Bose 4401 Preamp Restoration!

[ants2au]

The manual states -20dB muting, so its not completely shut off.

I am sorry to have to do this to you, but you are going to have to buzz out the offending channel, with a multimeter, all the components from Cx09 (where x = 1...4)
All of them. you need to make sure all are connected properly to each other. thats basically verifying the tracks are still ok.
I maintain you have a floating connection somewhere.

I had to do that on the 4401 I had..

Having the cover on didn't help the noise. The noise waveform that I saw was from my bench light. I'll try the bypass caps on the op-amp supplies next.

I still can't figure out what is wrong with the muting circuit. How is it supposed to work? Attenuation, or shorting the signal to ground. Looks like the muting switch shorts the gates of the fets to ground. The headphone jack has a switch that does that too. Here's an odd observation: For one jack, it cuts the sound entirely. The other one produces the same distorted waveform shown in figure 52. Grounding the gates of the fets with a wire also produces the distorted waveform.

Ideas?

 

[Raccoon1400]

The manual states -20dB muting, so its not completely shut off.

Good to know, I couldn't find that.

I am sorry to have to do this to you, but you are going to have to buzz out the offending channel

All channels are affected.

Also, back to the bypass caps at the opamps - there are there by design. But they are 0.01uf ceramics (and were replaced), as opposed to the 0.047uf poly ones laatsch55 suggested. I have 0.04uf poly caps ready to go.

 

[Raccoon1400]

Changed the power supply bypass caps for the last opamp, and added an extra ground wire going to the chassis so that part of the circuit could ground better. Before it was only grounded through one of the molex connectors to the other board.

No change.

 

[Raccoon1400]

The scope shows there is significantly more noise in the -ve 15V rail than the +ive one. But the noise on the 25V volt rails are equal. This could be the cause of both issues. But what is the cause of this? All parts in the power supply are new. I just checked the values, and replaced the 4148 diodes I hadn't previously replaced. No change.

 

[ants2au]

Ahh, ok, then its back to a power supply problem. (sorry, hard to keep track where we were up to)

Ooops, you have already responded...
Can you start disconnecting the supply from the preamp, and work backwards from there?

All channels are affected.

Also, back to the bypass caps at the opamps - there are there by design. But they are 0.01uf ceramics (and were replaced), as opposed to the 0.047uf poly ones laatsch55 suggested. I have 0.04uf poly caps ready to go.

 

[Raccoon1400]

Ahh, ok, then its back to a power supply problem. (sorry, hard to keep track where we were up to)

Ooops, you have already responded...
Can you start disconnecting the supply from the preamp, and work backwards from there?

Disconnected the preamp from the power supply, and the noise went down by half. But it was still there.

Everything in the power supply has been replaced.

Attached is the power supply schematic.

 

[ants2au]

Ok this next bit will be tricky, but important to do.
you need to isolate the 15V circuit from the 25V output.
We need to identify if the issue is being caused by the bridge and C5/C6, or the 15V circuit.

if the noise disappears, then we have to diag the 15V supply.
I say tricky. because there are multiple connection points to the 15V supply from the 25V outputs.

<Everything in the power supply has been replaced.>....This bit is a huge problem for me

 

[Raccoon1400]

Ok this next bit will be tricky, but important to do.
you need to isolate the 15V circuit from the 25V output.
We need to identify if the issue is being caused by the bridge and C5/C6, or the 15V circuit.

if the noise disappears, then we have to diag the 15V supply.
I say tricky. because there are multiple connection points to the 15V supply from the 25V outputs.

<Everything in the power supply has been replaced.>....This bit is a huge problem for me

Not exactly sure what you are referring to here. The 25V output goes to the 15V regulator circuit, I can only see one connection for each rail. There is noise on the 25V circuit, but it is equal on both rails. Seems the +ive 15V regulator is doing a better job than the -15v one.

