Hello,
I trying repair an amplifier
Problem is treble amp offset reads -32 VOLTS when COLD!!
amps bias is fine
- When amp heats up dc offset drops to +- 50mv, this is withing manufacturers specs
- Dc offset bounces a lot when hot, from 50mv to 10mv then to 70mv and back to 50mv
Bias is stable when hot
Manufacturer specifies that amp should be measured hot
- when this amp is working normally, offset is max 200mv when cold. I have excatly the same amps which work normally
- offset trimmer pot is tested and ok
- the whole amp board was resoldered just in case, looking for bad connection, no changes
- When amp is hot and its turned off for a couple of seconds, DC offset is immediately -5.00 Volts from -50mv. And starts super slowly rising back to 50mv
What are the main causes for DC offset to be so high when cold?
I trying repair an amplifier
Problem is treble amp offset reads -32 VOLTS when COLD!!
amps bias is fine
- When amp heats up dc offset drops to +- 50mv, this is withing manufacturers specs
- Dc offset bounces a lot when hot, from 50mv to 10mv then to 70mv and back to 50mv
Bias is stable when hot
Manufacturer specifies that amp should be measured hot
- when this amp is working normally, offset is max 200mv when cold. I have excatly the same amps which work normally
- offset trimmer pot is tested and ok
- the whole amp board was resoldered just in case, looking for bad connection, no changes
- When amp is hot and its turned off for a couple of seconds, DC offset is immediately -5.00 Volts from -50mv. And starts super slowly rising back to 50mv
What are the main causes for DC offset to be so high when cold?
Attachments
Could be anything that affects the balance of Q13/Q14, or these transistors themselves. Some freezer spray might be useful to identify which component(s) are the most temperature sensitive.
It would be helpful to draw transistor in convential way, not inventing your own symbols.
Likely lots of cold solder joints.
Likely lots of cold solder joints.
You dont really need to start two threads for the same thing do you ?
https://www.diyaudio.com/community/...-and-doesnt-react-to-the-bias-trimmer.404204/
https://www.diyaudio.com/community/...-and-doesnt-react-to-the-bias-trimmer.404204/
D
Deleted member 550749
Every amp behaviour same offset , high or low to sudden adjusted offset, that's why speaker creates popup sound it's normal
I have 12 of these amps, when this amp is working normally, offset is max 200mv when cold. I have excatly the same amps which work normally
Two of the 12 amps have very high offset when cold, one of 12 has no bias, thats why another thread
Two of the 12 amps have very high offset when cold, one of 12 has no bias, thats why another thread
R30 doesn't have a series capacitor so the DC gain of the amplifier is the same as the AC gain - this normally causes offset issues unless a DC-servo is used to drive the output to zero at DC.
Adding a series capacitor (normally a large value electrolytic) causes the DC gain to be unity which is much more stable for offset, although your case seems extreme so there's something else going on, one of the components probably has temperature-dependent leakage or temperature-dependent oscillations are taking place.
Adding a series capacitor (normally a large value electrolytic) causes the DC gain to be unity which is much more stable for offset, although your case seems extreme so there's something else going on, one of the components probably has temperature-dependent leakage or temperature-dependent oscillations are taking place.