Amplifier (JVC-SA22) : blows external fuse. Transformer Problem ?

[Jim Nomad]

Initially after the amp stopped working, I checked the external fuse, which was fine. Then after opening her up I found the two slow blow internal fuses had gone. These I replaced, and now the external fuse keeps blowing whenever I switch the power on. Could both problems be symptomatic of a faulty transformer, and how could I trouble shoot the problem. I love this amp !!

I have been told it may be a diode short in the bridge rectifier : I think I have identified the diodes of the bridge rectifier, both on the circuit diagram (attached and circled red and on the circuit board itself. I have tried to test these on the board with a multimeter - they show high resistance in one direction and low in another (as expected I think although two of them in the opposite direction to that show on the diode). Can I test them in line and would this be the correct test ?

Secondly I have been told that the capacitor on the supply side of transformer may be at fault. (circled blue on attached diagram and also photograph) . I do not have a capacitor tester, so do I have to remove these from the board to test with a multimeter, and if so would it be the restistance changing that I'm looking for?

Finally if it is these capacitors (which in circuit reed a sready resistance of 1 / infinite) where could I get a couple ?

Many thanks for any idea/ help/ cleaaring up my ignorance of the whole thing

 

[Jotto]

Remove the two internal fuses, does it still blow? Probably not now, since you have a stereo unit the fuses are probably to protect the speakers on the output transistors.

Since your probably using a analaog meter, you might see a slight deflection but as long as they not shorted, don't worry about them. Look at your output transistors. This is for the capacitors.

On the bridge, you should read infinity one direction an mid scale in the other direction. Trying to remember if the setting is Rx1 or R x10, been a long time since I used my old Simpson meter. Yes its a good test for you to use for diodes.

The capacitors look to be Vishey Sprague, you should be able to find a suitable sub. If you want NOS you will have to search a bit.

 

[Jim Nomad]

Thank you Jotto.

You were right after removing the internal fuses and power up the extrnal fuse did not blow. I also tested voltage across the removed fuse terminals and got 30VAC on each which is what the transformer should be supplying i think.

I went back to the diodes in the bridge rectifier and tested them again:
Power off, and DMM set to diode :
Direction:with against
D601 : 618 634
D602 : 627 577
D603 : 633 619
D604 : 577 627

and with DMM set to read resistance in ohms
with against
D601 : 498 508
D602 : 504 469
D603 : 509 498
D604 : 466 505

This doesn't make a lot of sense to me I'm afraid, but i would have expected a much higher resistance in one direction than the other. Is there an issue about testing them while in circuit (ie I'm measuring the resistance of the rest of the circuit) or could they have all gone? I've been told that I should remove them from the circuit to test them properly. Also while looking for transistors (the amplification on the circuit board seems to come from 2 IC's attached to a heat sink) , I noticed a black residue on the circuit board around the base of two big capacitors (C603 and C604) , same colour as the outside of the capacitors and looks like a melt. Could this be the problem ? Have attached photo and circuit diagram.

Many thanks again

 

[Jotto]

That is epoxy to hold the caps in place while being manufactured. Those won’t be the problem, very seldom will you lose a cap of that size, but when you do you will know it.

I was wrong about the analog meter, you should have a diode checker on the meter, but your readings are right, so don’t worry about that part. Shorted would be what you are looking for. On your fuses on the inside, at the input you had 30vac, now check the other side of the fuse holder to ground, your probably going to find a short or near a short that is blowing the fuses. You will need to find what is common to both left and right channel, that is where I would start.

Post a full schematic for us to look at, this is all generalization at this point, partial schematics only give us what you think is the problem area, theory is great in classroom but only a building block to learning to troubleshooting.

Old stereo equipment is well built and will last a long time. I run 2 Harmon Kardon Citation II amps which I mono blocked, and the preamp which I modified to accept digital input so I could use my DVD player for music.

 

[Jim Nomad]

Thanks,

I have attached the full schematic, hope its readable as had to paste two bits together. Yes , I love this old amp, and haven't heard better from anything I've tried thats newer. It really suits my deck and speakers too, the work well together,

 

[Jotto]

Jim,

Sent PM to you.

The part I like about vintage equipment. My amps cost 160.00 new, now its worth about 2500.00. Just hope I never lose the transformer, they have to be hand made now and only a few people are doing that.

 

[Jotto]

Can now read it, thanks. I believe that F604 & F605 were the two that were blown when you started out.

I would be looking in the area X601 and X602 to start with. Check both transistor and the zener diodes. You said you had 30vac at the fuse holders, by the schematic is should be 36..8vac. Did fuse F603 blow or was that F801? One is on the primary side the other is on the secondary.

Test D404 also, common to both sides

 

[Jim Nomad]

F601 and F602 were the two that first blew (F604 and F605 not present on my board - see last section of power supply schematics - i think they manufactured differently for different regions)

F603 (0.8A) - remained intact throughout

After that F801 (on the primary side) blew every time and the others stayed ok.

