I'm working on a pair of MC30s and one has got some problems.
It runs hot and distorted.
The voltage at the rectifier (pin 2) starts appropriately and then swiftly drops to under 300vdc. At start up, sometimes there is a fad in/out hum like a power tube imbalance that goes away.
Same story with voltage at the power tubes.
Filament voltage at the rectifier is 5.2vac, 6.6vac at the power tubes.
One issue is that pin 5 on the power tubes is measuring +2vdc, and the manual states that it should be -42vdc.
Any direction would be much appreciated.
It runs hot and distorted.
The voltage at the rectifier (pin 2) starts appropriately and then swiftly drops to under 300vdc. At start up, sometimes there is a fad in/out hum like a power tube imbalance that goes away.
Same story with voltage at the power tubes.
Filament voltage at the rectifier is 5.2vac, 6.6vac at the power tubes.
One issue is that pin 5 on the power tubes is measuring +2vdc, and the manual states that it should be -42vdc.
Any direction would be much appreciated.
First check the bias supply diode and cap. Sometimes the cap will fail short and take out the diode with it.
If it's fine, check the coupling caps and the 12AX7 driver tubes.
If it's fine, check the coupling caps and the 12AX7 driver tubes.
Replace the bias diode first. It sounds like it is shorting as it warms up. It might measure fine cold. Make sure the caps are good too under load. I would just replace them as they are cheap and don’t last forever.
If the grid bias supply is shorted/open/bad, . . . or Dead, there is no negative voltage at g1 of the output tubes.
But if the bias supply is good, and g1 is +2V, then . . .
The tube is very gassy / leaky
Or
The coupling cap is very leaky
Or
The wipers on the adjustment potentiometers from the grid bias supply are not making contact.
P.S. I wish posters would describe the product in question more completely.
Marantz XYZ
Fender 123
Etc.
I am lazy, and I do not want to have to open a separate window/web page to see what a model NAV or a model 795 is.
But if the bias supply is good, and g1 is +2V, then . . .
The tube is very gassy / leaky
Or
The coupling cap is very leaky
Or
The wipers on the adjustment potentiometers from the grid bias supply are not making contact.
P.S. I wish posters would describe the product in question more completely.
Marantz XYZ
Fender 123
Etc.
I am lazy, and I do not want to have to open a separate window/web page to see what a model NAV or a model 795 is.
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Best practice is to replace the selenium bias rectifier and its 10uF capacitor on both chassis'. Parts are cheap, unless you wait too long and melt the output valves and/or the output transformer. Retest first with the output valves removed - Pin5 must have reasonable bias voltage (maybe -30VDC or -40VDC or so). Then install output valves and recheck.
All good fortune,
Chris
All good fortune,
Chris