Help. Accidentally dropped needle on record. Blown amp result

[Homer4beer]

I worked it out. It's these ones.

I don't think these can be tested properly without removing them as I'm getting a reading on all of these types of bipolar transistors no matter what combination of pos/neg on any combination of leg (base, emitter, collector) I try.

 

[DaveVoorhis]

I don't think these can be tested properly without removing them as I'm getting a reading on all of these types of bipolar transistors no matter what combination of pos/neg on any combination of leg (base, emitter, collector) I try.

The main thing you're looking for is evidence of shorts -- zero ohms or very near to it.

 

[Homer4beer]

The main thing you're looking for is evidence of shorts -- zero ohms or very near to it.

Yep understood.

Don't the transistors have to have only 1 way flow between the base and collector though?

https://www.electronics-notes.com/articles/test-methods/meters/multimeter-diode-transistor-test.php

Eg if you get a measurement in both directions it's faulty? From my reading you should not have a reading when the neg is on the collector and the pos on the base. It should be an open circuit.

Negative to base and pos to collector should give a reading.

I'm getting readings no matter what combination I try.

 

[petehall347]

you have to study the circuit diagram if testing in circuit to make an educated guess of expected reading . not foolproof but you will see if any shorts are present . if your meter has a beep test use it . if it beeps check reading then reverse leads .same reading says a short . pull the suspect and test again . etc etc

 

[Homer4beer]

No beeps using the meter. So my understanding is you really can't definitively test in circuit.

Hmmm I think they must be fine though. They are surface mount so no easy to get off otherwise I'd remove to test.

 

[OMGCat!]

Usually when they are shorted they are near zero ohms so you should get a beep. If you get a relatively low reading it's likely something else in the circuit.

I've never tried to fix a unit with surface mount stuff. I don't envy you but wish you the best of luck!

 

[sqlsavior]

@Homer4beer, congratulations for your courage to dive right into the deep end.

And thanks to everyone trying to help. It's part of what I like about AK.

I hope you persevere, and are rewarded in the end. Good luck.

 

[Homer4beer]

Thanks guys!

Yea it's forcing me into learning all about circuits and electronics in general.

I got pretty lucky in an auction recently and picked up a tektronix 3054B oscilloscope for AUS $1500, just been serviced. Now I am educating myself in how to use it lol.

They seem to sell for $2500 ish US.

I see this board as a blessing in disguise ( as long as I finally get the thing working again!)

Again, you guys have been great so far in your advice. I look forward to the day I get it going again with your help

Edit: I do appreciate help from professionals who do this all day for a living. It's great you are willing to help somebody out when they ask.

 

[Homer4beer]

Ok guys,

Some mixed results. The amp turns on and stays on now. With the speaker selector to "off" the volume knob does not blink and stays solid. The input selector works perfectly, the bass and treble knobs work perfectly as does the loudness button and pure direct button. I can listen to music through the headphone socket perfectly. With the speaker selector to any other setting other than "off" the volume knob starts blinking but I can still listen to music through the headphones and adjust the volume normally.

No music comes through the speaker outputs at the rear even after removing the headphones.

When I activate the "Power amp direct" function all the "input selector indicators" goes out. This is normal as it's now simply acting as an amp for a pre amp input through the "Main in" connection.

So when I used to turn the amp on before I damaged it, I would get a "click" when it powered on, then another "click" approx 5 seconds later as the amp turned on and music came out. This second "click" is not happening......


I have also checked a few items on the board and have uploaded what I've done so far. No issues yet. All checked ok so far.

 

[Homer4beer]

From the manual

If the Volume indicator flashes quickly, any speakers may not be connected to the A-9755/A-9555. Make sure that
the required connection is performed appropriately, and set the [POWER] switch to the ON position again (pages 10,
14)

That's what's happening along with no click after 5 secs. I only have one small speaker connected to Chan A left side while testing. Should I connect my normal speakers again?

EDIT: Could it be the speaker relay not working. Perhaps that is the "click" I should hear after 5 secs and why I don't get any sound?

 

[DaveVoorhis]

Try with both speakers. The speaker detection circuitry might cut off both if you're missing a speaker on either left or right.

As always when testing a freshly-repaired or unknown amp, hook up some disposable speakers before testing with good ones.

 

[Homer4beer]

Thanks Harry. Any chance u could give me more in depth instruction on how I would do that?

