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Borbely DC 100 Amp: Left Channel Troubleshooting
Borbely DC 100 Amp: Left Channel Troubleshooting
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Old 15th June 2018, 03:17 AM   #1
dbxdx5 is offline dbxdx5
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Default Borbely DC 100 Amp: Left Channel Troubleshooting

An acquaintance asked me to look at this DC 100 he built back in the day. The left channel is out. At power on, the right channel heat sink warms up nice and toasty, but the left is cold. Here are the voltages I'm seeing, left and right, for the K135 and J50 output transistors:


LEFT
K135 pair

Drain: 57.8V and 57.4V
Gate: 0.275V and 0.264V
Source: .003V and .001V


RIGHT
K135 pair
Drain: 57.0V and 56.9V
Gate: 1.1V and 1.09V
Source: .048V and .05V


LEFT
J50 pair
Source: .004V and .003V
Gate: -.6V and -1.1V
Drain: -57.5V and -57.5V


RIGHT
J50 pair
Source: .059V and .050V

Gate: 1.129V and 1.11V

Drain: 57.6V and 57.2


I thought the issue might lie with the bias circuit, so I replaced Q16 and Q17 (K216 and J79 respectively), the zener diodes, the 1N4148s, and Q13 (MPS-A05). No change. When I compare the voltages of Q13, left and right, here's what I get:



LEFT
Q13 (MPS-A05)
Collector: 1.6V
Base: 1.0V
Emitter: 0.9V


RIGHT

Q13 (MPS-A05)
Collector: 1.1V
Base: 0.4V
Emitter: 1.8V


I'm going to continue checking the various components on the board, but relative novice at troubleshooting that I am, I welcome any suggestions.
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Old 15th June 2018, 08:57 AM   #2
Mooly is offline Mooly  United Kingdom
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Borbely DC 100 Amp: Left Channel Troubleshooting
We would have to see the circuit details to be able to advise on this. Although your measurements seem to indicate a lack of bias, that alone would normally only give rise to a small increase distortion so it sounds like something else is going on.

As always... a circuit diagram is a must.
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Old 15th June 2018, 10:23 AM   #3
epicyclic is offline epicyclic  United Kingdom
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The bases of Q13 should be approx 0.7V more positive than their emitters . None of the voltage readings around Q13 either left or right channel instill any confidence in their accuracy . Have you identified the leads correctly . Trying to work with the supplied voltages have you checked the resistance/ continuity of all the components associated with Q13
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Old 15th June 2018, 11:03 AM   #4
dbxdx5 is offline dbxdx5
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Mooly: Here's the circuit diagram. (European designation for transistor used.)

epicyclic: I'll check the voltages of Q13 again.
Attached Files
File Type: pdf Borbely DC 100.pdf (219.9 KB, 223 views)
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Old 15th June 2018, 11:53 AM   #5
Mooly is offline Mooly  United Kingdom
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Borbely DC 100 Amp: Left Channel Troubleshooting
Thanks...

Quick thoughts and things to check:

1/ That you have correct positive supply voltage on each end of R34 (not easy to make out some numbers) and also the correct negative voltage on the similar 22 ohm on the negative rail below it.

2/ Check the 5 volt references are correct (T7 and T10) and make sure you have approx. 4.4 volts across each 560 ohm (next to T7 and T10 on diagram).

3/ Check the voltage across the two zeners (marked D3, D6 and D7 and D10). The diode voltage isn't marked on the diagram so you will have to compare with the good channel. All four should be similar.

4/ The bias generator Q13 can only really give a fault of excess current or no current in the output stage. High current would be destructive, low current would not stop the amp from working to the point that you may not even notice.

One reading that can be inferred but needs checking is that voltage on L1 is zero. The driver voltages suggest it will be but you should check.
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Old 15th June 2018, 07:34 PM   #6
epicyclic is offline epicyclic  United Kingdom
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You say the left channel is out , please describe what you mean...no sound output at all or its very distorted or mildly distorted...etc
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Old 15th June 2018, 08:45 PM   #7
dbxdx5 is offline dbxdx5
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Quote:
Originally Posted by Mooly View Post
Thanks...

Quick thoughts and things to check:

1/ That you have correct positive supply voltage on each end of R34 (not easy to make out some numbers) and also the correct negative voltage on the similar 22 ohm on the negative rail below it.

2/ Check the 5 volt references are correct (T7 and T10) and make sure you have approx. 4.4 volts across each 560 ohm (next to T7 and T10 on diagram).

3/ Check the voltage across the two zeners (marked D3, D6 and D7 and D10). The diode voltage isn't marked on the diagram so you will have to compare with the good channel. All four should be similar.

4/ The bias generator Q13 can only really give a fault of excess current or no current in the output stage. High current would be destructive, low current would not stop the amp from working to the point that you may not even notice.

One reading that can be inferred but needs checking is that voltage on L1 is zero. The driver voltages suggest it will be but you should check.



Thanks Mooly. Here's what I came up with.



1. Supply voltages
LEFT
R34: 59.9V and 59.2V

R41: -59.9V and -59.2V


RIGHT

R34: 57.8V and 57.2V
R41: -57.9V and -57.2V





2. Reference voltages
LEFT

T7: 53.8V and 58.8V


RIGHT
T7: 57.6V and 52.6V




LEFT
T10: -54.1V and -59.1V


RIGHT
T10: -52.8V and -57.8V


I have 5V across each 560 ohm resistor next to T7 and T10.




3. Zener voltages
LEFT
D3: 19.4V and 14.5V
D6: 4.8V and 0
D7: -19.1V and -14.3V
D10: -4.7V and 0


RIGHT
D3: 19.1V and 14.3V
D6: 4.8V and 0
D7: -19.0 and -14.3V
D10: -4.7V and 0


4. I did in fact err in my measurements of T13. They should be as follows.
LEFT
E: 0.9V
B: -1.0V
C: -1.6v


RIGHT
E: 1.8V
B: -0.4V
C: -1.1v


As for L1, yes, the voltage is zero.
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Old 16th June 2018, 09:57 AM   #8
epicyclic is offline epicyclic  United Kingdom
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What voltage do you have across R38 (150 ohm )
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Old 16th June 2018, 12:00 PM   #9
Mooly is offline Mooly  United Kingdom
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Borbely DC 100 Amp: Left Channel Troubleshooting
All those voltages seem reasonable to me.

I guess this is a valid question epicyclic is asking as I took the meaning of 'left channel out' to mean no sound. But does it ?

Quote:
Originally Posted by epicyclic View Post
You say the left channel is out , please describe what you mean...no sound output at all or its very distorted or mildly distorted...etc
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Old 16th June 2018, 06:08 PM   #10
dbxdx5 is offline dbxdx5
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Quote:
Originally Posted by Mooly View Post
All those voltages seem reasonable to me.

I guess this is a valid question epicyclic is asking as I took the meaning of 'left channel out' to mean no sound. But does it ?
No sound at all out of the left channel, and the heatsink is completely cold.

epicyclic: I'll check R38.
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