Bose 1800 Amp Spare Parts (Amp PCB)

[Jon_Logan]

You're right. I wasn't very thorough in probing voltages before I posted that. The negative DC is at all the output transistors because they're all connected to the output bus.

I did a more thorough probing of voltage across the board:


Key:
Purple = Incorrect Voltages
Blue = DC Offset Voltages
Yellow = Correct Voltages

I believe there is a typo on Q10, the collector should be +84 and the emitter should be +56V.

I'm curious about Q3's base. Why is it at -49V?

I think 2.18V across R9 is no good. That is 100mA. Why would that transistor need to be pulling 100mA with a full negative offset?

Before replacing anymore parts, study what is going on right now.
U1 has a positive offset.
The output has a negative offset. Does that mean that U1 is bad? No.
Q2 is conducting hard while Q1 is not. Is the diff amp reacting normally? Yes.

Let this information sink in and think about why Q3 and Q24 have so much current going through them (without replacing/swapping anything else).

 

[JamesDean59]

I think 2.18V across R9 is no good. That is 100mA. Why would that transistor need to be pulling 100mA with a full negative offset?

Before replacing anymore parts, study what is going on right now.
U1 has a positive offset.
The output has a negative offset. Does that mean that U1 is bad? No.
Q2 is conducting hard while Q1 is not. Is the diff amp reacting normally? Yes.

Let this information sink in and think about why Q3 and Q24 have so much current going through them (without replacing/swapping anything else).

The current DC offset was between -3 and -5 VDC. Not the full negative rail.

I'll ponder this new info and see what I can drum up. I have a degree in electrical engineering but my work was all in computer engineering. I only had one electronics course that really involved circuitry of this composition.

 

[sregor]

I believe there is a typo on Q10, the collector should be +84 and the emitter should be +56V.

I'm curious about Q3's base. Why is it at -49V?

Base of R3 is set by voltage divider R7 and R50. Wrong voltage suggests either R50 is bad or wrong value, or Q3 is bad or in wrong.

 

[JamesDean59]

Base of R3 is set by voltage divider R7 and R50. Wrong voltage suggests either R50 is bad or wrong value, or Q3 is bad or in wrong.

I thought I checked R50 last night. Turns out that it wasn't fully contacting the trace.

I powered it up and DC Offset was at 125mV (within Bose Spec). However, after a minute of on time. The offset climbs to +85V then flops all around, positive voltages only so far. Is this what the service manual labels as "oscillating" ?

I cant really take any more measurements while its doing this, all the voltages are changing. Its seems contained to the positive voltages only. I monitored the op-amp during all this and its going all over the place, presumable trying to correct things.

 

[JamesDean59]

Let this information sink in and think about why Q3 and Q24 have so much current going through them (without replacing/swapping anything else).

I would imagine this occurred partially due to improper voltage at the base. Q3/Q24 was trying to correct the DC offset as instructed by the diff amp. Otherwise Im not sure.

 

[Jon_Logan]

I would imagine this occurred partially due to improper voltage at the base. Q3/Q24 was trying to correct the DC offset as instructed by the diff amp. Otherwise Im not sure.

It sounds like fixing the resistor on Q3 (R50), the offset changed. That's good.

Now it sounds like the offset is drifting wildly. That is a good clue. This is where the diff amp comes in. The Base of Q2 is looking at the output signal. ...be it AC or DC voltages. If a difference between the Base of Q1 and the Base of Q2 exists, the differential amp will try to nullify the difference.

A drift indicates that the differential amp isn't/can't do it's job.

Note the voltage across R9 when the offset is +85V, and/or 125mV and report those values. See if there's a change in value and whether that change is indicating that the differential amp is trying to correct the offset.

 

[JamesDean59]

It sounds like fixing the resistor on Q3 (R50), the offset changed. That's good.

Now it sounds like the offset is drifting wildly. That is a good clue. This is where the diff amp comes in. The Base of Q2 is looking at the output signal. ...be it AC or DC voltages. If a difference between the Base of Q1 and the Base of Q2 exists, the differential amp will try to nullify the difference.

A drift indicates that the differential amp isn't/can't do it's job.

Note the voltage across R9 when the offset is +85V, and/or 125mV and report those values. See if there's a change in value and whether that change is indicating that the differential amp is trying to correct the offset.

