AU555 - Restoration progress and help needed

Hi,

I recently received an AU555 in need of some repairs from a member here, and thought it would be worthwhile starting a thread to chronicle the progress in restoring this amp.

It suffered the dreaded "pop then smoke" syndrome, and had previous issues with crackle on the left channel. Also the right channel treble pot has a busted shaft and the knob is missing, as is the power switch button.

Front panel



busted pot shaft



missing power button



Covers off, and it was evident that the left channel output coupling cap had failed (Both Left and Right channel caps had been replaced sometime in the past).

Cap removed



View of Cap base



Decided to start the restoration with a recap of the driver board

Driver Board Recap complete



A good look over the unit showed both channels' output transistors (2SD180) have been replaced with 2N3055's, and a lot of the signal transistors in the left channel (preamp, tone and driver) have also been replaced at some time in the past, possibly in an effort to fix the crackle problem.

Powered her up after the recap, and both channels worked, although the dreaded crackle was evident in the left channel, getting slighlty worse as the unit warmed up. Checked and set bias and AC balance, but left channel is a bit flakey and tends to drift/jump around a little (both bias and AC balance). Removed the pre-out/main in jumpers, crackle was unchanged so it appears that it is being generated in the driver circuit.

Tried changing out D801/D803 with 1N4148's but no difference. All NPN's in the left channel had been previously changed out for BC639's as note above, and the PNP for a BC640. Have also "de oxited" the bias and ac balance pots.

Left Channel driver schematic -



Any suggestions as to the next steps in resolving the crackle issue would be thankfully received. Also would be happy for thoughts/opinions on whether it would be worthwhile changing the output transistors from the 2N3055's to something closer spec wise to the originals, and whether the BC639/BC640 are acceptable alternatives to the original transistors in the driver circuit (2SC281, 2SC734 and 2SA561).

Progress on replacing the damaged Treble Pot will be posted shortly, and procurement of a new knob for treble pot is underway.

If someone with an AU555 could measure the diameter of the power button, and how far it protudes from the front panel that would be greatly appreciated, I am organising the manufacture of a replacement and need these measurements.

Cheers

John

Seein how it gets worse with heat, you may try to isolate with some freeze spray.

I would definitely replace those 2N3055s with something that has a better current gain characteristic at high amps. I would have recommended 2SD424, but that's apparently no longer possible. Maybe someone else can enlighten us.

If idle current is unstable even without a load connected, try shorting TR805 and TR807 bases together. Idle current should go to zero and stay there. If not, then you probably have trouble in the driver and/or output stage transistors.

I see that 2SD424 is currently available from MCM. It's a really great replacement for many of the older Japanese output devices in this class of amplifier. 2SB554 is the PNP complement type, but you don't need 'em because you have a quasi-complementary circuit.

paul79 said:

Seein how it gets worse with heat, you may try to isolate with some freeze spray.

Click to expand...

Going to try that, have to get some more freezer spray as I have just used the last of can I had.

BinaryMike said:

would definitely replace those 2N3055s with something that has a better current gain characteristic at high amps. I would have recommended 2SD424, but that's apparently no longer possible. Maybe someone else can enlighten us.

If idle current is unstable even without a load connected, try shorting TR805 and TR807 bases together. Idle current should go to zero and stay there. If not, then you probably have trouble in the driver and/or output stage transistors.

Click to expand...

Thanks for the tip re the 2SD424, they look like a pretty good subsitute, and have plenty of "headroom" specwise.

The basic 2SD180 specs are
Vceo - 70V
Vcbo - 80V
Ic - 6A
Pd - 60W
Hfe 30-120 @3A
Ft - 10Mhz

Was thinking of a 2SC1051 as a possible close alternative
Vceo - 100V
Vcbo - 150V
Ic - 7A
Pd - 60W
Hfe 40-320@1A
Ft - 8Mhz

Will be doing some more faultfinding on the crackle problem tommorrow and I'll post up progress (will try your shorting trick and see what happens).

