Yamaha C-4 preamp hum.

[Hifibythekg]

This amp appears to be working well in all regards except that there is a significant hum when Phono is selected.

The hum also comes through substantially on Aux and Tuner.
Hum appears to be stronger through the left channel.
Hum is lessened when ’Tone Bypass’ is selected, but disproportionately so I think.
When a source is connected to either Aux or Tuner the source is ‘perfectly’ amplified without hiss or crackle, all controls work and adjust the sound as they should but when the performance ceases the hum is present.

In an attempt to trace and fix this the following has been done:

Thoroughly cleaned the wafer switches (albeit in-situ).
Inspected, flexed and tapped the wafer switches to see whether the hum is affected. It isn’t.
Inspected, checked and tapped all solder points on the RCA board.
Tapped the all easily accessible boards wherever possible. No sounds or affect on the hum occurs.
Touching ‘Ground’ by the Phono RCA outputs, or any of the RCA outputs doesn’t produce even the smallest change in sound.

Can anyone advise?

 

[mbz]

Hum present in right channel but at lower level? eg, 10%...30%...60% or whatever of left channel
Tone bypass usually involves bypass of a gain stage (also) so hum may appears quiter.

Are you able to measure the frequency of the hum, ie, mains 50/60Hz or rectified 100/120Hz, possibly the
latter with a part in the power supply breaking down. Mobile phone apps to measure frequency or scope or...
Hum increases with volume?

EDIT: Hum also through headphones, unit has seperate headphone amp.
Headphone amp appears to be an additional stage so expect hum

 

[Hifibythekg]

MBZ to the rescue again, many thanks.

So,
Hum present in right channel?
50% of Left channel

Hum increases with volume?
Yes.

Can try to check frequency level..

 

[Hifibythekg]

No hum in headphones when they're plugged into the Yamaha C-4 headphone socket.
Hum just continues through the speakers.
When headphones are plugged into the power amp the hum through the speakers stops, but the hum is present in the headphones.
When the Yamaha C-4 was connected in another system yesterday the hum existed and presented in the same way.

 

[avionic]

Check all your ground ( E ) solder connections in this area. Inspect PCB for burnt traces.

 

[Hifibythekg]

Have a recording of the hum with three tones recorded on an iPhone (so an m4a file, but I don't think AK will allow it to be uploaded..) Let me know if you'd like it sent some other way.
It has C-4 set to 'stereo,' with balance dial centred.
Tone 1 is Aux
Tone 2 is Phono (with MM selected.)
Tone 3 is Phono (with MC selected.)
1 and 2 are identical, 3 is louder.

 

[Hifibythekg]

A clue..,
when the 'Tone Bypass' is partially depressed (between being activated and not activated) the hum nearly disappears and no matter how high the volume is turned up the hum doesn't increase in volume, it just remains as something quite faint.
If the Tone Bypass is fully depressed the hum decreases, but as the volume is turned up so the hum increases.

 

[avionic]

There is around a half dozen screws on four of the five circuit boards that need to be fastened down tight to complete the pre-amps grounding scheme.

 

[Hifibythekg]

Also, there is some crossfeed of source signal.
Source is plugged into Aux which plays and sounds ‘great’ apart from the low level hum.
Turn the dial to Phono and we get pure hum.
Turn the dial to Tuner and we get the same level hum as Phono but with a little bit of tinny source sound.

 

[Hifibythekg]

Despite having tried to clean it in situ, could the Input wafer switch be the cause of the hum symptoms?

Can this wafer switch have the Ground circuit (and, indeed, all of it) within the switch be tested with a DMM while in situ?
It’s a real pain to remove and it’s clearly already be done previously on this amp.
Could the right channel‘s readings be compared to the left channel within the switch to see whether there’s a difference in readings and a need to inspect further or repair it?

 

[Hifibythekg]

Also, on the big pair of heat sinks attached to the transistors.. the temperature is measuring 124 Fahrenheit on the heat sink attached to the transistor located closer to the centre if the amp.
The other one much less, maybe 90. I’ve read that this amp can run hot but is at least one of these readings too high?

 

[avionic]

Also, on the big pair of heat sinks attached to the transistors.. the temperature is measuring 124 Fahrenheit on the heat sink attached to the transistor located closer to the centre if the amp.
The other one much less, maybe 90. I’ve read that this amp can run hot but is at least one of these readings too high?

No .. That's about right. The two large sinks are for the pass transistors in the regulated +/- 30 vdc P/S. The single smaller centered sink is the relay driver transistor.

 

[avionic]

Pre-amp out 1 & 2 sound the same ?