R9 yielded, 97mV to 233mV when the offset was changing. 320mV when at the 125mV DC offset.

Does the drift indicate the diff amp isnt doing its job or that Q3/Q24 isnt doing what the diff amp is instructing?

 

[Jon_Logan]

R9 yielded, 97mV to 233mV when the offset was changing. 320mV when at the 125mV DC offset.

Does the drift indicate the diff amp isnt doing its job or that Q3/Q24 isnt doing what the diff amp is instructing?

It sounds like the diff amp is working and that it is "instructing" the current through R9 to change in sympathy to the DC offset. Although, one thing to watch out for is the possibility of an open loop situation. In this case, the diff amp may be working, or Q3 may be working, but maybe there's a disconnect to the rest of the circuit. The circuit is supposed to be a closed loop. If any part of the loop is opened, correction signals don't ever get to their destinations.

If 320mV is across R9, when the offset is 125mV, you should expect the voltage across R9 to get larger if the offset goes to +85. This would tell you that the diff amp is "instructing" Q24 to turn on harder to put more negative current into the signal path (that has gone waaaay positive).

 

[JamesDean59]

It sounds like the diff amp is working and that it is "instructing" the current through R9 to change in sympathy to the DC offset. Although, one thing to watch out for is the possibility of an open loop situation. In this case, the diff amp may be working, or Q3 may be working, but maybe there's a disconnect to the rest of the circuit. The circuit is supposed to be a closed loop. If any part of the loop is opened, correction signals don't ever get to their destinations.

If 320mV is across R9, when the offset is 125mV, you should expect the voltage across R9 to get larger if the offset goes to +85. This would tell you that the diff amp is "instructing" Q24 to turn on harder to put more negative current into the signal path (that has gone waaaay positive).

I'll have to go through the circuit path and verify that it is in fact closed loop. I didn't notice any voltage higher than 320mV across R9.

Tonight I'll get the Fluke and run it side by side with my meter and get better numbers.

 

[JamesDean59]

If 320mV is across R9, when the offset is 125mV, you should expect the voltage across R9 to get larger if the offset goes to +85. This would tell you that the diff amp is "instructing" Q24 to turn on harder to put more negative current into the signal path (that has gone waaaay positive).

Somethings not right. If 320mV gives us a 125mV Offset, then as you said, trying to put more negative in the path will require more than 320mV. However when the offset flails about between 0 and 85V, the voltage across R9 is even less than at nominal offset. It actually dips low.

Q3/Q24 isnt turning on hard enough to compensate for the positive.

It almost sounds like its operating backwards, cutting the negative instead of adding.

My next thought would have been to look at Q9 for either not receiving the signal to conduct or failing to act on the signal. The part Q9 is good.

The op-amp is outputting negative voltage. Clarification? If the op-amp is outputting negative does that mean its attempting to add more from the negative half? or cut the positive output? or both?

 

[JamesDean59]

I wasn't going to post this but someone might find it amusing:

When I first got the DC offset back to the mV range. I turned it on, let it sit there for a few seconds. Everything appeared ok so I turned it off. Hooked up my test speaker, and a signal from my mixer. Turned on frank sinatra's fly me too the moon. Listened for about 5-10 seconds, then turned the volume up and listened for another 5-10 seconds then speaker went poof.

THE SMELL WAS HORRIBLE. I was happy, then sad. But hopeful, because it made sound. Then had the speaker for lunch. This was a good motivator to order a DC protection unit for this amp. Glad it took out my test driver and not a full cabinet. I would have been supremely pissed.

This looks promising:
http://www.ebay.com/itm/Speaker-pro...ltDomain_0&hash=item3cbf14ebfd#ht_5149wt_1265

Would just need to tap 12-15VAC somewhere. Or use a transformer http://www.radioshack.com/product/index.jsp?productId=2103732

 

[JamesDean59]

Alright update:

I ordered the aforementioned DC projection circuit.
I ordered a new meter (Tripplett 9045) and some cheapo meters.

I'm still quite stumped on what to do next on this amp, if anyone has any ideas. I'm open to suggestions.

Here's a brief synopsis of whats going on currently:
1) Stable DC Offset for approx 5-10 seconds. I stupidly thought it was OK and hooked a speaker up. It played for 5-10 seconds.

