smurfer77
Super Member
*edit: adding this up front for future reference.
Summary of variations, errors and omissions in the manual/schematic and other points of particular note.
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Hi everyone. This will be the first time I've done a rebuild 'live' (posting on AK as I go). So here we go...
I picked up a 'no sound' 777A for ebuy. Here is what it looks like. You can see a missing speaker clip, some minor damage to the wood case and a bent switch or two.
And some shots inside.
Installed new fuses although old ones were okay. Biased no problem and voltages look good at the ~70 V and ~37 V points. Couldn't measure any AC getting through with my multimeter so I guess the filtering caps are okay.
There is no sound output. Cleaned the heck out of the switches and yes the pre-out is connect to the main-in . A slight hint of sound/music gets through when the volume knob is at a certain position.
So then I hook-up my iphone directly to the main-in and nothing on both channels. So that is where I am at tonight. Will have a deeper look tomorrow night hopefully. Any suggestion welcome of course.
Did pull the output transistors (although I didn't expect the problem to be there since it is on both channels, I wanted to see what needs replacing for the rebuild anyway). All output transistors test okay.
I have ordered parts for a full rebuild over the xmas break, but I guess I would prefer to find the source of the problem before I go to town (easier for error checking if the amp works when doing a rebuild).
Hopefully this account of the rebuild and the many many pictures will not be too boring for everyone.
Summary of variations, errors and omissions in the manual/schematic and other points of particular note.
- R005/R006 (protect.): 220 Ohm 5W resistors on protector block PCB hit a temperature of over 100 C.
- R611/R612 (phono): 280kOhm in amp (390 kOhm in schematic). Either value is okay
- C725/C726 (preamp): manual says 47 uF, 6.3 V but 47 uF, 10 V found in amp
- C823, C824 (main): 500 pF ceramic (measure voltage and update here), not listed in manual
- C813/C814 (main): 100 pF, 50 V ceramic not listed in manual
- R841 (driver): 3.3k Ohm in parts list. 2.7k Ohm installed and service schematic. Not a big deal.
- R861 (driver): 3.3 (!) Ohm in parts list. 2.7k Ohm installed and on service schematic. A big deal.
- Note there is a service bulletin on the bias resistors R851/R853 and some nearby caps. Not mandatory unless "the trouble is found" with TR811-TR814
----------------
Hi everyone. This will be the first time I've done a rebuild 'live' (posting on AK as I go). So here we go...
I picked up a 'no sound' 777A for ebuy. Here is what it looks like. You can see a missing speaker clip, some minor damage to the wood case and a bent switch or two.
And some shots inside.
Installed new fuses although old ones were okay. Biased no problem and voltages look good at the ~70 V and ~37 V points. Couldn't measure any AC getting through with my multimeter so I guess the filtering caps are okay.
There is no sound output. Cleaned the heck out of the switches and yes the pre-out is connect to the main-in . A slight hint of sound/music gets through when the volume knob is at a certain position.
So then I hook-up my iphone directly to the main-in and nothing on both channels. So that is where I am at tonight. Will have a deeper look tomorrow night hopefully. Any suggestion welcome of course.
Did pull the output transistors (although I didn't expect the problem to be there since it is on both channels, I wanted to see what needs replacing for the rebuild anyway). All output transistors test okay.
I have ordered parts for a full rebuild over the xmas break, but I guess I would prefer to find the source of the problem before I go to town (easier for error checking if the amp works when doing a rebuild).
Hopefully this account of the rebuild and the many many pictures will not be too boring for everyone.
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