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Buzz after recap

zebulon1 said:
I had replaced the filter caps on a 750 and soldered the Output rail wires to soldered ring terminals. It was fine for a few months before it began to buzz.
One lead was not soldered in the group in that ring terminal. Only a slight connection. Enough to work but not enough to keep the AC at bay.
Click to expand...

Hmm... I did solder the wires to crimp connectors but didnt crimp, thinking just soldered thinking that should suffice. Maybe I will re-heat and crimp....thanks!
 
Get a chopstick or non conductive item to prod the solder joints and wires while listening to see if you find a bad solder joint or loose connection.
 
Alright....I'm about to freak out...I just realized that flux Ive been using is a zinc chloride. I must have grabbed the wrong flux puck out of my tool box when I started this project and did not realize it until just now! ...no lables on the lid....I think I may have screwed the boards.....
 
Do you have a picture of the ground bar between the filter caps before you started changing things? You may have altered the carefully constructed star ground (or T ground) by connecting ground wires differently than original. Power supply, transformer, and other high current paths should be connected to the ground bar, small signal grounds (star) should be connected together away from the high current ground, and then one connection to the ground bar made so they are common.

Also zinc chloride is very water soluble, a hot water rinse may well wash it all away.
 
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Thanks Merliski. Yep, I took many pics before and after got it as exact as possible. It saved my but many times on other projects.

Thanks Petes Pick , Zebulon, I found a capacitor solder bleed to a nearby resistor leg on the power supply. I also re soldered a lifted cap trace on the protection assembly. I went back and loosened the ring connectors and re soldered those as well. Then I cleaned both boards with a toothbrush, deoxit then some MAF spray with a new clean toothbrush.

Fired her up....no hum, no buz, and no cutoff when up halfway. Fingers crossed it stays that way. Going to let it run for a while.:music:

I'll keep you posted!

Stay tuned. :bigok:
 
This is why it's best to do some work, test, do some more, test, etc.

Imagine if you'd replaced all the caps and reworked the amplifier boards before checking! Way to many things to go wrong, and it can take forever to find the faults.

With just one change, it's pretty easy to look over what you just did to find errors.

Glad it was something simple. My current project is a Kenwood all the magic smoke came out of, a lot more troubleshooting!
 
Well it was good for a bit, sounded great! Turned it off.
Was buttoning her up and put the bottom pan back on, sat it down on its bottom (it was up on its side) and was about to put the face back on, so I had to switch power back on and mute in order to slide the face on. .....Now there is nothing. No relay "click". Came back later, nothing...

LOL....Oh brother... I have a feeling Im gonna learn this amp one way or another! No magic smoke or anything like HPMGuy, but still not home free yet.

Anyone have that happen before?
 
EvilDingo said:
Well it was good for a bit, sounded great! Turned it off.
Was buttoning her up and put the bottom pan back on, sat it down on its bottom (it was up on its side) and was about to put the face back on, so I had to switch power back on and mute in order to slide the face on. .....Now there is nothing. No relay "click". Came back later, nothing...

LOL....Oh brother... I have a feeling Im gonna learn this amp one way or another! No magic smoke or anything like HPMGuy, but still not home free yet.

Anyone have that happen before?
Click to expand...
Did you put a long screw in place of a short screw when reattaching the bottom plate, maybe short something out?
 
HPMguy said:
This is why it's best to do some work, test, do some more, test, etc.

Imagine if you'd replaced all the caps and reworked the amplifier boards before checking! Way to many things to go wrong, and it can take forever to find the faults.

With just one change, it's pretty easy to look over what you just did to find errors.

Glad it was something simple. My current project is a Kenwood all the magic smoke came out of, a lot more troubleshooting!
Click to expand...

Well I'm back at it... no magic smoke yet.. hopefully I avoid it.

merlynski said:
Did you put a long screw in place of a short screw when reattaching the bottom plate, maybe short something out?
Click to expand...

Nope, all the screws are the same. I checked voltage at the fuses, all good.

I
 
May have had a mains fuse fail too, from old age. Check the fuses, not unheard of for them to fail by cracking from the filament flexing. I have some ancient equipment at work, only time a fuse fails is a crack. Some of it has been around since 1972.
 
Main fuse is reading 75 volts.

Took some readings on the pins for the protection assembly board
1: 20 v
2: -51vdc
3: ground
4: 1.28 vdc
5: -.029 vdc
6: -.052 vdc
7: -.086 vdc
8: -.029 vdc
9: -.35 vdc
10: 72 vdc
 
Do you have a schematic with the correct values on it? I have one line not showing the correct voltage on my KA-80 and am in the process of tracing back from the protection circuit to the amp to see why -- if something is off, likely at failed component or two.

Might want to take a look at the circuit board traces in the power supply -- high heat for decades does bad things to thin copper traces on circuit boards. A cracked trace or a lifted one that makes intermittent contact could result in power while standing on end and not when sitting flat.
 
HPMguy said:
Do you have a schematic with the correct values on it? I have one line not showing the correct voltage on my KA-80 and am in the process of tracing back from the protection circuit to the amp to see why -- if something is off, likely at failed component or two.

Might want to take a look at the circuit board traces in the power supply -- high heat for decades does bad things to thin copper traces on circuit boards. A cracked trace or a lifted one that makes intermittent contact could result in power while standing on end and not when sitting flat.
Click to expand...

Exactly what I was looking. Its pretty toasty in there and has been reworked by the PO, so I am going to take a look again.
 
I don't think the original engineers anticipated we would still be using these amps 40 years on -- most high power amps should have had heat sinks on the large value resistors, and most of them should probably have been higher wattage if they'd been designed for decades of use instead of years.

Power supply transistors can fail too with no obvious external signs.
 
I plugged it back into my DBT, the bulb keeps surging for a second after it sits a while and then goes dim (out). Does not do it if I turn it off, then back on......only after it sits a while. I imagine that is filter caps charging?
 
HPMguy said:
I don't think the original engineers anticipated we would still be using these amps 40 years on -- most high power amps should have had heat sinks on the large value resistors, and most of them should probably have been higher wattage if they'd been designed for decades of use instead of years.

Power supply transistors can fail too with no obvious external signs.
Click to expand...

Yep. good point. I would love to find a used mint AWR156 so I can plan for the next 40 years. I want to hand it down to my son eventually. I think larger heat sinks will be a given on the rebuild.
I'm gonna search posts to see if I can find someone with same problem and possible pins to take readings on.
 
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