Issue with SA-7800

[Ryan Walton]

I appreciate all your help. Are all these measurements telling you anything though?

 

[paul_ral]

Take a brush and clean that board. It would be easy to find parts and connections on PCB.

 

[paul_ral]

Okay, so I think I may have found some (4?) diodes in this picture... which one is d27 though? You can also see the fuses at the top of the pic. Also, what is that white rectangle directly below the bottom most fuse with the metal piece on top that says "577" on it? Because it looks like from the schematic d27 should be right about there?

That "rectangle" is VFD filament transformer. It feeds tube filaments with AC voltage (arround 3V). In order to identify components on PCB follow the schematic from test point to near part using your digital multimeter in diode or continuity test mode.

 

[petehall347]

see if this makes sense to you .
http://audiokarma.org/forums/index.php?threads/sa-7800-relay-wont-click.200669/#post-2356408

 

[Markoneswift]

@petehall347 as someone you helped out several times in the past I just wanted to say your input in this thread is awesome. I hope the OP appreciates your considerable help and remembers that a little patience goes a long way

 

[Ryan Walton]

Take a brush and clean that board. It would be easy to find parts and connections on PCB.

I've got some deoxit on the way... anything else you recommend to clean up the board while I wait for that?

 

[Ryan Walton]

@petehall347 as someone you helped out several times in the past I just wanted to say your input in this thread is awesome. I hope the OP appreciates your considerable help and remembers that a little patience goes a long way

I can't thank Pete enough and I do hope he knows that. I've already learned way more than I ever knew and even if we dont get it fixed i'm eternally grateful for the knowledge.

 

[Ryan Walton]

Okay so here are the correct AC readings for the diodes now that i know where they are:

D27 (cathode) - 141.5
D27 (anode) - 47.3
D28 (cathode) - 141.8
D28 (anode) - 47.3
D29 (cathode) - 47.4
D29 (anode) - 0.00
D30 (cathode) - 47.3
D30 (anode) - 0.00 and -0.00 (fluctuates between the 2)

 

[petehall347]

check d30 again ..
D28 (anode) - 47.3 .. tells me d30 should have same voltage as they connect together ..

 

[petehall347]

am also thinking the higher voltages are millivolts . no way they can the be 141.5 volts .

 

[paul_ral]

Clean with soapy water.
Could you post a picture with your DMM readings?

 

[Ryan Walton]

d30 remains the same... d28 anode and d30 cathode are both 47.3v

Here is the setting I have my DMM when testing these diodes

 

[Ryan Walton]

see if this makes sense to you .
http://audiokarma.org/forums/index.php?threads/sa-7800-relay-wont-click.200669/#post-2356408

In this thread, Mark is asking the OP to take DCv of those diodes... why are we taking ac?

 

[petehall347]

In this thread, Mark is asking the OP to take DCv of those diodes... why are we taking ac?

one side AC the other DC . as soon as AC is restored to d30 there is a very good chance the DC voltages will come back ok and the unit will work again .

 

[petehall347]

d30 remains the same... d28 anode and d30 cathode are both 47.3v

Here is the setting I have my DMM when testing these diodes

find where the bad connection is between d30 cathode and fuse 3 .

 

[Ryan Walton]

find where the bad connection is between d30 cathode and fuse 3 .

And how do i do that Pete?

 

[petehall347]

And how do i do that Pete?

with your meter set on low ohms and follow the path from d30 to fuse 3 . mains power off and unplugged .its probably bad solder . your meter will show you where it loses connection .

 

[Ryan Walton]

I'm not good at reading these schematics... they seem to be right next to each other on the highlight path... is this correct? Also do I measure in ohms on the same side i measured the diodes or on the joint side?

 

[Ryan Walton]

Also, which probe do (red or black) do i use for ohms? Do I ground with the black?

 

[petehall347]

you go point to point . hold the probes together to see what reading you get . then you will know what reading to expect between 2 points .
yes that is the correct points you marked out . go on the solder side then you can visually see the copper traces .