 

[Jim Nomad]

ALSO , on the top of the actual transformer housing itself is a little schematic which give the voltages on the secondary coils, that go to the fuse as 29V ? This tallies more closely with the readings I got. Could there be a difference sometimes between the schematic and the gear they actually fit?

 

[Jim Nomad]

Some test I have done since yesteday :

From other advice : To check for shorts after the bridge rectifier I measured the resistance between the +ve connection of C603 to 0v and between the -ve connection of C604 and 0v. I completed this check (with negative probe of the MM on the 0V in each case?) and it reacted as resistance starting low and rising.

Then checking resistance between the collector or x601 and 0v (used fuse holder and circuit point 24 for the 0v) , gave a similar result to that when I tested the two capacitors - started at about 100ohms and then continued to rise. Same for x602.

More tests... I removed one leg of each diode on the bridge rectifier and all they show around 500 in the direction of flow and infinite in the other , so all good there.

Next i notice burn type marks around the leg of resistor R341 (and to some extent R342) - these resistor are as far as I can make on either side of the split from the 0v to each of the amp ICs.

They are both rated 6.8 ohm , 1 W (oxide metal film)

I removed them from circuit board and tested resistance. R341 gave 4.6ohm and R342 gave 25.4ohm . Is this a problem, should I replace with new ?

Many thanks

 

[Jotto]

Where are you at checking the components we discussed?

Yes they could be listed different, but not a big deal. Right by the fuse holders in the schematic are the 2 transistors and 2 zener diodes that should be looked at. D404 is a bit below that area. Physical location might not be right there but you can look at the drawing and track it down.

Also, have you checked the fuse holder to ground? If so what was the reading.

 

[Jim Nomad]

I have just tested the other end of the fuse holder (F601)to ground and the resistance started about 300 and steadily went up until infinite - a bit like a capacitor. The other fuse holder end (F602) to ground behaved in the same way.

I have found the transistors and zener diodes. What would be the best way to test / check these ? Thank you

 

[Jotto]

diode test on your meter, just like the bridge you tested. on the transistors hook to the base and the other lead to the emitter, and then the collector. should just read like a diode from base to each other leg. sometimes in circuit you great strange readings but I don't think you will on these two. then test emitter to collector to make sure there are no shorts.

 

[Jim Nomad]

Hi . I ended up testing as you suggested , but also removing the transistors and testing isolated from the circuit. They behaved as good transistors.

I have had to leave for work after that, so the diodes remain untested. will do them when i get back from the boat on wednesday. thanks again for your help so far. where are the test pointing at the mo? to the amplifier ic's ?

Cheers

 

[docb]

I haven't looked at your schematics, bu if it uses an output IC these are suspect. Desolder it, and see if the fuses still blow.

 

[Jim Nomad]

Hi, back from the boat and had a chance to do a quick check on zenner diodes D605 and D606 , removed and tested good, replaced.. Should I go onto desoldering the ICs (301 and 302) when I gert home tonight. Do these 10 leg beasts test like a lot of transistors?

 

[Jotto]

IC301, IC302 you can remove if you like, testing will be a bit difficult with just a VOM, but there is very little options since your blowing fuses. Pin 2 is ground pin 9 is Vcc, use that as your reference to test the other pins. Datasheet for this component is a bit hard to find. STK0040

http://www.elenota.pl/datasheet-pdf/131975/Sanyo/STK0040-II

http://doc.chipfind.ru/html/etc/stk0040.html

Not very good drawings, but will work.

If both of those test good, look at IC401. This circuit is for protecting the amp and speakers. TA7317P

http://www.datasheetcatalog.com/datasheets_pdf/T/A/7/3/TA7317P.shtml

 

[Jim Nomad]

Thank you Jotto, Would you recommend detatching the heat sink first , or leaving the whole thing connected and removing the lot. ?

 

[Jotto]

I would do it in circuit without desoldering anything. If I have a pin that I question the reading, then I would remove the pin in question from the circuit. You have two identical ones, I don't think both would be bad (but I have been wrong before) . Something is drawing more then it should to blow both fuses which is very low amperage.

I made a mistake on pin 9, that is Vcc, pin 2 is ground for IC 301 and IC 302

http://www.electronica.ro/audio/STK0040.shtml

this is a bit clearer, but not a lot of information

 

[Jim Nomad]

I have tested the 2 ICs (301 & 302) in circuit. I tested all pins (except 4,5,6,7 which are not connected I think) in both 'directions' and the results for each IC differ. I have tabulated the results , but can't seem to attach the file because of size constraints. Will try and get another format going, but until then , I hope yo don't mind but I have PMed them to you James (JOTTO) , if you have time could you have a look ? I'm still trying to decipher what they should be in relation to the schematic you kindly found for me. All best. Jim