After removing the front panel for the first time, I found I had a problem with the tape copy switch. The knob was glued on and I used just a bit too much force to get it off - result - the tape copy 'Source' switch wouldn't 'lock in' anymore, although it would cancel either of the other 2 buttons like it should.
So I removed the upper board and spent around 20 minutes working out how the mechanism worked and what I needed to do to get it working properly. I eventually found that a bit of extra spring power to push the locking bar to the right (in the below picture) was needed - accomplished very nicely with a short length of (blue) neoprene sleeving - see below between the right hand pair of push-button switches.
Here is a picture of a completed driver board (requested by AK member AUD101)
And a picture of the sacrificial output transistors for one channel - looking a bit lonely
When I remounted the original output transistors I treated them to a new set of stainless steel screws (I had to drill out 2 of the screws to get the precious original OP transistors out
)
Here is a picture of the Flat Amp F-2819 board - with 2 (of 4) extra resistors, a electrolytic capacitor (1 of 2) and a 'Black Flag' (1 of 2) mounted on the print side. When I renewed the electrolytics I mounted them on the topside. I am guessing that the original components were so big they fouled on the 'sandwich mounted' Buffer Amp F-2774 board, hence the mounting on the print side of the Flat Amp.
Loss of one channel after re-installation of output transistors.
I experienced what I felt was quite an unusual problem, right at the end after I had verified output from both channels. I refitted the original output transistors, this meant removing the driver modules yet again, and desoldering 3 connections at the top of each board and several underneath. So I get this done and refit the modules - no output from the left channel
. While I was doing this I also took apart the tape copy switch - and I thought I must have disturbed something, so my initial searches were around the very complex input and tape switching. Then my oscilloscope threw that 'wobbly' so I was a bit in the dark. So, I thought about it for a day, and composed in my mind a 'run-book' of tests. And I found that the signal input post on the driver module was not making contact with the (tortured) track at it's base, having been de-soldered and re-soldered many times. The solder joint at the base of the input post was fine visually, and the track wasn't lifted, but something about it was preventing contact with the board. Of course hooking a scope probe to the input post indicated 'signal'
. A quick fix for that and I had both channels again - phew!
A few observations or things to note.
The Driver Amp boards are a nightmare to repair if you get faults - because accessibility in running condition is so poor. Be prepared to get very good at removing the 2 modules (L&R), and testing components with your multimeter, on Ω range or using the Diode Test function. I found those nearly open 390Ω resistors - I don't think I was that lucky, or clever
- I tested them because they looked like fuse resistors.
When you have the front panel of a Sansui amplifier removed - often the power LED goes with it, get yourself a spare LED and plug it into the LED power indicator cable socket - very useful.
Removal and replacement of all Black Flags is an absolute must, both 'Silver Mica' or 'Ceramic C0G/NP0' are valid choices for this. However I consider the ceramic option to be superior, not only for audio reasons, but also due to their very small size. By comparison the Black Flags are huge, and I can't help feeling that by their very size (behaving like an aerial), and lead length (inductance), they are partly contributing to any instability problems.
The small 'signal relays' are cited as being a problem, so far (famous last words
), I haven't found this to be the case. However Ron (
Ronito6) has worked out a nearly 'drop in' solution with modern replacements and I may yet change all mine regardless. (I have the replacement relays
).
Although my model was a very late production one, I did install the Martin Kuehne, anti-oscillation grounding modification to the Flat Amp. Easy to do and it couldn't hurt, so in it went.
I have received considerable help with this project, on-line and off-line from AK members
Ronito6 and
Harolda for which I am extremely grateful. If it had not been for the publication on AK of their ground breaking (for me), work on this model of amplifier I might have found it a lot more difficult than it was. - thanks Ron and Harold.
I will be doing much listening and testing for the time being, with thinking about 'Phase 2' ('can' capacitor replacement), quite a long way in the future.
That's all for now, thanks for the encouragement along the way and reading this.
Here it is...
