Cyrus 2 bias, please help

[sbcellis]

Hi.
I have blown 2 output trans+drivers.
I have change output ti TIP42C and drivers to MJE340 and MJE350.

It plays, but get hot. I have 55mv over the emitter resistor, on the other channel i have 5 mv.
How can i adjust the bias or is there somthing else i can do.

Thanks for reading.

Soren

Scematic: http://www.tcaas.btinternet.co.uk/cyrus1pwr.gif

[sbcellis]

Hi
It's me again.
I have read the post "Cyrus 1 project"
There is a post that show how to put a biaspot in.
It tells that the q.current is 75ma.
It is the same for cyrus 2
If i measure over 1 emitter resistor (0.22 ohm) is 0.75v the right value.
Thanks for reading
Soren

[Geoff]

Soren

Assuming that you have the later 07 version of the Cyrus (the earlier ones had a preset for adjusting the quiescent current) you can adjust the output stage current by varying the value of R81 (or R82 for the other channel).

Increasing the value of this resistor will reduce the quiescent current. For an Iq of 75mA you would have 16.6mV across one of the 0R22 resistors.

The TIP42C is perhaps not a good choice as a replacement output transistor because typically it has a far higher gain than the type originally used (BUV48A) which is actually a high speed switching transistor.

Geoff

[richie00boy]

TIP42C is on OK choice as far as gain goes, it will be about the same gain as the original device. However it's a PNP device which will definitely not work. So what did you use really?

[hacknet]

i have a dead cyrus 2 and i have the same problem. one side keeps burning itself out. the outputs are not dead but the drivers are all in a mess.

ive been scratching my head on how to fix it.

[richie00boy]

Did you replace drivers and outputs at the same time? Did you replace the bias transistor?

Have you checked DC offset and stability and quiescent current?

[sbcellis]

Sorry, you are right richie00boy. The trans is Tip 41c. Sorry.

Yes, Geoff it is a 07. The resistor r81 is 130 ohm. I will try a pot across the r81.
Do you think, 75ma as q-current is right ?

Thanks for reading.

Soren

[Geoff]

Originally posted by sbcellis
Yes, Geoff it is a 07. The resistor r81 is 130 ohm. I will try a pot across the r81.
Do you think, 75ma as q-current is right ?

If your Iq is too high, as indicated in your first post, you need to increase the value of R81. Adding a preset across it will reduce the effective resistance.

The notes I made some years ago when taking some measurements on one of my Cyrus 1s indicate that the quiescent current was about 75mA. I have not seen a service manual so I cannot confirm that this is correct.