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Yamaha C4 preamp fault. Hum. Phono switches.

Hifibythekg said:
In the UK we have adapter plugs to convert a two pin to three pin mains socket.
Click to expand...
Maybe this is the source of your hum. The adapter plug itself.. Is there a dimmer switch in your work area ? CFL lighting ?
 
If you have hum, check first all (soldering)points and connections with ground. Sometimes it has on soldering points a hair crack or not well soldered.
If you have repleced parts, do they in correct position?
 
avionic said:
Maybe this is the source of your hum. The adapter plug itself.. Is there a dimmer switch in your work area ? CFL lighting ?
Click to expand...
Thanks, but just tried three different adapters and the hum continues. No dimmers or CFL lighting etc but I get you drift. The other amps plugged into the same sockets have no hum whatsoever.
Is there anything I can do to isolate or rule out a particular board or area of the amplifier which would allow me to know that the hum definitely isn’t generated from, say, the function board, or any board.
Any other analytical tools? Freezer spray..?
 
Also, which, if any, of these tasks would you prioritise? (Was going to recap the caps on the Function board in any event.) If I can’t obtain Nichicon Muse for every value on the Function Board please could you name three other Brands/series name to try for?
The infamous Flat Amp C313 33uF 16V.. I can use a Nichicon UPW or a ChemiCon EKRG.. which is better here?
“-Wholesale replacement of all diodes on the PSU and flat amp?
or
- Wholesale replacement of all caps on the Function board?
or
- Check wafers again?”
 
Cartridge Load...

At all settings PHONO is clear as a bell although with underlying hum (these switches were dismantled and cleaned) BUT at '100' setting the sound from the turntable goes super muffled almost distorted. All solder joins are fresh and look good.
Could there be a faulty component nearby? If so, which one(s) to test?
Phono cartridge is a MM.
 
Cartridge Load...

At all settings PHONO is clear as a bell although with underlying hum (these switches were dismantled and cleaned) BUT at '100' setting the sound from the turntable goes super muffled almost distorted. All solder joins are fresh and look good.
Could there be a faulty component nearby? If so, which one(s) to test?
Phono cartridge is an MM.
 
With turntable connected and the ground from turntable to amp disconnected there is a clear buzzy hum which disappears, after some scratchy noises as the connection is made. Should point out that the ‘ground hum’ noise is a proper ‘buzz’ as you described. Very different to the amp’s own hum. If the turntable provides a 120Hz hum it might suggest that the amp’s hum is actually a 50Hz?
Also, the turntable hum and audio bleed through doesn’t occur between PHONO and AUX/TUNER when a record is playing.
 
Hifibythekg said:
Cartridge Load...

At all settings PHONO is clear as a bell although with underlying hum (these switches were dismantled and cleaned) BUT at '100' setting the sound from the turntable goes super muffled almost distorted. All solder joins are fresh and look good.
Could there be a faulty component nearby? If so, which one(s) to test?
Phono cartridge is a MM.
Click to expand...
Sorry, this has already been explained to you at least two times: The 100 Ohm position is for high output MC cartridges only. Any typical MM cart optimized for a load impedance of 47kOhm will sound “super muffled”. This is so, accept it.
 
Thanks Dr Ear, fair comment and point taken.
Well, at least I’m doubly reassured that it’s working correctly. I remember being advised that the effects of all the other settings on the cartridge load switches are very subtle, and this is exactly how the switches in my C4 behave even after a thorough disassembly and servicing. (It’s almost as though the only audibly operational setting is the 100ohms one!) Hope others agree that it should be so. If that’s the case then it would appear that the adjacent components (polyesters?) are functioning correctly.
At this point of an unexpectedly long marathon, I’m grateful for every part of the amp that gets a ‘pass.’
Given the amp’s symptoms please feel free to suggest anything that might move me a step closer to solving this issue. Thanks again for the contributions to date, appreciated.
 
