AU-777A fix-up

F-1170 driver amp boards are done. New metal film resistors 1W and down. 2W and up resistors have been replaced, bumped up a watt, and lifted off the boards. C817 is a Nichicon KZ. The original pots are in good shape and I think they look cool, so I left them alone. Idle current set quickly to 30 mA and is rock solid. All solder joints were reflowed.

Service Manual Error: Main-Amp Section Current Adjustment, Page 26 of the AU-777A service manual, incorrectly refers to VR804, a non-existent part. The AU-777A has two identical F-1170 boards, one for each channel, each with a VR803. The reference should be to VR803 in Step 8.
 

Attachments

  • F-1170 Done.jpg
    F-1170 Done.jpg
    103 KB · Views: 77
  • SERVICE MANUAL ERROR.jpg
    SERVICE MANUAL ERROR.jpg
    66.9 KB · Views: 54
Last edited:
F-11831 Driver Amp Block, half done. Left side is original; right side, new. The original 3W concrete resistors (R819/821 and R820/822) run hot, are flush on the board, and almost touch the adjacent electrolytics. So they're getting replaced with 5 watters, elevated off the board (teflon tubing has the added benefit of adding stiffness), and gently bent toward each other and away from the caps.
 

Attachments

  • F-11831 Half Done.jpg
    F-11831 Half Done.jpg
    116.9 KB · Views: 66
  • Resistors on Stilts.jpg
    Resistors on Stilts.jpg
    63.4 KB · Views: 66
Last edited:
F-11831 Driver Amp Block is done. The six original electrolytics that were tight up against the hot running 3W cement resistors for 45 years - and which got warm to touch - measured as follows:

1000uf originals: 1459, 1508, 1659, 4.36

100uf originals: 154, 157

The new electrolytics run cool after 45 minutes with the new 5W resistors off the board and further away.
 

Attachments

  • F-11831.jpg
    F-11831.jpg
    72.2 KB · Views: 60
F-1194 board stencil error

F-1194 board stencil error

My F-1194 board has a 4.7K 1W black resistor labeled "R771". The service manual board drawing and parts list show it as R753.
 

Attachments

  • R751_R753 error photo.jpg
    R751_R753 error photo.jpg
    87.6 KB · Views: 45
  • R751_R753 error board.jpg
    R751_R753 error board.jpg
    34.8 KB · Views: 36
Amp and power supply are done. Bias set to 28ma, center voltage to 36v. Both are steady. Heatsink transistors in the back run just barely warm after an hour at moderate volume. It sounds great. Groovin' to Dexter Gordon as I write this.

Big signal path caps are Nichicon Gold Tunes. Small ones are Nichicon FG, Elna Silmic II, and WIMA films. Non SP are Nichicon PW and one Chemicon I had laying around. I replaced all small resistors with metal films, although most carbon comp originals were within 10% when pulled. 2SC458s have been replaced with KSC1845FTAs.

Waiting on a Mouser order to start the F-1194 Head Pre Amp board.
 

Attachments

  • IMG_5999.jpg
    IMG_5999.jpg
    167.8 KB · Views: 49
Nice work resurrecting !! Have you noticed how the imaging of the 777a is special ? - in my rig, the seperation and detail is amazing, coupled with a lush voicing - love this amp.
 
. . . Have you noticed how the imaging of the 777a is special ? - in my rig, the seperation and detail is amazing, coupled with a lush voicing - love this amp.

Absolutely. The skeptic in me keeps saying it's confirmation bias, but the detail is really something.

I'm taking my time and being real careful with this beauty.
 
R611 and R612: 270K originals pulled. Service Manual parts list says 390K. I doubt this would make much difference. I put in new 270Ks.
 
Good move, I would not deviate from the board itself we know the manuals are not correct. There were a lot of revisions that I believe we do not have proper updated literature for. Likely changes made as production runs went on.
 
Good move, I would not deviate from the board itself we know the manuals are not correct. There were a lot of revisions that I believe we do not have proper updated literature for. Likely changes made as production runs went on.

Correct. I put orders together based on a visual inspection of what's installed, not schematics or parts lists.
 
