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Marantz PM66KI amp - resistor keeps dying!
Marantz PM66KI amp - resistor keeps dying!
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Old Yesterday, 01:34 PM   #81
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
You're measuring in the correct place

We need to nail this one...

If you can achieve 4.756 volts across C752 then things should be happening. Remember that is the voltage across the cap as measured with a meter lead on each end.

Here is what we do. This should show us why the output transistors are still not conducting.

1/ Connect your meter negative lead to the middle pin of the resistor network which is the main amplifier output and leave it there.

2/ We now measure and record the voltage on each transistor base as shown. Each time we move back toward C752 we should see another (approximate) 700mv added to the total.

Measure initially where it is easiest to get at, for example on the resistors that connect to the base's.

So six readings in all. Your measured 4.756V should allow an average of almost 0.8 volts per transistor which should be enough for it to bias correctly. So lets see where it falls down.

The voltages on the upper three transistors will be positive in value, those on the lower negative.
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Old Yesterday, 05:58 PM   #82
dfear is offline dfear
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Quote:
Originally Posted by Mooly View Post
You're measuring in the correct place

We need to nail this one...

If you can achieve 4.756 volts across C752 then things should be happening. Remember that is the voltage across the cap as measured with a meter lead on each end.

Here is what we do. This should show us why the output transistors are still not conducting.

1/ Connect your meter negative lead to the middle pin of the resistor network which is the main amplifier output and leave it there.

2/ We now measure and record the voltage on each transistor base as shown. Each time we move back toward C752 we should see another (approximate) 700mv added to the total.

Measure initially where it is easiest to get at, for example on the resistors that connect to the base's.

So six readings in all. Your measured 4.756V should allow an average of almost 0.8 volts per transistor which should be enough for it to bias correctly. So lets see where it falls down.

The voltages on the upper three transistors will be positive in value, those on the lower negative.

Upper transistors:
Q762 = 333mV
Q758 = 333mV
Q754 = 8.33V

Lower transistors:
Q764 = 334mV
Q760 = 334mV
Q756 = 3.76V

Additional info:
key for below: ref, part number, transistor type, left to right pin numbering and configuration.
Q758 = BD139 - PNP (123,BCE)
Q760 = BD140 - NPN (123,BCE)
Q762 = MJL21194 - NPN (321, ECB)
Q764 = MJL21193 - PNP (321, ECB)
Q754 = 2SC2240 (from QN01) - NPN (123, ECB)
Q756 = 2SA970 (from QN03) - PNP (123, ECB)

Last edited by dfear; Yesterday at 06:22 PM.
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Old Yesterday, 06:56 PM   #83
dfear is offline dfear
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Quote:
Originally Posted by dfear View Post
Upper transistors:
Q762 = 333mV
Q758 = 333mV
Q754 = 8.33V

Lower transistors:
Q764 = 334mV
Q760 = 334mV
Q756 = 3.76V

Additional info:
key for below: ref, part number, transistor type, left to right pin numbering and configuration.
Q758 = BD139 - PNP (123,BCE)
Q760 = BD140 - NPN (123,BCE)
Q762 = MJL21194 - NPN (321, ECB)
Q764 = MJL21193 - PNP (321, ECB)
Q754 = 2SC2240 (from QN01) - NPN (123, ECB)
Q756 = 2SA970 (from QN03) - PNP (123, ECB)
IGNORE THIS POST AS RESULTS ARE INCORRECT AND I AM UNABLE EDIT THE POST ANYMORE.
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Old Yesterday, 07:09 PM   #84
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
Hmmm... we have multiple problems here. So one step at a time.

Q754 is the one to work with at this stage. Your reading shows the base is almost 8 volts higher than the emitter.

Can you do a double check and measure the voltage between base and emitter of that device. That's reading between the two top left arrows in the above diagram.

Your readings above suggest there is around 8 volts present across the junction. That's very odd, it should limit at under a volt by the diode action of the junction.

(Be sure all the semiconductors really are correctly placed. The BD139 is an NPN type, the BD140 is the PNP)
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Old Yesterday, 07:10 PM   #85
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
LOL

No worries. I was really trying to make sense of those results.
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Old Yesterday, 07:16 PM   #86
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by Mooly View Post
(Be sure all the semiconductors really are correctly placed. The BD139 is an NPN type, the BD140 is the PNP)
I bought in an amp that wasn't working and had a similar problem with BE voltages. Turned out someone had previously tried to repair it and put in a pnp instead of an npn transistor. To help the number had rubbed off the transistor. It "diode" checked ok but when I looked into it further the diodes were backwards on the check.
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Old Yesterday, 07:18 PM   #87
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
That sounds like fun
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Old Today, 06:39 AM   #88
pitbul is offline pitbul
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@Mooly, You have strong nervs :-)
You pointed investigation to right directions.
@nigelwright7557, right, that is sometimes first thing to look after some attempted to repair ;-)
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