Thanks everyone for all your help.
This amp has had such a horribly persistent and hard to solve fault that at times it’s felt insoluble or that a solution would require the replacement of every single component. It had certainly taken the ‘fun of a challenge’ out of the project. The usual and sensible solutions for curing a hum just weren’t achieving a result in this instance so there were many, many false dawns!

Anyway... a recommendation by MBZ to check and provide the AC voltages on the Flat Amp board at +45, -45, 12, +30, -30 led to the answer.

The data provided led him to diagnose that a short had occurred c-e to the transistors in the area of TR 626 and TR628.

Testing in circuit, TR628.. did indeed have this short!

I replaced this transistor (which must itself have been previously replaced, it was something other than a 2SA777 in this location) with a TTA004B, Q.

Result: no hum!!

DC Offset readjusted and pretty stable at 0mV +/- 40mv.

Still got some audio bleed between Aux and Tuner, rocking/operating/deoxit-squirting the switches has not yet achieved a cure to this.
Any suggestions welcome.

PSU, Flat amp and Tone Control board have all been fully recapped.
Am going to try it out in the system with the mostly original Silmics on the Function Board, but very tempted to replace all these caps with Nichicon Muse or equivalents at some point. Again, comments welcome. At this point though it’s sounding wonderful and I’m happy to have it working.

So glad and, again, sincere thanks to everyone who helped inch me closer to a solution with every contribution.
 
Thanks Blue Shadow. Yes, so relieved.
Thanks also for the simple fix but, yes, in practice the bleed isn’t an audible problem in practice as I don’t have anything via ‘Tuner.’
 
Actually I have also hum in the right phono channel. In MM-stage or Reverse-RIAA. Also I found, that instead like circuitmap R257/R258 were replaced with VD1212. I will change all VD1212 to 1N4148.
 
From memory.., does the VD1212 require replacement with a double diode joined in series? Hope others will comment.
Good luck with your task.
 
Thanks @avionic , phew..
Maybe I’ll stick with asking questions rather than attempting to answer them... : )
So, while you’re there.. can you see anything wrong with using a JFE2140 (SMD) on an adapter pcb to replace a 2SK100?
Apparently the specs should match well.
Both channels would get replaced in an A-1.
 
Hifibythekg said:
Thanks @avionic , phew..
Maybe I’ll stick with asking questions rather than attempting to answer them... : )
So, while you’re there.. can you see anything wrong with using a JFE2140 (SMD) on an adapter pcb to replace a 2SK100?
Apparently the specs should match well.
Both channels would get replaced in an A-1.
Click to expand...


Why are they bad? Which ones are you wanting to replace? In the recent A1 I repaired I just removed them, cleaned the legs and reinstalled them and the issues I was having went away, could not adjust some biases on the phono amp section.

Nashou
 
Thanks Nashou.
My A-1, after servicing was finally running great but...
Was getting an occasional ‘static, crunching cardboard at volume’ sound through the left channel. Swapped the 2SK100’s and the problem disappeared on the left channel but occurred on the right channel!
Tested the likely faulty 2Sk100 on the transistor tester and one set of three pins gave a very different reading.
I will look again at those legs though, good point.
Any view on the JFE2140?
Apparently it’s very low noise, the internal FETS are very tightly matched, and thermally very tightly coordinated.
 
Hifibythekg said:
Thanks Nashou.
My A-1, after servicing was finally running great but...
Was getting an occasional ‘static, crunching cardboard at volume’ sound through the left channel. Swapped the 2SK100’s and the problem disappeared on the left channel but occurred on the right channel!
Tested the likely faulty 2Sk100 on the transistor tester and one set of three pins gave a very different reading.
I will look again at those legs though, good point.
Any view on the JFE2140?
Apparently it’s very low noise, the internal FETS are very tightly matched, and thermally very tightly coordinated.
Click to expand...

Is there a board to mount them to that would fit the current package the A-1 uses?

Athanasios
 
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