1979 sanyo receiver blowing fuses

[Zenbrewer]

i just picked up a 1979 sanyo plus 75 receiver, i opened it up and noticed a blown fuse, wired in an inline fuse housing and it popped that one two. It pops the second the power switch is plugged in and a few of the panel lights light up as well for a second.
I have a voltmeter, and another website has copys of the diagrams that i could submit if someone is willing to help.
I have a long history of mechanical troubleshooting with my manufacturing background but small scale electronics requires different skills.
Can someone piont me in the right direction as far as what i should start testing and how to do it?
Thank you.

 

[davenn]

hi there
welcome to EP

yes post diagrams also post some pix (sharp and well lit) of the insides of the unit particularly close-ups around the PSU and output sections.
Show us where the fuse is that is blowing

cheers
Dave

 

[KrisBlueNZ]

Hi there and welcome to Electronics Point

Yeah, what Dave said. Also, do you have a proper model number? Or is it just called "plus 75"?

And can you post a link to the service information you found?

 

[Zenbrewer]

Thanks guys here we go:

the link to the manual is here, i tried to upload it to the forum but its 14 MB so it was denied.
http://www.hifiengine.com/manual_library/sanyo/plus-75.shtml

these are screenshots of the pages i though relevant.




Kris:
The model number does seem to be "plus 75" as shown here



dave:
i located the PSU and its so buried closeups are difficult i can however dig deeper if needed.
i am too new at this to know exactly what you mean by "output sections"



The first thing i noticed about this is that the preamp input/outputs has jumpers installed.
can you you guys explain why?


this is the transformer with the fuse buried to the right:
the cut blue wire is the hot that runs back from the power switch to a switched outlet on the back.
i cut it because i noticed that someone had done a messy solder job on the main power cord and was thinking this might be grounding the circuit (i was wrong)


this is the fuse in question it is soldered to the board as you can see i wrapped an inline fuse to it to make it easy to change
the fuse i used is a Fast Blow 5A - 125V. smaller size than the original but radio shack did not have the full size.
would a slow blow be better?





the power switch has some questionable coloration on a few terminals but i'm not sure if this is an issue, almost looks like rust.



here is the messy solder on the main cord



this is the back of the power amp



close ups of the odd parts, according to a rebuild thread i thumbed through these were factory mods.



the best shot i can get of the top of the amp, and whats listed as the left and right I.C.


the rest of the pictures are what i can see of the PSU




Where do i start?

 

[Zenbrewer]

hi there
welcome to EP

yes post diagrams also post some pix (sharp and well lit) of the insides of the unit particularly close-ups around the PSU and output sections.
Show us where the fuse is that is blowing

cheers
Dave

Ive posted detailed pictures and diagrams, is it possible to send you the whole manual in a personal message? .. It would seem that it is too large for the forum

 

[shrtrnd]

What's the white stuff in the second picture from the bottom? Is that thermal grease for a power component that's missing?

 

[alfa88]

I would be inclined to stock up on fuses remove the Darrlington Power packs 1 at a time. If it's not them then remove the Power Supply wires to the Amp boards to see if the Power supply is to blame. It looks like someone kludged in a fuse holder.

 

[davenn]

What's the white stuff in the second picture from the bottom? Is that thermal grease for a power component that's missing?

Yes, he's lifted the power packs off the heatsink

 

[davenn]

OK
first possibility is that its a shorted turn in the transformer either primary or secondary
second possibility is one of the bridge rectifiers on the secondary outputs is short circuit
third possibility is one or more of those power modules as mentioned by alfa88


To see if it was the transformer, you would have to disconnect the 3 pairs of wires coming from the transformer to the bottom of the PSU board as shown in your 3rd image

with those 3 pairs disconnected, replace fuse and switch on. If fuse blows then its the transformer
if not, the fault is further on.
Then connect one pair of those wires at a time and switch on each time and see if fuse blows
That will narrow down to which rectifier set could be the problem

 

[Zenbrewer]

I would be inclined to stock up on fuses remove the Darrlington Power packs 1 at a time. If it's not them then remove the Power Supply wires to the Amp boards to see if the Power supply is to blame. It looks like someone kludged in a fuse holder.

I just twisted the fuse holder on there as the original is soldered to the board and i did not want to redo it that way if it was going to pop again.
I plan to solder the fuse holder in place if i can resolve the problem.

 

[Zenbrewer]

OK
first possibility is that its a shorted turn in the transformer either primary or secondary
second possibility is one of the bridge rectifiers on the secondary outputs is short circuit
third possibility is one or more of those power modules as mentioned by alfa88


To see if it was the transformer, you would have to disconnect the 3 pairs of wires coming from the transformer to the bottom of the PSU board as shown in your 3rd image

with those 3 pairs disconnected, replace fuse and switch on. If fuse blows then its the transformer
if not, the fault is further on.
Then connect one pair of those wires at a time and switch on each time and see if fuse blows
That will narrow down to which rectifier set could be the problem

Thank you.
I will work on it this weekend, And let you guys know.

