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Old 22nd February 2012, 02:40 AM   #1
kotofei is offline kotofei  United States
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Default Need help with power amp in Marantz 2240

Hello everyone,

I have a problem with left channel power amp section of a Marantz 2240 receiver. Initially I couldnít 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 thatís 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 didnít 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? Iím lost; any help will be greatly appreciated.
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Old 22nd February 2012, 03:30 AM   #2
Wakh is offline Wakh  Russian Federation
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The problem is that there is a plus power.
Check the H714, H003, C704, C711.
J714 & J705 = 34B
?
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Old 22nd February 2012, 02:51 PM   #3
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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.

Last edited by sgrossklass; 22nd February 2012 at 03:11 PM.
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Old 22nd February 2012, 03:41 PM   #4
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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?
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Old 23rd February 2012, 01:12 AM   #5
kotofei is offline kotofei  United States
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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.
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Old 23rd February 2012, 05:54 AM   #6
Wakh is offline Wakh  Russian Federation
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Quote:
Originally Posted by kotofei View Post
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.
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!
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Old 23rd February 2012, 08:40 AM   #7
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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
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Last edited by east electronics; 23rd February 2012 at 08:42 AM.
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Old 25th February 2012, 02:14 AM   #8
kotofei is offline kotofei  United States
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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.
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Old 25th February 2012, 04:54 AM   #9
Wakh is offline Wakh  Russian Federation
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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!
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Old 27th February 2012, 01:58 AM   #10
kotofei is offline kotofei  United States
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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.
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