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[jakub421]

Hi everybody. I recently bought a vintage Yamaha RA-200 from a  guy, he said its about early to mid 70's. This amp actually has 3 amplifiers. 1 for rotating speakers, and two for the top and bottom half of the 4x12s. Now the problem is that in one of the amps for the 4x12, one transistor (2SC783) started getting really hot and the sound started distorting. Now I have no idea what the problem is, and I want to get it fixed. I checked the resistors going to the base, and they are good, so I don't know what else could be the problem. Thanks for any help!

The transistor is labeled TR4 on the schematic.

[Roly]

TR4 is the driver for output transistor TR6 and it getting hot suggests that TR6 has failed.  You need to check TR6 and TR7.  To do this properly you will either have to remove them from the circuit, or at least isolate two of the three connection of each.  If you remove them mark the assembly with the transisor number so they go back in the right place.

Making Electrical Measurements Part 3:
Testing Diodes and Transistors
http://www.elexp.com/t_test.htm

Meter check of a transistor
http://www.allaboutcircuits.com/vol_3/chpt_4/3.html

Rudimentary transistor test (diodes)
http://www.youtube.com/watch?v=wFHAGd_Tcvw

Google how to test transistors
http://www.google.com/q=how+to+test+transistors

Do not power up your amp until you get this fault sorted because there is a very reall risk of doing more damage, such as burning out one of those unique loudspeakers.

[J M Fahey]

Yes, probably TR4 is trying to drive the speakers on its own, instead of through TR6, and it can't (too small).
Besides TR6 I suspect the emitter ballast resistor (0.22 ohms)
1) disconnect the speakers going to that amp *NOW*.
As Roly says, you'll kill them.
2) build a lamp bulb limiter (search here) and use it until the amp is working fine.

[jakub421]

From the testing I did on the transistors, both of them seem to be working fine. Connections only from B to E and B to C on both of them. The 0.22 ohm emitter resistor is reading 0.6 ohms off circuit. Any other ideas? Oh yeah, I do have the amp always disconnected ever since it started distorting. The rotary amp is able to function independently.

[Enzo]

While TR6 was a good guess, don't forget that TR4 itself could be bad.   Diode function tests are great for locating shorted transistors, but they can also be leaky or have other failure modes.   

Yamaha documentation is usually really good, and this one has DC voltages all over the schematic.  How do yours compare?

Open resistors can be just as deadly as shorted transistors.  You 0.22 ohm guys may be OK, but there are other resistors in the circuit.


And don't limit your transistor test to "both of them."  You could have trouble on the other side making TR4 overheat trying to compensate.

[Roly]

With the speaker(s) disconnected;
- does TR4 still get hot?
- can you measure the voltages (numbers in square boxes) given on the circuit around TR3 to TR7 and post them please?

[jakub421]

So I got the amp to work. I re-soldered the transistors back into place and turned on the amp without the speakers. TR4 did not get hot. Out of curiosity I hooked up the speakers and the amp was working fine. It sounded clean and loud. I don't know what fixed it, maybe a bad solder joint was fixed, but it works.

[Enzo]

Bad or broken connections are pretty much the same thing as an open resistor.   Glad you got it working.

[Roly]

Bravo!   

Have a look at it from the point of view of small movement or stress, say between where the power transistor is mounted and the PCB itself.  Even small movements over time can fracture a solder joint and make it go intermittent or open circuit, and re-soldering will fix it.  This is a *very* common fault where input or output connectors are mounted on a PCB and not otherwise supported, and lead wrenches break the solder joints.

It would be a good idea to go over all the other similar joints to the power transistors with a magnifying glass and renew any others that look less than ideal.

[joecool85]

It would be a good idea to go over all the other similar joints to the power transistors with a magnifying glass and renew any others that look less than ideal.

Or do like me and spend 1/2 hour and renew ALL solder joints on the board.  Saves from headaches later.