It surely could be. If the hum didn't suddenly appear after making some parts swaps, the power supply caps would be where I would look (good practice to replace anyways if they're that old).
They started long before I swapped out any parts. I was beginning to wonder if some of the PC board traces were bad so I pulled a couple and hard-wired them. I also tried bypassing the cans but of course that didn't do anything.
Did the hum start suddenly or gradually over some months? If the former then look for a grounding connection come loose. If the latter then it could be the smoothing caps.
It happened over time. Basically everything except for the little tantalum caps, ceramic and the can cap have been replaced. The diodes are original too.
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OK. Given a gradually worsening hum the main PSU caps should have been the first things to replace. Probably nothing else needed changing.
Can you tell me the value of the smoothing or PSU caps by the schematic? I am not a technician but I can replace parts well. If the abbreviation for PSU is power supply unit then yes, I know. I am thinking of just getting a few 400V solens. It will be hard to fit them but I will make it happen.
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Shane...the probability is that one of those can caps is bad. If the can cap is (multi section electrolytic capacitor), get some caps for each section of the can, and wire those caps across the corresponding section of the can (watch polarity and markings) and see if the hum disappears. There has to be a source for the can caps....Be very careful working with the can caps...the bakelite wafer support is brittle and it will break easily.. If the can cap is actually bad and no replacement can be found, some "purists" will save the terminal strip from the can, and gut the contents, wire replacement single value capacitors for each section inside the can, and reseal it...
Thanks RMB. I was considering 4 solen 30mfd black caps. Putting the top of the chassis is not a big deal to me.
Would you advise "not" to go in this direction?
http://solen.ca/products/capacitors/fast-capacitors-400v-pb-series-metallized-polypropylene/pb3000/
Would you advise "not" to go in this direction?
http://solen.ca/products/capacitors/fast-capacitors-400v-pb-series-metallized-polypropylene/pb3000/
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Shane...you could probably get 4 of those 30 uF caps in that can. Surprised the preamp has a BJT (bipolar junction) output...I would also check the 10,000 uF cap on the low voltage heater supply...look at it with a scope for AC ripple, or substitute another capacitor...the 10,000 uF cap may also be a single section metal can unit....
Okay. I don't have a scope but since a 10k uf cap is cheap, I will do it anyway.
And no, the 30uf solens are pretty big.they are almost the size of the can cap, each.
And no, the 30uf solens are pretty big.they are almost the size of the can cap, each.
If you check on tube vendor sites, JJ makes some can caps with multiple sections and they are fairly affordable. Clamps available, too.
Are those caps mounted through the top of the chassis?
Are those caps mounted through the top of the chassis?
No Soda, they're the original caps so they're internal.
What is the abbreviation JJ?
Oh, never mind... I found them
What is the abbreviation JJ?
Oh, never mind... I found them
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REPLACE the voltage regulator on the heaters also . Those thing are much improved in 35 years and very inexpensive. If you are up to it replace it with an adjustable voltage reg it is quieter do to the cap bypass on the adjustment leg as well as setting the voltage closer to the 12.6 volt heater point . The last is not that big a deal as 12 is close enough given it is regulated.
Those should work, but yeesh at the price! There are 32+32 or 50+50 500V JJ caps on the same site for about half the cost. Don't get me wrong, I'm sure the Mundorfs will work. Just don't know if the price tag is going to gain you anything here.
Yeah, there's always the diminishing returns.
So, Soda, judging by the schematic. those are the right caps for me to buy? I am going to pay someone to put them in for me. I just wanted to check in with one of you guys before I spend the moolah.
So, Soda, judging by the schematic. those are the right caps for me to buy? I am going to pay someone to put them in for me. I just wanted to check in with one of you guys before I spend the moolah.
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