Kenwood monoblock L-07M Low power supply voltage for differential amp

diepbtt · Nov 6, 2014

Hi all,

I just got a pair Kenwood L-07M( one with 3 pair PT Sanken 2SA747A/2SC1116A) and one with 3 PT: Marantz A747A and 3 Sanken 2SC1116A) and tried to re-cap it.

After powering it up using DBT i plug both direct to the 110V source both pieces come out of protection successfully then i can adjust one piece(with all Sanken PT) for power supply voltage for differerntial amp , DC offset and bias as recommended by Kenwood

But by other piece i only can reach to 57V for power supply voltage for diff. amp ( vs. reference: 63V) and not further. Exhange the VRf1 1K from Bourns does not help.

I have checked all components of the power connection board and do not find any unnormality.

I have measured following:
By Qf1 (2SC1669):
C (for B+) : + 84,5V instead of 87V
B: 57.3V
E: 56,6V

By Qf2 (2SA839)
C (for B+) : - 84,5V instead of 87V
B: -72.8V
E: -72.2 V

Rf1/2: 4.7K
R17: 56K
R21: 4.3K

Suspected defect Qf3/4(2SA850/2SC1735) I replaced it with new KSA1220YBUKSC2690AYBU but change with same voltages at Qf1/2 and power supply voltage for differential amp.

I have measured the voltage at zener diodes :
Df1(YZ-140): around 14V almost same as in service manual
Df2(YZ-140): around 14.6V a bit higher
Df7(EQA01-5S): around 5.2V
All are good with diode test

Checking Qf1/2(2SC1669/2SA839) , Qf5/6(2SC1213A/2SA673A) with diode testing mode they are OK.

Nevertheness with acceptance of the highest voltage I can get 56V i was able to adjust DC offset to around 0.0mV and and bias to 25mV

After power it off and 30 minutes later i powered it up again to re-check DC offset & bias but this time the protection relay does not come out. Checking the power amp I found one driver transistor Qe11(2SC1913) was shorted (B-C).

Checking all power transistors( 3 Marantz A747A/ 3 Sanken 2SC1116A) with diode test and they still are OK
Currently I took this power board out and will replace the driver transistor pair (original 2SA913/2SC1913) through 2SA1837/2SC4793.

Without power amp board plugged i tried to power it up the relay does come out of protection but I got same voltage results as before.
Measured the voltages at RC4558T with following results:
Pin 1: 2.13V (Opamp output A- should be 3.7 V)
Pin 2: 5.06 V (Opamp in put A- should be 4.5V)
Pin 3: 5.06V (Opamp input A - should be - 4.5V)
Pin 4: -14.7 V (should be -14.5V)
Pin 5: 0 V
Pin 6: 0 V
Pin 7: - 3.97 V (Opamp output A- should be -3.7 V)
Pin 8: 14.22 V(should be 14.5 V)

So my questions someone can chime in what will cause this problem so that i can not adjust the power supply voltage for differential amp to the spec.( only from 52 V to 56.6 V as maximal vlaue and no further to the reference 63V)?

1. My first doubt: Maybe the IC RC4558T is defect? And its replacement JRC 4558D with modified pinout ?

2. I also suspect the control transistors Qf1/Qf2 (2SC1669/2SA839) for the cause due to aging?
What should I replace for this pair for best result for examples MJE 15032/15033G( but low Hfe compared to the original 2SA839/2SC1669)?

3. Other possibilities like ?

Thanks

sregor · Nov 6, 2014

I took a look a the schematics - the power supply is relatively simple. The negative tracks the positive supply. With 5 volts reference (from the zener), if anything the supply voltages should be hard to adjust lower, not higher. My semi-educated guess would be leakage through cap c15 (1uf, 63 v). If they will fit, I would change all the 1 uf caps to film types with higher voltages - they are rated at 63 volts, and have around 60 volts across them which leaves no room for error.
I am presuming that there is a - sign on pin 7 voltage (measured).
SInce you can adjust the supply voltages, and they aren't way out of line, suggests the IC may be okay, but that still certailnly could be a problem. Other possibilites may be the 6.8 ohm resistor (rf11) and the smiconductors. My 2 cents. Good luck with it,