Cheers mate

 

[Soundphile]

Hi Home4beer,
Admire your guts and willingness to carry out this repair. The AK experts are also fantastic in the help they are giving.
You started out confused and regretting the "accident" that caused the problem.
In this short time you have rapidly progressed to fixing the preamp and from the looks of it, will soon complete the repair with continued help from the AK community. Not many here do smd repairs ... but you're in good hands,
What a great forum to be in.
Kudos to you.
Robert.

 

[Homer4beer]

Thanks Harry,

I'll have a crack at figuring out what's going on soon. I'm scared I'll blow up my Oscilloscope through having no idea what I'm doing!

I've connected two speakers to the terminals now with no change to the blinking power volume light. Still no sound.....

 

[DaveVoorhis]

It sounds like the speaker relay and speaker protection/detection circuitry may have a fault. On your schematic, it should be the circuitry connected to the wires I've highlighted in red. They go to the speaker relays, which are shown below on the right-hand side of the diagram. The circuitry that drives them will be off the diagram to the left.

 

[Homer4beer]

It sounds like the speaker relay and speaker protection/detection circuitry may have a fault. On your schematic, it should be the circuitry connected to the wires I've highlighted in red. They go to the speaker relays, which are shown below on the right-hand side of the diagram. The circuitry that drives them will be off the diagram to the left.

View attachment 972843

Thanks Dave.

I am now thinking it's the relays to the speakers.

http://www.mouser.com/ds/2/307/g5pa_2-298840.pdf

There are two of them for each channel A and B.

RL501 and RL502 in the schematics. Any way to test these?

Edit: Going by the circuit diagram the driving comes from a plug (To NAAF -8708(2/2)) (SD-2 : D4)

I take it that's schematics diagram SD-2 grid referance D4?

 

[DaveVoorhis]

I am now thinking it's the relays to the speakers.

http://www.mouser.com/ds/2/307/g5pa_2-298840.pdf

There are two of them for each channel A and B.

RL501 and RL502 in the schematics. Any way to test these?

Supply 12 volts DC across the relay coil and it should click in.

Edit: Going by the circuit diagram the driving comes from a plug (To NAAF -8708(2/2)) (SD-2 : D4)

I take it that's schematics diagram SD-2 grid referance D4?

Very likely.

 

[Homer4beer]

Supply 12 volts DC across the relay coil and it should click in.

.

Ok I did as you suggested. I bought a power supply and set it at 12V 40mA and both of those relays kicked in. I also went through and VERY carefully checked all the inputs into the board for AC voltage (Red, blue, orange and brown). They all tested working. I also tested (again) the bridge rectifier coming off the blue connector which puts out 12V DC. I put my multi meter to the + and -- terminals coming off it while the amp was powered up and got no voltage coming off it. I think I may now know why the amp isn't working properly and the speaker relays were not working.

There isn't any 12V power to the board due the bridge rectifier being stuffed (I'm pretty sure).

On a scarier note.....I had my power supply set to 12V 40mA to test the relays and then upped the amperage to 950mA to test elsewhere. For some reason I went to double check the relays again and when I put the leads on the circuit to test they didn't click and I saw some smoke! I turned the power supply off immediately and wept (well felt like crying) as I thought I'd now stuffed the amp up due not turning the amperage down (Although won't the circuit only take the amperage required? I wasn't forcing amps through it?)

So I can't see and damaged parts and I have no idea where the very small amount of whispy smoke came from...only the general area. I hope I haven't stuffed something else up

Anyhow... I just shouldn't have mucked around putting voltage through parts of the 12V circuit just because I thought I knew what the problem was. I tested the small diodes in blue and the relays I've circled in Red.

The suspect Bridge Rectifier is D519 part number S1NB60-4062

http://au.mouser.com/ProductDetail/Shindengen/S1NB60-4062/?qs=/ha2pyFaduhQpnubSiXYNQT21Sy4XtCsah0%2b%2bxP96emZJvwlDhAcfg==

Of course it's not available

 

[DaveVoorhis]

You're right, you're not forcing amps through the circuit but the power supply to the relays might be current limited, in which case something drew more current than it should. However, the smoke might have come from the dud bridge rectifier.

Bridge rectifiers are non-critical. Any bridge rectifier of similar specifications that you can solder in will work.

 

[Homer4beer]

Thanks Dave.

The bridge rectifier was not working prior to my stuff up and is quite a way away. The smoke did appear to come from the relay area.

It's a smd Bridge Rectifier which makes it that much harder to;

a) get the bugger off, and
b) find a replacement

If u know of one or find one let me know