Help me fixing my Bose 901 EQ!

davidro

Lunatic Member
G'day all

I have recently picked up a Bose 901 Series V sound system complete with the speakers, pedestals and the matching EQ. Unfortunately the EQ 'popped' when I plugged it in the mains outlet and smoked for a minute. It still powers on (the red bulb is on) but when connected to the amp I only hear weird frequency noises instead of the music. The speakers sound fine when played without the EQ.

I post the schematics and parts list here. Can you please help me fixing this? I know how to solder but that's about it. Where do I begin? I'll post the photo of the internal when I get home tonight.

pg12.jpg


series4Bandseries5partslist.jpg


series5schematicsection2.jpg


series5schematicsection1.jpg
 
Thanks man I'll have a look but what do you mean by 'Check the v+ and v- voltages'? I'm so green with this!
 
Ok, for orientation, here's the typical process used to determine what _might_ be wrong.

1) We going to want to know some things before we proceed with a diagnosis.

You're going to examine the board closely and, hopefully, share some pictures with the 'armchair bench techs' here. This is a very effective way to help us see what you see.

Then you'll be taking some voltage measurements to eliminate some of the basic stages of the circuit.

2) Once those are established, we can either eliminate some parts of the circuits or not. The key to diagnosing is confirming what's working right first.

3) Once those major building blocks are eliminated, then we'll be proceeding to render some logical diagnosis.

So, let's get started:

* Did you see smoke or just smell it? What part of the cabinet did you see the smoke coming from? If smelled, what did it smell like? Any evidence on the lid to indicate heat or smoke?

* Are you getting this on both channels or just one? Which one?

* Examine the circuit board, post some pics of it here with closeups of the components and the lid if its appropriate. Any evidence of heating of the fiberglass board? What about discoloration of the larger components? Any evidence of liquids or leaking from round components?
- Push and prod on the board while watching connections like the RCA jacks. Anything appear to move when it shouldn't be? Wiggle the connections and see what they do.
- Smell around with nose close to circuit board. Any whiff of the smell you smelled? What part of the board?
- If possible, lift the circuit board away from the chassis to see what the bottom looks like. Use a magnifying glass to examine the solder joints. You're looking for ones that might not have been well soldered.


* The next step involves taking some voltage measurements - I assume you have a volt meter or a digital volt meter (DVM).

- Do you know how to use it to take readings? This is critical as we need to be confident in the readings that are taken.
- The first set of readings are intended to eliminate the power supply as being the problem. Here goes:

There's printing on the board. Locate TP-1, TP-2, TP-3 and TP-4.

With your meter set for 50 volts DC or higher, clip the ground test probe, usually black, to the chassis of the equalizer. Then, with the Red one touch the probe to each of the points on the circuit board marked TP-1/4.

Take a reading from each of those points and post them here. Be prepared to record both 2 positive (TP-1 and 2) and 2 negative (TP-3 and 4) voltage readings. TP-1 and TP-3 voltages will be higher than the other 2.

So, we need:

- Pictures
- Readings from TP-1 through TP-4

Cheers,

David
 
* Did you see smoke or just smell it? What part of the cabinet did you see the smoke coming from? If smelled, what did it smell like? Any evidence on the lid to indicate heat or smoke?

The smoke came from the slide bars (treble and bass) of the EQ. Yes I could smell it well and it still smells. Just the smell of some PCB stuff burning.

* Are you getting this on both channels or just one? Which one?

Both channels don't sound! Just weird noises!

* Examine the circuit board, post some pics of it here with closeups of the components and the lid if its appropriate. Any evidence of heating of the fiberglass board? What about discoloration of the larger components? Any evidence of liquids or leaking from round components?
- Push and prod on the board while watching connections like the RCA jacks. Anything appear to move when it shouldn't be? Wiggle the connections and see what they do.
- Smell around with nose close to circuit board. Any whiff of the smell you smelled? What part of the board?
- If possible, lift the circuit board away from the chassis to see what the bottom looks like. Use a magnifying glass to examine the solder joints. You're looking for ones that might not have been well soldered.

Will do this when I get home tonight thanks!


* The next step involves taking some voltage measurements - I assume you have a volt meter or a digital volt meter (DVM).

This I have!

- Do you know how to use it to take readings? This is critical as we need to be confident in the readings that are taken.

Sort of but never done it on PCB boards.


- The first set of readings are intended to eliminate the power supply as being the problem. Here goes:

There's printing on the board. Locate TP-1, TP-2, TP-3 and TP-4.

With your meter set for 50 volts DC or higher, clip the ground test probe, usually black, to the chassis of the equalizer. Then, with the Red one touch the probe to each of the points on the circuit board marked TP-1/4.

Take a reading from each of those points and post them here. Be prepared to record both 2 positive (TP-1 and 2) and 2 negative (TP-3 and 4) voltage readings. TP-1 and TP-3 voltages will be higher than the other 2.

So, we need:

- Pictures
- Readings from TP-1 through TP-4

Cheers,

David

Will attend to this shortly.

Thanks for being extremely helpful mate. Cheers

David
 
Hi Rez photos

I've uploaded Hi Rez photos. They're rather big so I'd just link the photobucket album here.

I seem to smell the smoke strongly from the large black cap with a marking on it (not my marking). You will see some ash powder sort of things around it on the close up photo (#CIMG 4932). But it could also be the power supply next the the large caps.

And what now on?
 
The smoke came from the slide bars (treble and bass) of the EQ. Yes I could smell it well and it still smells. Just the smell of some PCB stuff burning.
Sliders (potentiometers) don't usually smoke. Likely they provided the air holes for the smoke to escape. Smell the components on the board closer to localize the smokage.

Look close at R141 and R341. They are directly behing the mid pot. The pic, while very good is dark in that area but it looks like those two resistors may have blown. That would also explain the smoke from the pot.
 
With your meter set for 50 volts DC or higher, clip the ground test probe, usually black, to the chassis of the equalizer. Then, with the Red one touch the probe to each of the points on the circuit board marked TP-1/4.

Take a reading from each of those points and post them here. Be prepared to record both 2 positive (TP-1 and 2) and 2 negative (TP-3 and 4) voltage readings. TP-1 and TP-3 voltages will be higher than the other 2.

  1. So do I keep the PCB sitting in the chassis when I test them?
  2. Or do I take it off from the chassis?
  3. Do I touch the bottom of the PCB or the leg of the resistor/caps?

TP2 = LK6 See if you have +15VDC here
TP4 = LK5 See if you have -15VDC here

Ok. Will have a look tonight thanks.

Sliders (potentiometers) don't usually smoke. Likely they provided the air holes for the smoke to escape. Smell the components on the board closer to localize the smokage.

Look close at R141 and R341. They are directly behing the mid pot. The pic, while very good is dark in that area but it looks like those two resistors may have blown. That would also explain the smoke from the pot.

I smell it strongly around the black caps and the power supply/voltage converter. I will have a look at R141/341. Do resistors blow too? lol
 
I will have a look at R141/341. Do resistors blow too? lol
Yes, resistors blow all the time if too much current is run through them. So usually it's something else that fails or is demanding too much power that fries resistors but this helps find the culprit.
 
They have such a high resistance that they will dissipate 4mW, at most. I wouldn't worry about them.
 
Suggest you inspect closely for signs of overheating/venting around the bottom of the power transformer where it's mounted on the PCB.
 
Suggest you inspect closely for signs of overheating/venting around the bottom of the power transformer where it's mounted on the PCB.

Will do. do you think the transformer might have blown up? If so how is it I get the power on lamp and all? It's not gonna be an easy fix if I have to replace the transformer as it appears to be Bose own stuff.
 
Wow you may be right. Perhaps I should just replace that cap then?
That looks like C1 on the parts list being 0.0047 uf SW/lie (220V).
Is that a common cap that I can get? I should just be able to get one from the parts shop with "0.0047 uf SW/lie"? Sorry I don't know much about caps.
 
Um, what do you know about that equalizer? I see from the pics that it has the right kind of plug on it. Do you know whether the former owner brought into the country from somewhere else in the world? Or might it have been used elsewhere and then brought back?

I ask in that the symptoms you describe, the smoke you've seen/smelled tends to indicate a potential over voltage issue. Note that the schematic you posted (thanks for that!) calls for 2 different power transformers depending on where in the world it will be operated. If there's been overheating of components that caused the smoke, this angle needs to be ruled out.

We still need to know what the voltages are on those test points. I wouldn't recommend swapping components until we have an idea of where the problem is.
Do you have any old guitar repair/restoration places in Sydney? - there must be some specialists in a city that size. If so, they have a variac and I'd strongly suggest you find out of you can gain access to one. Also do a search here for Dim Bulb Tester. This is an alternative tool to limit current into a device on power up. Funny the equalizer doesn't have a fuse visible....

Cheers,

David
 
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Um, what do you know about that equalizer? I see from the pics that it has the right kind of plug on it. Do you know whether the former owner brought into the country from somewhere else in the world? Or might it have been used elsewhere and then brought back?

I ask in that the symptoms you describe, the smoke you've seen/smelled tends to indicate a potential over voltage issue. Note that the schematic you posted (thanks for that!) calls for 2 different power transformers depending on where in the world it will be operated. If there's been overheating of components that caused the smoke, this angle needs to be ruled out.

Here we use 240VAC and that's what it says on the EQ. Plus I understand the seller bought the speakers in Sydney and the original packaging bears the Sydney address too.


We still need to know what the voltages are on those test points. I wouldn't recommend swapping components until we have an idea of where the problem is.
Do you have any old guitar repair/restoration places in Sydney? - there must be some specialists in a city that size. If so, they have a variac and I'd strongly suggest you find out of you can gain access to one. Also do a search here for Dim Bulb Tester. This is an alternative tool to limit current into a device on power up. Funny the equalizer doesn't have a fuse visible....

Cheers,

David

Ok I'll refrain from replacing the caps yet. I don't know no guitar repairs... And I too thought it's funny that there was no fuse.

On a side note I've had a couple of transformers blowing up at my place.
 
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