Hi,
My 7200 goes into protection. The symptoms are; upon initial switch on from cold the amp plays happily for a few minutes then a thud is heard through both speakers and goes into standby. If the amp is left for 5 or 10 minutes it will switch itself back on for a few seconds then cut out again until it won’t do anything unless powered down overnight.
Having researched known issues for the 7200 it seemed likely the capacitors are failing after many years of use. So I’ve changed all caps on the protection board and all the caps on the power supply board but the issue remains. All the boards have been flexed while powered up and a dry joint hasn’t shown up.
Any ideas to move forward appreciated.
David.
My 7200 goes into protection. The symptoms are; upon initial switch on from cold the amp plays happily for a few minutes then a thud is heard through both speakers and goes into standby. If the amp is left for 5 or 10 minutes it will switch itself back on for a few seconds then cut out again until it won’t do anything unless powered down overnight.
Having researched known issues for the 7200 it seemed likely the capacitors are failing after many years of use. So I’ve changed all caps on the protection board and all the caps on the power supply board but the issue remains. All the boards have been flexed while powered up and a dry joint hasn’t shown up.
Any ideas to move forward appreciated.
David.
Thanks Hugo,
These are DC offset measurements in both class A and AB; Right channel between 0 and 2mv, Left channel -56mv. So it appears the fault is probably on the output board ?
David.
These are DC offset measurements in both class A and AB; Right channel between 0 and 2mv, Left channel -56mv. So it appears the fault is probably on the output board ?
David.
-56mV is not excessive but if it fluctuates up to the protection circuit's trigger voltage this could explain the problem.
The power supply will most likely be OK because only one channel seems affected.
That's why you should be able to monitor for a longer time and record the voltage when it goes into protection.
Has you DMM a record function?
If bad solder joints are ruled out, I would look at the bias current.
But gently tapping the board and components with the back of an isolated screwdriver could reveal a possible overlooked bad joint.
Hugo
The power supply will most likely be OK because only one channel seems affected.
That's why you should be able to monitor for a longer time and record the voltage when it goes into protection.
Has you DMM a record function?
If bad solder joints are ruled out, I would look at the bias current.
But gently tapping the board and components with the back of an isolated screwdriver could reveal a possible overlooked bad joint.
Hugo
Unfortunately my DMM doesn’t have a record capability, but in between being in protection when the offset is monitored it starts at -50mv rises to -56 then the amp cuts out.
However, I’ve now adjusted the Left channel offset pot to a few mv but the amp is still cutting out. I’ll look into measuring the bias current.
Using a wooden spoon the output board has been comprehensively checked with no obvious dry joints.
David.
However, I’ve now adjusted the Left channel offset pot to a few mv but the amp is still cutting out. I’ll look into measuring the bias current.
Using a wooden spoon the output board has been comprehensively checked with no obvious dry joints.
David.
The most crude way a Philips technician ever learned us was to disable the protection circuit (in some specific TV sets) a look what's going up in smoke. 🙂
I wouldn't advice this with this amp.
Hugo
I wouldn't advice this with this amp.
Hugo
😀
I'm not advising disabling protection, only using it as a pointer to the defect. OP seems to have already exercised prolonged shutdown. Are heatsinks hot as they shutdown?
Bias current deserves inspection, but be very cautious about adjusting bias current, and bias current should have very little influence on DC offset, anyway.
If you suspect DC offset is the issue, you might consider possible leakage in cap 2255, left channel, in the feedback to input pair. Just 3.7uA leakage would shift amp output 100mV; it would show as DC across resistor 3271, with 5.5mV corresponding to 3.7uA. Increasing leakage pushes offset in the plus direction.
I'm not advising disabling protection, only using it as a pointer to the defect. OP seems to have already exercised prolonged shutdown. Are heatsinks hot as they shutdown?
Bias current deserves inspection, but be very cautious about adjusting bias current, and bias current should have very little influence on DC offset, anyway.
If you suspect DC offset is the issue, you might consider possible leakage in cap 2255, left channel, in the feedback to input pair. Just 3.7uA leakage would shift amp output 100mV; it would show as DC across resistor 3271, with 5.5mV corresponding to 3.7uA. Increasing leakage pushes offset in the plus direction.
Thanks for the advice on cap 2255, I’ll check the voltage across 3271 to confirm correct cap operation (and compare results with its opposite on the right channel).
The heat sinks are cool upon shut down.
I’d prefer not to begin adjusting sensitive bias pots as you rightly say they shouldn’t affect offset hugely and suspect the fault lies with one of the components on the left channel on the output board.
The amps been switched off since yesterday so I may have several minutes for testing before shut down 🙂
David.
The heat sinks are cool upon shut down.
I’d prefer not to begin adjusting sensitive bias pots as you rightly say they shouldn’t affect offset hugely and suspect the fault lies with one of the components on the left channel on the output board.
The amps been switched off since yesterday so I may have several minutes for testing before shut down 🙂
David.
Sure 🙂
David,
I think we can assume there is no potential short at the ouputs?
As BSST said, it might be worth checking at the protection circuit connector to see what's going on there.
Here the datasheet of the TA7317P protection IC:
Hugo
Unfortunately upon first switch on today the amp came out of protection for about a second before shutting down and now is stuck in standby so I’m unable to check the 3271 voltage.
Thanks for the data sheet for the protection IC but I fear the depth of my ability has been reached in as much as testing this component in situ as it appears impossible to access the pins safely without accidentally shorting between them. It would be possible to check the pins on the multi connector which the protection board fits into.
I’ll see if a circuit diagram for the output board & protection board is available and try to find voltages.
Thanks for the data sheet for the protection IC but I fear the depth of my ability has been reached in as much as testing this component in situ as it appears impossible to access the pins safely without accidentally shorting between them. It would be possible to check the pins on the multi connector which the protection board fits into.
I’ll see if a circuit diagram for the output board & protection board is available and try to find voltages.
Look for the Marantz PM68 / PM78 service manual.
It's drawn in the Philips style and shows voltages.
Its' on Hifiengine if you can get it.
Hugo
It's drawn in the Philips style and shows voltages.
Its' on Hifiengine if you can get it.
Hugo
Unfortunately HiFi Engine is not accepting any new members and all the other available circuit diagrams don’t show the voltages so I’m a bit stuck.
It may be time to seek professional help with the amp, can anyone recommend a competent technician in the Derbyshire/South Yorkshire area ? Or would anyone have a 7200 for spares with a known good output board ?
It may be time to seek professional help with the amp, can anyone recommend a competent technician in the Derbyshire/South Yorkshire area ? Or would anyone have a 7200 for spares with a known good output board ?