Few weeks ago the amp stayed in the protected mode when I turned it on. I switched the speaker selections between A, B, A+B and Off a few times then the fuse blew. It will blows the fuse every time I turn the amp on ever since.
I found a pair of output transistors that were short (SK A1186 and C2837) so I replaced them with a new set. I made a dim bulb tester before the parts arrived. The 60 watt bulb was bright when I turned the amp on after replacing the transistors. I guess it still has an issue somewhere.
The transistor I ordered are from the same maker, have the same number. The old transistors have 87Y under the main number but the new transistors have " 10 12" and "12 21" instead. Does it matter? The ohm readings are different between the new and the old ( still have a set of good transistors from another channel).
If I disconnect the wire from transformer to the main board then fuse will not blow. I have the schematic downloaded. Can someone suggest where I should check?
Thank you.
I found a pair of output transistors that were short (SK A1186 and C2837) so I replaced them with a new set. I made a dim bulb tester before the parts arrived. The 60 watt bulb was bright when I turned the amp on after replacing the transistors. I guess it still has an issue somewhere.
The transistor I ordered are from the same maker, have the same number. The old transistors have 87Y under the main number but the new transistors have " 10 12" and "12 21" instead. Does it matter? The ohm readings are different between the new and the old ( still have a set of good transistors from another channel).
If I disconnect the wire from transformer to the main board then fuse will not blow. I have the schematic downloaded. Can someone suggest where I should check?
Thank you.
There are 2 parallel pairs of these Sanken LAPT output transistors in each channel so I guess you checked them all over. There are also a few legitimate "copies" of these, such as by ISC, plus several poor performing faked Sanken branded versionsof these available too, so you need to check your sources and expected markings from the datasheets, as you can't just mix them ad hoc. If the output transistors have blown, it's quite possible that their driver transistors have also blown when they were forced to supply the current as the outputs failed short. Check these, 2SC2275/2SA985A drivers, also. Unless you have the same output types, Hfe rank and manufacturer, there will be current hogging problems there.
Note there are a number of Sony's notorious fusible resistors around the drivers for protection (ahem!) so there were issues there right from the outset. You may need to check these over and replace with the right types as necessary to restore proper operation.
Sony TA-F700ES Manual - Stereo Integrated Amplifier - HiFi Engine
Allegro MicroSystems - Sanken Audio Transistors
Note there are a number of Sony's notorious fusible resistors around the drivers for protection (ahem!) so there were issues there right from the outset. You may need to check these over and replace with the right types as necessary to restore proper operation.
Sony TA-F700ES Manual - Stereo Integrated Amplifier - HiFi Engine
Allegro MicroSystems - Sanken Audio Transistors
Driver transistor seems fine. Anything else?
Can I turn the amp on with the driver disconnected on one channel?
Before I disconnected the driver, I checked the voltage on each leg of the transistors ( amp on, in the dim bulb tester). All values seems match between two channels except one showing almost zero volt. Not sure if that tell me anything.
Can I turn the amp on with the driver disconnected on one channel?
Before I disconnected the driver, I checked the voltage on each leg of the transistors ( amp on, in the dim bulb tester). All values seems match between two channels except one showing almost zero volt. Not sure if that tell me anything.
The drivers (and the pre-drivers preceding them) are all arranged as complementary pairs, like the output transistors. Are they both OK, and the fusible resistors around there check out too? Which leg of which transistor (Q number as shown on the schematic and overlay) do you say is different at almost zero?
After your dim bulb, the main indicator that something is wrong with the output stage semis and their associated resistors, connections etc, is the DC offset, a tiny residual DC voltage between the output terminals. A high offset triggers the protection circuit as it should but you can just as easily measure the output before the protection relay. It's still there. If offset is below + or - 50mV, likely fine. Then measure bias current according to the manual. If that is more than 50% in error, the problem will require more investigation as to why.
Note that you need to remove all inputs and speaker connections to the amp, maybe even turn down volume and minimise noise to allow a useful DC measurement.
After your dim bulb, the main indicator that something is wrong with the output stage semis and their associated resistors, connections etc, is the DC offset, a tiny residual DC voltage between the output terminals. A high offset triggers the protection circuit as it should but you can just as easily measure the output before the protection relay. It's still there. If offset is below + or - 50mV, likely fine. Then measure bias current according to the manual. If that is more than 50% in error, the problem will require more investigation as to why.
Note that you need to remove all inputs and speaker connections to the amp, maybe even turn down volume and minimise noise to allow a useful DC measurement.
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