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Marantz PM66KI amp - resistor keeps dying!
Marantz PM66KI amp - resistor keeps dying!
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Old 17th February 2019, 10:03 PM   #11
dfear is offline dfear
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Join Date: Feb 2016
Quote:
Originally Posted by Mooly View Post
Is Q704 dead short? Have you checked this out of circuit ?
Results of todays testing:

With the device powered on (through the series lamp limiter):

Q704 -
c-e = 000.0 ohm resistance and no continuity.
c-b = 000.0 ohm resistance and no continuity.
Q702 -
c-e = 127 ohm resistance and continuity.
c-b = 20.32-20.43 M ohm resistance and continuity.
With the device powered off and the following components removed from the pcb:

Q704 -
LCR tester showed that the component is: NPN. hFE = 545.
(first test indicated 2x diodes, but subsequent tests showed as NPN).
c-e = O.L. on both resistance and continuity modes.
c-b = O.L. on both resistance and continuity modes.

Does these results indicate that Q704 has a dead short? and should be replaced?

Should I replace R764 (as it's dead (it glows!)) as the same time as replacing Q704?
or should I replace Q704, power on the amp, test (Q704); then replace R764?
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Old Yesterday, 04:02 AM   #12
Ian Finch is offline Ian Finch  Australia
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Location: Coffs Harbour, NSW
Your "LCR" tester is giving you a meaningful measurement for Q704 that says the transistor is apparently OK because it works as it has a high current gain. Resistance measurements require that the circuit is unpowered so that the meter's battery alone supplies the small test voltage from which resistance is derived. Don't waste your time trying to measure resistance or continuity in a powered circuit and likely damaging your meter. Measure voltage drop instead and of course, zero voltage difference would mean a short in circuit where you expect at least some DC differential.

All bipolar transistors though, have a small fixed diode voltage of about 0.65V between B and E and reverse polarity for NPN or PNP type. This is a quick check of function, along with a check for a short between C and E. Use the low voltage range for this on a simple multimeter.

As you found with your micro based component tester, they need to identify what you connect to them to give a meaningful readout and its easy to confuse them, perhaps because the manufacturer didn't do enough research or install the appropriate firmware for the latest chip revision. I have a few that consistently tell me that JFETs are a pair of diodes too and I had to pay like 10 times as much for a better version that is still prone to the occasional error.
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If only I had listened....
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Old Yesterday, 08:47 AM   #13
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
The only way to get a definitive result on Q704 is to remove it and test it out of circuit, however as Ian suggests, your reading from the component tester suggest it is not short circuit.

Just for reference, a short circuit means there is conductivity. 0.00 ohm on the meter is a dead short but the meter can give results like this due to residual voltage around the part being tested confusing things, not to mention interaction with surrounding components.

You should be seeing approximately 1.3 volts across the two series connected diodes D702 and D704 (approx 650 millivolts across each) and you should also see approx 650 millivolts across R722 which is the 68 ohm connected to the emitter. If that is OK then Q704 is OK.

There are a couple of possibilities as to why R764 is glowing.

Firstly, the fact its glowing mean that there is high voltage across it. This means (no doubt whatsoever on this) that there is a problem with the output transistors and their associated components.

Put simply, this high voltage across the resistor must also be present between the base's of the two output transistors, and that in turn will turn them fully on which will apply a near short across the rails.

So you will find that some combination of parts in the final output stage will have failed.

These parts are the 2.2 ohm base stopper resistors, the double 0.1 ohm emitter resistor connected to the output transistor and the output transistors themselves.

At least one (and probably more) of those resistors has to be open circuit and likely one (or even both) the transistors are short circuit C to E.

The above are all collateral damage and replacing these on there own won't fix the fault unless you have the incredibly unusual situation of a B-C short on one of the outputs.
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Old Yesterday, 11:04 AM   #14
DF96 is online now DF96  England
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Join Date: May 2007
Quote:
Originally Posted by dfear
I was following instructions from:
Is that your usual 'repair' technique?

Anyway, you can now charge all the work you have created for yourself to the 'education' budget of your business.
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Old Yesterday, 12:40 PM   #15
dfear is offline dfear
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Join Date: Feb 2016
Quote:
Originally Posted by DF96 View Post
Is that your usual 'repair' technique?

Anyway, you can now charge all the work you have created for yourself to the 'education' budget of your business.
Not the usual technique; then again I don't usually work on amplifiers.

Definitely an education experience.
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Old Yesterday, 12:45 PM   #16
dfear is offline dfear
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Join Date: Feb 2016
Quote:
Originally Posted by Ian Finch View Post
Your "LCR" tester is giving you a meaningful measurement for Q704 that says the transistor is apparently OK because it works as it has a high current gain. Resistance measurements require that the circuit is unpowered so that the meter's battery alone supplies the small test voltage from which resistance is derived. Don't waste your time trying to measure resistance or continuity in a powered circuit and likely damaging your meter. Measure voltage drop instead and of course, zero voltage difference would mean a short in circuit where you expect at least some DC differential.

All bipolar transistors though, have a small fixed diode voltage of about 0.65V between B and E and reverse polarity for NPN or PNP type. This is a quick check of function, along with a check for a short between C and E. Use the low voltage range for this on a simple multimeter.

As you found with your micro based component tester, they need to identify what you connect to them to give a meaningful readout and its easy to confuse them, perhaps because the manufacturer didn't do enough research or install the appropriate firmware for the latest chip revision. I have a few that consistently tell me that JFETs are a pair of diodes too and I had to pay like 10 times as much for a better version that is still prone to the occasional error.
Results of testing Q704 out of circuit:

Diode mode:

original = 0.725 V (+ve on B).
new = 0.694 V (+ve on B).
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Old Yesterday, 02:16 PM   #17
dfear is offline dfear
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Join Date: Feb 2016
Quote:
Originally Posted by Mooly View Post
The only way to get a definitive result on Q704 is to remove it and test it out of circuit, however as Ian suggests, your reading from the component tester suggest it is not short circuit.

Just for reference, a short circuit means there is conductivity. 0.00 ohm on the meter is a dead short but the meter can give results like this due to residual voltage around the part being tested confusing things, not to mention interaction with surrounding components.
Results of initial out of circuit testing, for Q704 are in post #11.

I have some new results.

Out of circuit testing results for Q704:

Resistance:

New

c-e (NEG/POS) = O.L
c-e (POS/NEG) = 6.459 M ohm
c-b (NEG/POS) = O.L
c-b (POS/NEG) = 6.815 M ohm

Orig
c-e (NEG/POS) = O.L
c-e (POS/NEG) = O.L
c-b (NEG/POS) = 5.488 M ohm
c-b (POS/NEG) = O.L


Quote:
Originally Posted by Mooly View Post
You should be seeing approximately 1.3 volts across the two series connected diodes D702 and D704 (approx 650 millivolts across each) and you should also see approx 650 millivolts across R722 which is the 68 ohm connected to the emitter. If that is OK then Q704 is OK.
D702 and D704 - are you implying voltage drop? if yes, I should be testing the diodes in circuit (with the amp powered on), correct?
R722 - are you implying voltage drop?


Quote:
Originally Posted by Mooly View Post
There are a couple of possibilities as to why R764 is glowing.

Firstly, the fact its glowing mean that there is high voltage across it. This means (no doubt whatsoever on this) that there is a problem with the output transistors and their associated components.

Put simply, this high voltage across the resistor must also be present between the base's of the two output transistors, and that in turn will turn them fully on which will apply a near short across the rails.

So you will find that some combination of parts in the final output stage will have failed.

These parts are the 2.2 ohm base stopper resistors, the double 0.1 ohm emitter resistor connected to the output transistor and the output transistors themselves.
which components are you referring to?
are the "double 0.1 ohm emitter resistor" = R768.
is the "output transistor" = Q762 or Q764
2.2 ohm base stopper resistors = ? & ?

Quote:
Originally Posted by Mooly View Post
At least one (and probably more) of those resistors has to be open circuit and likely one (or even both) the transistors are short circuit C to E.

The above are all collateral damage and replacing these on there own won't fix the fault unless you have the incredibly unusual situation of a B-C short on one of the outputs.
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Old Yesterday, 03:16 PM   #18
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
Q704 results look basically OK. If you use the 'Diode' range on your meter and place the red lead on the base and the black on the emitter you should see a reading of around 0.650. That is actually the meter measuring and recording the volt drop across the junction. It will be similar for the B to C. Reverse the leads and it should read whatever the meter displays when the leads are not connected.

All the voltages I mentioned earlier are with the amp on. Q704 is used as a simple current sink with a fixed voltage reference on its base provided by the two diodes. So around 0.65 volts across each diode and the same 0.65 will appear across the 68 ohm.

There are two output transistors, Q762 the upper NPN device and Q764 the lower PNP one.

R768 is labelled as a single device which implies it is a three terminal part consisting of two series connected 0.1 ohm resistors. Individual resistors can be used as replacements if needed.

The 2.2 ohm stoppers are R766 and R774 which couple the base of the output transistors to the driver stage in front.
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