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Marantz PM66KI amp - resistor keeps dying!
Marantz PM66KI amp - resistor keeps dying!
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Old 12th June 2019, 01:35 PM   #71
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
I'm never sure whether you are talking of DC offset or bias currents (voltage across the 0.1 ohms).

-22mv as an offset is fine and offset is not related to bias current adjustment. Offset is the voltage you measure across the speaker terminals.

-22mv measured across the two 0.1 ohms would equal 110 milliamps which is already on the high side. That would not need any alteration to components.

Have you actually tried the amp on a speaker yet?
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Old 12th June 2019, 07:03 PM   #72
dfear is offline dfear
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Quote:
Originally Posted by Mooly View Post
I'm never sure whether you are talking of DC offset or bias currents (voltage across the 0.1 ohms).

-22mv as an offset is fine and offset is not related to bias current adjustment. Offset is the voltage you measure across the speaker terminals.

-22mv measured across the two 0.1 ohms would equal 110 milliamps which is already on the high side. That would not need any alteration to components.
When measuring voltage on R768 (0.1x2 resistor), I put the positive probe on one of end pins of the resistor and the negative/black probe on on of the (pcb) heatsink ground points. At the moment i am using two multimeters to measure the voltage on both end pins of R768 at the same time. I put the positive lead of multimeter A on one of the end pins of R768 and the positive lead of multimeter B on the other end pin of R768; then the negative/black probes of each meter is placed on one of the (pcb) heatsink ground points.

-22mV is what I am measuring on one of the end pins of the resistor R768. Both of end pins measure the same.

Also when I adjust R756 trimmer, the voltage on either of the end pins of R768 doesn't change.

I have tested the voltages with and without the series lamp limiter limiting the current.

Hope this answers the above question(s).

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Originally Posted by Mooly View Post
Have you actually tried the amp on a speaker yet?
Not yet.


Thanks
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Old 12th June 2019, 07:18 PM   #73
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
That explains what you doing

So... all you are doing with the two meters is really measuring DC offset.

The trimmer doesn't adjust offset (although in practice it might appear to change it a little) but we are not really interested in that.

What you need to do is use just one meter and connect one lead to one outer end of the resistor network and the other lead to the other end of the network.

Doing that means you are measuring directly across what is effectively a 0.2 ohm resistor.

The voltage you see here will enable you to calculate the current. The manual suggests a rather precise figure of 38.9 milliamps which means you would see just 7.78 millivolts across those combined series resistors (or 3.89 millivolts across each).

You will never get the bias current set that accurately as it drifts naturally with temperature. Aim for no more than 10 millivolts across the pair (which would be 50 milliamps) as a maximum. Lower is fine.

Does that make sense?
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Old 12th June 2019, 07:21 PM   #74
dfear is offline dfear
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Quote:
Originally Posted by Mooly View Post
That explains what you doing

So... all you are doing with the two meters is really measuring DC offset.

The trimmer doesn't adjust offset (although in practice it might appear to change it a little) but we are not really interested in that.

What you need to do is use just one meter and connect one lead to one outer end of the resistor network and the other lead to the other end of the network.

Doing that means you are measuring directly across what is effectively a 0.2 ohm resistor.

The voltage you see here will enable you to calculate the current. The manual suggests a rather precise figure of 38.9 milliamps which means you would see just 7.78 millivolts across those combined series resistors (or 3.89 millivolts across each).

You will never get the bias current set that accurately as it drifts naturally with temperature. Aim for no more than 10 millivolts across the pair (which would be 50 milliamps) as a maximum. Lower is fine.

Does that make sense?
Yes that makes sense. I will retest tomorrow and report back.

Thanks for the clarification.
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Old 12th June 2019, 07:25 PM   #75
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
OK

And remember no speakers attached for setting bias current.

Try it with the bulb first so that you know what to expect... these are very tiny voltages and you might feel happier soldering wires to the test points and bringing those out to the meter, then you can concentrate on the readings and turning the trimmer.

One slip of the probes would be a disaster.
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Old 12th June 2019, 09:28 PM   #76
Mark Tillotson is offline Mark Tillotson
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A nice set of test lead ends is useful to have, like this: K-arm Multimeters Test Leads - Threaded Extension Hook Probe Testing Clip 4mm Aperture - - Amazon.com
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Old 17th June 2019, 09:45 PM   #77
dfear is offline dfear
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Quote:
Originally Posted by Mooly View Post
That explains what you doing

So... all you are doing with the two meters is really measuring DC offset.

The trimmer doesn't adjust offset (although in practice it might appear to change it a little) but we are not really interested in that.

What you need to do is use just one meter and connect one lead to one outer end of the resistor network and the other lead to the other end of the network.

Doing that means you are measuring directly across what is effectively a 0.2 ohm resistor.

The voltage you see here will enable you to calculate the current. The manual suggests a rather precise figure of 38.9 milliamps which means you would see just 7.78 millivolts across those combined series resistors (or 3.89 millivolts across each).

You will never get the bias current set that accurately as it drifts naturally with temperature. Aim for no more than 10 millivolts across the pair (which would be 50 milliamps) as a maximum. Lower is fine.

Does that make sense?
Here are my results:

With the series lamp limiter on:

Left channel
R767 = 6.79mV across the resistor

Right channel
R768 = 0mV & no change when R766 trimmer is adjusted from MAX to MIN resistance.


With the series lamp limiter off:

Left channel
R767 = ~10.97mV across the resistor

Right channel
R768 = 0mV & no change when R766 trimmer is adjusted from MAX to MIN resistance.

FYI:
no speakers are connected during the above tests. Its the RIGHT channel that we are working on.
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Old 17th June 2019, 09:50 PM   #78
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by dfear View Post
Here are my results:
Right channel
R768 = 0mV & no change when R766 trimmer is adjusted from MAX to MIN resistance.

Right channel
R768 = 0mV & no change when R766 trimmer is adjusted from MAX to MIN resistance.

FYI:
no speakers are connected during the above tests. Its the RIGHT channel that we are working on.
So it looks like your Vbe stage has a fault somewhere.
Maybe Vbe transistor gone short.
Maybe fault in constant current source or VAS.

Start by checking al those components in situ.
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Old 18th June 2019, 06:36 AM   #79
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
So we are back to post #69, that the available bias voltage is possibly marginally low for the modern semiconductor replacements.

Follow post #69 and swap the resistor mentioned. Use the bulb at all times for this.
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Old 18th June 2019, 01:07 PM   #80
dfear is offline dfear
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Quote:
Originally Posted by Mooly View Post
So we are back to post #69, that the available bias voltage is possibly marginally low for the modern semiconductor replacements.

Follow post #69 and swap the resistor mentioned. Use the bulb at all times for this.
I have swapped R752 for a 3.9K and here are my results:

R768 = 0 V across the resistor and no change wheen R766 is adjusted from MAX to MIN resistance.

C752 = 4.756 V to 3.731 V (when R766 is adjusted from MAX to Min resistance).

FYI:
I have attached photo showing how i'm testing both R768 and R767.
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File Type: jpg 20190614_145913.jpg (1,020.0 KB, 45 views)
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