I have a Marantz 240 amp that I have recapped the electrolytic, and all the transistors following the addendum gain matching. All the components except 1 transistor has be replaced on the relay board including the relay. My left amp board seems to be working well. I am using a dim bulb tester while I am checking voltages. The right amp board is having an issue. I was checking the right amp board voltages and when I touched the positive multi meter probe to the base of Q519 there was a buzzing noise and the DBT light comes on and the relay engages. I checked Q519 and I get correct diode readings with no shorts.
I checked the left amp board and there was no issue checking the base voltage on Q519. I replaced Q519 on the right board and the issue is still there. I checked the resistors and caps before the base of Q519. All were in spec but C501 which should be 150pF is reading 129pF. Then when I checked the voltage on the C501 leg it shorted the DBT. If I power the amp down, wait two minutes and power it back up it seems fine until I check voltages from the base of Q519 and back.
Powering the left board the DBT meter shows 113Vac with 0.3A and the rails measure +/-53.26dcv
Powering the right board the DBT meter shows 111Vac with 0.3A and the rails measure +/-51.25dcv
I also replaced the bridge diodes, pulled them today and they all checked ok with no short when I swap the leads. CR601, CR602, CR603, CR604
Any suggestions? Let me know if more information is needed.
I checked the left amp board and there was no issue checking the base voltage on Q519. I replaced Q519 on the right board and the issue is still there. I checked the resistors and caps before the base of Q519. All were in spec but C501 which should be 150pF is reading 129pF. Then when I checked the voltage on the C501 leg it shorted the DBT. If I power the amp down, wait two minutes and power it back up it seems fine until I check voltages from the base of Q519 and back.
Powering the left board the DBT meter shows 113Vac with 0.3A and the rails measure +/-53.26dcv
Powering the right board the DBT meter shows 111Vac with 0.3A and the rails measure +/-51.25dcv
I also replaced the bridge diodes, pulled them today and they all checked ok with no short when I swap the leads. CR601, CR602, CR603, CR604
Any suggestions? Let me know if more information is needed.
You are injecting noise in Q519, the input transistor, with the meter probe. This is the highest gain point of the amp. There are a few components meant to filter out noise, like C501 R051 etc, but they are not working. You probably have a bad solder joint on the base of Q519. WIth power off, check that the lead of C501 & B Q519 are connected, with the ohmmeter. Could be a bad solder joint either place or you have broken the land.
Why I don't agree with replacing parts that you haven't determined with voltage or music checks are defective. Especially front end transistors. Meltdowns from a shorted speaker wire or driver cone shorted turn, tend to propogate from the back frowards. Usually they don't get past the VAS Q503 Q504 although it has happened.
Why I don't agree with replacing parts that you haven't determined with voltage or music checks are defective. Especially front end transistors. Meltdowns from a shorted speaker wire or driver cone shorted turn, tend to propogate from the back frowards. Usually they don't get past the VAS Q503 Q504 although it has happened.
Another possibility is that the gain is excessive because the feedback through Q502 is not working. Gain is limited by the negative feedback from the output. No negative feedback, gain at input is 100000 X or more.
I check with an ohm meter and C501 and B Q519 are connected.
When I checked the left amp board B Q519 for voltage it gave me a reading and did not cause a short. I can pull Q519 on the left board to check the gain and compare it to the right board resistor gain.
Why would the rail voltage to the right amp board be 2 volts lower than the left amp board?
When I checked the left amp board B Q519 for voltage it gave me a reading and did not cause a short. I can pull Q519 on the left board to check the gain and compare it to the right board resistor gain.
Why would the rail voltage to the right amp board be 2 volts lower than the left amp board?
Gain is not controlled by the Q519, unless seriously defective. Gain is determined by the resistor ratios of collector, emitter, and the feedback loop.
Is outlet 19 at 0 volts as indicated by the schematic.Also plus of C516. If not, can the zero offset pot RS02 push it there? Both with the probe off of base of Q519, and probe on base of Q519.
K301 should not go into protection if outlet 19 is <80 mv. + or -
Is outlet 19 at 0 volts as indicated by the schematic.Also plus of C516. If not, can the zero offset pot RS02 push it there? Both with the probe off of base of Q519, and probe on base of Q519.
K301 should not go into protection if outlet 19 is <80 mv. + or -
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To compare I measured the left and right amp board.
Left board - connection 19 should be 0V but reading -0.649 vdc and plus C516 reading -0.0650 vdc
Right board - connection 19 should be 0V but reading -0.011 vdc and plus C516 reading -0.013 vdc
Left board - connection 19 should be 0V but reading -0.649 vdc and plus C516 reading -0.0650 vdc
Right board - connection 19 should be 0V but reading -0.011 vdc and plus C516 reading -0.013 vdc
Well adjust R502 on left board until plus C516 is zero. If can't be done measure & report voltage emitter of Q502 and base and collector of Q520.
I just adjusted the DC offset on the left amp board so it is now reading 0v. I have no problems measuring the B Q519 leg on the left amp board.
I moved back to the right amp board and by accident my finger touched the long cap at the bottom, C517, there was a buzz sound followed by DBT lights glowing so I powered it down.
I moved back to the right amp board and by accident my finger touched the long cap at the bottom, C517, there was a buzz sound followed by DBT lights glowing so I powered it down.
Check continuity lead to land both ends of C517. Up near where the problem was. at base of Q519. I don't know why c17 would be long, it shows as only 1.0 uf. Maybe some difference between models 240 and 250m.
When I replace all e-caps I check the board for function after every two. Then if a problem occurs I know just about where the bad solder joint, incorrect part, or reversed cap is.
When I replace all e-caps I check the board for function after every two. Then if a problem occurs I know just about where the bad solder joint, incorrect part, or reversed cap is.
I re-soldered connections around B Q519 as I found C502 was lacking continuity. When put the probe on the B Q519 I could measure -0.065v but there was a slight humming and DBT lights start to light slowly. I found Q511 was getting hot.
Disconnect speaker, or protect it from DC with - to - electrolytic capacitors 2200 uf or larger. The used rail capacitors will do fine. Prefer a trash speaker from the junkyard for this position, myself. Helps to hear pops as you push on components with bad solder joints. but don't want anything good damaged to hear them.
What voltage are tabs 18 & 21, rail voltages, with the AC mains lamp lit up? Under same conditions what are voltages of collectors of Q602 & Q603? What is + of C516 at the same time? This will tell you by subtracting, what voltages across R531 & R532, which dividing by 0.1 will give you idle current of output transistors. This is adjustable by pot R526, idle bias current. Should be about .02 A cold.
What voltage are tabs 18 & 21, rail voltages, with the AC mains lamp lit up? Under same conditions what are voltages of collectors of Q602 & Q603? What is + of C516 at the same time? This will tell you by subtracting, what voltages across R531 & R532, which dividing by 0.1 will give you idle current of output transistors. This is adjustable by pot R526, idle bias current. Should be about .02 A cold.