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Blown 767. No sound


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#1 ranter2887

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Posted 05 January 2018 - 03:46 PM

Hey everyone. I wanted to permanently keep the 767's switch to LINE IN by soldering because I kept having to mess with the switch to make the line work again. Before soldering I was testing with a multimeter and accidentally made one part contact the other and the music stopped.

I turned the system off and still no sound from FM, AM, SW, Line/Aux or Tape.


I checked both amps (HA1392) and they seem to be working by getting a voltage reading on the multimeter. Could it be the preamp that's down? Caps? Also the Stereo LED stays off while turning the dial. So I was thinking it could be a cap problem. The 5 contacts you see in the first pic is what I was dealing with.

Wish I was more careful.

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#2 caution

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Posted 06 January 2018 - 02:16 AM

I traced out that cable on the free 767 service manual from hifiengine.com. Pin 1 is on the left side.

1 - Right channel audio path output, prior to bass/treble. Goes through R334(4.7K) to line out right (J101-B).
2 -  Left channel audio path output, prior to bass/treble. Goes through R333(5.6K) to line out left (J101-A).
3 - Ground
4 - Ground
5 - Power(~12V) - originates at fusible resistor R224(4.7), the other side of which ties to Q110, a ripple filter tied to 15V.
As soon as pin 5 enters the IO board, it goes through R323(560) which supplies ~10V to the line-in preamp(IC401 pin 4).

The only damage I could see happening from shorting pins on that connector together is pin 5. If you're sure you shorted together pins then they were probably adjacent to each other. So, I'm guessing you grounded the power supply pin (4+5).

R224 is probably fine, fusible resistors take time to blow. But such a small value means that it's possible it allowed Q110 to blow, essentially cutting power to a number of circuits.

 

Measure the voltage on pin 5, if it's not at least 11V, you may have a bad ripple filter. Measure the voltages on Q110, you should see 14V at the emitter, 15V at the collector, and 14.7V at the base.

 

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#3 Superduper

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Posted 06 January 2018 - 12:21 PM

I recall assisting another fellow about maybe 7 or 8 years ago that did some “live” work and had a mishap causing similar symptoms. If I recall, based on the symptoms, I concluded that it was this same transistor and the OP on that thread at the time confirmed after swapping it out that it was fixed. That thread is buried deep in our archives and I’m loathe to go looking but I believe you’re probably correct on Q110 or the associated fuse resistor being blown. Again most likely the transistor however because shorting pin 4/5 will cause that transistor to instantly pass 3+ amps. The resistor is intended to protect not the transistor but the downstream components.

#4 caution

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Posted 06 January 2018 - 01:15 PM

A forum search for "Q110" popped it up. It was a 777. You're right, that user had the same problems and fixed it with a new Q110.

 

Had a disaster with my GF-777 HELP!



#5 ranter2887

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Posted 06 January 2018 - 02:55 PM

Before I saw replies from both of you, last night I was already doing research on the problem. I found a pdf service manual for that box, learned a little about its parts and did my best to figure this stuff out myself. I mean I'm not very new at messing with electronics but this is another level for me.
 
I did find a post here that's very similar to this issue. On that thread, Superduper gave simple instructions on what to do and mentioned Q110 (photo 1). And yeah I did notice Superduper's frustration in that post lol. That's what pushed me to not give up and go deeper into the details and learn something.
 
I researched on how to test transistors, BCE and all that. AGAIN, my clumsy, shaky self, tapped another contact with my + lead of the meter. BUT, I heard FM music for 1 second when I made that accidental contact. I momentarily bridged between Q110 (E) and a resistor (Photo 2. Below C203) next to it for few seconds and I hear music from FM Tuner, Music from Tape and Line in connected to my ipod. That confirms that the tuner and others are fine.

Then today, I see the 2 replies here about Q110.

  • No voltage on Q110. I tested another ripple filter (Q703) in the cassette area and that gave me a reading above 11V. So I believe it's safe to say that Q110 is down.
     
  • There is Voltage on Pin 5, but it's measuring 18v.

 

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#6 Superduper

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Posted 06 January 2018 - 03:43 PM

What do you mean no voltage at Q110, you need to be more specific, b/c/e.

#7 ranter2887

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Posted 06 January 2018 - 06:55 PM

Ok Here's the readings. Sorry got lost for a bit and kept focusing until I fully understand what to do  :blush:  :lol:

+B & -E = 0.45v
+B & -C = 0.00v
-E & +C = 0.45v



#8 Superduper

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Posted 06 January 2018 - 09:01 PM

I don’t know what those readings are.... are you testing that transistor out of circuit using a diode test?

I’m asking what are b/c/e readings with the transistor (in-circuit), and the neg probe at a good ground location and the positive probe at a transistor lead, provide voltage readings. This tells us not only if the transistor is working properly but also provides other clues too on how well the circuit is working.

#9 ranter2887

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Posted 07 January 2018 - 05:30 PM

No it's stil in circuit. I placed the negative lead to the cleanest ground location and on the positive lead of the transistor, I got:

B 18.50v
C 18.50v
E 18.05v

The readings below probably doesnt matter but while the 767's off, but I got:

B 0.54v
C 0.54v
E 0.24v

#10 caution

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Posted 07 January 2018 - 05:55 PM

You momentarily got audio because the side of C203 you shorted to Q110's emitter effectively jumpered across R224.

 

Seeing readings that high at Q110 makes me wonder if R224 really burned out. Put your leads across it and see if it gives more than 5 ohms.

 

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#11 Superduper

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Posted 07 January 2018 - 08:16 PM

The circuit specs are based on a 15v steady supply so 18.5v is surely on ac mains. If C and B has the same voltage, that sort of suggests that Q110 is not passing any significant current through it, otherwise there should be some droop at the base as its being supplied through a resistor. Good voltage at E suggests it’s working properly and the lower voltage is consistent with the approx forward voltage drop through that junction. Why isn’t it passing current? Probably R224 is blown. Simple test is to check both sides powered on. This tells how well the resistor works under load. Sometimes, resistors that have marginal internal junctions will test well with low current but fail under load. Anyhow like Eric says, R224 is highly suspect and can be confirmed through a resistance or voltage check.

#12 ranter2887

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Posted 14 January 2018 - 12:51 AM

Sorry for the late reply. Adult duties. Yes I was thinking about the R224 being suspect as well. I had no measurement or even a beep on the meter. I removed a similar resistor from a maggy d8443 that im currently modifying. It's only slightly more resistant than the R224's by 3 ohms, but I finally got music from all sources on the 767. I put the resistor back into the maggie and bought new resistors that matches with R224's (1/4W 4.7ohm 5%) and I replaced Q110's transistor (D471) with a fresh one too. The box sounds good again. 

 

The rest of the issues I can easily take care of and the box will be good to go. This was a learning experience especially about resistors and transistors. Thank you Eric and Norm for your help.  :thumbsup:



#13 caution

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Posted 14 January 2018 - 01:21 AM

Glad to help! That resistor should be fusible, not sure if you caught that. You can still obtain them here if you want to play it safe.



#14 ranter2887

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Posted 14 January 2018 - 01:33 AM

Glad to help! That resistor should be fusible, not sure if you caught that. You can still obtain them here if you want to play it safe.

I'm aware that the old resistor was fusible. But I should have asked where can I get one lol. I thought it stopped being manufactured after not being able to find them on ebay and amazon. So I bought a non fusible one  :lol: Thank you for the link though!