Hello all
In search of a schematic for the item listed above - I believe it's 70's era -turntable AM/FM receiver unit. Wired for 240 sold in Australia from Japan. The power amp section is 4 Toshiba 2SC490 (two per side). It arrived with one side down in volume and a bad hum. Recapped the power supply but it didn't fix the problems. While checking voltages on one of the transistors in the final audio power stage my hand slipped and it must have shorted (?) and started smoking resistors (2 1ohm 1 watt on emitters on audio LEFT). Each time the power was then applied the two resistors would smoke after a few seconds. I removed the transistors from that side and I'm waiting on replacements, but as is the other side (RIGHT) works fine. Steep learning curve as I'm more of a tube guy but checking if anyone A) has access to schematic or one close from that era and B) ever heard of anything like that happening as a result of a momentary short (to ground I'm assuming). Thanks in advance!!
In search of a schematic for the item listed above - I believe it's 70's era -turntable AM/FM receiver unit. Wired for 240 sold in Australia from Japan. The power amp section is 4 Toshiba 2SC490 (two per side). It arrived with one side down in volume and a bad hum. Recapped the power supply but it didn't fix the problems. While checking voltages on one of the transistors in the final audio power stage my hand slipped and it must have shorted (?) and started smoking resistors (2 1ohm 1 watt on emitters on audio LEFT). Each time the power was then applied the two resistors would smoke after a few seconds. I removed the transistors from that side and I'm waiting on replacements, but as is the other side (RIGHT) works fine. Steep learning curve as I'm more of a tube guy but checking if anyone A) has access to schematic or one close from that era and B) ever heard of anything like that happening as a result of a momentary short (to ground I'm assuming). Thanks in advance!!
Ah, the classic slip with the probe. You know how it is with transistors and fuses, the transistor will protect the fuse by going first. You're not the first to have that kind of experience by any means. (This is what I mean.)
Looks like 2SC490s came in three gain groups, B, R and Y. Which one is used here (or none)?
Finding TO-66 parts could be fun in general.
In any case you will have to check for shorted transistors - outputs, probably driver stage, too.
Is the hum still there? If not, the now-defective left side may have drawn excessive idle current to begin with.
If you do not have a variac, at least using a bulb tester may be advisable. Probably little more than a 40 W bulb for what I assume can't be too powerful if it uses 50 V Vceo outputs.
Looks like 2SC490s came in three gain groups, B, R and Y. Which one is used here (or none)?
Finding TO-66 parts could be fun in general.
In any case you will have to check for shorted transistors - outputs, probably driver stage, too.
Is the hum still there? If not, the now-defective left side may have drawn excessive idle current to begin with.
If you do not have a variac, at least using a bulb tester may be advisable. Probably little more than a 40 W bulb for what I assume can't be too powerful if it uses 50 V Vceo outputs.
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Thanks for the reply and glad I'm not the first! and thanks for the tips in the link.
They are 2sc490Y. I found a few and have ordered them (which includes spares)...
I do have a Variac but the light bulb safety seems like a very practical and handy tool.
The hum is still there. I have an MK203 transistor checker. I had one spare 2SC490Y that reads good as a BJT NPN hfe 58.6 Vbe 608mv. The two I pulled out of the unit only read as "resistors" from pins 2 (collector) to pin 3 (emitter) as .42ohms and .43ohms? I was assuming that was not a good sign.
They are 2sc490Y. I found a few and have ordered them (which includes spares)...
I do have a Variac but the light bulb safety seems like a very practical and handy tool.
The hum is still there. I have an MK203 transistor checker. I had one spare 2SC490Y that reads good as a BJT NPN hfe 58.6 Vbe 608mv. The two I pulled out of the unit only read as "resistors" from pins 2 (collector) to pin 3 (emitter) as .42ohms and .43ohms? I was assuming that was not a good sign.
You can also use resistors around 100 ohms between power supply and amp boards for current limiting during testing - you can monitor the voltage across one with a multimeter
and they will act as fuses if you slip-up. In quiescent conditions an amp with 30mA will
lose 3V across each resistor, 100mW dissipated, but the amp won't see more than a fraction of an amp if somethings wrong, should save output devices and drivers.
Most importantly such resistors are a barrier between the big PSU filter caps and the output devices, something a dim-bulb tester cannot protect against.
and they will act as fuses if you slip-up. In quiescent conditions an amp with 30mA will
lose 3V across each resistor, 100mW dissipated, but the amp won't see more than a fraction of an amp if somethings wrong, should save output devices and drivers.
Most importantly such resistors are a barrier between the big PSU filter caps and the output devices, something a dim-bulb tester cannot protect against.
Hi Bhelonious,
I have just picked one up myself. They seem to be quite rare (around here anyway), with very little information available. Have you resolved the issues with yours? Mine seems to be fully functional so if you need any measurements let me know.
These are a solidly built unit - wonder what they are worth now. This one needs a bit of a clean, otherwise in very good condition.
Cheers
Q
I have just picked one up myself. They seem to be quite rare (around here anyway), with very little information available. Have you resolved the issues with yours? Mine seems to be fully functional so if you need any measurements let me know.
These are a solidly built unit - wonder what they are worth now. This one needs a bit of a clean, otherwise in very good condition.
Cheers
Q
