I will try to make this walkthrough as clear and concise as one with a disorganized mind and a very short attention span possibly could. I will do what I can to answer any questions.
I have this SA-7800 and TX-6800 that I took in on a trade a couple years ago and left sitting on the shelf until the other day. Regular yard sale cast off items if ever there were.
So, I drag it down and have a little look. It seems that some time ago, I had pulled off the top cover and blew out the inch or so of sawdust that it had accumulated while apparently living in a cabinet shop, but nothing else.
As per standard procedure, I plugged it in to my handy DBT, a simple, cheap piece of gear that is pretty much mandatory if are going to fiddle with old stereos.
And nothing. After a couple of preliminary checks (fuses), it was determined that the power switch was bad.
For now, we bypass....
It came on, and the relay clicked. Good news! Throughput was good, typical dropouts from dirty switches, but solid on both channels.
Time to pull the covers and inspect.
Someone has been here before.
This was the only easily observable repair, but looking at the board there had been other failures.
For now, the amp is working, so we're going to start at the beginning and work from there. This is kind of a board by board walkthrough of the bare minimum of what needs to be done on any vintage piece, and this one in particular.
The first thing is the regulated power supply.
On this one there are two regulator transistors on heat sinks. They need to be removed, tested, cleaned (if good) have new heatsink compound applied, and then re-installed. They can also be replaced if you feel like it, with little no danger of installing a fault condition.
Since the the ones in this amp had very corroded leads, they were replaced. Also replaced were some small capacitors that were in close proximity and obviously cooked.
Everything else was left alone. The entire board was scrutinized for cracked solder joints, which were re-flowed with brush on flux and a good hot iron. If they won't reflow, you will have to remove the old solder first, then re-solder.
Afterwards, the board is washed on both sides with isopropyl alcohol (at least 91%) and patted dry with denim and allowed to fully air dry.
Test board (on DBT always!) and verify correct function before proceeding. And while we're at it, let's double check that ground.
Now on to that amp board.
Note that there are two pair of trimmers on the board. Neither of them is for DC offset, which is not adjustable on this amp.
There two very important things to do before you ever touch a trimpot in an amp circuit.
1) Read the service manual and find out what it does.
2) This is critical--have it plugged in on the DBT. On many of these vintage amps, if anything in the bias string goes open for even a millisecond, such as a dirty pot cutting out, it will smoke the outputs faster than a fuse can open.
This is a non-switching amplifier (do a search), the 100 ohm pot sets the main bias, the 68K pot sets up current flow for the non-switching circuit.
The 100 ohm MUST be replaced immediately, not optional. Set the thing aside until you can procure some new Bourns sealed trimmers. The 68 should also be replaced as well, but is slightly less critical.
Since I had the 100s on hand, the 68K are still in the pics so that I could proceed.
To be continued.....
I have this SA-7800 and TX-6800 that I took in on a trade a couple years ago and left sitting on the shelf until the other day. Regular yard sale cast off items if ever there were.
So, I drag it down and have a little look. It seems that some time ago, I had pulled off the top cover and blew out the inch or so of sawdust that it had accumulated while apparently living in a cabinet shop, but nothing else.
As per standard procedure, I plugged it in to my handy DBT, a simple, cheap piece of gear that is pretty much mandatory if are going to fiddle with old stereos.
And nothing. After a couple of preliminary checks (fuses), it was determined that the power switch was bad.
For now, we bypass....
It came on, and the relay clicked. Good news! Throughput was good, typical dropouts from dirty switches, but solid on both channels.
Time to pull the covers and inspect.
Someone has been here before.
This was the only easily observable repair, but looking at the board there had been other failures.
For now, the amp is working, so we're going to start at the beginning and work from there. This is kind of a board by board walkthrough of the bare minimum of what needs to be done on any vintage piece, and this one in particular.
The first thing is the regulated power supply.
On this one there are two regulator transistors on heat sinks. They need to be removed, tested, cleaned (if good) have new heatsink compound applied, and then re-installed. They can also be replaced if you feel like it, with little no danger of installing a fault condition.
Since the the ones in this amp had very corroded leads, they were replaced. Also replaced were some small capacitors that were in close proximity and obviously cooked.
Everything else was left alone. The entire board was scrutinized for cracked solder joints, which were re-flowed with brush on flux and a good hot iron. If they won't reflow, you will have to remove the old solder first, then re-solder.
Afterwards, the board is washed on both sides with isopropyl alcohol (at least 91%) and patted dry with denim and allowed to fully air dry.
Test board (on DBT always!) and verify correct function before proceeding. And while we're at it, let's double check that ground.
Now on to that amp board.
Note that there are two pair of trimmers on the board. Neither of them is for DC offset, which is not adjustable on this amp.
There two very important things to do before you ever touch a trimpot in an amp circuit.
1) Read the service manual and find out what it does.
2) This is critical--have it plugged in on the DBT. On many of these vintage amps, if anything in the bias string goes open for even a millisecond, such as a dirty pot cutting out, it will smoke the outputs faster than a fuse can open.
This is a non-switching amplifier (do a search), the 100 ohm pot sets the main bias, the 68K pot sets up current flow for the non-switching circuit.
The 100 ohm MUST be replaced immediately, not optional. Set the thing aside until you can procure some new Bourns sealed trimmers. The 68 should also be replaced as well, but is slightly less critical.
Since I had the 100s on hand, the 68K are still in the pics so that I could proceed.
To be continued.....
Last edited:
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