I bought Rotel power amp recently and it was not working; I found that C617 and R667 (in series ) were burned. I replaced them but protection light was still on. I replaced all capacitors (except large main rail caps, 10,000uF), but protection still on...also checked all transistors and they seem to be okay... Would anyone have any idea what can keep protection on?
I guess that C617 and R667 are forming Zobel network to protect against oscillations, strange it did not work at some point... In amp like this I would expect also inductive coil in series with output, strange Rotel did not implement it...
Check C607/8 and is there DC on the output before the relay?
Yamaha had a design problem back in 1990 with one of their top range amplifiers. It took off at HF during loud music and would eat HF drivers. We cured it at factory level with the addition of a couple of 120pF capacitors slowing down the output stage. The Zobel network was damaged on the faulty amplifiers. As a former Rotel Authorised repairer we were told to tell the customers the Rotel amplifiers did not fail on their own, it was caused by customer abuse and Rotel would not honour warranties on blown output stages! I removed my service when I was told that by the Technical Rep back in 1993.
Yamaha had a design problem back in 1990 with one of their top range amplifiers. It took off at HF during loud music and would eat HF drivers. We cured it at factory level with the addition of a couple of 120pF capacitors slowing down the output stage. The Zobel network was damaged on the faulty amplifiers. As a former Rotel Authorised repairer we were told to tell the customers the Rotel amplifiers did not fail on their own, it was caused by customer abuse and Rotel would not honour warranties on blown output stages! I removed my service when I was told that by the Technical Rep back in 1993.
Hello Jon, thanks for advice, as I said I changed all capacitors, including C607/8...also I found R813 to be open (but not burned) and replaced it with new one; still protection light is on. I found rail voltage ( 51 V )on collector of Q802, where it is supposed to be only 0.4 V and around 7 V on base, which is supposed to be 1 V. Transistor seems to be okay...
@Tomom
Check or better replace styroflex cap of 330pf C612-611 exchange if you like to see how it will go
@ John Snell
At this point rotel was right about that and you where wrong ( at least for the specific line and era of amps ) obviously you lost a lot of money !!!
Check or better replace styroflex cap of 330pf C612-611 exchange if you like to see how it will go
@ John Snell
At this point rotel was right about that and you where wrong ( at least for the specific line and era of amps ) obviously you lost a lot of money !!!
I did most of those changes, still on junction of R818 (4K7) and C802 there is +54 volts instead 21 (goes to relay coil)...and protection light still on, any idea? It seems that problem is inside protection circuit itself, but measuring components did not show fault (the ones I found I rectified), even changed all 3 transistors...
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