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Borbely DC 100 Amp: Left Channel Troubleshooting
Borbely DC 100 Amp: Left Channel Troubleshooting
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Old 20th June 2018, 09:12 AM   #41
Trollet is offline Trollet  Sweden
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Borbely DC 100 Amp: Left Channel Troubleshooting
Zero volts on the gates sounds like they are in contact with ground in some way.
Otherwise I belive it would not be possible ?

Just a thought - N and P devices aren't swapped ?


Figge
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Old 20th June 2018, 09:36 AM   #42
epicyclic is offline epicyclic  United Kingdom
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A simple check to try , it might get us closer . With the amp off for at least 10 mins connect your multimeter directly across the left hand output terminals and measure the resistance , you should see approx 2300 ohms ( continuity all the way through to the feedback network ) . You can obviously compare it with the other channel .

There are a lot of red wires connecting the heat sink assembly to the pcb check each red wire is going to the correct pcb pad , include the green ground wire .
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Old 20th June 2018, 12:19 PM   #43
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Originally Posted by Mooly View Post
The three 0.1's (C15, C21 and C23) would not cause this problem. If C15 was open circuit then the amp might be unstable but the scope is saying no.

The 33pF. I'd have to say no for the simple reason that even if one of them were faulty (leaky or short circuit), you would still have the 1.28 volts applied to the other one and that FET pair would work normally.

I've changed a simulation I had of a lateral FET amp to match your output stage. This is what you should be seeing. Do you see how the FET gates draw no current. There is no volt drop across the 220 ohms. The voltage between the 220 ohms should be identical at the left and right sides (your 1.28 volts). That kind of voltage causes around 200ma to flow in the lateral FET's. You should be seeing similar to this on each parallel pair of FET's.

(Ignore the 33pF on the lower FET as the designer hasn't fitted them on the lower P channel FET's.)
Thanks for mocking this up. The only part I'm having trouble following (and perhaps indicates I've been taking these measurements incorrectly) is the bolded part. I had interpreted "between the 220 ohms" to mean the voltage between the two 220 ohm resistors connecting the K135 gates (and then between the two 220 ohm resistors linking the J50 gates).
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Old 20th June 2018, 01:14 PM   #44
dbxdx5 is offline dbxdx5
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Originally Posted by epicyclic View Post
A simple check to try , it might get us closer . With the amp off for at least 10 mins connect your multimeter directly across the left hand output terminals and measure the resistance , you should see approx 2300 ohms ( continuity all the way through to the feedback network ) . You can obviously compare it with the other channel .

There are a lot of red wires connecting the heat sink assembly to the pcb check each red wire is going to the correct pcb pad , include the green ground wire .
So the right (good) channel does measure exactly 2300 ohms. The left channel measures 2000 ohms.
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Old 20th June 2018, 02:24 PM   #45
Trollet is offline Trollet  Sweden
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Borbely DC 100 Amp: Left Channel Troubleshooting
Looking back at measurements in post 1 it appears that the right channel has a similar problem, but at least one pairs working.

Gate voltages indicates that it should draw current in some way but the 6V (!) Means full on wich would give full rail on output, but it isn't.
The low values across source resistors indicates that no current flows, that they are broken, or that the fets don't conduct.

I would have checked all fets and it's wiring first.
Then source resistors, they might be broken when higher current flows.
And I'm still suspicious about those sockets

Figge
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Old 20th June 2018, 03:19 PM   #46
mmerig is offline mmerig  United States
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Originally Posted by phase View Post
There used to be a good schematic along with a manual of sorts that was on a Russian website, canít find it now however...
In 1984, Erno Borbely published a two-part series of articles on the DC 100, and its related Servo 100, in Audio Amateur. I have hard copies and can make pdf's out of them if you like. I'll send it via PM.

The DC 100 is similar to the Hafler DH220 driver boards, which uses similar Hitachi MOSFETS. I have the Hafler, and it is prone to "loss of bias" (the MOSFETS) stop drawing current). As I recall, there was a broken driver transistor.

The DH220 draws about 270 mA across the output MOSFETS, similar to the DC 100.
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Old 20th June 2018, 03:26 PM   #47
mmerig is offline mmerig  United States
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Regarding broken MOSFETS, the Hitachi's are pretty tough, and on the DC 100, the set of diodes between the two MOSFET gates (D11 to D14) protect the MOSFET gates from over-voltage. See if the zener, especially, is broken. This could be a hint for problems at the MOSFETS. There are some quick tests for broken MOSFETS using a meter and low voltage supply - see Nelson Pass's site.
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Old 20th June 2018, 04:02 PM   #48
epicyclic is offline epicyclic  United Kingdom
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Originally Posted by dbxdx5 View Post
So the right (good) channel does measure exactly 2300 ohms. The left channel measures 2000 ohms.
Can you repeat the measurements maintaining the same polarity for each channel and then do the same with reversed polarity for both channels .
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Old 20th June 2018, 05:15 PM   #49
dbxdx5 is offline dbxdx5
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Originally Posted by epicyclic View Post
Can you repeat the measurements maintaining the same polarity for each channel and then do the same with reversed polarity for both channels .
LEFT: 2000 ohms
RIGHT: 2200 ohms

LEFT (Rev. polarity): 1600 ohms and rising
RIGHT (Rev. polarity): 700 ohms and rising
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Old 20th June 2018, 05:17 PM   #50
dbxdx5 is offline dbxdx5
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Originally Posted by mmerig View Post
Regarding broken MOSFETS, the Hitachi's are pretty tough, and on the DC 100, the set of diodes between the two MOSFET gates (D11 to D14) protect the MOSFET gates from over-voltage. See if the zener, especially, is broken. This could be a hint for problems at the MOSFETS. There are some quick tests for broken MOSFETS using a meter and low voltage supply - see Nelson Pass's site.
Thanks for directing me toward the FET testing info at Nelson Pass's site. Re the diodes D11 to D14, I replaced them last week. I just checked them again, and they're good.
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