Hello. Have you ever experienced any issues with the left amplifier channel?
I also installed Toshiba transistors, type 2SC5359 NPN and 2SA1987 PNP, but I’m having a problem with the left channel.
Actually, I’m not even sure if it’s the left amplifier — the orientation is reversed for testing, so what appears to be the left side from the front becomes the right side when the amplifier is flipped over (viewed from the bottom, with the legs up).
Anyway, here’s how I’ll explain it:
I slowly raised the voltage using a variac and measured the bias points TP1 and TP2.
I’ll describe it as seen when the amplifier is upside-down.
On the right-hand side (which would be the left channel from the front), everything worked fine. The relay clicked on, voltages were around 85.5V, maybe a bit higher, measured between TP4 and TP5.
The right channel works fine, and the bias was around 30mV.
Then I connected the AC voltage to the left channel.
Even during the slow power-up using the variac, I noticed some anomalies:
At just 30V AC input, the bias already jumped to 60mV.
At 50V AC input, the relays clicked on, and the bias jumped to 160mV — which is way too high.
The VU meter for that channel also jumped to the middle. Something clearly isn’t right.
I quickly powered it off to avoid burning the main output transistors.
What do you think I should do? Should I swap the driver boards from left to right to test?
I suspect this issue might have damaged the original DLPT transistors.
Before installing the Toshiba ones, I had installed new old stock DLPTs, which the technician burned out — the same issue occurred back then. He wasn’t able to find the cause.
That’s when I decided to go with the Toshibas, but now I’m facing the same problem.
I really don’t want to burn out another set of transistors.
Where should I start troubleshooting?
Thank you for any help you can provide.
I also installed Toshiba transistors, type 2SC5359 NPN and 2SA1987 PNP, but I’m having a problem with the left channel.
Actually, I’m not even sure if it’s the left amplifier — the orientation is reversed for testing, so what appears to be the left side from the front becomes the right side when the amplifier is flipped over (viewed from the bottom, with the legs up).
Anyway, here’s how I’ll explain it:
I slowly raised the voltage using a variac and measured the bias points TP1 and TP2.
I’ll describe it as seen when the amplifier is upside-down.
On the right-hand side (which would be the left channel from the front), everything worked fine. The relay clicked on, voltages were around 85.5V, maybe a bit higher, measured between TP4 and TP5.
The right channel works fine, and the bias was around 30mV.
Then I connected the AC voltage to the left channel.
Even during the slow power-up using the variac, I noticed some anomalies:
At just 30V AC input, the bias already jumped to 60mV.
At 50V AC input, the relays clicked on, and the bias jumped to 160mV — which is way too high.
The VU meter for that channel also jumped to the middle. Something clearly isn’t right.
I quickly powered it off to avoid burning the main output transistors.
What do you think I should do? Should I swap the driver boards from left to right to test?
I suspect this issue might have damaged the original DLPT transistors.
Before installing the Toshiba ones, I had installed new old stock DLPTs, which the technician burned out — the same issue occurred back then. He wasn’t able to find the cause.
That’s when I decided to go with the Toshibas, but now I’m facing the same problem.
I really don’t want to burn out another set of transistors.
Where should I start troubleshooting?
Thank you for any help you can provide.