Very nice work and well thought out. I am looking at picking up a 555a or a 777a. What are the sonic differences you are hearing? I think yours would be a good comparison as you have/are rebuilding the 2 in a similar manner.:scratch2:
 
Phono preamp on F-1194 board is done.

New metal film resistors. So far, every original carbon composition resistor I have pulled has been within 10% of marked value, so replacing them all is probably overkill. But, in for an inch, in for a mile.

KSC1845FTA’s in place of 2SC458 transistors. This is the first project where I’ve used the Peak Atlas semiconductor analyzer. It makes replacements easy and screw ups hard. Well worth the investment.

I replaced the 1.5uf input electrolytics (C601, C602) with 2.2uf WIMA MKS2 films. The other signal path caps (C611, C612) are now 10uf Nichicon Fine Golds. Nichicon PW’s and HE’s elsewhere.

I am listening to the phono inputs with a line level source into a reverse RIAA attenuator, since I have no turntable. Both inputs sound fine.

Service manual discrepancy: The parts list shows R611 and R612 as 390K. Schematic shows 270K. I found 270K originals, and replaced with the same value.
 

Attachments

  • Phono Stage F-1194 done.jpg
    Phono Stage F-1194 done.jpg
    63.1 KB · Views: 59
Last edited:
Triac power switch relocation.

As noted in Post # 25, I initially located the triac power switch modification at the back of the unit by bolting it to the underside of the power transformer, where the cord enters. This was about 2 inches from the inside of the speaker terminals. In previous units, I had put the triac right at the power switch, but there was no easy place to do this on the AU-777A without drilling a hole.

The result of the triac near the back panel was some high frequency hash in the speakers (but not into headphones), probably due to radiation onto the speaker leads. Not strong enough to notice when playing even low-level music, but still not acceptable. This was confirmed by the fact that the hash was only in one speaker, but not in the other, which was several feet farther away.

Nothing I tried got rid of the hash, so I have relocated the triac to the chassis, as close as possible to the power switch. Rather than drill a non-reversible hole, I glued it on with a few drops of silicone. Gluing the triac (Littelfuse Q4040J7TP, TO-218x package) directly to the chassis is OK, since the case is isolated.

Voilà, dead silence! Putting it right at the switch (or at least far from the speaker terminals) appears to be the better practice.

I’d feel better with a proper nut and bolt, but this way seems pretty solid, and it is reversible if I ever sell the unit to a purist.
 

Attachments

  • Triac relocated.jpg
    Triac relocated.jpg
    105.6 KB · Views: 219
1W per amp of primary current. Unless that device is making good contact with the chassis, it'll destroy itself if the amp is run hard for an extended period.

Can't say I recommend this method of attachment.
 
1W per amp of primary current. Unless that device is making good contact with the chassis, it'll destroy itself if the amp is run hard for an extended period.

Can't say I recommend this method of attachment.

I'm not crazy about it, either. I take your recommendations seriously. Once I get the preamp back together, I think I'll fire up into dummy loads, put a scope on the terminals, shoot it a sine wave, set it to just below clipping, let it run, and keep a finger on it. I think this would be a good way to test it. Do you?

I still don't want to drill a hole, so I am looking at various clips that would allow solid metal-to-metal contact, with a little heatsink compound. If I can figure out a way to do this, I will, heat or not.
 
There is a large ground bus foil between the two main preamp channels on the F-1194 board, with lots of shields and resistors attached to it with huge globs of solder. Just too much heat sinking metal for my trusty Weller WES51 iron, especially when using braid. Plus, it’s a little tight at places. I melted some insulation and got frustrated to the point where I was thinking of breaking out the 100W iron. This would have been like giving a chimp a machine gun.

Fortunately, I walked away for a few hours, cooled down, and ended up snipping the leads flush on the top side, then carefully drilling the holes out with a pin vice. No excessive heat, no lifted foils, no fried components. Worked great. Lesson learned.
 

Attachments

  • Preamp Ground Foil.jpg
    Preamp Ground Foil.jpg
    142.9 KB · Views: 134
Back
Top Bottom