• 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

MR67 rectifier replacement

Childof60

Childof60

AK Subscriber
Subscriber
I have a MR67 that has very low voltage from the pair of selenium rectifiers - 128 volts instead oh the expected 150 volts. Normally I would replace the selenium rectifiers with diodes and a dropping resistor. I recall seeing a post cautioning this this mod can damage other parts of the receiver due to the power on in-rush. Does anyone have suggestions on the best approach to replace the selenium rectifiers. The low voltage is present across the down chain. Thx
 
Register to hide this ad
Hello

did you made any measurement at the output of the power transformer with an accurate AC voltmeter ?
Do you know also the power consuption after the diode selenium bridge ?
 
Yes. Interesting as I noted on start up the voltage starts at 155 and then rapidly drops to 123. The restvier seem to test ok. I am starting to suspect the four 50 UF filter capacitors. For the first 10 minutes or so the stereo output is highly distorted then cleans up at a lower volume than with the mode set to mono. I am guessing but it might be the 12au7 does not like the low plate voltage.
 
If the MPX section has otherwise been restored, then distorted MPX stereo usually indicates problems with the 38 kHz oscillator not being properly locked to the pilot signal. Or, it can also be due to a poorly aligned tuner section as well.

Does the problem clear up instantly when it does, or gradually?

Dave
 
Hi Dave. The problem clears suddenly after 10 to 15 minutes. When it clears the output signal level is still lower than the mono setting.
 
Check C503 in the MPX circuit. These caps seem to fail at a fairly regularly rate in these MPX units, and typically become intermittent or noisy when they do. When intermittent, they are affected by heat as well. When they fail, they upset the phase of the 19 kHz pilot frequency that the 38 kHz oscillator locks onto, and when that happens, the stereo audio is of course shot. If you replace it, use a close tolerance mica cap as a replacement.

Functioning properly, the stereo output setting will always produce a slightly lower sound level than that appearing at the mono output setting -- this being a product of the slight loss in signal when passing through a properly aligned MPX section. If the stereo setting output is louder than the mono setting, that is a sure sign that the MPX section has issues in the phase relationship between the 19 kHz and 38 kHz frequencies.

I hope this helps!

Dave
 
Yeah, I was looking at an MR65B schematic, but I thought the component numbers in the MPX section would have all been the same. The schematics basically are. It is the 2700 pF cap in the cathode circuit of the 6U8 pentode section that's connected in series with the 19 kHz peaking coil (L501 on my schematic).

Sorry about that.

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