Bias adjustment for "click off" on CR-2020

jdg123

Active Member
I have only the faintest notion of what I am doing here, no on-site mentor to show me things so forgive my trepidations but I am very hopeful for the life of my CR-2020 continuing and I am oh-so-close to getting it up and running…I'd hate to make a stupid mistake now.

I have replaced almost every cap in the unit and some of the key resistors. I have executed the service bulletins. The unit comes on/plays for about 30 seconds but then it clicks off. It has been suggested that the bias/idle current may be way off and thus enacting the "protection circuit".

Taking a reading will have to be quick and I am unsure of how to set up my DMM.

On page 12 of the service manual it talks about connecting points TP1 and TP0. That is where I am unsure. TP1 is clear; it is marked on the board even. Is "TP0" (in the picture) that first solder connection? I am really, really new to this stuff

I know voltage measurements are supposed to be done with polarity in mind and between two specific points but I am never sure what two points! I need arrows; I need explicit directions…I need help, please...


When I am sure of where to clip my leads I will try to get a reading on the idle current in the few seconds I get before the protection clicks on and then I guess a second time to get the turn direction of the adjustment resistor correct and then some more adjustments until I get to the specified 10mA. I just need a little help here.:thmbsp:
 

Attachments

  • bias adjust on CR-2020.pdf
    91 KB · Views: 85
Hello jdg123

Don't panic ! it is easy to do if you are carefull !
TP1 is indicated on the board yes and TP0 is the first solder connection ! o like OUTPUT .
See my picture if you are not sure

:thmbsp:
 
Last edited:
set your meter on mv dc .. if the reading stays steady and it still clicks off then i suspect the dc offset is going high or there is a problem in the protection circuit .
 
Just what I needed

:banana:Thank you clinic-audio and petehall347, I will take the information to heart!
 
if you end up having to take voltages at speaker terminals before and when relay clicks off post them as plus or minus .it doesn't make as much sense otherwise .and of course which channel .
 
OK, will do.

The voltage at the speaker terminals at idle is called DC offset no? I recall testing the unit when I bought it as a "health check", it was under 20mV I'm sure. I'd have to look up the numbers.

Seems I know just enough to get myself in trouble...but it is still fun! Thanks for the help!
 
using E and TPO measure the dc offset after the protect circuit relay has tripped. If you still have less than 100mv of dc offset then you have a problem in the protect circuit itself or a power supply problem .

Have a close look at the solder connections for the relay driver transistors
TR702 and TR706. Wouldn't hurt to reflow some fresh solder on them. TR702 is a TO-220 style and tends to run rather hot by design.This weakens and cracks solder connections over time. ie heat/cool cycles.
 
Last edited:
When you replaced the capacitors on the Electrolytic board,did you pay attention to the polarity of the caps you removed or depend on the polarities as silkscreened on the circuit board? On the 2020 there is a polarity marking mistake on that board.
 
Bias adjustment for "click off" on CR-2020
Bias will have little to do with your intermittent protect relay issue.To much bias and the output transistors overheat.Prolonged overheating at some point will eventually short out the output transistors .Once there shorted, there permanently destroyed and will definitely trigger the protect circuit. To little bias causes distortion. However, DC offset has a larger role in triggering the protect circuit.
When the protect circuit detects DC voltage at the speaker connections it disables the protect relay - disconnecting the speakers from the amplifiers. Excessive DC offset will trigger the protect circuit.
 
Just a question for my education...if you have reason to suspect too much bias, should you remove the output transistors and get your measurement (and adjust)?
 
Just a question for my education...if you have reason to suspect too much bias, should you remove the output transistors and get your measurement (and adjust)?

Bias is the current flow through the outputs collector/emitter at quiescent (idle). If you remove the outputs how can you measure current passing through them? To answer your question --No.
 
Last edited:
Bias is the current flow throught the outputs collector/emitter junction at quiesent(idle). If you remove the outputs how can you measure current passing through them? To answer your question --No.

Collector/emitter junction? What kind of transistors are we talking about here? :scratch2: :D
 
Bias is the current flow through the outputs collector/emitter at quiescent (idle). If you remove the outputs how can you measure current passing through them? To answer your question --No.
OK, now that makes sense. Bias is used as a term here like it is with tubes. Not that I know any more about that, but I see it frequently referred to with tube gear. So output tranny is akin to output tube?
Sorry to move the thread slightly askew, just trying to learn something.:scratch2:
 
OK, more data, very cool. I had to put it all aside for the weekend; now I can get to the testing.

avionic: I caught one mis-oriented cap on the electrolytic board (the vertical board with the giant caps) when I went back to look at the "before" images. That would be the 1000/50 on the far left looking from the cap side. I mistook a (+) meant for the main cap as a guide for that 1000/50. After a photo review I saw that the (+) goes up, toward those big diodes.

I found a mistake in the service manual on the power supply board (pictogram, p16): TR707 is pictured with the emitter in the opposite side relative to its neighbor TR708 but the board is marked and the unit is built with them both in the same direction. This shatters my illusions of perfect paperwork! :-(

I am going to try to get a future post to be sticky-noted mirroring your advice: Mark the polarity of every cap BEFORE you photograph the rig!

Short answer: no I did not pay strict attention to every cap as I pulled it out for replacement. I wish I would have marked and photographed. I think all the other caps match the board markings, I have been over them twice...

Do you have knowledge of which caps are mis-marked on any of the boards?


I will check out the solder work specifically on the TR702, TR706.

I did not specify that I have changed the output transistors and should have. I will check bias first thing today.

Thanks to all for the input. I will post data from my tests later today. :yes:
 
I will check bias first thing today.
Not really interested at the bias reading. DC offset/voltage is key!
Measured at TPO and E E = chassis ground..while the unit is in protect mode.
You will still be able to check DC offset at this point when the relay is set or reset because its at a point before the relay.
 
Last edited:
OK I will check potential between TP0 and E (earth)...

Almost ready. I had a lot to button up.

jdg
 
Back
Top Bottom