 

[ants2au]

Yes.. the 25v has its own output, as well as feeding the 15V circuit.

I need the 15V isolated from the 25V, so you can measure only the 25V, on its own..
I want to know if the 25V, on its own, is clean...its the primary source for everything.


Thats why I said this is tricky, because yo have to lift the following components out:
CR5, R3, R6, Q3, Q4, R1, R4, R5, Q2.

Not exactly sure what you are referring to here. The 25V output goes to the 15V regulator circuit, I can only see one connection for each rail. There is noise on the 25V circuit, but it is equal on both rails. Seems the +ive 15V regulator is doing a better job than the -15v one.

 

[Raccoon1400]

Yes.. the 25v has its own output, as well as feeding the 15V circuit.

I need the 15V isolated from the 25V, so you can measure only the 25V, on its own..
I want to know if the 25V, on its own, is clean...its the primary source for everything.


Thats why I said this is tricky, because yo have to lift the following components out:
CR5, R3, R6, Q3, Q4, R1, R4, R5, Q2.

Ah, I see what you mean. I can see now that there is noise on the 25V rails, but equal on both rails. The 15v regulator does a good job smoothing the noise on one, but not on the other. Does that not suggest the regulator circuit?

 

[ants2au]

To be sure, we need to know that the 15V is not causing any funny things to feedback to the 25V.

Id be happy if the problem is in the 25V part..easier to find
but wont know that until the 15V is disconnected.

Ah, I see what you mean. I can see now that there is noise on the 25V rails, but equal on both rails. The 15v regulator does a good job smoothing the noise on one, but not on the other. Does that not suggest the regulator circuit?

 

[Raccoon1400]

A new observation!

Some of the signal seems to be finding its way onto the -15V rail too. Much more when the signal is under 500Khz, starts to drop off after that. I can't find anything similar on any of the other rails. It doesn't happen when I pull the load. (I pulled the load by pulling J7) Pulling out the first op-amp in the signal path stops this. It also decreases the noise to the same level as when I remove the load from the power supply. (remember pulling the load almost halved the noise)

 

[BilboBaggins]

Update to this thread. I got a call from Raccoon1400 to come out and review the work he has done on my 4401.

I went out there, and heard the noise he has been talking about in this thread. Really miniscule. We looked things over in the scope, and listened to them, and I decided, why not bring it home and see how I feel about it?

So I did. First, I would like to thank Raccoon1400. It sounds really good! No more noise that I was hearing before. It is very clean. I have done a few upgrades to my system since leaving my 4401 in Raccoon1400s hands. And I have to say those upgrades improved sound, but showed some limitations in my current set up. The same week I got the 4401 from Raccoon1400, I also got a pair of Kef 104/2s. It all sounds so good now together. I am listening to records, and CDs, and Apple Lossless, and loving it!

I'm not hearing limitations now.

 

[ants2au]

So the noise is only at Racoons place?

sorry couldnt resist..

But well done nonetheless..

Update to this thread. I got a call from Raccoon1400 to come out and review the work he has done on my 4401.

I went out there, and heard the noise he has been talking about in this thread. Really miniscule. We looked things over in the scope, and listened to them, and I decided, why not bring it home and see how I feel about it?

So I did. First, I would like to thank Raccoon1400. It sounds really good! No more noise that I was hearing before. It is very clean. I have done a few upgrades to my system since leaving my 4401 in Raccoon1400s hands. And I have to say those upgrades improved sound, but showed some limitations in my current set up. The same week I got the 4401 from Raccoon1400, I also got a pair of Kef 104/2s. It all sounds so good now together. I am listening to records, and CDs, and Apple Lossless, and loving it!

I'm not hearing limitations now.

 

[BilboBaggins]

The noise is present, however I don't hear it the way I have my system configured. Raccoon1400 only heard it when he cranked the power amp to max. I never have the power amp past 2/3s and try to keep the preamp volume under 50%. I don't hear it at all.

You can see the noise through the scope, but it has low amplitude. I believe within engineering tolerances, but could not say for certain.

 

[Raccoon1400]

The noise is present, however I don't hear it the way I have my system configured. Raccoon1400 only heard it when he cranked the power amp to max. I never have the power amp past 2/3s and try to keep the preamp volume under 50%. I don't hear it at all.

You can see the noise through the scope, but it has low amplitude. I believe within engineering tolerances, but could not say for certain.

I could hear the noise until I turned the power amp down to 2/10. I was just at Bilbo's place listening to the system, with this and the bose 1801 I restored last. I still heard a bit of noise. Maybe it just sticks out more to me. But Bilbo is happy so I am. It sounds very good. I think it comes pretty close to my kenwood 700C after this treatment.

Thanks for the help everyone!

 

[EchoWars]

If someone wanted to get industrious, those FET's could be replaced with reed-relays. Muting FET's (and BJT's) just seem to have too many problems.

 

[BilboBaggins]

ears and years

I could hear the noise until I turned the power amp down to 2/10. I was just at Bilbo's place listening to the system, with this and the bose 1801 I restored last. I still heard a bit of noise. Maybe it just sticks out more to me. But Bilbo is happy so I am. It sounds very good. I think it comes pretty close to my kenwood 700C after this treatment...

Your ears are about 30 years less overexposed

 

[BilboBaggins]

I thought I would resurrect this thread to give you my thoughts after 15 months. First of all, quite a bit of gear has gone through my system, just to hear the differences and how the various components interact together. Part of the hobby that I love doing.

My critical listening system;

Bose 1801 fully restored by Raccoon1400 with safety mods and speaker protection circuit
Bose 1801 partially restored by oreo382 with fused speaker protection on output 1
Bose 4401 fully restored by Raccoon1400
Kef 105 on Raccoon1400 Bose
JBL L26 on oreo382 Bose
Thorens TD-160 stock in great condition
Yamaha PX-3
Denon CD player
Apple airport express with analog out into AUX

My impressions after over a year of use;

I love this system! It is my reference system, and at the moment doesn't get as much use as my McIntosh system, but sounds phenomenal! When I first got the McIntosh gear, I compared the 4401 with a C32 and a C26. I couldn't hear a difference between the three of them, so I sold the c32 and I'm using the C26 in my casual listening system with a MC2205 and an MR74, airport express, pioneer pd-m501 cd cassette player, a Yamaha kx-260 cassette deck, and Kef Concerto speakers. About 3 months ago, since I was reorganizing my Mc Rack, I decided to lug the MC2205 down to the reference system to see how the Kef 105s sounded with the Mc. They sounded good, but the Bose sounded much better. This was also the impression of a good friend of mine who came by that afternoon for some beer and tunes. I believe it is the synergy between the components. I know the Kef 105 are very power hungry speakers and the Mc is 200 watts per channel into 4 ohms and the Bose is at least 400 watts per channel into 4 ohms. So not really a fair comparison, but I do love the contrast and irony that I love the Bose system more than the McIntosh system.

I find it interesting, because when I first bought the 1801, it was because I wanted big VU meters in my system, and I didn't think I would ever own a pair of big blue meters. I bought the Bose because it was affordable at the time, whereas the MC2205 was beyond my means. Now, three years after buying my first Bose Amp for $300 not working, going through the expense of getting it repaired and restored (it was missing all 14 power transistors in one channel), finding and restoring a 4401, my Bose system cost less than the McIntosh system, and now sells for more, if you can find a working restored Bose 1801/4401 combo. I am very content with this system and have no desire to upgrade any more components in the system, except perhaps the CD player. I would like to find a B&O CD player for it, just because they are so beautiful and the Denon is starting to have tracking problems.

 

[Thierry]

Hi there,

It's always nice to read such nice stories from people who have been persevering and as are rewarded with an happy ending.

Good old Bose amps (and preamps) never die.