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Marantz PM66KI amp - resistor keeps dying!
Marantz PM66KI amp - resistor keeps dying!
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Old 9th March 2019, 06:04 PM   #31
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
OK, lets go through that bit again

Quote:
I removed Q758 and Q760 As per your instructions and shorted c-e on Q752. WITHOUT replacing any other components R764 will SMOKE/GLOW/DIE
That shows that both output transistors must be faulty (or there is a physical short somewhere). The only way that the resistor can get voltage across it in this configuration is via the collector/base junction in the output transistors.

Quote:
NEXT I replaced Q764 with the original one, then powered on and tested the amp. THEN I replaced R764 with BRAND NEW one and re-tested the amp.
So just Q764 is replaced along with the resistor.

Quote:
During the previous TWO tests the following happened:
I powered on the amp and the "series lamp limiter bulb is *not on* or is imperceptibly dim.
R764 is cool to the the touch and doesn't move".
AND with the BAD resistor in, R764 = 155.5 ohm (when amp is on); but with a BRAND NEW one in, R764 = O.L (false reading?) when the amp was on and R764 = 328.8 ohm when the amp was subsequently powered off.
Replacing either of the output transistors with a new one would now not burn up the resistor. So Q764 is now assumed good, and that means the collector/base junction is now blocking the current in the resistor. Thats why it doesn't smoke and burn.

Q762 still has to be faulty though because the resistor burned originally. In the post above I mentioned that both output transistors have to be faulty for that to happen.

Note... you can only test resistors when there is no external voltage across them, and no external influence from other parts fitted around them and so that means for reliable results they normally have to be checked out of circuit)

Now you move on to refit Q758/760, however Q762 is still very much suspect. This means that Q760 is now pulling current through Q762 collector/base junction, through the 330 ohm and out to the negative rail via Q760.

Q760 may or may not be faulty, either scenario would burn the resistor in this state.

Hope that makes some sense

What you really need to do is just replace the output, drivers and pre drivers.

The MJL21194/21193 are the output transistors. For the drivers (Q758/760) you could try BD139 and BD140 or MJE340 and MJE350. The pre drivers could be MPSA42 and MPSA92.

You would have to look at the physical packages and make sure these look suitable. The pinouts of the MPSA device would be different although not an issue in practice. ML21194G and ML21193G are T0264 flatpack packages.
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Old 14th April 2019, 07:59 PM   #32
dfear is offline dfear
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Quote:
Originally Posted by Mooly View Post
OK, lets go through that bit again



That shows that both output transistors must be faulty (or there is a physical short somewhere). The only way that the resistor can get voltage across it in this configuration is via the collector/base junction in the output transistors.



So just Q764 is replaced along with the resistor.
Yes, that's correct


Quote:
Originally Posted by Mooly View Post
Replacing either of the output transistors with a new one would now not burn up the resistor. So Q764 is now assumed good, and that means the collector/base junction is now blocking the current in the resistor. Thats why it doesn't smoke and burn.

Q762 still has to be faulty though because the resistor burned originally. In the post above I mentioned that both output transistors have to be faulty for that to happen.

Note... you can only test resistors when there is no external voltage across them, and no external influence from other parts fitted around them and so that means for reliable results they normally have to be checked out of circuit)

Now you move on to refit Q758/760, however Q762 is still very much suspect. This means that Q760 is now pulling current through Q762 collector/base junction, through the 330 ohm and out to the negative rail via Q760.

Q760 may or may not be faulty, either scenario would burn the resistor in this state.

Hope that makes some sense

What you really need to do is just replace the output, drivers and pre drivers.

The MJL21194/21193 are the output transistors. For the drivers (Q758/760) you could try BD139 and BD140 or MJE340 and MJE350. The pre drivers could be MPSA42 and MPSA92.

You would have to look at the physical packages and make sure these look suitable. The pinouts of the MPSA device would be different although not an issue in practice. ML21194G and ML21193G are T0264 flatpack packages.
So just to check?
-----------------
MJL21194/21193 = Q762 and Q764
MPSA42 and MPSA92 = Q754 and Q756

Also I am correct in thinking that you are saying that the 3 pairs of components (Q762/764, Q754/756, Q758/760) should all be replaced at the same time. Should I be replacing R764 at the same time too?


P.S. Thanks for all your help so far.
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Old 15th April 2019, 07:23 AM   #33
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
You only need replace the resistor if it reads obviously out of tolerance (due to overload) or is physically burnt up.

The MJL21194/3 replace Q762 and Q764 and the MPSA's Q754 and Q756.

It's really a case of just rebuilding the output stage with known good components as faults like this are very common. And remember to use the bulb tester during testing
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Old 15th April 2019, 03:37 PM   #34
dfear is offline dfear
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Quote:
Originally Posted by Mooly View Post
You only need replace the resistor if it reads obviously out of tolerance (due to overload) or is physically burnt up.

The MJL21194/3 replace Q762 and Q764 and the MPSA's Q754 and Q756.

It's really a case of just rebuilding the output stage with known good components as faults like this are very common. And remember to use the bulb tester during testing
Any recommendations for places to buy components?

I normally use RS components or Farnell. Backups are Mouser/Digikey/Rapid.

Are Cricklewood Electronics trustworthy and reliable?
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Old 15th April 2019, 05:51 PM   #35
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
I normally use CPC for a lot of stuff (part of Farnell) or RS if CPC don't cover it. Can't recall ever using Cricklewood over the last twenty years or more so no idea what the stuff is like tbh.
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Old 16th April 2019, 01:30 PM   #36
jaycee is offline jaycee  United Kingdom
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RS, CPC and Farnell are all fine.. though Farnell have deteriorated somewhat since they became part of Avnet!

A lot of the parts in this amp will be obsolete and hard to get, so modern equivalents is the key. Fairchild/Onsemi do a good range of parts.

For small signal/predrivers KSA992/KSC1845 will replace 2SA970/2SC2240 very well.
For replacing the drivers MJE243/253 would be acceptable, KSC2690/KSA1220 would be better.
For the outputs, NJW0281G/NJW0302G would be a better fit than those MJL21193/4 devices... or even better, 2SC5242/2SA1962.
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