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BOSE 901 EQ Restoration

6 weeks? Wow. Not sure I've ever waited more than a week from them. Was something on backorder?
 
Yeah, that seems like a long time. Perhaps a brand substitution of a part might have got that going faster. i like to use all the same brand resistors or whatever when I do something but it's not always possible due to value or backorder. In such cases I'll check if there is equivalent part... eg. instead of a Brand X resistor maybe I'd get a Brand Y or Brand Z instead.
 
Hi ToastedMaple, so I have a question... I’ve got all the pieces removed from this board finally and have the replacements at the ready... except for C9 and C49. Those don’t appear to be on the pdf you provided... what components do you recommend for those caps? They appear to be the same to me. 1 side reads “1KV S3N” and the other side reads “68 10%”. Do I want to replace it with something similar or do you have a better upgraded option. I noticed that many of the components here were upgraded and I want to make sure to keep in sync with what we’re already doing.

also, which is C74? Is that the one that connects to the power transformer? I think so, just confirming.
 
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Hey! Good morning. Lee Stereo said to leave these two ceramic caps be. They’re not in the signal path and are usually reliable. (He said - I know nothing ) I believe C74 is the safety cap on AC supply. I will double check later and confirm.
 
E9ABE4FC-CB9B-4008-ABA4-86B9C59E50E3.jpeg

this is my progress so far. I still have yet to pull the transistors. I’ll do those next. If you feel that C9 and C49 are fine, then I’ll leave them be. It would be nice to do all of them of course, but I can’t find specifics on what those caps are exactly
 
ToastedMaple said:
Good Day Fellow AK Members

Recently, Leestereo completed restoration of my Marantz 2270. Success of this project prompted me to revive a project to restore the Equalizer for my beloved 901 Series II. I have three, sets of 901's. Pity me if you must. The active set have the Walnut Facing kit for Series I/II. I'm restoring another set for my daughter.

For context on the restoration, consider the Bose 901 Active Equalizer as a supplement to your existing tone controls. Without any cross-over for the 9 drivers in each channel, it is an essential component of the 901 sound. Now, after 40+ years, the EQ electrolytics are most certainly suspect, but also the many carbon resistors that shape the tone curve will likely have drifted and affected the equalization curve.

With Leestereo’s analysis in hand, I repopulated the board with higher-value electrolytics, film caps and 1% resistors. Some electrolytics were replaced with bipolar film capacitors for better quality audio. New transistors (there are 5 per channel) were selected for low noise and matched for hFE. Phono jacks were upgraded and respected for modern connectors. Tape lop was left intact. The complete parts list is attached with Mouser PN, analysis and recommendations, again, courtesy of Leestereo's deep experience in vintage restoration.

Most of us won’t need the full range of +18dB boost but I have tinnitus and a bit is most welcome. Bass is superb, tight and detailed and getting better, possibly as the caps (in both my 2270 and the EQ) bed in.

This was my first electronics restoration project. It was daunting to depopulate and re-install nearly 100 components. I am very grateful to Leestereo for his advice, enormous patience with a newbie, and support. I learned a lot about restoration and trouble-shooting as well (after I installed the &^*% transistors backwards!).

This board was not unmolested, prior restoration work, and that posed some additional challenges. Pictures attached of original board, intermediate and semi-final project, as well as parts list, noted above.

Please feel free to pose any questions.View attachment 1810657View attachment 1810710View attachment 1810718

Best Regards, DGC
Click to expand...
Nice work! I recapped my Series II and left all else alone. The sound definitely improved, and I eventually plan to recap a Series I and a Series IV. But now this thread is tempting me to go for the full monte on these units.
 
kyledp said:
...If you feel that C9 and C49 are fine, then I’ll leave them be. It would be nice to do all of them of course, but I can’t find specifics on what those caps are exactly
Click to expand...

The stock C9/C49 capacitors are 68pF ceramic types; they are the Miller capacitors for Q3 and Q43, respectively.
 
Beware kyledp. Do not ask what a miller capacitor does. Many AudioKarma denizens will be all too wiling to explain Leestereo's reference!
I dipped my toe in to find that it acts to reduce oscillation, or to stabilize the circuit.

I did replace mine in the first try with Mouser P/N 75-561R10TCCQ68 68 pF 5% 1K.
https://www.mouser.ca/ProductDetail...EWYaMVRoBiIUXWJtYOM%2BxfJ2OqDrkmNRoWYaFzvoA==

Probably money wasted (price has doubled). Saved the old ones in round 2. Np problems so far.

VR/DGC
 
Hi All, so I found the caps for C9\C49 here locally. 2 for $1.09. So I’ve got everything I need now!

and for the record, the legs on the transistors aren’t crossed, just trick photography.

I know what I’m doing this weekend!
 
57B174B7-9F16-401A-9742-866865644354.jpeg BAC7677F-3145-448C-BA62-59FF0FC6C338.jpeg 7342A70B-6050-48AE-9567-7A3DA4D72695.jpeg

Well All, I think I got the power supply installed okay. It was a bit of a challenge. The mounts on the original were just tabs that were bent and soldered to the board. The new one had mounting ears. I had to grind some screw heads way down and bend the ears over to be under the chassis. Then I cut the screws short and was able to bolt it in the same holes. It’s a good thing I replaced the power supply, the old one was leaking, but I couldn’t determine this until it was off the board.

also, for C10/C2/C12/C50/C42/C52, (all require the same cap specs) we are replacing polarized caps with bipolar ones. I trust y’al know what you’re doing, but just wanted to inquire if that was intentional.
 
kyledp said:
...also, for C10/C2/C12/C50/C42/C52, (all require the same cap specs) we are replacing polarized caps with bipolar ones. I trust y’al know what you’re doing, but just wanted to inquire if that was intentional.
Click to expand...

The polarized capacitors in the signal path were upgraded to film or bi-polar electrolytic types, as appropriate.
 
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