Yamaha CR620 Power Transistor question

[Texas Tom]

Greetings all,

I have a question regarding a Yamaha CR620 that is showing a short. It blows the main fuses when turned on.
The first thing i read is that it is a bad power transistor.

I have tested them and they all they have acceptable voltage drops on them all.

I looked at them more closely and one channel has an original transistor pair and one has a replacement pair.

Original:
TR723 - NPN: 2SD371
TR725 -PNP: 2SB531

Replacement:
TR724 -NPN: MJ21194G
TR726 -PNP: MJ21193G

Screws are out or TR724 b/c I just tested it.

In another thread: https://audiokarma.org/forums/threa...sest-outputs-to-use-for-2sb531-2sd371.325670/

The replacement combination is discussed as an acceptable replacement.

When asking Copilot: "2sd371 transistor is mj21194 equivalent"

The MJ21194 is not a direct drop-in equivalent for the 2SD371. While it can often serve as a heavier-duty, high-performance substitute in vintage audio amplifiers, its electrical limits are vastly higher, which can cause circuit instability if used carelessly. [1, 2]​

​

Key Differences​

• Voltage Rating (\(V_{CEO}\)): The 2SD371 is rated for 80 V, while the MJ21194 handles up to 250 V.
• Current & Power: The MJ21194 is a 16 A, 250 W device, whereas the 2SD371 is only an 8 A, 50 W device.
• Frequency Response (\(F_{T}\)): The MJ21194 has a significantly higher frequency cutoff, which can lead to high-frequency oscillation (parasitic oscillation) in amplifier circuits designed for the slower 2SD371. [1, 2]

​

Usage & Sourcing​

• When upgrading: If used as an upgrade, resistors and capacitors in the bias and driver circuits may need to be adjusted to prevent oscillation and handle the higher gain. [1]
• Repairs: If replacing a blown 2SD371 in one channel of a stereo, it is highly recommended to replace both the NPN and PNP pair (the 2SB531 is the original 2SD371 PNP complement) with modern equivalents to prevent channel imbalances. [1, 2]

My question is whether mixing the two sets can be causing my problem?

Thank you for your assistance,

Tom

 

[ivandezande]

No. But if you’re blowing the main fuse, there is something shorted in the output transistors.

 

[avionic]

My question is whether mixing the two sets can be causing my problem?

As Ivan has already opined. I concur with him.

Main fuse could also be a power supply problem. I would check D812 and D813 as well as C823 and 824.

D812 and 813 are dual diode rectifiers. Test them like two diodes. Center pin is common. Looks like D812 is common cathode. And the other is common anode.

BTW I have used those Onsemi subs without any issues.

Do all 4 of the outputs have a mica insulator between the case and the heatsink? <----- KEY

Looks to be more than ample amount of thermal compound used.

 

[Texas Tom]

My apologies to you for not responding. Life has distracted me from my hobby. I hope to report back in the near future.
I greatly appreciate your observations and recommendations!

Tom

 

[mbz]

You should be using a Dim Bulb Tester, a current limiting device, numerous plans on AK.
Will allow you to power up without blowing fuse so you can measure/trackdown fault.
Certainly do checks as outlined above as a first step.

 

[Texas Tom]

MBZ, I am using a DBT. Thank you for the reminder.

 

[Texas Tom]

I tested the voltage drop on all diodes. They all tested a little over 0.5V.
I tested all the ceramic capacitors (0.01 µF / 500V). They tested >0.013 and < 0.015 µF.

This seems to be ok.