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Yamaha C4 preamp fault. Hum. Phono switches.

H

Hifibythekg

Super Member
Have done a lot of work on this preamp and am starting a fresh thread because several faulty components have already been replaced or repaired and am hoping it may now be much easier to isolate and identify the problem.

The issue:

Hum is present on all inputs.

-Playing vinyl on Phono 1 and 2, the hum is present BUT, despite the underlying hum, the audio is super clear and dynamic and evenly balanced on both speakers.

-When the Cartridge Load switch has ‘100 ohms’ selected the clear dynamic music is reduced to a muffled/bass and overwhelmed by the hum.
-When MC phono input is used only the sound of “muffled/bass and overwhelmed by the hum” is produced and turning both Cartridge Load dials makes no difference to this muffled sound quality. At no point does the clear music (as produced when MM Phono 1 and 2 are utilised) come through.

Repairs to date.
Full disassembly and servicing of:
  • Wafer switches.
  • Cartridge Load switches
  • Reflow of solder and checking for solder bridges on all boards
  • Replacement of eight 820 ohm carbon resistors that appeared to have previously suffered heat damage.
Replacement of all caps on:
  • Power circuit board
  • Flat amp board
  • Tone / headphone amp board
  • RCA jack board checked and solder reflowed.
...Obviously, I’ve still overlooked something... but given the symptoms described above, can anyone deduce what the fault might be?

(A clue? When audio is playing there is some bleed through to the other inputs. Feels like about 10% of the volume bleeds through.)
 
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I believe bleed through is caused by dirty switches mostly.

What carts are you using to test, that is MM and what MC .

Athansios
 
There are 3 or 4 circuit board grounding screws that need to need secured to the metal chassis.
 
Thanks Nashou.
Only using an MM cart for all tests.
(As expected, it plays louder with the volume control at the same position.)
Other than that, I’m hoping that the behaviours described in my opening post might point to a location where the fault most probably lies.
Given these behaviours, does it suggest that there’s a bad connection near a wafer switch, or the cartridge load switches, or a component nearby?
Tapped the area in and around these switches with a wooden stick but it has no effect at all..
 
The only board I haven’t yet recapped is the Function board.
But before embarking on this I’d just like to try and locate the whereabouts of the fault.
Do the symptoms I’ve outlined suggest anything I should check, or even revisit..?
 
Metal case solder connections on the two large slide switches on the function board. The metal cases complete a circuit ground for the function board.
 
Check and reflow all ground connections on the rear RCA jack board.
 
Must emphasise the cartridge load switch phenomenon.
From bright sparkly music it goes to a quarter of the volume and totally muffled
 
Nashou66 said:
Ahh, I always try to fix the issue before any restoration. Because I might make the issue worse and then I'd be chasing my tail trying to figure it out.

Hope its something simple that was just overlooked.

Nashou
Click to expand...
Thanks Nashou, yes, I really feel that it will be something ’simple’ but nothing I do has found it yet
 
avionic said:
Check and reflow all ground connections on the rear RCA jack board.
Click to expand...
Yes...
Although, I’ve already done this... actually spent a long time cleaning and testing for solder joins on the RCA board..
Before I revisit, could it be anything else?
Appreciate your advice, and of course I may well have missed something.
 
Hifibythekg said:
Must emphasise the cartridge load switch phenomenon.
From bright sparkly music it goes to a quarter of the volume and totally muffled
Click to expand...
100 ohm is usually for MC cartridges. I bet that loading is for them not MM. Only connect a MM cart to a MM input, and MC to only an MC input unless there is a switchable input that can take both but those are very rare.

Nashou
 
Thanks Nashou.
But it’s strange the way it sounds when the MM cartridge is connected to the MC input. So muffled and in such contrast to the clarity through MM 1 and 2. Makes me think that Dave’s suggestion to recheck the RCA board’s ground connections might be at fault.
 
Hifibythekg said:
Thanks Nashou.
But it’s strange the way it sounds when the MM cartridge is connected to the MC input. So muffled and in such contrast to the clarity through MM 1 and 2. Makes me think that Dave’s suggestion to recheck the RCA board’s ground connections might be at fault.
Click to expand...


Thats how it will sound usually . The voltage of the MM might be too high for the MC section and cause it to be over driven, hence the crappy sound.

Athanasios
 
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