Hello everyone,
I have a problem with left channel power amp section of a Marantz 2240 receiver. Initially I couldn’t adjust both bias and DC offset, both were not sensitive to corresponding pots. The bias was -18 mV and the DC offset jumped from -0.5 to -10 V activating the protection relay.
For bias problems, the service manual suggests checking bias diodes H005/H006 and open L004. The diodes tested OK in circuit and the L004 is not open. For DC offset problems, the SM suggests checking the pot R707 and diodes H701 and H704. The pot seems OK, not open, not shorted and resistance changes if it is turned. I soldered out one leg of each of the diodes and H701 seems OK. The voltages at the voltage regulator for H801-H804 were within specs.
I replaced all electrolytes in the power amp section and H710 and H711 transistors. Upon powering on, speaker relay clicked in as it should; however, DC offset and bias jumped for both right and left channels. I set DC offset to zero in both channels then relay clicked and the left channel DC offset, measured at relay input terminals, went to -33.5 Volts! Now the DC offset in this channel is insensitive to the R707 pot.
I measured voltages at left channel transistors and that’s what I got:
H702 base: jumps around -14V (should be 0.1V)
H702 /H703 emitters: Jumps around -13V (Should be 0.7V)
H702 collector: -33.3V (Should be -34V);
H703 base: -13 V (should be 0V);
H703 collector: -34.5V (Should be -34.5V)
H707 collector: -34.7V (Should be -0.6V)
H707 emitter: -35V (Should be -34.5V)
H710 base: -33V, jumps (Should be 0V)
H711 base: -12 .. -13V, jumps (Should be 0V)
H715 collector: -36.3 V (Should be -35.1V)
Tested out of circuit H707, H714, H715, and both outputs- all OK by diode test.
I removed both outputs but it didn’t change the DC offset and most of the voltages shown above. Base of H702 became -24V, emitters of H702/H703: -17V and others were the same. The only electrolyte that could be damaged at -33V is C705, 47 microF/16V but voltage at its positive terminal is -6/-8 V. Other electrolyte are 35V or higher in specs.
What could be wrong here? I’m lost; any help will be greatly appreciated.
I have a problem with left channel power amp section of a Marantz 2240 receiver. Initially I couldn’t adjust both bias and DC offset, both were not sensitive to corresponding pots. The bias was -18 mV and the DC offset jumped from -0.5 to -10 V activating the protection relay.
For bias problems, the service manual suggests checking bias diodes H005/H006 and open L004. The diodes tested OK in circuit and the L004 is not open. For DC offset problems, the SM suggests checking the pot R707 and diodes H701 and H704. The pot seems OK, not open, not shorted and resistance changes if it is turned. I soldered out one leg of each of the diodes and H701 seems OK. The voltages at the voltage regulator for H801-H804 were within specs.
I replaced all electrolytes in the power amp section and H710 and H711 transistors. Upon powering on, speaker relay clicked in as it should; however, DC offset and bias jumped for both right and left channels. I set DC offset to zero in both channels then relay clicked and the left channel DC offset, measured at relay input terminals, went to -33.5 Volts! Now the DC offset in this channel is insensitive to the R707 pot.
I measured voltages at left channel transistors and that’s what I got:
H702 base: jumps around -14V (should be 0.1V)
H702 /H703 emitters: Jumps around -13V (Should be 0.7V)
H702 collector: -33.3V (Should be -34V);
H703 base: -13 V (should be 0V);
H703 collector: -34.5V (Should be -34.5V)
H707 collector: -34.7V (Should be -0.6V)
H707 emitter: -35V (Should be -34.5V)
H710 base: -33V, jumps (Should be 0V)
H711 base: -12 .. -13V, jumps (Should be 0V)
H715 collector: -36.3 V (Should be -35.1V)
Tested out of circuit H707, H714, H715, and both outputs- all OK by diode test.
I removed both outputs but it didn’t change the DC offset and most of the voltages shown above. Base of H702 became -24V, emitters of H702/H703: -17V and others were the same. The only electrolyte that could be damaged at -33V is C705, 47 microF/16V but voltage at its positive terminal is -6/-8 V. Other electrolyte are 35V or higher in specs.
What could be wrong here? I’m lost; any help will be greatly appreciated.
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My primary suspects would be H704 or possibly a dud H702. What type is H704? Some of these stabilizer diodes (VDxxxx) were quite notorious.
Other than that, it probably isn't a bad idea to check for bad resistors or solder joints.
