• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Bose 1800 Amp Spare Parts (Amp PCB)

Just wondering - did you Measure R15 - you have the right voltages, but it could have opened or changed higher and would give similar problems to as described (not enough current from the positive rail).
Donberry's suggestion may help with your off -16V to the preamp - It was never answered if that was resolved or not. Also double check grounding. It is unusual for amplifier to sit at large voltage - most problems usually cause it to go to one of the rail voltages or draw current. :lurk:
 
sregor said:
Just wondering - did you Measure R15 - you have the right voltages, but it could have opened or changed higher and would give similar problems to as described (not enough current from the positive rail).
:
Click to expand...

I think that is a good call.
 
Jon_Logan said:
Okay. It gets interesting.

So, we know we have a -10mA "feed" from R9
We know that there is 15mA "feed" from R15
There is a -43V offset at the output
There is no currents detected at either set of emmiter resistors.
CR5 is currently open.
Cr7 is currently open.

It's true that this is kind of a "hrmmm" moment, but there's always a reason. It could be a more esoteric kind of a failure.

Is C9 leaky?
Is CR10 leaky?
Is Q3 leaky B-C?

There must be an additional negative current path. You've confirmed that the rail voltages are equal and opposite. These are "fighting" currents. There's more current coming down from R15, but the offset is negative.
Click to expand...

Alright back on this, took the weekend to work on some Kenwood/Technics stuff.

I double checked R15 and I'm still getting .82V, so ~16mA of current.

I pulled Q3 and tested it got conductivity in both directions on all pairs. BE/EB--55 Ohms both ways. BC/CB--96-98 Ohms both ways. CE/EC. 42-45 Ohms both ways. Not sure if this indicates failure or there are other items inside the transistor connected to the Base/Emitter/Collector.

C9 seems ok. I pulled a leg out of the circuit and tested resistance. It tested infinite.

CR10 seems ok as well. I tested across it, it only conducted one way, read about .54v in circuit.

Additionally R50 seems to be seeing quite a bit of current/heat. I replaced it a bit ago due to it looking cracked/black. It is now again, blackened a bit and has some stuff flaking off it.
 
Last edited:
JamesDean59 said:
<snip>
I double checked R15 and I'm still getting .82V, so ~16mA of current.
<snip>
Click to expand...

You only have 16mA of current if R15 is 51 ohm. You've measured 0.83V across the thing that is supposed to be R15. If R15 is open, what is the current? 0.83(V)/inf(ohms)= ?(I),

You'll need to measure ohms of R15. Power it off and see if you get something close.

I think sregor guessed right here. Things aren't adding up if we assume that all components are operating correctly. You're not pulling a lot of current. If an amp is failing buy conducting more current, due to a leak or short, a large correction current occurs and things get hot. It sounds like your failure is not pulling a lot of current, indicating an open. Further, the differential stage is turning off the negative conducting side, while the current source into the positive side is supplying a higher current, yet the offset is negative. R15 being open sounds real plausible about now.
 
Jon_Logan said:
You only have 16mA of current if R15 is 51 ohm. You've measured 0.83V across the thing that is supposed to be R15. If R15 is open, what is the current? 0.83(V)/inf(ohms)= ?(I),

You'll need to measure ohms of R15. Power it off and see if you get something close.
Click to expand...

Measuring R15 in circuit yielded 52.4 Ohms. So ~15.8mA. I even removed it from the board and got near 51.8 Ohms.


Jon_Logan said:
I think sregor guessed right here. Things aren't adding up if we assume that all components are operating correctly. You're not pulling a lot of current. If an amp is failing buy conducting more current, due to a leak or short, a large correction current occurs and things get hot. It sounds like your failure is not pulling a lot of current, indicating an open. Further, the differential stage is turning off the negative conducting side, while the current source into the positive side is supplying a higher current, yet the offset is negative. R15 being open sounds real plausible about now.
Click to expand...

What about R50 getting real hot? Or its that just a side-effect of the way the amp is operating at the moment?
 
Jon_Logan said:
R50 getting real hot? Q3 is bad. Leaky or shorted B-C.
Click to expand...

Its black in the center and has some flakes coming off it. I can only assume thats from lots of heat. Q3 is that Motorola MPS-U10.

According to the Bose PDF it was previously a 2N6175. Bose released a service bulletin having new production changed to the U10.

Any recommendation on a suitable replacement transistor?

For the purpose of testing I could pull Q3 off the working amp board and see where thats gets us...With any luck maybe Q3 was the only problem all along. Or would doing that be not recommended?

Additionally should I re-connect the lifted diodes once Q3 is replaced? Or leave them out still?

Would something like this work in place:
http://www.mouser.com/ProductDetail...=sGAEpiMZZMtn7beagP2hITDI/IpVDSPufhRiFrbgDWs=
 
Last edited:
JamesDean59 said:
------ Q3 is that Motorola MPS-U10.

--------------

Any recommendation on a suitable replacement transistor?

For the purpose of testing I could pull Q3 off the working amp board and see where thats gets us...With any luck maybe Q3 was the only problem all along. Or would doing that be not recommended?

Additionally should I re-connect the lifted diodes once Q3 is replaced? Or leave them out still?
Click to expand...


Does on-semi not have a MPS-U10? Sub? I don't have a sub in mind off the bat. You'll need a Vceo of better than 170V. I'd look for a fast (audio) driver from On-Semi with maybe a Pd of 5W~10W, Ic of .5A ~1A. If you take Q3 out, I'd expect a large + offset. You can leave CR5 and CR7 out until you check operation of your new Q3. I think in this case, you'll want to change one parameter at a time. Look for the offset change with Q3 out. If the replacement zeros the offset (or gets it near zero), reconnect the lifted diodes.
 
The ones I do find are within voltage/current spec but a low Ft (10MHz)... Or I find the appropriate voltage/Ft/current but inadequate Pd. Gr.
 
I'd try the 10MHz transistor. I would not go down in power (Pd). You might try something along the lines of a 2N3440. If the MPS-U10 is not heatsinked, you might not need a whole lot of Pd ability.

I haven't really skulled it out yet, to make sure the 2N3440 would be a safe choice yet, but it looks pretty promising. You might need to add a "hat" (heatsink) to it to keep it happy.
 
A couple more candidates for Q3:

MJE340 (might need a hat)
2N5655
BU406



<edit>

The key features that you're going to need from a Q3 are:
large breakdown voltage greater than the 85(V) rail times 2, or >170V
at least 30mA of current capability. That's 0.6(V)/20(ohms) (R9)
a Pd of max current (.03A* 170V = 5W)
and while a wide bandwidth is good, it seems there's pretty slim pickins in this high voltage class. Over rating these specs is a healthy thing to do. The calculated worst case Pd for Q3 tells me to buy a heatsink for it.
 
Last edited:
While I was looking up stuff for this I came across most of those transistors but wrote them off due to low Ft.

Bose's original transistor for Q3 the 2N6175 was billed in the datasheet as the "plastic 2N3440"

Q3 was mounted to the large heatsink that the output transistors use. Electrically isolated of course.

So unless I went with the TO-39 package I shouldnt need to buy any heat sinks. I can just attach to the main like Bose did. The only advantage the 2N3440 has over the rest appears to be a +5Mhz Ft.
 
Hey! Great work! Q3 was part of the problem! I swapped Q3 from the other working board. I didnt want to place an order to a parts house and end up needing to place a second.

I powered up the board with the "good" Q3 and DC offset is now at 10VDC. Still not fully functional but better than -44VDC

I reconnected the lifted CR7,CR5, no change in dc offset at output.

The op-amp is outputting 10VDC.
Its being supplied with +14.85VDC and -8.85VDC for the power connections.
I located a 3W 3KOhm Ceramic resistor and installed it at R3 negative voltage supply for the op-amp and it made no change to voltage.

I measured the supplies to the other amp board's op-amp and its getting +/-15.85. So something is up with this one...

The troubleshooting section of the Bose manual suggests with a >2V DC offset and same polarity voltage at the opamp's output that,
U1,CR1,2,6,15 could be defective or an open ground at the RCA input jack.

CR1,2 are 1N4148
CR6,15 are 1N5425 15V, 1/2 Watt Zener. I've got some 1N4745A 16V 1 watt Zeners, would those be a suitable replacement?
 
Last edited:
Congratulations!

So now, you have a +10V offset and +10V coming out of the opamp. It sounds like the voltage portion of the amp is working.

Now you've got to tackle the +10V coming out of the opamp. Start by fixing the -supply problem. Get that -8 back down to -15V. it's most likely a filter or zener diode.
 
Jon_Logan said:
Congratulations!

So now, you have a +10V offset and +10V coming out of the opamp. It sounds like the voltage portion of the amp is working.

Now you've got to tackle the +10V coming out of the opamp. Start by fixing the -supply problem. Get that -8 back down to -15V. it's most likely a filter or zener diode.
Click to expand...

I can get replacement 1N4148 from Radioshack and I have some 16v zeners. I wasnt sure about using them in place of a 15v zener.

IIRC zeners are like standard diodes but they let a certain amount of current to flow in the reverse direction when the voltage is above the zener voltage. So I dont think there would be any harm in using the 16 volt zeners due to the fact that the circuit calls for +16/-16... right?

I actually ordered replacements for the C2,C3 filters when I bought stuff from Mouser last. Ha. So probably just the zeners..
 
Last edited:
Yes, you can add zeners in series if you want to increase the voltage, but you can't reduce the voltage of a 16V zener. Check the data sheet for the IC. Some IC's top out at +/-15V.

Now the other side of the opamps minus rail. Just because the supplies are offset, doesn't mean that the opamp will put out anything due to the offset. So, this +10V out of the opamp may be related to the offset or, it may be more due to the voltage difference at the +/- inputs.

I'll check the schematic you posted and see about a plan be once the supply is fixed.
 
Make sure CR2 is not leaky. You can lift a leg. Those are there for ESD protection and possibly to avoid latch-up.

Yeah, I'd go along with "it's probably okay to go with +/-16V zeners".
 
Alrighty. I replaced Cr6,Cr15,C2 and C3. Q3 is out of circuit. Op-Amp shows +10VDC. 11-12v DC offset at speaker terminals.

Positive voltage is at 15.85, Negative is -8.85 at the op amp supply terminals.

Radioshack is closed tomorrow so I wont be able to replace Cr1,2 until Friday. Hopefully that restores proper supplies to the op-amp. Maybe that will correct output.
 


Yellow = Has Been Replaced
Red = To Be Checked/Replaced

I've also depleted my solder wick :(

I noticed on the bag label for the zeners, I had Bose 1800 market on it from when I placed the order. I must have intended on using these back in the day.

So RadioShack opens at 4AM Friday. I wonder how much of a mess that place will be.... I usually dont partake in the black Friday events.
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom