MC60 Hum

[garmadon]

I recently bought a pair of MC60. The seller said they were re-cap and checked in specs. However, when connected to my 92dB speakers, there's noticeable humming from 8 feet away. Power tubes are new Tung-Sol.
I search in the forum and I have been tried the suggestions I found:

1 - Adjust hum pots: hum doesn't change much
2 - The Gain knobs:
- when at 0 (all the way counter-clockwise) no hum, but also no sound from speakers
- Hum increasing when increase the gain, at 9 o'clock the hum is acceptable but I still can hear the hum at 4 feet away
3 - Un-plug the input: Hum still there
4 - flipping the plugs over: it doesn't help
5 - unplug the V1 12ax7 tube: the hum goes away, no hum at all

Any thoughts on what would cause this? Is it normal?
Both amps look identical inside, so they behave the same way with all the tests I tried.

 

[garmadon]

I changed rca cable to a good Bluejean cables, and connected common and ground ports, but still they still humming.

 

[tuberadios69]

Take a picture of the external caps and the inside of the amp showing both sides of the phenolic board and post it. We should be able to judge/confirm whether the caps have been replaced or not. Sounds like a power supply issue to me, however, so it is possible you have a failing electrolytic. By the way, did you try another 12AX7?

 

[garmadon]

Yes, I did tried with another 12ax7, but no luck.
Here're the photos of one amp, the other amp look identical.

 

[tuberadios69]

The Aerovox/Mallory multi-section electrolytic caps on the top of the amp (external near the tubes) are not very recent. I believe Mallory and Aerovox stopped making caps like that close to over 30 years ago. CE Manufacturing (tubesandmore.com) and a German manufacturer make new equivalents. There will be a 4 digit number on the caps such as "3472" which, for example, would equate to "34th week of 1972". See if you can find that info in order to date the electrolytic cans. Caps over 20 - 25 years old are potential problems. If they are failing, you would hear hum. Good news is that the caps on the phenolic board have been replaced as has the selenium rectifier (with a diode). The resistors, however, look original so many are potentially out of tolerance. I'd check then replace those that are more than 10% or so off from their rated impedance.

 

[garmadon]

Thanks for your info. I dont see any thing for the date code, all I found is FP247 677-8740, and they are 2-section caps 100uF and 40uF 450VDC.
By looking at the inside, do you happens to know what version the amps and where I can get the right schematics?

 

[tuberadios69]

Oops! I reversed the year with the week of production in my comment. The 677-8740 equates to a manufacture date of the 40th week in 1987 so your electrolytic multi-section caps are over 32 years old. They are more than likely near the end of their expected life. And, at 450 volts they are a bit under rated for today's typical 122 AC wall socket volts. They should be rated at 500 or more ac volts IMHO. And, it should be a three section cap, not two. Typical, older, MC-60's usually had 80uf/50uf/30uf (ideal) or 80uf/40uf/20uf or 80uf/35uf/15uf caps on the outside top of the chassis. Whoever worked on your amps apparently subbed two-section caps and then put 22uf caps underneath the chassis for the third section. I'd recommend you have someone replace the external caps (and remove the extra ones tied to them underneath) with modern day caps like the FT Caps from Germany (rated at 550 volts) or the US-made CE Manufacturing caps rated at 525 volts. You can buy these on ebay from a seller like ctech-electronic-store or on-line from www.tubesandmore.com - I have no affiliation with either) and maybe someone on here sells them, too. Those newer caps have 4-sections (80/40/30/20) and there is advice (from the ebay seller) on how to tie the 20uf section on the cap to the 80uf section for 100uf. No guarantee this will eliminate your hum but the caps currently in place, while younger than the amps, ain't spring chickens! Check the serial number of your amps and consult MC60 schematics from any number of sources like hifiengine.com and others to match your amps to the appropriate schematic. The MC60 schematics I've seen state which serial numbers they apply to printed towards the bottom of the page. Check ebay too for schematic sources; there is one seller who puts a package together with manual, service manual and multiple schematics for the MC30's and maybe MC60's from time to time (twofrie-56).

 

[garmadon]

Thanks for helping. I'll contact those seller and get those caps replaced.

 

[garmadon]

Thanks alot for your help, Tuberadios69. I bought the CE caps from tubesandmore.com and replaced all the can caps. I also changed the series resistor (it was 4.7K one) to 5.1K as Terry recommended, and I got the bias -45V right on the spot. All the hum gone, the amps now dead quiet.