Other than that, it probably isn't a bad idea to check for bad resistors or solder joints.
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PS: The biggest puzzle is how there can be -13 V on H702 base, i.e. the input. A bad ground connection somewhere? Dead R703?
Thank you everyone for suggestions,
I did some measurements on suspected components and that's what I got:
H704 is an MV-13 diode according to service manual. With one leg soldered out of the circuit it tested OL/1.065V by diode test.
H702 was soldered out of circuit and tested OK by diode test as well.
R703 was tested in circuit and showed 7.04 kOhm resistance while SM calls for 6.8 kOhm. I think it's good.
Ground connection for this power amp plate was tested and found OK.
H714 was soldered out and tested OK as I previously mentioned.
H003 ( and H004) were new output transistors; anyways, they were also tested by diode test after I removed them since I was afraid the -31 V could damaged them. Both found OK.
C704 was just replaced with Nichicon 47 microF/50 V electrolyte. Polarity tested and found OK. I don't think -31v should affect it.
C711 was tested with one leg soldered out of circuit and found not shorted (resistance =OL). My DMM showed it has 1.21 nanoF capacity while SM indicates it should be 100 picoF. However I am not sure I can measure such small capacitance values correctly.
Didn't see bad joints; testing of all resistors in circuit is in plans.
One more piece of data: I found that I measured voltages at J705-J712 connectors of this channel and H715/H714 after removing both output transistors. Here they are:
J 705 =+ 37V; J 706 =-30.8V; J708 = -31.1V; J 709 = -31.5 V; J 710 = -31.1V; J 711= - 31.5 V; J 712 = -36.9 V. Voltages at H 715: b = -32.2V, c = -31V; e = -31.4 V. Voltages at H 714: b= -30.3V; C= +37V, e= -31.4V.
I did some measurements on suspected components and that's what I got:
H704 is an MV-13 diode according to service manual. With one leg soldered out of the circuit it tested OL/1.065V by diode test.
H702 was soldered out of circuit and tested OK by diode test as well.
R703 was tested in circuit and showed 7.04 kOhm resistance while SM calls for 6.8 kOhm. I think it's good.
Ground connection for this power amp plate was tested and found OK.
H714 was soldered out and tested OK as I previously mentioned.
H003 ( and H004) were new output transistors; anyways, they were also tested by diode test after I removed them since I was afraid the -31 V could damaged them. Both found OK.
C704 was just replaced with Nichicon 47 microF/50 V electrolyte. Polarity tested and found OK. I don't think -31v should affect it.
C711 was tested with one leg soldered out of circuit and found not shorted (resistance =OL). My DMM showed it has 1.21 nanoF capacity while SM indicates it should be 100 picoF. However I am not sure I can measure such small capacitance values correctly.
Didn't see bad joints; testing of all resistors in circuit is in plans.
One more piece of data: I found that I measured voltages at J705-J712 connectors of this channel and H715/H714 after removing both output transistors. Here they are:
J 705 =+ 37V; J 706 =-30.8V; J708 = -31.1V; J 709 = -31.5 V; J 710 = -31.1V; J 711= - 31.5 V; J 712 = -36.9 V. Voltages at H 715: b = -32.2V, c = -31V; e = -31.4 V. Voltages at H 714: b= -30.3V; C= +37V, e= -31.4V.
I repeat the question J714 = Volts?
Check your meter to another 100 pF capacitance.
C703 and C707 check.
Check the correct polarity of the diode H005
Good luck! 🙂
check power supply voltage
isolate pre from main amplifier to trace the problem
replace related capacitors in auxiliary power supply
check for open or drifted resistors on amplifier board
isolate pre from main amplifier to trace the problem
replace related capacitors in auxiliary power supply
check for open or drifted resistors on amplifier board
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Checked all resistors in the power amp plate including potentiometers. All within specs, no open or shorted (in some cases, one leg was soldered out to measure resistance).
Soldered in H714 and H715 replacements as well as H702 that was removed for testing purposes.
Powered the Marantz on: DC offset still -30 v and not sensitive to R707 pot. The voltage on J 714 is + 34.95V.
Replaced C711 and C703 ceramic caps (200 pF and 100pF, respectively) with new polyprop 220 pF/100V WIMA. I believe it's OK for testing purposes; moreover, C711 body reads "220K KCK" so I'm suspicious it's also 220 pF. If this is so I'll just leave both 220 pF polypropylene caps there. Unfortunately, C711/c703 change had no effect on DC offset.
