beepre right channel hum [resolved]

[bgf]

so i bulit this about seven years ago. it goes with the two mono blocks and an eros phono preamp. i worked on getting rid of the ground loop hum but now i have developed and loud hum right channel only not dependent on volume control intermittent in nature.  comes on as tubes warm up then disappears and reappears randomly with time. three peaks measured by advanced spectrum smart phone app at 64 hz 129hz and 236 hz.  the loudest hum peaking at 129hz.  i might add that this noise changes in volume.  again only the right channel.  i have also played with switching tubes and it still is only coming from the right channel.  any help would be much appreciated.  thanks.

 

[Paul Joppa]

What is your power line voltage? A low voltage will cause a loud hum in the original BeePre. The whole USA seems to be pretty warm right now, and air conditioners can draw the voltage down.

 

[bgf]

line voltage if i measured it correctly at plug is 120 volts with air condition unit going.

 

[Doc B]

Next step is to check voltages and post any that are off.

 

[bgf]

ok you are working with a basic guy here who a long time ago i finished my degree in chemical engineering unfortunately not electrical so i need simple "how to instructions" to provide you with the numbers. thanks.

 

[Paul Birkeland]

These are in the construction manual we provided with the kit.  You would have had to complete them after building your kit originally. 

 

[bgf]

thanks found the construction manual and working on it. 

 

[Paul Birkeland]

The terminals of immediate interested would be POS IN and POS OUT on the noisy channel.

 

[bgf]

terminal      voltage
1                4.48
2                97.5
3                0
4                9.72
5                0
6                147.1
7                0
8                0
9                0
10                184.7
11                147.1
12                0
13                0
14                0
15                184.6
16                5.02
17                98.6
18                0
19                9.84
20                0
21                0
22                211.6
23                0
24                0
25                211.7
26                0
27                0
28                  (0) .041   
29                  6.62
30                   

6.62
31                  13.30
32                  0
33                  0
34                    (13.6) 11.8 ?
35                    (6.8)  5.17 ?
36                    (6.8)  5.16 ?
37                    0
38                    0
39                    0
40                    211.3
41                    0
42                    0
43                      211.3
44                    0
45                    0
46                      211.3
47
48                    0
49                    0
50
51                    0
A1
A2                    (0)  96.5    ?
A3                    (90-110)  0
A4

B1
B2                      (0) 98.3    ?
B3                      (90-110)  0
B4

c1,d1 
c2,d2               
c3,d3                  4.48    3.48
c4,d4
c5,d5
c6,d6
c7,d7                  147    147
c8,d8
c9,d9                  146.8  146.7

ok voltage check.

right channel  POSIN  13.11  POS OUT  9.93 this is the channel of interest
left channel      POSIN  13.33  POSOUT  9.85  for a check

 

[Paul Birkeland]

Can you post some build photos?  There isn't anything immediately obvious from the voltages.

 

[bgf]

build photos let me know if you need some other views.

 

[Paul Birkeland]

I would pop the power supply boards off and reflow the joints under there. 

I would also post pictures of the bottom sides of the PC boards on the noisy channel.

 

[bgf]

reflowed joints right channel no change added pictures

 

[Paul Birkeland]

It's very likely that there's a loose cap or some other connection in here that you need to track down.  One way that I do this is to run the preamp into a cheap amp and pair of speakers, then poke around in the amp with a wooden chopstick to see if I can change the nature of the noise.

 

[bgf]

so capacitor tied in at voltage check point 36 and 37 when moved up and down changes the hum.

 

[Paul Birkeland]

Then you've found what's loose.

 

[bgf]

on closer examination i was moving capacitor by prying off 1N5820 diode below it and at 37L that joint appeared to have no solder so i corrected it and so far no hum.  will keep you posted if it reappears.  thanks.

 

[Paul Birkeland]

Thank you for sticking with the process!