My D6c has slight audio hum on both channels when the volume is turned up above 4. Turning on Dolby will reduce the sound but will not eliminate it. I suspect it's one of the capacitors, but have no idea which ones to pin point. Otherwise the unit is fine.
Does somebody here with expertise on the D6C help me out here? I have the service manual.
Is this running on batteries or through an AC adapter?
does both on either.
However, record is fine so I can rule that out.
Anything to do with the playback/record switch can also be ruled out as I have already took it apart and cleaned the contacts which solved a previous crackling problem. Volume slider is also cleaned, heads demaged, etc.
It also comes through the line-out, but only at very high levels. So the cause may be before both the lineout and headphone amps.
The unit is a SMT board Serial 299541.
Does it sound as bad on left and right channels?
Looking at the schematics, if it is bad caps... there are several capacitors before the line amp that may be the cause. (C1xx for left replace with C2xx for corresponding right channels)
Note: there are quite a lot of caps to check, but seeing as it is the D6C, Sony felt the need to over-engineer the circuitry to get the very best sound out the device.
C110 (4.7uF) is the first cap in the playback path.
C112 (4.7uF) is used for noise filtering
C115 (1uF) is the second cap after the 3 stage EQ amp circuit
C131 (1uF) is a noise filter cap for the VCC line on the Dolby chip IC101
C135 (1uF) is the third cap after the Dolby chip, before the line amp IC301
In addition it may be worth also checking these :-
C136 (1uF) is the fourth cap after the the line amp IC301, before the headphone amp IC201
C138 (100uF) is the fifth cap after the headphone amp IC201
If it is a noise filter cap failure, then it may also be worth checking :-
Around the Dolby chip there are a number of caps involved in noise filtering
C130, C132, C127, C122, C123
OMG thanks alot. I'll most likely try to check the noise filter caps first and yes, it is the same on both left and right channels. Does this mean that there are 2 identical caps that are failing at the same rate on both channels, or a single cap before both? Or something before that?
What is the best method to check the caps? Is it possible to check them in circuit without desoldering?
EDIT: Checked vs my manual and it seems most of them are tantalums which have a low chance of failure. Should I try only perhaps replacing the electrolytics first?
Only 3 seem to be electrolytics out of the list.
The elect caps are: C138 (100uf), C130 (0.33uf), C122(1uf).
Yes, would seem unlikely that the tants are failing. Check the electrolytics first.
What is the best way to check them? In the past for other units I just replaced them all in one shot. Is it possible to check them without desoldering?
Since the hum is identical on both left and right channels, and the listed caps are channel respective it makes me wonder if it can be a capacitor that comes before them or a cap that is shared by both channels.
I realise that if I increase or decrease the speed tune the hum's pitch goes along with it.
quote:
Originally posted by bub:
What is the best way to check them? In the past for other units I just replaced them all in one shot. Is it possible to check them without desoldering?
Since the hum is identical on both left and right channels, and the listed caps are channel respective it makes me wonder if it can be a capacitor that comes before them or a cap that is shared by both channels.
I realise that if I increase or decrease the speed tune the hum's pitch goes along with it.
The first thing to check for is electrolyte leakage around the rubber seal. Best results for capacitance measurement are gained from removing the cap from the board, but you can get a rough measurement in situ.
I had a thought about your latest piece of info above. If since the noise is throughout and changes in pitch with speed control, then it may well be to do with the actual power line feed, in which case it may be worth checking C316 and C317. These are two relatively meaty caps used for voltage conditioning.
A visual check of all the caps shows that they all seem fine with no evidence of leakage/bulging of any sort.
This hum cannot be heard when music is playing, and the hum is significantly lessened when playing a blank tape rather than no tape at all. (The analog tape hiss is louder) And it only becomes slightly audible at levels too loud (4-5 and above), so technically I can live with it. (Heads are demagged with a TDK HD01.)
I am inclined to be less invasive on my D6C as it is my only cassette recording unit that can pull off quality recordings (My nakamichi suffers from... you guessed it. Orange Cap disease! woooo). The D6C sounds amazing- better than anything else I have.
Sorry only just found this through the advanced search tool that we now have, as it was buried deep.
You could test the ESR (equivalent series resistance) of the suspected capacitors. If they are out by a factor of 5 times or more of their nominally rated resistance then it is likely they are faulty. Typical values of capacitors can be gleaned from component manufacturers data sheets or comparing with an equivalent good part.
ESR meters can be bought or constructed from kit form. Some of the best cheapish ones are the Anatek Blue or the EVB ESR meter. Both are available to buy ready built or in kit form and start from a reasonable €49.
ESR is best measured when the device is cold, as when the capacitor warms up the ESR will fall and could give a false reading. The capacitors can generally be tested in-circuit negating the need to remove them from the PCB first.