diepbtt · Nov 6, 2014

sregor said:
I took a look a the schematics - the power supply is relatively simple. The negative tracks the positive supply. With 5 volts reference (from the zener), if anything the supply voltages should be hard to adjust lower, not higher. My semi-educated guess would be leakage through cap c15 (1uf, 63 v). If they will fit, I would change all the 1 uf caps to film types with higher voltages - they are rated at 63 volts, and have around 60 volts across them which leaves no room for error.
I am presuming that there is a - sign on pin 7 voltage (measured).
SInce you can adjust the supply voltages, and they aren't way out of line, suggests the IC may be okay, but that still certailnly could be a problem. Other possibilites may be the 6.8 ohm resistor (rf11) and the smiconductors. My 2 cents. Good luck with it,

Thank you for chime in.

1. I compile the circuit description of Power supply voltage for diff. amp from Kenwood L-07M service manual for easy understanding.

2. I will check voltage at pin7 again and correct it. It should be -3,9V maybe my type mistake.

3. I already have replaced Cf5 1uf/63 v (so not C15) with Nichicon FG 1uF/100V but it would be good trial with Fim capacitor 1uF/100V or 250V

4. I already checked also Rf 11(6,8ohm) it is OK in range it should be.

5. I will check voltages of Qf5(2SC1213A) and Qf6(2SA673A)

diepbtt · Nov 12, 2014

diepbtt said:
Hi all,

I just got a pair Kenwood L-07M( one with 3 pair PT Sanken 2SA747A/2SC1116A) and one with 3 PT: Marantz A747A and 3 Sanken 2SC1116A) and tried to re-cap it.

After powering it up using DBT i plug both direct to the 110V source both pieces come out of protection successfully then i can adjust one piece(with all Sanken PT) for power supply voltage for differerntial amp , DC offset and bias as recommended by Kenwood

But by other piece i only can reach to 57V for power supply voltage for diff. amp ( vs. reference: 63V) and not further. Exhange the VRf1 1K from Bourns does not help.

I have checked all components of the power connection board and do not find any unnormality.

I have measured following:
By Qf1 (2SC1669):
C (for B+) : + 84,5V instead of 87V
B: 57.3V
E: 56,6V

By Qf2 (2SA839)
C (for B+) : - 84,5V instead of 87V
B: -72.8V
E: -72.2 V

Rf1/2: 4.7K
R17: 56K
R21: 4.3K

Suspected defect Qf3/4(2SA850/2SC1735) I replaced it with new KSA1220YBUKSC2690AYBU but change with same voltages at Qf1/2 and power supply voltage for differential amp.

I have measured the voltage at zener diodes :
Df1(YZ-140): around 14V almost same as in service manual
Df2(YZ-140): around 14.6V a bit higher
Df7(EQA01-5S): around 5.2V
All are good with diode test

Checking Qf1/2(2SC1669/2SA839) , Qf5/6(2SC1213A/2SA673A) with diode testing mode they are OK.

Nevertheness with acceptance of the highest voltage I can get 56V i was able to adjust DC offset to around 0.0mV and and bias to 25mV

After power it off and 30 minutes later i powered it up again to re-check DC offset & bias but this time the protection relay does not come out. Checking the power amp I found one driver transistor Qe11(2SC1913) was shorted (B-C).

Checking all power transistors( 3 Marantz A747A/ 3 Sanken 2SC1116A) with diode test and they still are OK
Currently I took this power board out and will replace the driver transistor pair (original 2SA913/2SC1913) through 2SA1837/2SC4793.

Without power amp board plugged i tried to power it up the relay does come out of protection but I got same voltage results as before.
Measured the voltages at RC4558T with following results:
Pin 1: 2.13V (Opamp output A- should be 3.7 V)
Pin 2: 5.06 V (Opamp in put A- should be 4.5V)
Pin 3: 5.06V (Opamp input A - should be - 4.5V)
Pin 4: -14.7 V (should be -14.5V)
Pin 5: 0 V
Pin 6: 0 V
Pin 7: - 3.97 V (Opamp output A- should be -3.7 V)
Pin 8: 14.22 V(should be 14.5 V)

So my questions someone can chime in what will cause this problem so that i can not adjust the power supply voltage for differential amp to the spec.( only from 52 V to 56.6 V as maximal vlaue and no further to the reference 63V)?