Cheers

John

2SC1051 looks acceptable as a replacement, but for improved performance, you really want a transistor with current gain as linear as possible up to the highest current level it sees in operation. 2SD424 probably achieves that goal, but I couldn't verify it in a quick search for specs. It's the hFE vs collector current plot that tells the story, and that graph is missing from all of the abbreviated data sheets that I found.

I did find a couple of very interesting offerings from On Semi, however: MJ21194 and MJ21196. Both look like a good bets up to 10A, which should nicely accommodate 4-ohm loads in an AU-555.

Any time output devices have to be substituted, it's a good idea to verify that parasitic oscillation hasn't crept in. You can do this with a scope and square wave generator, or have a tech do it for you.

Hi,

Understand your comments re the linearity, and will check out the MJ21194 and MJ21196 specs.

I have a scope and function generator so I can do the necessary checks when the outpuits are replaced.

Thanks again for your help.

Cheers

John

Success!!!!! Crackle is gone.

I tried a variation on BinaryMike's shorting TR805 and TR807 bases together. I simulated the protection activation - two diodes in series from "BL" (junction of R921 and R923) to ground, and the crackle persisted. Checked the base of TR805, and voltage was varing from 4 to 7 volts, when it should have been around 1.2-1.4 volts and steady. C813 (220pF Mica) appeared to be the likely culprit.... swapped it for a ceramic I had and bingo.... no more crackle. Bias is now stable, as is AC balance. Looks like the previous replacement of all the left channel transistors was in vain. Will change out the other mica caps on the driver board.

Next is the replacement of the right treble pot, and making up the new power switch button.

Cheers

John

My compliments on a difficult diagnosis. Cheers!

skippy124 said:

Success!!!!! Crackle is gone.

Click to expand...

Well done!

I'm looking forward to hearing this amp in action soon :thmbsp:

You have cleaned up all that spooged electrolyte oil from the circuit board and chassis haven't you? When that cap let loose did it crack the PC board under it or is it just a shadow in the photo?:thmbsp:

I have cleaned up the electrolyte from the failed cap since the pics were taken, and fortunately there was no damage to the PCB (just a shadow).

Giving it a complete clean over the weekend (dont get home till after dark weeknights, and I am banished outside when doing cleaning duty :sigh: !!)

Cheers

John

Finished off the cleaning today, came up pretty well

Internals (with rear cover over the inputs removed, and don't ask what happened to the missing PCB mounitng screw......)



Front Panel



Rear panel



Right Treble pot and power switch button replacents to come.......

Cheers

John

Looks great!

From the link that bluesky posted today a 1969 review by Gramophone, fun read:

http://www.gramophone.net/Issue/Page/August 1969/84/769009/

Hi,

Dragging up an old thread, but I finally got around to completing the restoration on this nice little amp.

Procured a new knob to replace the missing one, and then set about obtaining a new power switch button, and sorting out the repair of the right channel treble pot.

Luckily I have a good friend who has a lathe, so I drew up a power button and he turned up one out of Delrin



I was able to find a donor pot with a long shaft, and had a bush turned up on the lathe as well



Measured up and cut both shafts, and then glued on the bush to the shaft of the original pot



Then fitted the new front shaft to the bush with glue



Repaired pot back in place



Front panel fitted with new power switch button and knob





Power Button came up pretty well




I can now sit back and enjoy this great little amp!!

Cheers

John

Congratulations on a sweet restoration and terrific documentation.
Good job on that pot shaft!

Great Job!

Thanks for sharing that project with us....
Casey

Hi John,

As the donor, I was very interested to read your story of the superb restoration. I can't imagine a better home for this lovely amp than with you and your son - you've earned the enjoyment,

--Toby

qu1j0t3 said:

Hi John,

As the donor, I was very interested to read your story of the superb restoration. I can't imagine a better home for this lovely amp than with you and your son - you've earned the enjoyment,

--Toby

Click to expand...

Many thanks again for your generosity Toby!

Great job Dad... looking forward to seeing and hearing this in the flesh tomorrow :thmbsp: