Marantz SR-640 SR640 Idle current

[vinyltocdr]

hello,

i just got done restoring a Marantz SR-640. on the amp board there is a idle current adjustment for each channel (along with a dc offset adjustment).

does anyone have a service manual or know where to put the meter probes and what mA to set the idle current to for the outputs ?

if anyone could tell me this information, it would be greately appreciated.

thanks for any help.

Steve

 

[kcollins4]

Steve, I wish I could help you, but don't have the answer. I do have a question, though. Twenty posts in 5 years? Don't be such a stranger.

 

[vinyltocdr]

marantz sr 640 idle current

thanks for the invitation ! lots going on...as always.

 

[SB Marantz]

You restored the receiver and don't know how to adjust the bias?

What have you done to it?

Bias can be messured over the emitter resistor's.
DC over the speakerterminals.

 

[vinyltocdr]

marantz sr 640 idle current

i replaced all the caps in the amp and powersupply, realigned the am, realigned the fm discriminator, osc, mpx, etc and all that with an audio gen, rf gen, and a scope. i can do that stuff in my sleep. however, SS amp settings concern me because transistors are certainly not as forgiving as tube gear.

i just need to know what setting to set the idle current to. on each set of outputs, there is one emitter resistor with three legs. a center, a left, and a right. do i put the probes on the center pin and pick a right or left pin ? if so, what current reading to i adjust the idle current pot to ? 8-10mA like other marantz's ? i don't have the service manual and i want to get this right.

i have restored vacuum tube receivers, amps, radios, etc and am very fluent on all those procedures for the past 30 years. i don't have much experience on SS stuff and want to be sure on this. as we all know, misaligned solid state gear is not as tolerant and forgiving as vintage tube gear, especially in the outputs :tears:

any advice would be appreciated.

thanks.

 

[SB Marantz]

5 to 8 mV is more then enough.

 

[Richard C.]

mV is what you actually read & adjust...

The .2 ohm to .47 ohm power emitter resistors are mostly there to access emitter idle current. In some models, they are also used to trigger the output protection circuit in case too much current is flowing in the outputs.(If there are parallel output transistors - BIG amps!, these will also help balance the sharing of the load current).

You will either have to get a service manual, have someone tell you, or pick a safe value of idle current in mA, which would require ohms law to determine mV to mA conversion for the particular value of resistor.

In your case, I think that you would measure mV from one emitter to the other emitter (3 term series emitter resistors = common collector).

8 to 10 mA should be about right. DC output should be very close to zero (10 mV or so, but varies with temperature).

Richard C.

 

[Mike Stone]

hello,

i just got done restoring a Marantz SR-640. on the amp board there is a idle current adjustment for each channel (along with a dc offset adjustment).

does anyone have a service manual or know where to put the meter probes and what mA to set the idle current to for the outputs ?

if anyone could tell me this information, it would be greately appreciated.

thanks for any help.

Steve

You put a millivolt meter across one of those white emitter resistors and adjust each bias control (left and right) until you read 10mv across one of the left emitter resistors and across one of the right emitter resistors. Good luck! Oops! This is a really OLD thread.