Good evening to all,
Obviously, this is my first post on this forum, and although I have been looking at the threads for a long time, please excuse me if I am unclear or incomplete. Thanks to many of you, I learned a lot about the DIY audio.
The story is as follows..
I clone/restore amplifier by the attached schematic. The original PCB is in very bad condition and is not repairable, so I decided to clone the board as close as possible to the original.
Once all the elements were soldered in place, I started to check the amp. DC offset is about 4mV.
However, the amplifier oscillates at 1.4MHz, 1V RMS near to pure sine. The volume control is fully turned left, no load at the output. The idle current is affected by the oscillation. The oscillation is also present at the output of the VAS (L501 II R533).
I cut the link between L501, R533 and the next stage. That stops the oscillation at the output and the adjustment of the idle current become possible. No oscillation at the output of the VAS either.
I reconnect the link and check with 1kHz at the input to see the reaction. I started from 10mV to 1V. The clipping point is about 1.1V. On the low levels the 1kHz is affected by the oscillation. Rising the amplitude suppresses the oscillation and at 0.5-0.6V it disappears.
I noticed that the oscillation was affected when I tapped the body of the C523 or C524 with tweezers. The level is halved and sometimes completely disappears. The oscillation is also affected by the Volume potentiometer. In the fully left and fully right position it is about 1V, but when it is in the middle the level drops to 0.5V.
Very strange behavior. Any idea or advice on what to focus on to solve the problem? It will be the worst if it turns out that the reason is in the design of the board ...
Many thanks in advance!
Obviously, this is my first post on this forum, and although I have been looking at the threads for a long time, please excuse me if I am unclear or incomplete. Thanks to many of you, I learned a lot about the DIY audio.
The story is as follows..
I clone/restore amplifier by the attached schematic. The original PCB is in very bad condition and is not repairable, so I decided to clone the board as close as possible to the original.
Once all the elements were soldered in place, I started to check the amp. DC offset is about 4mV.
However, the amplifier oscillates at 1.4MHz, 1V RMS near to pure sine. The volume control is fully turned left, no load at the output. The idle current is affected by the oscillation. The oscillation is also present at the output of the VAS (L501 II R533).
I cut the link between L501, R533 and the next stage. That stops the oscillation at the output and the adjustment of the idle current become possible. No oscillation at the output of the VAS either.
I reconnect the link and check with 1kHz at the input to see the reaction. I started from 10mV to 1V. The clipping point is about 1.1V. On the low levels the 1kHz is affected by the oscillation. Rising the amplitude suppresses the oscillation and at 0.5-0.6V it disappears.
I noticed that the oscillation was affected when I tapped the body of the C523 or C524 with tweezers. The level is halved and sometimes completely disappears. The oscillation is also affected by the Volume potentiometer. In the fully left and fully right position it is about 1V, but when it is in the middle the level drops to 0.5V.
Very strange behavior. Any idea or advice on what to focus on to solve the problem? It will be the worst if it turns out that the reason is in the design of the board ...
Many thanks in advance!
Attachments
Information about the components used:
All transistors except drivers are original ones. The drivers I use are Sanken 2SA1668/2SC4382, it’s impossible to find original Toshiba 2SA968B/SC2238B.
Both operational amplifiers are also the original ones.
All diodes 1SS270A are replaced with 1N4148. D527-1S2076A (temperature compensation) is also replaced with 1N4148.
Zenner diodes: HZ15-2 replaced with BZX55 C15, HZ5C-1 replaced with BZX55 C5V1
The varistor MV-1YH, I found the original.
All ceramic capacitors are replaced with the same values silvered mica.
All film capacitors are replaced with FKP and FKS types.
All 0.25W resistors (originals are carbon film 5%) replaced with metal film 1%.
All transistors except drivers are original ones. The drivers I use are Sanken 2SA1668/2SC4382, it’s impossible to find original Toshiba 2SA968B/SC2238B.
Both operational amplifiers are also the original ones.
All diodes 1SS270A are replaced with 1N4148. D527-1S2076A (temperature compensation) is also replaced with 1N4148.
Zenner diodes: HZ15-2 replaced with BZX55 C15, HZ5C-1 replaced with BZX55 C5V1
The varistor MV-1YH, I found the original.
All ceramic capacitors are replaced with the same values silvered mica.
All film capacitors are replaced with FKP and FKS types.
All 0.25W resistors (originals are carbon film 5%) replaced with metal film 1%.