Is a fast blow fuse ok to use, or should i use a slow blow?
Also do the jumpers in the pre amp outputs make sense? Should they stay in?

 

[davenn]

Also do the jumpers in the pre amp outputs make sense? Should they stay in?

they just connect the preamp to the main amp. without the jumpers the system wont work
the idea of them is that you can either use the preamp in that unit and take its output to a bigger external amplifier, or feed an external preamp/mixer system into that power amp

Dave

 

[Zenbrewer]

they just connect the preamp to the main amp. without the jumpers the system wont work
the idea of them is that you can either use the preamp in that unit and take its output to a bigger external amplifier, or feed an external preamp/mixer system into that power amp

Dave

That is what i was thinking, it is certianly not something you would see theses days.

 

[davenn]

still occasionally on amplifiers for commercial use
Its a very handy feature, wish more had it

 

[Zenbrewer]

OK
first possibility is that its a shorted turn in the transformer either primary or secondary
second possibility is one of the bridge rectifiers on the secondary outputs is short circuit
third possibility is one or more of those power modules as mentioned by alfa88


To see if it was the transformer, you would have to disconnect the 3 pairs of wires coming from the transformer to the bottom of the PSU board as shown in your 3rd image

with those 3 pairs disconnected, replace fuse and switch on. If fuse blows then its the transformer
if not, the fault is further on.
Then connect one pair of those wires at a time and switch on each time and see if fuse blows
That will narrow down to which rectifier set could be the problem

so i just removed all three sets of transformer output wires and the fuse held up.

transformer output sets delivering the following voltage.
orange = 40.8 V
brown = 10.2 V
yellow = 37.7 V

replaced the orange set. no visual response from unit but fuse did not blow.

replaced brown set, fuse did not blow. receiver panel lights lit up, tuned to a known station and all the little stereo bulbs etc came on.

replaced yellow set, fuse popped.

thinking i may be able to further isolate things. i removed the pre-amp jumpers and not only did the fuse pop but the orange wire as show in the first picture below arced and melted the solder from the board as show in the second picture below.




I than replaced the jumpers and removed the power pack that arced, fuse popped.
removed the other power pack, fuse popped
removed jumpers, fuse popped.

so i'm guessing we are looking at a rectifier set as mentioned?
how do i start further testing the circuit in question?


also since i saw an arc at the power pack how would i go about testing them before i put them back on the board?
the pack is below its on a simple little board all of the components are visible in the picture

(newbie questions and somewhat separate)
what are they? capacitors? how do i test them? ohms? what will the diagram tell me about them? should i get a "circuitry for dummies" book?

 

[davenn]

They are the Audio power output stage modules
this is the inside circuitry .....




OK for narrowing down which rectifier stage was causing the fuse to blow
show me good pics top and bottom of where those yellow wires go

OK on the schematic below I have drawn and labelled 3 blue circles around the
3 pairs of wires from the transformer can you please identify which was the Orange, Brown and Yellow pairs

Be prepared, I may also need a larger version of that schematic as well as it is a little difficult to follow at that size

 

[Zenbrewer]

They are the Audio power output stage modules
this is the inside circuitry .....

View attachment 17010


OK for narrowing down which rectifier stage was causing the fuse to blow
show me good pics top and bottom of where those yellow wires go

OK on the schematic below I have drawn and labelled 3 blue circles around the
3 pairs of wires from the transformer can you please identify which was the Orange, Brown and Yellow pairs

Be prepared, I may also need a larger version of that schematic as well as it is a little difficult to follow at that size

View attachment 17011

The yellow wires were attached to circle 1, the brown was circle 2, the orange circle 3. Now below is two better pictures of the schematics.
I will take pictures of the board itself tomorow.

 

[davenn]

thanks mate
will look over them whilst awaiting the pics

Dave

 

[Zenbrewer]

ok heres the back of the power supply with different angles





here is the top as close as i could get pictures.



Here's a few things i noticed
the first is the Zener diode in the picture below




tested it in the diode setting of the volt meter and got a reading of 670 one direction and the other direction started around 670 and moved all the way up the scale to stop at 1 (I'm assuming that means max)
tested the nearby diode on the board and got the same readings. a quick look on how to test them leads me to believe it is fine.
should i be concerned about the discoloring? the back of the board looks fine.


the second thing i noticed is what appears to be something missing in the board #2

which lead me to look at the amp pictures where i saw #3

the final thing is that what i think is a small heat sink is melting the board a bit #4

should i be concerned with these?

 

[davenn]

OK thanks
first thing we need to prove is if that large bridge rectifier is the cause
That's the large black block immediately to the right of the yellow wires in pic's 1&2