are no speakers on there as of yet. There is some electrical leakage within the system that would probably damage the speakers. Just using a headphone (tested the resistors and the chain up to the headphone jack and it's fine), if that breaks it's no big deal.

 

[Bert Beukema]

Tonight I decided to change the main board capacitors using Silmic II and Nichicon Gold KG. That also didn't solve the problem, it did introduce a new one. There is now clearly residual electricity on the line, it translates in a slow 'hump'. I connected my sacrificial speakers and the telltale signs of leakage could be clearly seen; with each 'hump' the woofers would react massively. I already noticed this visually on my DBT setup where the lamp would phase in and out just barely lighting up with each 'hump'.

So more troubleshooting needs to be done, but this surely is one of those amps that doesn't want to play nice.