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Marantz 8B red plate

Mike Chan said:
The voltage in between R31 & R32 is 470volt instead of 408 volt.
The voltage on the other end of R32 is 450volt instead of 390 volt.
Can you tell me why these voltages are so high? Is it normal?
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I am losing track. Were these voltages measured with the output tubes removed?
 
With no output tubes, all of your B+ voltages will be elevated. I don't think its anything to be concerned about.
 
Improper bias adjustment and higher line voltage can both cause this. Low current draw in the output tubes makes for high plate voltage. No tubes at all means no current draw, so the voltages go way up.

Voltages on this old stuff were often specified +/- 15% and assumed input voltage of 117 vac. Its usually spelled out on the schematic somewhere.
 
I try to measure the resistance of my Marantz 8B by following the Resistance Chart of Service Manual. Basically, all the readings are close to the manual except that the V1 A & B 6BH7 pin 2 shows that it is 1.3 K Ohms instead of 13 K Ohms ??? It seems that there is something wrong there. Please comment. Thank you
 
Mike Chan said:
Thank you for your reply. I already order two set of 5 K Ohm potentiometer ahead and waiting for their delivery. I will also try your method to see if it can be solved or not.
BTW, do you still remember at the early beginning I also found that:
The voltage in between R31 & R32 is 470volt instead of 408 volt.
The voltage on the other end of R32 is 450volt instead of 390 volt.
Can you tell me why these voltages are so high? Is it normal?
Click to expand...
The power transformer may have taps on its primary (I do not know if that amp as them or not) to handle somewhat different AC line voltages, and is it possible that you have a higher line voltage connected to a tap meant for lower line voltage? That would cause heightened voltage readings you are experiencing.
 
Mike Chan said:
I try to measure the resistance of my Marantz 8B by following the Resistance Chart of Service Manual. Basically, all the readings are close to the manual except that the V1 A & B 6BH7 pin 2 shows that it is 1.3 K Ohms instead of 13 K Ohms ??? It seems that there is something wrong there. Please comment. Thank you
Click to expand...
My 8B (Serial # 8-6862) has a 6CG7 rather than 12BH7. The chassis marking near the tube says 6CG7. Pin 1 plate resistor is 15K, and pin 6 plate resistor is 18K, both measured and listed on the schematic.
 
Mike Chan said:
I try to measure the resistance of my Marantz 8B by following the Resistance Chart of Service Manual. Basically, all the readings are close to the manual except that the V1 A & B 6BH7 pin 2 shows that it is 1.3 K Ohms instead of 13 K Ohms ??? It seems that there is something wrong there. Please comment. Thank you
Click to expand...
13K is the correct reading (R4 12K plus R27 and R28 1620 ohms + 178 ohms in parallel). Have you replaced C3 A and B yet?
 
ramjet said:
Define "separate". Electrically, physically, mechanically?
Please answer my question "Have you replaced C3 A and B yet?"
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Not yet replace C3, I am still waiting for my parts to delivery. How to physically separate these two potentiometers.
 
I notice that R4 A, B its physically color code is Brown/Red/Red, so it is only 1.2 K Ohms instead of 12 K Ohms from the printed Service Diagram. If this is true, the pin 2 measurement should be 1.3 K Ohms instead of 13 K Ohms from the printed manual. Right?
 
Mike Chan said:
I notice that R4 A, B its physically color code is Brown/Red/Red, so it is only 1.2 K Ohms instead of 12 K Ohms from the printed Service Diagram. If this is true, the pin 2 measurement should be 1.3 K Ohms instead of 13 K Ohms from the printed manual. Right?
Click to expand...
Right!
 
Mike Chan said:
Weeks ago, my Marantz 8B turned to red plate (EL34 V4A) suddenly while listening to music. The fuse blowout also.

I found that R20A 100 ohm resistor was burned, I replaced it with a new one. The music plays again for about 5 minutes. The El34 V3A turned red again and the fuse blowout again.
I checked that the selenium diode was open circuit. So I replace it with 1N4007. I adjust the bias, however after 2 minutes the El34 V4B start getting red. I switch off the Amp and take four EL34 out.

I measure the bias is -50.5 volt.
The voltage in between R31 & R32 is 470volt instead of 408 volt.
The voltage on the other end of R32 is 450volt instead of 390 volt.

Are these voltages normal or too high?
Can anybody help me to fix this problem? Thank you so much.
Click to expand...

Two most likely causes are either a leaking coupling cap, or a bad connection between the octal socket and the tube pins.
Either can cause the socket to lose neg bias which turns the tube full on.

You may want to pull the tubes, bring up the amp on a variac to about 90VAC and look for your neg bias voltage, look for DC leakage at the coupling cap for that suspect socket.
Then power off the amp and check the snugness or each octal receptacle in the suspect socket to insure its a snug fit with the tube pin. If you have a dead power tube you can cut off a pin and use that as a test gauge for each socket pin.
 
After replacing the two 5 K Ohms potentiometers (replace the two 5 K Ohms AC Bal potentiometers at the same time as I can't separate them). It looks that the bias are back to normal. It can be adjust in between -2V to -3.2V with the power from a variac set at 10V AC. I am going to replace some old capacitors after that before put the EL34 tubes back to make a test. One question is that how can I adjust the AC Bal if I don't have a 50Hz signal Generator and a telescope to complete this step?
 
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