Thanks.
Don´t you have higher resolution pictures to post?
Maybe as-taken by the camera.
I usually ask people to reduce them to avoid clutter and excessive download times, when, say, they send 22 pictures of a cabinet they built or something where it´s not really needed, but here we want to enlarge them on screen to read parts values, see cracked solder, etc.
These boards "should" match the schematic I posted, which is a cleaned/sharpened version of the fuzzy one in the service manual, but you check and confirm that.
Just curiosity, it would be nice to see the whole power module and how it´s mounted in the cabinet.
I know the big horn loaded ones, there they had free open air space on the floor
but your 260 looks like a closed (or bass reflex) cabinet from the front.
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I suggest an immediate recap of the amp. Those bluish caps may have over 40 years. And the big bottle too.
I am going to, I did pull one side and check them, they did meet the rated value but I know that does not tell you everything. I looked and did not enough proper value caps laying to change them.
I think I'm about to order a set of outputs and caps and see where that goes.
you could just check the driver transistors and make sure they are OK.
Otherwise it could be something simple.
There is a few open frame potentiometers which adjust bias and DC offset.
If the finals are not actually blown, they might just be getting nailed hard.
Meaning it is possible, just the open frame bias pot is bad.
or there is a issue with the simple diode string.
or DC offset pot is not so great and causing issues.
could also be basic insulators or mounting hardware for the T03 outputs.
looks like there is fresh solder spots on that board.
might be previous work done on the head, or just your work for testing.
The point is more often than not if previous work was done.
most the time it is where you find problems. check traces/ semiconductors for continuity.
if something is open / shorted could be massive DC offset just nailing the outputs fully on.
likewise noticed various switches for channel routing/ channel outputs. could be as simple as old panel
mount switches for channel A/B in the pre. or apparently switches controlling speaker outputs might have gone bad.
its a rather simple bootstrap current source for the bias/ 2nd gain stage.
bootstrap cap is rather small 10uF could be increased to more standard 100uf
Otherwise it could be something simple.
There is a few open frame potentiometers which adjust bias and DC offset.
If the finals are not actually blown, they might just be getting nailed hard.
Meaning it is possible, just the open frame bias pot is bad.
or there is a issue with the simple diode string.
or DC offset pot is not so great and causing issues.
could also be basic insulators or mounting hardware for the T03 outputs.
looks like there is fresh solder spots on that board.
might be previous work done on the head, or just your work for testing.
The point is more often than not if previous work was done.
most the time it is where you find problems. check traces/ semiconductors for continuity.
if something is open / shorted could be massive DC offset just nailing the outputs fully on.
likewise noticed various switches for channel routing/ channel outputs. could be as simple as old panel
mount switches for channel A/B in the pre. or apparently switches controlling speaker outputs might have gone bad.
its a rather simple bootstrap current source for the bias/ 2nd gain stage.
bootstrap cap is rather small 10uF could be increased to more standard 100uf
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I've done the simple test on the outputs and drivers with a multimeter they all passed that test.
I did pull the pots and measure them on both sides of the wiper and that seemed to add up, put a couple drops of Caig Lab control lube in them gave them few rotations and set them back to where they were at least really close going by the resistance measurement.
Changed the insulators and checked for shorts to the chassis
The fresh solder spots are from me pulling parts for testing.
It did not look anyone has done anything to the board as far as the solder connections look.
There had been some "work'' to the input and output 1/4' jacks, the speaker and input jack need to have
insulating washers on them and they were gone!
This unit is actually just the 260 amp module version and does not have all the extra switches.
Have you ever thought about just leaving the old man dead ? Just say RiP and goodbye my old friend ! lol
/me joking 😉
Seriously, that's one ugly, crusted poor sod ! As the doctors would say, he has not died yet but clinically yes. Good luck mate ! 🙂
/me joking 😉
Seriously, that's one ugly, crusted poor sod ! As the doctors would say, he has not died yet but clinically yes. Good luck mate ! 🙂
Hey 😉 This one is incredibly clean for a 50-55 y.o. amplifier.
Doubly so by a M.I. one!!!! 😱
Normal is finding incredibly hard encrusted dust. gypsum/portland cement crusts, saline water, dead mice, dead or alive cockroaches, mouse droppings, urea crystals from dog/cat/mouse dried pee, the works.
Bonus points for the cockroach nests/nurseries with hundreds of tiny eggs, which when disturbed leave hundreds of little cockroaches free running around and hiding everywhere. Ugh!!!
Some amps , specially when used/stored near a kitchen (even in very expensive restaurants and clubs) must be put while still in the street inside a large trash bag, fumigated (think half a can) and left there for hours so the nasty chemicals do their job.
Ugh!!!!
Doubly so by a M.I. one!!!! 😱
Normal is finding incredibly hard encrusted dust. gypsum/portland cement crusts, saline water, dead mice, dead or alive cockroaches, mouse droppings, urea crystals from dog/cat/mouse dried pee, the works.
Bonus points for the cockroach nests/nurseries with hundreds of tiny eggs, which when disturbed leave hundreds of little cockroaches free running around and hiding everywhere. Ugh!!!
Some amps , specially when used/stored near a kitchen (even in very expensive restaurants and clubs) must be put while still in the street inside a large trash bag, fumigated (think half a can) and left there for hours so the nasty chemicals do their job.
Ugh!!!!
I'm getting close to that, going to make a final shotgun repair, with a set of output, diodes in the stack and re placing
the caps on the board. This was not a must fix repair job but just a see what you could do with it type thing.
If this fails when I redo the cabinet I'll make it passive.
At one point or another I have seen most of that,
Back when I worked for a local cable company the customer cable boxes that were infested were bagged up in the house and dumped into the dumpster at the office.
Working on video tape machines that were used for automated program playback for that same company that were out in various headends I found a dead snake in one and a bird in another.
Guess you are not much in contact with the Musical Instrument amplification world, maybe you only see audiophile amps sitting in nice rooms,surrounded by fine furniture, daily dusted by a Maid in uniform 🙂
Here, no roaches nor mouse droppings, just the "house" Bass amp at a beachside Restaurant, breathing all that pure invigorating Sea breeze for a few years 😉
You are invited to enlarge the image for the fine details 😱
Incredibly it worked for years until "just one more cabinet" was added, causing the mini explosion and fire in the short protection area above the main filter caps.
Ermm...cough... there is some truth there. 😊 Most of my life I have dealt with relatively clean amps, a bit dusty, blocked fans, some rust here and there, yeah, but no cockroaches and dead mice in them. I like to have a clean bench. Btw, I have a terrible OCD when it comes to a dirty bench.
I had seen several cocroaches burned inside Danfoss AC to AC inverters for squirrel cauge motors. Inside there there are 3*380VAC rectified, say, 540DCV. Some few of them had survived magically.
I have high hopes.
I used a ACC 370 head with the 301 W Bin or SVT810E cabinet for many many years.
Little magic to cranking up these old Quasi Heads
Acoustic Control had lifetime warranty
Many of these amps have lasted 40 to 55 years of abuse.
That warranty lasted way way past the company lifetime.
The huge screw terminal can caps last forever.
Mine tested fine, and no hum.
Very early heads are full of slaughter proof Hometaxial T03
I used a ACC 370 head with the 301 W Bin or SVT810E cabinet for many many years.
Little magic to cranking up these old Quasi Heads
Acoustic Control had lifetime warranty
Many of these amps have lasted 40 to 55 years of abuse.
That warranty lasted way way past the company lifetime.
The huge screw terminal can caps last forever.
Mine tested fine, and no hum.
Very early heads are full of slaughter proof Hometaxial T03