Sansui 5000A 31v in speaker outputs, help!

[FerrisBuellr]

Hi there y’all, first post here as I ask the wizards for assistance. Thank you in advance for the help.

I have recently replaced all caps and transistors on my Sansui 5000A. I’ve had a few interesting steps on this journey and am looking forward to adjourning it with a happy receiver.

Brought her home and cleaned controls to the point that I had sound but had noisy transistors. Replaced all of the caps and resistors but the output transistors. Happily had great sound….for two hours at a time until the right channel fuse popped. Checked it and found I had 31v to each of the positive speaker outputs, both left and right for whichever speaker was active at the time. Then replaced the output transistors just in case with no change. Final hint, the dim bulb tester is not bright when in line with the unit and everything is on.

I understand I’m getting line voltage to the outputs and this is dangerous for my speakers until I get this fixed. I have a schematic and the F6103 boards in my receiver. Unfortunately the schematic does not list prescribed voltages across the components for testing and I don’t know where to go from here.

1. Anybody have thoughts or things I could check out that would allow me to solve this?

2. Is there a trick to inspecting and detecting issues on boards without schematics with voltages included?

 

[Hyperion]

The 5000A is a 'capacitor coupled' amplifier, so having a voltage on the output (of such receivers/amplifiers), with no load (i.e. speakers) would be considered normal. However, you say that you have voltage on the outputs WITH speakers connected, which would be a danger sign for a 'direct coupled' amplifier, and could result in severe damage to the connected speakers. No bright DBT bulb with everything connected seems to indicate that things are normal, and the blown fuse could be due to bias current not having been reset?- especially after replacing all the resistors - you don't mention re-setting bias current - this is important.

With speakers disconnected for the time being, you should definitely double check your bias current settings.

2. Is there a trick to inspecting and detecting issues on boards without schematics with voltages included?

No 'trick' you just have to know your way around such a unit, and have sufficient knowledge to know what voltages should be where - a skill usually built up over years of working on these units.

Let us know how you get on.

 

[FerrisBuellr]

The 5000A is a 'capacitor coupled' amplifier, so having a voltage on the output (of such receivers/amplifiers), with no load (i.e. speakers) would be considered normal. However, you say that you have voltage on the outputs WITH speakers connected, which would be a danger sign for a 'direct coupled' amplifier, and could result in severe damage to the connected speakers. No bright DBT bulb with everything connected seems to indicate that things are normal, and the blown fuse could be due to bias current not having been reset?- especially after replacing all the resistors - you don't mention re-setting bias current - this is important.

With speakers disconnected for the time being, you should definitely double check your bias current settings.


No 'trick' you just have to know your way around such a unit, and have sufficient knowledge to know what voltages should be where - a skill usually built up over years of working on these units.

Let us know how you get on.

Thank you sir for your reply, I have been leery about hooking up a set with the voltage high. I’m split between hooking some up and giving it a shot or (probably would be worthwhile anyways) going to find some cheapos to run as test units until I find somewhere to buy a set of dummy loads. I had set my bias current once prior after the rebuild but it’s probably worth double checking and resetting again.

I’d conservatively guess I’m in the youngest 5% of folks on this board and am appreciative of those with experience passing the torch of knowledge.

 

[Hyperion]

You could obtain a couple of 8 or 10Ω (say 2 watt) resistors and use them as a substitute load (instead of speakers) - to see if the voltage drops when you connect them. If the voltage does drop then that settles it, it's normal. (don't play any music with the resistors connected, you'll probably smoke them!).

If the voltage doesn't drop then you very likely have more than usually leaky output capacitors, which need to be replaced.

 

[FerrisBuellr]

Hey there sir, reaching out once more.

Good news, I got a set of resistors and can report the 5000a keeps the dim bulb dim.

I’ve been trying to set the bias however and am finding .40 A across the fuse holder rather than the .023 A that’s prescribed. The bias pots were replaced and are functional yet adjusting them does not change the amperage number. It is the same on both boards. I checked the 5000x service manual and the service bulletin for a procedure specific to the F6013 boards and found it to be the same.

Has a component failed perhaps or am I missing something obvious? I’m testing it with the speakers set to A, dummy loads in place, and fuses removed. Still, a high value and no adjustment ability on either side.

 

[Hyperion]

Providing you are sure you have the correct resistance value trimmers. And if both channels behave the same, the most common reason for this fault is a blown multimeter fuse.

Blowing the fuse is common due to the surge of current to the output stage when the unit is turned on. This surge is normal but is often enough to blow the fuse.

The solution (after replacing the meter fuse), can be to power up the unit with the unit fuse in place, and with multimeter leads connected, power on, then remove the fuse and adjust.

Do not be tempted to increase the current rating of the meter fuse as the accuracy of the meter is affected by the resistance of the fuse. (a function of the fuse current rating).

 

[Bruno Primas]

I'd add one more thing:

If your DC Balance is LOW, adjusting the balance voltage higher will lower your bias and vice versa.