• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Marantz 2238b power supply adjustment

Willarado

Willarado

Active Member
I am just finishing up restoring a Marantz 2238b and attempting to set the power supply voltage. I can only get it to 29 volts instead of the 35v stated in the manual. Bias and DC offset are spot on. Does anyone have suggestions? And it is a concern given that it is working and sounds fantastic?
IMG_7855 copy.jpg
 
Register to hide this ad
Are you adjusting this voltage while you are on DBT or connected directly to the
wall? Measure the voltages at the collector and base of the Q750.

The power supply board does not supply voltage to the main amp section so you will not have any problem with Dc offset or bias. It supplies power to Pre-amp, tuner, and phono board. you ill have problem there.
 
Last edited:
At J722 and J723 you should have 35V. At J724 you should have 14V. J722 supplies voltages to the phono and tone board. J724 supplies voltage to the tuner.
What is the voltage at J724?
 
Last edited:
I don't know whether there is any mistake on the SM or not. If the schematic is correct, I would rather connect the Red DMM port to J722 and Black to the chassis ground and adjust the voltage to 35V.
 
If you have set the voltage to 29V at J722 wrt J724 then I guess at J722 the voltage is almost 43V wrt ground and it is too high. Before switching it on, rotate the R787 to the previous position. Then adjust the voltage at J722 wrt the chassis ground to 35 V.
 
Ok, all good now. The 2238b service manual was definitely wrong. It should be J722(+) and J728 (-/ground). Sheesh, that's a quick way to mess up a rookie like me. Thanks for your help sanjib! And you were correct, the voltage using the wrong info was very close to 43V.
 
sanjib said:
The (+35V) power supply board does not supply voltage to the main amp section so you will not have any problem with Dc offset or bias. It supplies power to Pre-amp, tuner, and phono board. you ill have problem there.
Click to expand...

Wrong! The +35V supply also feeds the input differential stage and the current source for the driver stage in the power amplifier section. This +35V supply is important for the main amp.
 
Willarado said:
I can only get it to 29 volts instead of the 35v stated in the manual.
Click to expand...
Yup, same here. Thanks for this thread. The SM is definitely wrong.

Last week I was tweaking the power supp[y on a 2220.

The same error appeared in the same section in the model specific service manual.

Wrong power supply pins.

Again!

BTW: If you set the PS voltage to 29V on pins J722 and J724 as the service manual says (it won't go higher than that), you'll end up with dangerously high bias current which translate to about 130mV across the emitter resistors, instead of the desired 23mV. Thank god for the DBT!
 
Last edited:

Marantz Service Manual error thread

This is a compilation of errors in Marantz service manuals. These corrections will likely save you from smoking your receiver. Remember to use some good mini grabber leads when you're adjusting bias, I've read way too many horror stories of slipping probes. Big thanks to Tom (catrafter) for...
audiokarma.org

bookmarked

Thanks!
 
You must log in or register to reply here.
Back
Top Bottom