• Check out the Annual AK Secret Santa exchange Program, signup before November 23rd in the AK Charities & Christmas forum!

CR 2020 Differential Pair Replacement

mjdupuis

mjdupuis

Be not rash
In this case I may have just enough information and knowledge to be dangerous.
Before performing Service Bulletin Mods the left channel performed at a slightly weaker level than the right based on observation of the sp meters, though there was no audibly discernible difference.
After performing the Service Bulletin Mods I set the bias with readings at Left- 10.5mV and Right- 10.0mV and checked DC offset with readings at L- 1.1mV and R- 21.1mV.
While the amp was apart and accessible I decided to do some research on correcting DC Offset on the CR-2020. (I must note: I am not doing this out of necessity; I don't believe this is a serious issue but a slight one I believe can be easily fixed. I just noted that something was a little off and wanted to optimize everything while it was on the bench.) From what I have gathered replacing the differential pairs on the corresponding Main board (Left in this case) is the best way to correct the DC offset. If I am going to replace the differential pair should I replace the pair on the Left and Right Main boards together? The Zetex ZTX795A was mentioned as a possible replacement but is this the best match for this application? Does anyone have any experience with a more exact replacement? Since this application calls for closely matched transistor pairs, does anyone have any advice on pairing them up? Thanks in advance
 
Register to hide this ad
Yes the ZTX795A will do. These need to be closely Hfe matched. You will need a means to measure the Hfe and purchase a handfull of these transistors ,check /measure the Hfe of them all until you find a pair with the highest Hfe that are within 1 or 2 % of each other. Preferably exactly the same.The closer the better.:thmbsp:
Diff pair--- OEM are 2SA872 --TR602 and 603.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ZTX795A-ND

More exact replacement...
http://www.bdent.com/2SA872-p/2sa872.htm

Remember your still going to need to "match them" which means purchasing 15-25 pieces - maybe less,maybe more.

Zetecs are cheaper and higher gain(Hfe)..
 
Last edited:
left channel performed at a slightly weaker level than the right based on observation of the sp meters
Click to expand...
Meters are just eye candy and are probably in need of alignment.Might be a procedure in the service manual- need to be adjusted for equal output display with a known input source ie. 1 Khz tone with a audio signal generator.
 
Last edited:
Thank you very much for the information. That would be exactly what I needed to know. While on the subject, by what means should the transistors be measured?
 
Last edited:
In an attempt to avoid having to purchase a new piece of equipment (a new multimeter or other test apparatus) I'm trying to find a way to test hFE. I stumbled across the following formula:

base current IB = VRB / RB

collector current IC = VRC / RC

hFE = IC / IB

hFE = ( VRC / RC ) * ( RB / VRB )

Knowing very little about this to begin with, would this be a viable alternative to using a dedicated piece of equipment?
 
Most likely close enough..:thmbsp:

That is cheap..:D LOL


I used something very similar for a long time..wore out the transisor test socket in it.:D
 
Last edited:
mjdupuis said:
In an attempt to avoid having to purchase a new piece of equipment (a new multimeter or other test apparatus) I'm trying to find a way to test hFE. I stumbled across the following formula:

base current IB = VRB / RB

collector current IC = VRC / RC

hFE = IC / IB

hFE = ( VRC / RC ) * ( RB / VRB )

Knowing very little about this to begin with, would this be a viable alternative to using a dedicated piece of equipment?
Click to expand...
Read here: http://www.diamondstar.de/transistor_matching_bjt.html
 
I must be on the right track then, that's the very page I copied the formula from. Thank you, gentlemen, for your help.
 
avionic said:
Most likely close enough..:thmbsp:

That is cheap..:D LOL


I used something very similar for a long time..wore out the transisor test socket in it.:D
Click to expand...

One of many reasons I still keep a cheap Radio Shack hand-held meter around.
 
Output Transistor

I have a related question (related to the area of the amp I'm working):

I have purchased replacement output transistors for the Left and Right channels: MJ21193G & MJ21194G. Of these two, which one is the direct replacement for the 2SD424 and which one is the replacement for 2SB554?

I'm on the final leg of this rebuild and have loved the process so far. Any general advice about finishing it up?
 
Ztx795a

The layout of the ZTX795A seems to differ from the 2SA872, the former being Emitter, Base, Collector; the latter being Emitter, Collector, Base. Do these have to be modified to be installed?
 
OK. You just need to position the leads making sure the correct lead gets inserted into the correct location on the circuit board without shorting any of the three against each other.:D
 
Complete

Just to give everyone an update: the CR-2020 is fully functional and sounding fantastic. I replaced the differential pairs on both channels and while in there replaced all the electrolytics on those boards as well as the output transistors. I couldn't have done this without the help of the community; you guys are great. I am satisfied with the results. I'm just sitting back enjoying the music right now (Rush- A Farewell to Kings).

bias: L- 9.9mV
R- 9.9mV
offset: L- 6.0mV
R- 4.3mV
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom