Ear Yoshino 834P hum problem

[Alan UK]

Hello everyone.

I have not used the amp for years so decided to sell it. The chap who bought it complained of an audible hum from both speakers and he returned the amp for a refund.

The year of manufacture is 1986. The amp has a gain control and a MC / MM switch. The hum increases as the gain is increased. Inputs short circuited.

According to a spectrum analyzer app on a phone the frequency emitted by my speakers is 50Hz which is the UK mains frequency.

I decided to change all the electrolytics. The circuit diagram that came with the amp showed different values to those actually fitted so I messaged Ear Yashino for advice. The engineer replied suggesting the following values to use. C8 100uf 200v. C9 & C10 47uf 400v. C11& C12 100uf 200v. C13 & C142200uf 35v. I also changed C 101 & C106 for 100uf Elna silmics. N.B. C106 is denoted as C6 on the diagram. C101 is the corresponding capacitor for the left. channel.


The hum diminished but it was still audible. As suggested in a forum discussing a similar problem which I can not now find I fitted new Sovtek 12AX7WA tubes to reduce the gain slightly. The hum is now much less but I can still hear it when the gain is on max and my Benchmark pre and power amps are turned well up. Using a DMM I could see what appears to be a 50Hz frequency on the heater suppliy.

One strange thing is that if I remove the top cover from the Ear (just a flat metal sheet) the hum reduces, Tried earthing the cover to the single earth point on the rear but no changeI can not find out if


this level of hum is normal for the Ear / Benchmark combination. Should I just have a hiss from the Ear or would you expect a hum at these levels.



I want to sell the amp again but I am not confident that it is fixed.



Help please. Alan.

 

[Keith Flanker]

50Hz hum that tracks the gain control with the inputs shorted points to coupling before or inside the first gain stage, and the cover behaviour is the big clue — if removing the metal cover reduces the hum, it's mains-transformer stray field coupling into high-gain wiring or the input tubes, not a grounding problem (earthing the cover would have helped if it were shield-to-ground). With AC heaters you'd typically see 50Hz; check the heater wiring is tightly twisted and routed away from the input triode grids, and try rotating or shielding the mains transformer. It's also worth verifying the hum is on the heater rail itself rather than riding on the B+ (a scope at the output with gain maxed will show which). For a 1986 834P some residual hum at max gain is normal, but audible through a Benchmark chain suggests the field coupling fix above is worth doing before you sell.

 

[Redacted]

An old transformer can lose internal rigidity, and the hum could be acoustically coupled.

 

[Alan UK]

Sorry about the late response. Been away for a few days with grandchildren and it's all go.

Thank you both for making the effort and taking the time to respond. The transformer is a torroid. I can lift it out of the case and see what happens.
Going on hols abroad for 6 weeks on Sunday so can't check things out for a while.

 

[Redacted]

Should be better if it's a toroid. Valves can react badly to acoustic coupling, but from the lid thing, it sounds like a grounding issue or some kind of pickup. Hard to say - try selling it as a project, and get solid state!

 

[Alan UK]

Tried today with the transformer outside the case and the hum was worse. Refitted the transformer and found that the hum increases or decreases as the transformer is rotated about it's central mounting bolt. Tried wrapping the transformer in aluminium foil but no difference. I got a Michael Fidler LO PRO phono amp for my birthday a couple of weeks ago. Photo of the Ear in case it gives anyone any clues.