1. My first doubt: Maybe the IC RC4558T is defect? And its replacement JRC 4558D with modified pinout ?

2. I also suspect the control transistors Qf1/Qf2 (2SC1669/2SA839) for the cause due to aging?
What should I replace for this pair for best result for examples MJE 15032/15033G( but low Hfe compared to the original 2SA839/2SC1669)?

3. Other possibilities like ?

Thanks

Update:
I have re-checked the voltage at pin 7 of RC4558T and it is -3,97V so your correction is right. I have corrected it.

Measured voltages further at:
Qf3(KSC2690AYBU)
B: 2,1V
C: 72.1V
E: 1.6 V

Qf4(KSA1220AYBU)
B: -3.9V
C: -56.7V
E: -3.4 V

Qf5(2SC1213A)
B 71.6V
C 72.1V
E 71.7V

Qf6(2SA673A)
B -56.2V
C -56.4V
E -56.1V
So the regulated Voltages at

Point 4 on power amp board : - 56.1V(should be + 63V)

Test point 23 on the connection board or Point 22 on power amp board : + 71.6V (should be - 63V)
It is strange for me why I can not get minus voltage here , but plus ________ not as in the sm descripted (black probe stuck to Test point 23, red probe to GND as suggested).

After replace Qf1/Qf2((2SC1669/2SA839) with new MJE15032G/15033G as a trial it remains with above results, no change.

Re-checked all resistors in circuit,( Rf17, Rf21 out of circuit), solderjoints, again new caps of this connection board they are all in range as it should be.

I have replaced Qf5/Qf6( 2sc1213A/2SC673A) with original pair from my stock and IC 4558T with pin- modded JRC 4558D , all zener diodes and Df3-Df6 but the situation does not improve.

So I have took a look at that good ones, no unusual thing compared to the sick one except at the good one I found R20 =2,2K/ 1W in stead of 1K but this good one runs well. it is easy to adjust voltages to +/-63V

It is really puzzles!

sregor · Nov 12, 2014

Do some intelligent trouble shooting. First address the positive supply. The way the circuit works is the output will change until pin 3 matches pin 2. Pin 2 is the reference voltage which you measured as 5 volts. With the resistors (4.3k and 56K) and a 1k pot, the range should be from 57 to 72 volts. (Originally you had 56 volts - now its 72) You should adjust for 63 volts. Originally you said you couldn't adjust it high enough, but that no longer seems to be, as now you have 72 volts. Once you get 63 volts on the positive side, the negative side should track to the same voltage. Pin 6 is ground, the voltage should adjust until pin 5 is at ground. Pin 5 is the same resistance to the + supply as to the - supply, so the minus supply should be at - the same potential. Resistors should be checked, but the caps or more suspect because of the voltage ratings. Good luck with it. (note on trouble shooting - if pins 2 and 3 match (5 volts) and pins 5 and 6 match (0 volts) then the circuit is probably OK except for problems in the resistor chains)

diepbtt · Nov 12, 2014

I have pulled out R16 & R18 ou91t of circuit and measured with DMM:
R16 Metal film resitor : 91K
R18 Metal film resitor : 71.2 K

Oh man they drift too much from the references in the service manual R16/18: 56K, 01% tolerance!

Are they now the culpits?

Updated:
Replaced both with Dale RN55D 5602F(56K, 1% tolerance) i easily can now adjust +/-63 V where it must be .

So they are the causes for wrong voltages i hardly fighted.
Lession learnt: never measure resitors value in circuit!

Again thank Steve for helping and assuming that he supposes it may semiconductors in the circuit may be defect i follow!

sregor · Nov 13, 2014

:thmbsp: Ohm's law still works.

Kenwood monoblock L-07M Low power supply voltage for differential amp

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