Bose 901 series i eq starts buzzing once warmed up

[hrtattckcity]

Rebuilt this bose eq. Series II I believe.

it sounds great at first and then once warmed up the volume drops out and a terrible humming/buzzing starts.

Ant thoughts on where to start troubleshooting?

 

[hrtattckcity]

schematic attached if that is helps.

I will post some photos.
Most of the leads were pretty corroded, in a previous life this probably was not stored in an ideal setting.

Everything sounds fine for 1/2 to a full side of an LP, then it goes nuts--I was wondering if that trait would point to a specific component or connection--that only goes haywire once it has warmed up?

 

[hrtattckcity]

Photos attached.

I replaced the transistors with, I believe, some 2n5088’s I had.

and all the film caps with wimas. Some of the wimas have shorter than ideal leads but I find it hard to imagine it would get hot enough for the joint to work fine initially and then open up?

have not replaced the disc caps and there is some surface bubbling (see photo of c8) If anyone has a recommendation for suitable replacements for
C19,c69,c18,c68,c10, c60, c8, c5 That would be helpful—and d1 and d2

thank you

 

[Robisme]

Could be a diode. I had a Marantz receiver that would hum after it warmed up and it was the rectifier diode on the power supply.

What is the number on the diodes? Looks like 1N400 and cant see the last number. 1N4004 maybe.

Rob

 

[hrtattckcity]

looks like a 4001 to me.

I have some n4007’s from a previous project—are those suitable replacements?

Could be a diode. I had a Marantz receiver that would hum after it warmed up and it was the rectifier diode on the power supply.

What is the number on the diodes? Looks like 1N400 and cant see the last number. 1N4004 maybe.

Rob

 

[Aptone]

Is this the sort-of-working EQ I sold you? Sorry it is still causing you trouble.

 

[hrtattckcity]

No worries. I got two responses from my wtb and picked up both. Decided to do a deeper restoration on this to see if I noticed a difference.

Replaced the diodes with some 4007’s I had and just listened to a full album with no issues.

I’ll write back if anything changes but I might be good.

Is this the sort-of-working EQ I sold you? Sorry it is still causing you trouble.

 

[Robisme]

Yes, the 1N4007 will work.

Rob

 

[Aptone]

Replaced the diodes with some 4007’s I had and just listened to a full album with no issues.

Fingers crossed!

Decided to do a deeper restoration on this to see if I noticed a difference.

Do you think you can hear a difference? Too soon to tell?

 

[hrtattckcity]

I was trying to leave this running today to test stability. I don't have 901's in the house right now--I had been rebuilding this for a friend, so I had it going through some small bookshelves and/or headphones, but periodically checking in on it, but leaving it powered on with a signal running through it.

Not conclusive, but it still seems somewhat unstable--like maybe the volume is dropping out or its getting a little distorted. But it seems like turning it off and back on resolves these problems for a while.

Any guesses on what could cause that--only thing left to change would be the transformer, the disc caps and the resistors--or if there is a problem with a capacitor connection or transistors or diodes that I have already replaced?

 

[whoaru99]

IMO, the likelyhood of it being the transformer is about as close to zero as you can get without actually saying zero.

 

[whoaru99]

FWIW, before you get too much time/money invested (maybe too late??) Deer Creek Audio sells miniDSPs pre-programmed with 901 curves. They have one for each of the three variants; I/II, III/IV, and V/VI.

 

[hrtattckcity]

Oh yeah, definitely too late! = P We are in point-of-pride/learning experience territory here!

Or it will be completely rebuilt and fantastic sounding.

FWIW, before you get too much time/money invested (maybe too late??) Deer Creek Audio sells miniDSPs pre-programmed with 901 curves. They have one for each of the three variants; I/II, III/IV, and V/VI.

 

[hrtattckcity]

Back to this....

I had wrongly identified it as a series ii--it is a series i, later production run.

the diode swap fixed the buzzing problem but it does still have a problem where occasionally the volume drops out after a while, and it becomes muffled and distorted--but a quick turn-off and then turn back on takes that problem go away.

Any guesses as to what would cause that?

Schematic attached--
does anyone have recommended replacement parts for the following:
L1 - 22mh
L51 - 22mh
C19 - .01\100v
c69 - .01\100v
c18 - .01\100v
c58 - 10pf
c68 - .01\100v
c10 - .001/1kv
c60 - .001/1kv
c8 - 10pf
c9 - .015/5%

And, finally, does anyone know where the .0047 capacitor near the switch and bulb assembly is? I was assuming the cylindrical component on the back for the back of that was the resistor?

Thanks for any help in tracking this down..

 

[hrtattckcity]

Switch assly

 

[hrtattckcity]

Picking this back up.

going to rework it one side at a time so I have a reference.

replacing all the remaining except the inductors.

 

[hrtattckcity]

Does anyone know what the voltage rating requirement of c8 and c58 is?

two of the ceramic caps.

Thanks.

 

[hrtattckcity]

Does anyone know what the voltage rating requirement of c8 and c58 is?

two of the ceramic caps.

Thanks.

Reached out to WestMassGuy / DHS;
He said the fadeout problem that resets by turning it off and on sounded like a tape monitor switch problem--and normally he likes to jumper bypass that switch unless a client needs it.
He also said the greatest voltage any part of the circuit should see is 22v.

 

[hrtattckcity]

Done and running all day to test of the fade out is still an issue.

 

[hrtattckcity]

Left it running for over 12 hours with no fade. might consider this case closed.