2) After 5-10 seconds it will oscillate from +85 rail voltage to 0V, up and down.
-During this "oscillation" op-amp voltage vary from 0 to -7VDC.
-The front LED meter for this channel flicker all on (including clip) and off depending on the offset. There is no signal input at the moment, I've disconnected the RCA line
-Current draw appears to be minimal/nominal. I dont have an AC ammeter right now to check however when the unit would draw +1AMP the variac would make clear audible noises and there are no such noises.

Any ideas gents?

 

[Jon_Logan]

Can you PM me?

 

[JamesDean59]

Well. This was a surprise.

Its fixed. It was so simple. I didnt secure Q3 down to the main heatsink.

I dont want to speak to soon but I think its fixed.

I'm going to leave it on for a good amount of time and see what the offset does. Right now I'm looking at around 5 min of on time. I have a signal connected to it but no speaker (not fooling me again) and DC offset is around 50mV with the volume all the way up. Around 120mV with volume off.


EDIT: I dug out an old 8" driver I had in the closet. Its pretty beat up, but it still works and I'll just run here for a while see where we get.
EDIT: ~15min up time.

I managed to break the thermistor by accident as well. Its connected in parallel with R11, recommended for thermal improvement as per the service manual. I'll have order a replacement as well as another Q3,4,5. Q4/5 are still avail. Q3 needs a new transistor entirely.

So this leaves us with:
Remaining:
1) Replace missing components on left channel amplifier
2) Figure out why it still outputs to speakers when main power switch is turned off. Until the main caps are drained, it continues to output sound.
3) Install speaker protection relay whenever it arrives.
4) Repair right channel 1/4 input jack. Maybe consider installing RCA jacks in parallel for ease of use.
5) Should probably perform bias adjustment as well. I think I have some replacement pots around here too.

 

[Hyperion]

I suspect "2)" will be largely sorted out with the installation of "3)" (when it arrives) but also you can fit bleeder resistors across the main caps with a value chosen to be high enough in value not to affect them in operation, but sufficiently low in value to bleed off excess charge after the power switch is turned off.

 

[JamesDean59]

My concern with 2 was that even without the relay (which is aftermarket) it shouldn't remain on like that. I dont think thats normal operation. It was suggested by a few of my friends that perhaps it needs both boards connected to drain the caps naturally (meaning without adding resistors/relay)

 

[Hyperion]

Well, adding the other board + "5)" bias adjustment might make all the difference. (extra current draw)

Well done indeed to you and Jon for getting this far - I don't expect to ever own a Bose 1800/1 - but have enjoyed this trouble-shooting journey very much.

 

[JamesDean59]

Sadly the journey is not quite over yet. It just toasted my test speaker. It wasnt full rail voltage it was more like 3-4V. I actually watched it suck the cone downward and spew a bit of smoke.

I suspect that Q3's heat is still at fault, the board was working for 30+ minutes, outside the chassis. With it being not heatsinked for the past month, perhaps the heat has weakened it. It might be needing replacement...

Might be time to pick up a test load from Parts-Express too...

 

[JamesDean59]

Alright. Parts Lists assembled. Kudos to aokman for posting up a parts list of replacements he is using. I wasnt sure what thermistor and pot to use.

Can someone double check my sub for Q3? I think it should do. The pin out is different. That will be fun dealing with. Will have to find some way of extending the legs. Bose also mentioned this might need to be done with replacement Q4/5s too.

MJE340 might also be a suitable sub although i cant seem to find a rating for its operating speed.

The other thing I was thinking of doing was switching the 1/4" TRS jacks out for RCA style connections..or adding them in parallel.

Additionally I need to source 12-15VAC from somewhere for the speaker protection relay. Any ideas on that?

 

[Jon_Logan]

Ah shoot, I hadn't read this for a couple days.

So, this is a little late in saying but, there's no real reason to connect a load to an amp to troubleshoot voltage problems.

It's better if there's no/light load to troubleshoot voltage stage problem. The drivers and outputs are generally followers. They are in the Bose. Here's a really good spot to put a scope on the output You can view the AC and DC waveforms and look for signs of instability.

Did that U1 power supply issue get worked out?