Recently a customer of mine brought a Marantz PM66 KI amp in for repair. I was told that one of the channels wasn't working. I was told that the left channel wasn't working and that the volume knob was also loose. The customer also wants to upgrade the two huge caps to Mundorf AGs, but that's not relevant to the current problems I am having.
I tightened the nut on the volume knob, which seemed to cure most of the problems with the amp.
STEP 1:
-------
as per:
https://www.diyaudio.com/forums/solid-state/70830-marantz-pm66ki-amp-post805742.html
I changed Q764 Q762 Q752 Q754 Q758 Q756 Q760 QN02 QN03 anyway.
Also I did:
"once rebuilt set volume to minimum , balance to center position , set the bias VR to center position (R756 or R755).
Set your digital (not analogue) multimeter to mV and connect to the two outer test points of the output resistor (R767 or R768). "
For the first minute or so every thing was OK, then the problems started. First R760 went up in smoke, then R764 started glowing red hot in the center of the resistor. I switched off the amp ASAP.
STEP 2:
-------
I tested R760, which was dead and R764 which started at around 200 ohms then quickly rose to 100 ohms plus and then the meter showed O.L.
I replaced R764 with a 330 ohm 5% 2W resistor (PR02000203300JA100 | Vishay PR02 Series Axial Metal Film Fixed Resistor 330Ω +-5% 2W +-250ppm/degC | RS Components) and R760 with a 100 ohm 5% 1/4W resistor (FRN25J100R | TE Connectivity FRN25 Series Axial Thin Film Fixed Resistor 100Ω +-5% 0.25W +-350ppm/degC | RS Components) as I couldn't find a 1/6W 100 ohm one.
Also I removed the new transistors Q756 and Q760 and replaced them with the original ones I removed from the amp. I tested both with my LCR tester. The new 2SA970 (which is Q756) was shown as a 2 diodes, where as the original one shows up as PNP transistor.
STEP 3:
-------
I turned on the amp and R764 glowed, then went up in smoke; straight away. R760 *didn't* fail and hasn't failed yet.
I replaced R764 for the 2nd time and removed the new 2SD3033A (which is Q758) and replaced it with the original one.
STEP 4:
-------
I turned on the amp and R764 glowed, then went up in smoke; straight away.
I have checked Q760 and Q756, which seem overly hot.
What should i do next?
---------------------
I plan to replace the new 2SD3033 (Q758) with the original one and replace the burnt out R764. I this the correct next step? or should I be checking and replacing other components?
Thanks for any help anyone can give.
The service manual: http://testing.dfear.co.uk/pm66ki.pdf
I tightened the nut on the volume knob, which seemed to cure most of the problems with the amp.
STEP 1:
-------
as per:
https://www.diyaudio.com/forums/solid-state/70830-marantz-pm66ki-amp-post805742.html
I changed Q764 Q762 Q752 Q754 Q758 Q756 Q760 QN02 QN03 anyway.
Also I did:
"once rebuilt set volume to minimum , balance to center position , set the bias VR to center position (R756 or R755).
Set your digital (not analogue) multimeter to mV and connect to the two outer test points of the output resistor (R767 or R768). "
For the first minute or so every thing was OK, then the problems started. First R760 went up in smoke, then R764 started glowing red hot in the center of the resistor. I switched off the amp ASAP.
STEP 2:
-------
I tested R760, which was dead and R764 which started at around 200 ohms then quickly rose to 100 ohms plus and then the meter showed O.L.
I replaced R764 with a 330 ohm 5% 2W resistor (PR02000203300JA100 | Vishay PR02 Series Axial Metal Film Fixed Resistor 330Ω +-5% 2W +-250ppm/degC | RS Components) and R760 with a 100 ohm 5% 1/4W resistor (FRN25J100R | TE Connectivity FRN25 Series Axial Thin Film Fixed Resistor 100Ω +-5% 0.25W +-350ppm/degC | RS Components) as I couldn't find a 1/6W 100 ohm one.
Also I removed the new transistors Q756 and Q760 and replaced them with the original ones I removed from the amp. I tested both with my LCR tester. The new 2SA970 (which is Q756) was shown as a 2 diodes, where as the original one shows up as PNP transistor.
STEP 3:
-------
I turned on the amp and R764 glowed, then went up in smoke; straight away. R760 *didn't* fail and hasn't failed yet.
I replaced R764 for the 2nd time and removed the new 2SD3033A (which is Q758) and replaced it with the original one.
STEP 4:
-------
I turned on the amp and R764 glowed, then went up in smoke; straight away.
I have checked Q760 and Q756, which seem overly hot.
What should i do next?
---------------------
I plan to replace the new 2SD3033 (Q758) with the original one and replace the burnt out R764. I this the correct next step? or should I be checking and replacing other components?
Thanks for any help anyone can give.
The service manual: http://testing.dfear.co.uk/pm66ki.pdf
I'm afraid that replacing a whole bunch of semiconductors wasn't a great move. You should have proceeded to do normal checks and tests on the amplifier in its original faulty state rather than replace parts in hope.
At this stage, using the DBT that mbz advices above is the first thing to do.
At this stage, using the DBT that mbz advices above is the first thing to do.
I now have a series lamp limiter.
R764 still goes up in smoke and glows. Q756 and Q760 no longer hot to touch.
Q704 has c-e short (beeps on continuity mode - bear with, i'm still learning).
R764 wobbles like crazy when the amp is on, so is it getting so hot that it's melting the solder on the legs?
What's the next step?
------------------------
I assume that because Q704 has a c-e short, it should be replaced. Am i correct?
As R764 is dead it also will be replaced.
Anything else to test before replacing the failed components?
Last edited:
Just done a little testing on components near/connected to R764.
results:
R774 = 2.2 ohms
and
R766 = 2.4 ohms
The obvious thing to do is change Q704.
I would be checking all the transistors as one going can take others with it.
I don't understand why you changed so many semiconductors.
Its electrolytics that age in an amp not the transistors.
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?
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.
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.
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.
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.
Not the usual technique; then again I don't usually work on amplifiers.
Definitely an education experience.
Results of testing Q704 out of circuit:
Diode mode:
original = 0.725 V (+ve on B).
new = 0.694 V (+ve on B).
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
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?
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 = ? & ?
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.
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.
todays results:
Diode test - Q704
b-e = 0.640 (forward), 2.454 (reverse)
b-c = 0.001 (forward), O.L (reverse)
Voltage drop test:
R722 = 0.47 V
D704 = 0.54 V
D702 = 0.74 V
Side note:
The lamp in the series lamp limiter has gone from a medium brightness to dim or very dim.
AND R764 is no longer very hot when the amp is powered on. also R764 = O.L ohms
R768 - left and centre pins and right and centre pins = 000.3 ohms
R766 = 2.3 ohms
R774 = 2.3 ohms
OK, so based on the actual measurements...
D702 and D704 should have virtually identical volt drops as they are type similar devices (and probably from the same batch) and they are series connected. If they are really that far apart then something is amiss. The ISS254 is a general purpose diode and can be replaced by 1N4148 types.
As confirmation of that you can do the same voltage check on the good channel. Each diode should have similar voltage dropped across it.
That said, the values you have are workable and the voltage across R722 is in the correct ballpark. Assuming the voltages were measured accurately then 0.54v +0.74v is 1.28 volts. Subtract the B-E volt drop of approx. 0.65v and we get 0.63 volts. That would be closer to the expected value of voltage across R722... however
I would recheck those voltages and compare with the good channel. The theory is sound, its just your measured values are a little unexpected.
Something is definitely amiss there. 0.001 would be almost a short circuit which doesn;t compute with it acting as a diode in the reverse direction.
The fixed resistors seem to be OK.
When you say the lamp is now dim, is that because you are running it with Q704 out of circuit, or has something else happened ?
If it is because Q704 is removed then that is actually a good sign and does indicate some issue with or around that transistor.
D702 and D704 should have virtually identical volt drops as they are type similar devices (and probably from the same batch) and they are series connected. If they are really that far apart then something is amiss. The ISS254 is a general purpose diode and can be replaced by 1N4148 types.
As confirmation of that you can do the same voltage check on the good channel. Each diode should have similar voltage dropped across it.
That said, the values you have are workable and the voltage across R722 is in the correct ballpark. Assuming the voltages were measured accurately then 0.54v +0.74v is 1.28 volts. Subtract the B-E volt drop of approx. 0.65v and we get 0.63 volts. That would be closer to the expected value of voltage across R722... however
I would recheck those voltages and compare with the good channel. The theory is sound, its just your measured values are a little unexpected.
Something is definitely amiss there. 0.001 would be almost a short circuit which doesn;t compute with it acting as a diode in the reverse direction.
The fixed resistors seem to be OK.
When you say the lamp is now dim, is that because you are running it with Q704 out of circuit, or has something else happened ?
If it is because Q704 is removed then that is actually a good sign and does indicate some issue with or around that transistor.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.