Audio Marantz PM500

Hello,

You say that on the pins 8 of the QF01 and QF02 is -13 Volts.
The schematic states a value of +19 Volts.
The value of -25 Volts on the emittor of Q801 is stated wrong, it should be +25 Volts.

I see two 100 Ohms resistors between the +25 and +19, and -25 and -19 of QF01 and QF02.
Are these resistors OK.



The picture is a snapshot from a manual downloaded from eservice.

Bertus

 

Hello,

Did you replace CF11?
If so, did you measure the voltage on it?
When the upper resistor ( RF09 in the schematic ) is broke, the voltage on it can be floating and causing the strange readings on the chips QF01 and QF02.

Bertus

 

Sir eytonbranhan . . . . .

If your power supplies outputs are ~ +25 and -25 at those stages that they feed, and are being there, all must be well and in accordance to this CORRECTED power supply print at the bottom.

My suspicion is . . . .

You have QF3-12 which are arranged in 5 sets of complementary /symmetry arrangements and should balance out in their voltage swings and current demands in their handling the 5 sections of frequency selective nodes of the equalizer.
If their is an imbalance (leaky or shorted transistor) that throws the common demand of all of them off, it will swing the power supply balance askew.

Now, it's blurry, but looks like ? RF13 ? . . . a 100 ohm resistor, couples in the +25 supply to its companion CF11 + filter .
Likewise an also even blu r r r r r ier ? ? ? RF09 ? ? ? is coupling in the -25 v supply to its CF12 - filter .
You can make a quick check of any power supply drain imbalance by measuring the voltage across each of those 100 ohm resistors .
An imbalance will be shown by one of the voltages being appreciably higher.
You could have a transistor, of those 10, that is shorted or one that has its emitter open . Creating a big time imbalance then.

Reference Schematic of + and - 25 Supply Source . . . . .





73's de Edd . . . . .

Why should I waste time learning from my past, when I am all too busy now, worrying about my future?

 

Hello,

You are right, the resistors are indeed 47 Ohms.
Both readings are off.
0,810K is 810 Ohms
0,L is open leads.
I would replace bothe with the in the manual mentioned 100 Ohms resistors.

Bertus

 

Hang on.
I wouldn't change any values. The parts listed in the manual doesn't match what you have. Notice RF01 and RF02 are 33kΩ and manual says 22kΩ? This thing was working for years the way it was and you shouldn't have to alter it.

Try reading resistance values again with your meter in the 1x scale. (If your fingers touch both test leads it can throw off an ohm reading in the KΩ range) You only need to unsolder one lead at a time for testing.
Which resistor reads open?

I would hold off ordering/replacement of parts until you have evaluated all the parts you may need.

Parts are cheap, shipping is expensive.

 

If you look at the node on the left (neg) side of your bad cap (cf11) you see it connects to pairs of 47k resistors feeding the base of transistors. I'd try pulling them out to try and isolate a short circuit. As Edd said, probably one of the transistors is shorted.
Once resistors are out, compare ohm readings to ground in each grouping. They should have similar readings.

If you find low resistance, you know there's a bad apple somewhere.

 

Sorry, I wouldn't power it up yet.

It's hard to follow blurry on my phone screen.

I think your voltage measurements are out of kilter in part because of the open resistor.

It's possible that cap could be failing because of wrong polarity, rather than overcurrent from a short. 47k would not alow enough current to flow for a short.
The resistor on the other hand must have had serious current flowing through it to blow it apart. I presume RF09 was the one open circuit.

Anyway, a more practical approach instead of chasing circuit measurements, might be to pull and check components that are suspect.

I would start by checking diodes and transistors for shorts.
Yes, you can do a continuity test between pins to look for a short but it only tells you the component is shorted. Not that it's good.
It's better to use the diode check feature of a Dvm. Try googling how to check transistor.

Keep track what component came out of where on the board.

The good news is that you have other similar components to compare with.

 

Hey, great progress!
I don't think R21 would cause those problems. More than likely a transistor.

Does the volume transition quiet to loud fairly evenly (linearly) and then jump up in volume only at the spot at the extreme top position? This could be a bad pot.
If accessible, I would just transplant it with a 50k pot next to it. If the problem follows, you know it needs replacement.

If the level is quieter than normal and then jumps up, it its likely something else.

Looks like RF21 feeds the base of QF03 to control that slider frequency. I would check it and the surrounding components. It also controls input to opamp QF01.
Ceramic caps usually don't go bad, but it wouldn't be a bad idea to just replace all electrolytic caps do to their age.

I would also verify that the voltage rails are now reading correctly. Check voltage with Rf21 at top position, and again sliding it down.

 

Sir eytonbranhan . . . . .


Your RF21 pot is being associated with your QF03 stage which is likewise related to the lowest frequency aspect of your audio spectrum control . . .which I woulld suspect to be marked as being 50 or 60 Cycles / Hertz.
SO . . . that pots adjustment, should be more noticable than either a mid range RF23 or treble end RF25 of the audio spectrum, pots end to end adjustment.

Your two 47 ohm resistors being involved were related to being series isolating resistors for their associated CF11 and CF 12 positive and negative power inputs to the PF00 board for filtering and decoupling functions..
They were even being more robust METAL FILM resistors, rather than all of the the nearby CARBON FILM resistors.
See their assigned triangular / exclamation mark . . . .special part / exact type replacement . . . designation.

Was your failure fault of the CF11 being a short or leaky condition, as an opening of one of those resistors could result in a potential opposite polarity voltage being across one of the opposite side filters.

If you look at your post #5 photo at the dead center of the RF9 and 10 resistors, they show a discoloring, transitioning from grey/towards/ white discoloration, from referencing to the original case coloring at the ends.
Those have been heat stressed /overloaded. The RF09 seeming a bit moreso.
I would guess that your . . ." found" . . . . . presetly installed resistors, would be CARBON FILM or old composition types . . . . does a finger temp test applied to them, reveal any excess warmth ?

You might further do a DC voltage reading across each of them and use the received voltage and resistance values to then use OHMS law to compute the wattage that each is now presently handling.
There is usually a 400% overloading to have blown open the ORIGINAL METAL FILM resistor..

I REALLY was expecting your problem to be failure / breakdown within QF01-QF-02 pre amp I.C. 's ,of their pins 4 and 8, as they DIRECTLY connect to those RF9 and 10 resistors .

If you analyze all of the QF03-QF12 transistors, circuitrys, related resistors, none of them are being of such a LOW enough resistance to permit the level of power that would have been neccessary to blown RF9 and 10 resistors.

Also place volume all the way down and test DC voltage across all of the 100 ohm collector resistors of QF03-QF13 transistors, logging each down, then comparing for sameness / balance.

Also do the same test across the 4.7K emitter resistors of those transistors.
(Now you can see how even a dead shorted transistor(s) and those series 100ohm + 4.7K resistors could not even slightly start to create the overload situation to blow one of those RF9 and 10 resistors.)

Thaaaaaaaaaassssssit . . . . .

73's de Edd . . . . .


Illiterate?… write for FREE HELP!



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Hello,

When you say that there is a sharp transition 3-4 mm before the end, it sounds like a bad potentiometer.(the shift regulator).

Bertus