Finally I isolated the preamp from main amp by soldering out J701 pin. Same result: DC offset stays at -31V.
The diode H005 polarity should be right since I didn't remove it. Tested it in the circuit: if + is at J703 and - at J 704 the DMM shows "OL"; if + is at J704 and - is at J703 the DMM shows 1.277V.
The power supply voltage was OK when I started work on this Marantz but I'll re-check it. I will also replace electrolytes in the aux. power supply and relay driver , they are located at the same board.
Otherwise, it's still a puzzle.
Soldered in H714 and H715 replacements as well as H702 that was removed for testing purposes.
Powered the Marantz on: DC offset still -30 v and not sensitive to R707 pot. The voltage on J 714 is + 34.95V.
Replaced C711 and C703 ceramic caps (200 pF and 100pF, respectively) with new polyprop 220 pF/100V WIMA. I believe it's OK for testing purposes; moreover, C711 body reads "220K KCK" so I'm suspicious it's also 220 pF. If this is so I'll just leave both 220 pF polypropylene caps there. Unfortunately, C711/c703 change had no effect on DC offset.
Finally I isolated the preamp from main amp by soldering out J701 pin. Same result: DC offset stays at -31V.
The diode H005 polarity should be right since I didn't remove it. Tested it in the circuit: if + is at J703 and - at J 704 the DMM shows "OL"; if + is at J704 and - is at J703 the DMM shows 1.277V.
The power supply voltage was OK when I started work on this Marantz but I'll re-check it. I will also replace electrolytes in the aux. power supply and relay driver , they are located at the same board.
Otherwise, it's still a puzzle.
If J714 = +34,95V,
and the base H714 = -30,3V,
then R715 must be hot
and then you have to check the resistance of the collector-emitter of transistor H710.
Good luck!
and the base H714 = -30,3V,
then R715 must be hot
and then you have to check the resistance of the collector-emitter of transistor H710.
Good luck!
The R715 is not that hot after Marantz is on for 30 or so seconds. This is kind of strange since voltage on its end connected to R736 is -29 volts and at the other end connected to R712 is +17 volts. The capacitor C704 that is located near R715 at the PCB feels warmer... which is not a good sign. However, this capacitor was just replaced with Nichicon FG. This C704 is not shorted (by DMM). I'll test it out of circuit tomorrow.
The C-E junction resistance of H710 is 0.71 MOhm if +to e and - to C and OL if + to c and - to e. The H710 in the "good" right channel measured 0.66-0.72 M / 23.8 MOhm. The "OL" measurement in the left channel might be due to removed outputs.
I also tested diodes H708 and H709 in circuit and both looks good (0.61V/OL) by DMM diode test.
The C-E junction resistance of H710 is 0.71 MOhm if +to e and - to C and OL if + to c and - to e. The H710 in the "good" right channel measured 0.66-0.72 M / 23.8 MOhm. The "OL" measurement in the left channel might be due to removed outputs.
I also tested diodes H708 and H709 in circuit and both looks good (0.61V/OL) by DMM diode test.
if you turn to lock the forceps (connect) at the base to the emitter of transistors H702, H707 and H715, each time at the amplifier output voltage to appear +30 V.
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Hi,
The voltage at the base of H702 can not be -17 unless the transistor is breakdown when the voltage it is applied to the amplifier and it is feeding negative voltage from the collector. The only way also is that it is feeding negative voltage from the J701 but the capacitor need to be shorted. Check the voltage at the J701 to see if it is negative. Sometime transistors check OKAY static but under power may fail.
The voltage at the base of H702 can not be -17 unless the transistor is breakdown when the voltage it is applied to the amplifier and it is feeding negative voltage from the collector. The only way also is that it is feeding negative voltage from the J701 but the capacitor need to be shorted. Check the voltage at the J701 to see if it is negative. Sometime transistors check OKAY static but under power may fail.
I soldered out the wire going from the preamp to J701 upon Sakis advice. Thus, no voltage could be feeding from J701.
Update:
I thought about H702 breakdown (Leaky transistor) and then found that H702, 2SA763, could be replaced with KSA 992 FBU that I just received from Mouser with a bunch of other parts!
So I replaced H702 with KSA992 and Left ch. bias went back to normal. I easily brought it to 0 mV with the R707 pot. What should be noted here that the H702 was tested good in the diode test just after I replaced it and showed hFE of ca 350.
