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Bose 550 AM/FM receiver - dead power section?

pqrannes

pqrannes

Tubes warm your heart too
I picked this odd receiver up at a junk yard. It seems like it is kind of dedicated to the Bose 901 speakers – there are 901-/non-901-speaker-switches on the back and a special narrow/wide button combined with the low frequency changer (bass) on the front (see attached pictures).

It lightens up alright. With an antenna connected to it and it set to ‘FM’ a signal is detected by the signal-meter. But no sound comes from the speakers connectors or the head phone output.

But I got an output out of the tape rec. phones on the back and a useful signal (music) into another amp. As I see (and I’m not very skilled) it is the power section of the amp that is faulty. I took of the top. Not much in there compared to a (heavy) Sansui Eight Receiver sitting on the same bench in my workshop. Enclosed pictures show what I found there.

I’ve checked all the fuses (one on the board) and the two on the back (the lid is missing), but they all seem fine.

The 1000$ question: what is wrong? Or: how can I (still without mentionable knowledge or technical skills) but with a digital multimeter (see picture).

I hope someone have the patience to give me a hand here and maybe guide me through thise error finding.

Peter, Denmark
 
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Hi! I can't tell for sure buy the photo but it appears that it might have a speaker protect relay, if it does it probably is in protect and you have bad outputs. See if you have any voltage on the emmiter resisitors (the big ones in front of the output transistors), if you do you have bad outputs or a bias problem. Check for rail voltage on the collector of the outputs (35 to 55VDC), if you don't there has to be an open fuse, protector, fusable resistor or connection somewhere. Hope this helps.

Steve
www.acmepas.com
 
Thanks. I'll give it a try. Are the emitter resistors the two really big ones (kind of black) or the four grey ones (I told you: I'm starting from scratch)?

Peter
 
Using the multimeter

:yes: I measured the voltage of two big black ones (emitter resistors?) from the back side (from underneath) and it gave me resp. 41,7 and 41,9 VDC. I see no speaker protect relay (as I know it from my Sansui stuff at least, big and nice and recognisable as a obvious relay). Where does that leave me?

Peter
 
You should'nt have any votage on the 4 gray emitter resisitors. If you do there is a problem with the amp, probably bad out transistors. Check their emitter-collector junction and see if you have a pair of them shorted. See if you got the rail voltage on the collectors of the outputs (should be the center leg). Do have a schematic for it?

Steve
 
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Trial and error ... and some smoke

No, I don't have a schematic for the Bose.

I found no voltage on the 4 emitter resistors (as I shouldn't). Also checked the 4 transistors (?) for voltage between the three legs. About 42 V between the centre one and each of the others on the two first. Got to number three (from rigth and probably shorted it between two legs and the T1.6A fuse on the back blew up with a pop.

Replaced the fuse with another T1.6A fuse, powered up and two of the emitter resisters got really hot and started to smoke. :thumbsdn:
Power off, fast! Maybe my test speakers shouldn't have been connected (of course not)?

T1.6A fuse didn’t ‘pop’ but burned slowly. Where does that leave me?

Anyone wants to pick up a Bose 550 AM/FM receiver in Denmark before I destroy it totally by my trial and error?

Maybe I should use this fine Sunday to change the chain and chain wheels on my motorbike instead. 60-70’s Moto Guzzis aren’t that easy to destroy.

Peter
 
I beleive it has an amp problem, make your voltage mearsurements to ground.
Check them outputs with your meter set on diode test. Also check the four diodes (or bridge rectifier) in the power supply section.
 
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:scratch2: You're sure right about the amp problem, especially after I burned it down.

I’d like to make “the voltage measurements to ground” as you suggest, but how do I do that (I warned you, I am newbie in this) and can it be done with out powering up the old Bose thing (which must be impossible after the trial and error and smoke stunt this morning)?

Peter
 
Attach your black test lead to the chassis and use the red to take your measurements. You might want to check out the website http://www.bcae1.com/, although it is based on car electronics, electronics is electronics, it is a cool, interactive site that could be very educational for you.
 
Hi Eggman,

Thanks for the link to the www.bcae1.com/ webpage.
I took a quick look at it and it looks just like what I need. I''ll study it thouroughly and return to the Bose 550 (and you and AK) when I'm fully educated!

Peter
 
im good at discrete amps.

your output transistors are most likly shorted.

Need to put the meter on diode check, and do a B to C and a B to E test, and it should read around .4 to .6 something. if it reads 000 when the leads are on any of the legs, the output transistor is shorted. if you dont get any reading, reverse the leads. if still no reading, the transistor is open.

You need to test the nearby resistors and transistors for shorts. because if they are bad, especially in the VAS circuitry, it will cause the output to swing rails and blow them. Those emitter bias resistors smoking, means the DC voltage is being thrown through there, because the output transistors are bad.
 
whats happeneing, is one of the transistors have shorted, which is throwing 40 volt rails into that low ohm resistor.

Well, ohms law states, that volts/resistance = current. well, then you calculate for power with given current, youll see that your WAY over the resistors power rating, so thats why its smoking.
 
You should always replace outputs in pairs and be sure to check the bias resistors or the outputs will pop again.
 
trial and smoke - doing my homework

:thmbsp: Thanks for their comments to Eggman and mbates14.

I'm doing my homework and studying both the above mentioned webpage and these books (with quite promising titles) that the local (Danish) library got me: Delton T. Horn: How to test almost everything (TAB Books, 1993) and G.C. Loveday: Electronic fault diagnosis (Longman, 1994).

When I'm smart enough not to shorten the transistors with my multimeter I'll be back!

Peter, Denmark
 
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