Bose 1800 Amp Spare Parts (Amp PCB)

[JamesDean59]

Hello everyone,

This might be a somewhat odd request. I have a Bose 1800 two channel amp here. I got it a number of years ago with the right channel/B channel bad. The PCB has several damaged traces on it and might be beyond repair at this point so I was wondering if anyone out there might have one of these amps sitting around broken and was interested in getting rid of the B channel board..

I doubt I'll get any responses since there probably not too many of these around anymore, let alone in tact.

thanks!
Kris

 

[JamesDean59]

I decided that I've nothing to lose by working on the old pcb, so I decided to tear into it and see what I could do:

Heres some pictures:

Initial:



Removed Power Transistors / Heatsink:




Damaged Traces that need to be repair..somehow. :



All the output transistors are new too. I'll test em before i put them back on. I still have to find a replacement for R31 (.8 Ohm, 3W, 5%) and a 2N3904 for Q24.

 

[Jon_Logan]

I think you're doing fine with patching the existing PCB.:thmbsp:

 

[JamesDean59]

Hopefully that's the case. I've had this amp for a number of years. Tried to fix it when I first got it. Was impatient with it and never quite got it.

I threw a bunch of parts at it, shitt-ily installed them. Now, I think if I fix up the traces properly, install the components properly, check the existing, and take my time it might just work..

 

[Jon_Logan]

The PCB looks similar to the Phase Linear 400/4000 PCB's. They are known to have easily delaminated traces. Best bet for desoldering is to use fresh solder wick. Don't use a Solda-Pult or other suction device. Try to use a iron with a hotter (yes, hotter) tip. Just try to desolder/re-solder your connections quickly. Try not to leave the iron on a joint too long. If it looks like a component will damage the traces when removed, and that part is easily replaced, you could just cut the lead to help in a clean removal.

 

[JamesDean59]

I actually couldnt find my solder sucker, so I ended up using my solder wick. I think I've got all the parts off that I wanted. I just need to find a replacement for the one resistor, transistor.

I'm starting to look at where the traces blew out at.. it seems concentrated on the -85v side going to Q3/Q24. Hmmm..





On the left side, for a while today the DC offset was <125mV. Then I decided to put an equalizer in the signal path, after hooking it up I heard some noise out of the speaker, so I removed it and checked DC, and it was ~540mV. After I power cycled it, and let the caps drain, DC offset returned to 120mV~ and is slowly climbing upwards. Hits around .6-.7v at clipping signal.

Also when I turn the power off, it doesnt cut sound output off. Just power, so you can hear the main caps draining. lol.

 

[Jon_Logan]

I actually couldnt find my solder sucker, so I ended up using my solder wick. I think I've got all the parts off that I wanted. I just need to find a replacement for the one resistor, transistor.

I'm starting to look at where the traces blew out at.. it seems concentrated on the -85v side going to Q3/Q24.

This is a good amp to troubleshoot with a Dim Bulb Tester and a VariAC.

R-9 (20 ohm) should have about ~0.7 volts across it, supplying ~35mA. Q-3 has lot's of voltage across it and could create trace burning paths if it fails.

What I would do. I'd use a VariAC. I'd leave the drivers (Q-10 and Q-11) in and just leave a pair of outputs in/ One top and one bottom. Disconnect the load, run the Variac to about 1/3 and begin voltage measurements. Check for big offsets and larger than you like bias spread. If that all pans out, reinstall the outputs and do the same thing.

The protection stuff (Q-6 & Q-7) can sometimes mask the real issue and can be removed for troubleshooting, if desired.

 

[JamesDean59]

This is a good amp to troubleshoot with a Dim Bulb Tester and a VariAC.

R-9 (20 ohm) should have about ~0.7 volts across it, supplying ~35mA. Q-3 has lot's of voltage across it and could create trace burning paths if it fails.

What I would do. I'd use a VariAC. I'd leave the drivers (Q-10 and Q-11) in and just leave a pair of outputs in/ One top and one bottom. Disconnect the load, run the Variac to about 1/3 and begin voltage measurements. Check for big offsets and larger than you like bias spread. If that all pans out, reinstall the outputs and do the same thing.

The protection stuff (Q-6 & Q-7) can sometimes mask the real issue and can be removed for troubleshooting, if desired.

I think I might be able to get a VariAC off a buddy of mine. Its a 1kVA unit. Is that adequate?

if Q3 was in need of replacement, the sheet says 2N6175, however that isnt available, what would be the best replacement for it? Actually, theres an addendum to the service manual that states MMPS U10 was used in place along with some thermal improvements to be made on amps so equipped with the MMPS U10..I've no info on the U10 though.

 

[Jon_Logan]

I think I might be able to get a VariAC off a buddy of mine. Its a 1kVA unit. Is that adequate?

if Q3 was in need of replacement, the sheet says 2N6175, however that isnt available, what would be the best replacement for it? Actually, theres an addendum to the service manual that states MMPS U10 was used in place along with some thermal improvements to be made on amps so equipped with the MMPS U10..I've no info on the U10 though.

a 1kV VariAC is plenty to troubleshoot this thing. If you don't have an AC Ammeter rigged, you'll want a ~100W incandescent lamp in series with the amplifier.

The VariAc doesn't need to be huge because you won't be power testing when the VariAc is used.

So, with the VariAc in there and a way of monitoring the current, you'll be all set to make some voltage measurements. There's that IC in the front end, you'll just need the VariAc up high enough so that things are mildly powered enough for feedback loops to function properly.

The MPS-U10 is a Motorola number. On semi might have a recommended sub for that. It looks like you'll need a transistor with a VCEo of around 190~200V or greater. Does the existing Q-3 have an external heatsink on it? You might need a 5~10W (Pd) part. From On-Semiconductors site, look for a NPN transistor with a Pd of 5~10W and a VCEo of about 200V, and look for one that is for "Audio Applications". That will ensure that the transistor is designed for audio frequencies and be fairly linear (low distortion).

 

[JamesDean59]

Excellent, I'll grab the variac off him and ammeter.

IC in the front end?

The existing Q3 is located just above the first top on semi MJ transistor. It was kind of attached to the main heatsink. There was some plastic behind it so that it didnt conduct to the main heatsink..If



There are still one transistor and one resistor missing on the pcb. I've got a 3904 for the transistor as per spec sheet but I've got no .8Ohm,3w,5% resistors..anything I could sub in there or does it have to be .8 Ohm? Its not a big deal really. I gotta put in an order to Mouser. I'm going to be rebuilding a power supply for a Kenwood 600 in a few days here.

Also, any idea how much current is passed through those broken traces? I think I can repair them with just some simple wire, but I'm not sure how thick it will need to be. I've got some old solid core copper phone line laying around, and my buddy has transformer wire (1 amp max)...

 

[Jon_Logan]

Excellent, I'll grab the variac off him and ammeter.

IC in the front end?

The existing Q3 is located just above the first top on semi MJ transistor. It was kind of attached to the main heatsink. There was some plastic behind it so that it didnt conduct to the main heatsink..If



There are still one transistor and one resistor missing on the pcb. I've got a 3904 for the transistor as per spec sheet but I've got no .8Ohm,3w,5% resistors..anything I could sub in there or does it have to be .8 Ohm? Its not a big deal really. I gotta put in an order to Mouser. I'm going to be rebuilding a power supply for a Kenwood 600 in a few days here.

Also, any idea how much current is passed through those broken traces? I think I can repair them with just some simple wire, but I'm not sure how thick it will need to be. I've got some old solid core copper phone line laying around, and my buddy has transformer wire (1 amp max)...

Yes, the IC (U-1) looks like it's just used to buffer the input signal before sending it to the differential amplifier (Q-1, Q-2). Q-3 looks like it's mounted (thermally) to the main heatsink. If you do get a sub, make sure you get the appropriate insulator to keep it electrically isolated from the heatsink.

Are you sure you need to replace Q-3? There may be a pinout change and/or a case style change to get it in there. If transistors are okay, it's best not to sub them unless you know that the sub behaves well in the circuit. Circuits can be fussy that way.

For the 0.8 ohm/3W emmiter resistors.... ... yeah, they picked a non standard value. I'd go with 0.82 3W or 5W to sub. Bose chose a pretty high value for their emmiter resistors. It's good because the transistors current share very well, but it's bad because a fair amount of power is lost across the resistor. If you wanted to be really picky, you could replace all of the emmiter resistors with 0.82 or 0.75 ohm.

As far as replacing traces with wire goes, if it's a very skinny trace, thin wire is okay. If it's a fatter trace, you can just patch with Solder-Wick.

 

[JamesDean59]

U1 and Q1 are listed in the troubleshooting section for replacement if DC offset is less than 2V but higher than normal value. My A channel seems to do this after some interval of time. Most of the time its <120mV but it all of a sudden decided to go near 500-600mV
I'm not sure about replacing those two, maybe Q1 I could find a sub for but I don't know about U1.

As for Q3, my only inclination to replace was the fact that you mentioned if it were bad it could lead to blown out traces (which I do have) that are all leading up to it.

I think I will sub the .82 Ohm resistors. I was originally going to use a .8 5 watt, but its much more expensive and physically larger. I think I'll replace the two electrolytic caps on here as well, I have to put my mouser order in still, might as well.

I'm going to try to use this solid core phone cable on some of the thinner traces. It supposedly 22 awg, according to my chart it should take 7 amps... Due to the layout of this board, I actually might use it on the larger ones too. I cant see using the solder wick as a trace in some of these spots. They're pretty curvy.

As for testing, I dont think I should/could do anything until I have all the components in, really all that are left is Q24/R31. I can source a 3904 for Q24 locally though, so if R31 isnt needed for variac type testing...maybe i could mount the heatsink and a few of the main transistors? Any thoughts?

I appreciate all the help you've given me so far.

 

[JamesDean59]

Alrighty,

I repaired all but one spot of broken trace.




The remaining trace is at the Q24 connection. Once I install a 3904 I'll solder to the leg a small jumper wire to the nearest trace solder point. While I was taking this apart I noticed that instead of a 3904 there was a 2N5210 installed on the backside (due to heatsink and lack of access)

I wonder what effect using a 5210 in place of a 3904 was... The 3904 has a higher VBEO, higher continuous collector current and higher operating frequency.

 

[JamesDean59]

Cleaned and installed the missing 3904. I'm going to be putting an order in for a bunch of parts. I might as well replace all the 3904/3906s while I have it out.




After that, Variac/Ammeter testing. Not 100% sure what voltages I should be looking at/expecting but we'll get there.

 

[ouimetnick]

 

[petehall347]

some of them original solder joints may benefit from re-flowing .

 

[JamesDean59]

Alright, I hooked up the variac and ammeter and watched the good side power up alone, the ammeter never registered more than 1A as I slowly brought it to 120VAC.

I repeated the same with the "bad" side and it too never took in more than 1A. Good so far.

I installed two drivers and two power output transistors as recommended.



I measured voltage at the Base and emitter of each transistor and got the following values (with respect to ground): (I'll update this as I go along)

Q15: -43 / -43
Q10: -43 / -43

Q21: -83 / -84
Q11: -83 / -84

Q6: -44 / -44
Q9: -44 / -44

Q5: -44 E / 83 C / 83 B
Q4: -44 E / -44 C / -44 B

Q7: -44 E / .5 C / -43 B
Q8: -43 E / -43 C / -43 B

So the problem seems to be somewhere with the top section not getting the full potential. I verified that the board is getting both +85 and -85 from the caps.

It looks like Q5 might be problem. Its supposed to supply 2V to a number of other transistors, all of which seem to report -44 volts or there about.

Anyone have any other thoughts?

Effected Circuit Paths (Roughly)

 

[JamesDean59]

bump, any thoughts?

 

[Jon_Logan]

Measure the voltage across R-15 (51 ohm). I calculate approximately .7 volts across it. That would supply about 13mA of pullup current. If your getting the approx .7 volts across R-15, Q-5 is not the problem.

If it's not drawing a lot of current, the problem could be the front-end not sending the correct DC balance "instruction" to the following circuitry. Is there a DC offset at the output?

 

[JamesDean59]

Measure the voltage across R-15 (51 ohm). I calculate approximately .7 volts across it. That would supply about 13mA of pullup current. If your getting the approx .7 volts across R-15, Q-5 is not the problem.

If it's not drawing a lot of current, the problem could be the front-end not sending the correct DC balance "instruction" to the following circuitry. Is there a DC offset at the output?

Your calculations were correct, R-15 has .75 VDC across it. There is a DC offset at the speaker terminals of -43VDC. So the front-end circuitry? Where to start with that?

I started checking voltages in the front end area involving that op-amp U1. It doesn't seems to be getting the +16/-16 volts it should be getting.

The op amp getting -.75 on the +16 side and -11.38 on the -16 side. The +16 side seems to get +85 from the trace that Q17 is connected to. I don't have this output transistor installed, so that might be why its not getting +16...

Installed the transistor, now we've got +3.85v at the +16 side. This must be the wrong resistor installed... I found a 3k resistor in a bag here, it looks much larger than whats currently installed. I'll have to dissassemble the heatsink and mount the 3k. That *should* fix the +16v supply to the op-amp however that doesnt solve the -16 supply which is currently at -11.38v...