Just out of curiosity I decided to measure the bias in the right, supposedly good channel. It was around -20 mV a minute or so after powering Marantz on but was not stable, jumping from -20 to +10 mV. Then suddenly changes became bigger, to hundreds of mV, then they went in the Volts range, relay clicked... Now bias in the right channel jumps from ca. 0 to ca. -7 V just after powering on. Bias in the left channel is stable at approx. 0 mV.
I have few more KSA992s and I will replace H702 and/or H703 in the right channel; however, such behavior suggests it might be a failure somewhere else (voltage regulator?) that screws up a good channel if both channels seems fixed.
Any thoughts?
I thought about H702 breakdown (Leaky transistor) and then found that H702, 2SA763, could be replaced with KSA 992 FBU that I just received from Mouser with a bunch of other parts!
So I replaced H702 with KSA992 and Left ch. bias went back to normal. I easily brought it to 0 mV with the R707 pot. What should be noted here that the H702 was tested good in the diode test just after I replaced it and showed hFE of ca 350.
Just out of curiosity I decided to measure the bias in the right, supposedly good channel. It was around -20 mV a minute or so after powering Marantz on but was not stable, jumping from -20 to +10 mV. Then suddenly changes became bigger, to hundreds of mV, then they went in the Volts range, relay clicked... Now bias in the right channel jumps from ca. 0 to ca. -7 V just after powering on. Bias in the left channel is stable at approx. 0 mV.
I have few more KSA992s and I will replace H702 and/or H703 in the right channel; however, such behavior suggests it might be a failure somewhere else (voltage regulator?) that screws up a good channel if both channels seems fixed.
Any thoughts?
Hi,
I checked the specs for the 2SA763 and there is two specs. If you are using the 2SA763-Y the max voltage is -30 volts and if your are using the 2SA763-W the working voltage is -60. Means that if your amp is using the Y it may fail due to the voltage at the collector is about -34 it is too close to the working voltage. You should make sure the transistor that you replace be are least of a working voltage of -50 volts.
I checked the specs for the 2SA763 and there is two specs. If you are using the 2SA763-Y the max voltage is -30 volts and if your are using the 2SA763-W the working voltage is -60. Means that if your amp is using the Y it may fail due to the voltage at the collector is about -34 it is too close to the working voltage. You should make sure the transistor that you replace be are least of a working voltage of -50 volts.
That's what written on the transistor: "4x A763 WL 5". Most likely it is "W". The KSA 992 has working voltage of 100 V.
Update: Replaced H702 in the right channel to KSA992. Bias went back to normal, is not jumping as before and could be brought to 0 mV by R707. Left channel bias is also OK.
However all this was done with removed output transistors so I might be not out of the woods yet.
Should I change H703 which is also 2SA763, to KSA992? If so, should it be hFE matched with H702 in its channel?
However all this was done with removed output transistors so I might be not out of the woods yet.
Should I change H703 which is also 2SA763, to KSA992? If so, should it be hFE matched with H702 in its channel?
Hi,
I would replace both to KSA992 so both are of the same brand. Also since you have the output transistors removed check the base voltage going to the outputs to see if they read as the schematic 1.1 volt for the H714 and -.6 H715. If they match then you can install the output transistors.
I would replace both to KSA992 so both are of the same brand. Also since you have the output transistors removed check the base voltage going to the outputs to see if they read as the schematic 1.1 volt for the H714 and -.6 H715. If they match then you can install the output transistors.
Update: the Marantz is working now!
I replaced both H703s to KSA 992 and checked voltages at H714 and H715 bases. They were close to expected values (+1.26/+1.35 V for H714 and -0.63 V for H 715) so I decided to go ahead.
As output transistors, I used MJ15024G instead of the original 2SC1403 since one of them was shorted. Set bias and DC offset without a pboblem, connected suiside speakers and tuned to FM.
The Marantz played music, perhaps first time in many years atfer sitting broken in someone's closet. It sounded undistorted to my ears although I'll check inputs with generator and scope.
HOPE it's the end of this story.
I replaced both H703s to KSA 992 and checked voltages at H714 and H715 bases. They were close to expected values (+1.26/+1.35 V for H714 and -0.63 V for H 715) so I decided to go ahead.
As output transistors, I used MJ15024G instead of the original 2SC1403 since one of them was shorted. Set bias and DC offset without a pboblem, connected suiside speakers and tuned to FM.
The Marantz played music, perhaps first time in many years atfer sitting broken in someone's closet. It sounded undistorted to my ears although I'll check inputs with generator and scope.
HOPE it's the end of this story.
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