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Old 5th May 2016, 10:35 AM   #1
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Default Vintage Sansui restoration

Hey,

I bought relatively cheap, almost free of charge Sansui G-301/G-3500 stereo receiver in unknown condition. Visually unit is in really good shape. Components were not burnt or anything. I could tell they were old and dusty.
I just decided to get this unit to work on a new project. I have restored one unit before this, so I'm not very experienced.

So I tested the unit; in the beginning there was major crackle present on both channels. Just to know what I'm dealing with. So I've carefully recapped whole preamp, power output stages and majority on receiver stage.
After some component updates I tested the unit again. Not much to be said at this point, just power restored to both channels, but sound was distorted.

I went on measuring transistors legs soldered on the power output / driver stage. So I find out TR13 was blown. Measuring zero ohms between one leg pair. I popped these out of the stage to make sure they're blown and yes they were. I have soldered the new parts there and now left channel restored to crackle free condition and left channel seemed alright basically. No popping just clean sound, however right channel is still crackling and less power output there. After some more measurements TR12 seemed as bad as TR13.

So I have replaced these 2 transistors now and tested the unit again, well resistor R74 gave the "magic smoke". I turned off the unit. The unit was on perhaps 15-20 seconds and smoke came out when I tried to increase volume, but noticed right channel sounded good, no distortion present for that tiny moment, just lack of power and then smoke came out from the power stage.

Output transistors TR17 and TR16 measure both zero ohms between 1-3 legs. I popped these off the board. Now I have ordered the new components. I have visually inspected the resistor R74, there's tiny area of black residue due to "minor" heat damage. I measure approximately same readings from the resistor R73 and R74 soldered in unit. What you think should I replace the R74 even if it seems like "okay" by numbers?
Should I pop out more components for testing?

I am aware I haven't been careful enough, causing major damage to the new components possibly. However I feel there might not be too much damage done yet. I was able to turn off the unit as soon as I saw the smoke coming out.
Could be I'm just dreaming....

Anyway..
I'm aware of the dim bulb tester and basics of it. All the instructions on DIY bulb tester seem to be for american standard 120V and there's polarity in them. How should I wire it for 240V standard? Because the part which I don't understand is here I can insert AC plug in both ways to outlet in wall. Both AC socket plug pins are the same size. DIY diagrams show the bulb should be connected in series to the "hot wire". How in earth this is done, since I can insert the plug to wall outlet in both ways? To me it seems that diagram is polarized. Only the ground pin is always the same.

All help is appreciated.
Thanks.
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Old 5th May 2016, 12:50 PM   #2
Welcome is offline Welcome  France
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AC is not polarized, you've been reading too much american stuff.

Put the light bulb in series with one of the wires on the power cable. Just cut one of the wires off (doesn't matter which), and install the two cut ends of your cable on each side of a light bulb socket. Easiest to do this with an extension cord, so its reusable, and you don't ruin the equipment. Don't die here, do it properly and safely.

Click the image to open in full size.
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Old 5th May 2016, 04:10 PM   #3
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Thanks for the info and diagram.

I was able to find out similar diagrams, for example:
http://www.radiolaguy.com/images/misc/TestSocket.jpg

Now that's easy to build. I have a bulb socket available from old lamp and I guess 60W bulb is good to go.
I wasn't so sure until now, since there was no voltage rating in the diagram or picture description. American bulb tester diagrams show different size AC socket pins and couldn't find 240V mentioned anywhere and I'm no electrician, so it seemed very puzzling.

Now waiting in for the new parts and starting this thread decided to test few more resistors and I can say R67 is also shot. Multimeter giving OL. R68 could be shot as well, readings jump heavily. To be 100% sure, I should desolder one leg free. Aware of that.
Meanwhile good time to build that bulb tester, since I don't want the new output transistors to fail at first thing and get some silicon grease for the transistors
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Old 5th May 2016, 10:00 PM   #4
Gusser is offline Gusser  United States
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Quote:
Originally Posted by Welcome View Post
AC is not polarized, you've been reading too much american stuff.

Click the image to open in full size.
Well it is and it isn't depending on the context. When we speak of AC power being polarized, it means one conductor is hot and the other at ground potential. Because in the American electrical system as well as many other countries as I understand tie the neutral to the ground at the service entrance. This provides a fault path to trip a breaker should enough current flow in the ground conductor. Otherwise a dangerous ground fault would not trip the breaker.

So if building a series light, you would want the lamp in the hot lead not the neutral lead. Just a bit safer shock wise that way in case you contact with an external ground. At least then the current flow though YOU is limited by the bulb resistance.
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Old 6th May 2016, 03:05 AM   #5
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As I understand from information documents, the electrical supply system in Finland is similar to many European countries where a "Shuko" plug system is used for up to 16A domestic appliances. The polarity of the AC socket connectors is random, i.e either could be called "Live" or "Neutral" because there is no MEN system where one AC supply cable would be strapped to earth at multiple points in the supply grid and at the meter box.
Power plug & outlet Type F (Schuko) - World Standards

In other words, you don't have a "Live", "Active" or "Neutral" pin. Both are "Live" and equally dangerous. In any case, lightbulb wiring should be connected to a standard, approved type of light socket that is fully insulated and enclosed or surface mounted to an approved type of insulated box for the 220V supply used.

We all need to know what we're doing when we take mains electrical safety in our own hands - not like when we have a transformer that isolates the mains supply from our electronic device
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Old 6th May 2016, 09:42 PM   #6
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Being one of the channels OK, compare all drivers and output transistors (as well as neighbour resistors) from both channels, without the need of removing any component. Also check voltages, as stated on the schematic (only shown on the right channel/lower part).
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Old 10th May 2016, 09:03 PM   #7
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Quote:
Originally Posted by Ian Finch View Post
The polarity of the AC socket connectors is random, i.e either could be called "Live" or "Neutral" because there is no MEN system where one AC supply cable would be strapped to earth at multiple points in the supply grid and at the meter box.
Power plug & outlet Type F (Schuko) - World Standards
That's what we have here, true.

I am certain the problem lies on the output stage. I started measuring resistors near output transistors. Resistors from R69 to R72 all 4 (220 ohm) were toasted or heavily out of range. Also R75 near output transistors (6.8 ohm) and resistors closer the power source R67 and R68 were toasted. I replaced all these with correct wattage resistors. Thanks for the tip.

I received the new output transistors from Ebay. The replacements came from UK. The old ones underlined red on the picture are definitely toasted.

I am worried a bit since on the schematics appropriate transistor TR17 replacement is a transistor 2SA771 with Q or Y, but off the Ebay comes something "09H" and it's not Sanken. Not printed at least. The other shipped, TR16 is Sanken and is alright with correct letter. What you think is TR17 going to be a problem?

However, now I have conducted the first test run with the amp wired to the dim bulb tester without speakers. It's not very pretty looking, but obviously it did what I made it for. 60 watt bulb gets bright for 1 second as I turn the unit on and then it dims down and stays dim. No smoke! Good sign.
I noticed some warmth over the heatsink and the unit was on maybe for one minute tops. No speakers connected and volume very low. Is it all good?

Should I let the amp stay powered with the dim bulb tester for longer perioid of time or should I just test the unit with speakers connected?
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Old 10th May 2016, 09:16 PM   #8
Welcome is offline Welcome  France
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Your Ebay transistors are very clearly fake (pretty much all semis on Ebay are), so be careful. They'll blow up very easily, and might take the speakers down with them.
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Old 11th May 2016, 04:19 AM   #9
wg_ski is offline wg_ski  United States
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Yeah... finding outputs is gonna be fun. I'd start by looking through what Digikey has for Sanken TO-220's, see if you can find a 60 to 80 W device pair that's good for at least 6A. That supply voltage is a bit out of range of generics like the TIP41/2, which would likely overstress the drivers due to low Hfe at high current. If you never never ever ran 4 ohms they would be ok, but even then distortion would be higher than with the originals. Might even try the Fairchild C5200/A1943 clones which are available in a variety of packages - including the TO-220.
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Old 11th May 2016, 04:39 AM   #10
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Sure are fake They don't even have the insulated package so even if they work, only a desperate person would try to use them and certainly not for a quality audio repair.
Here's Sanken's current replacement types: http://www.semicon.sanken-ele.co.jp/...4511_ds_en.pdf
http://www.semicon.sanken-ele.co.jp/...1725_ds_en.pdf
Don't buy from Ebay sellers - use reputable distributors of traceable stock components like Digi-key, Mouser and 2SC4511 - Power Transistor 80V 6A.
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