I've poked around the site looking for schematics with no luck but here's my issue ... and i could certainly use some help
The fuse is blown on the left channel.
When i swapped that fuse with the good right channel fuse, the left channel has a hum without any input signal. With an input signal the hum stays the same, and there is no audio at any volume level (beyond the hum).
One interesting thing that i noticed when i cracked open the amp...was that the circuit boards for the left and right are not only different, but the circuits differ also.
This is a unit i purchased in 1980...and has been in a box for most of the last 20 years.
So any thoughts?
i had read that it uses an lm391 op amp. Do those suckers go bad...and if so, are they still available anywhere?
The fuse is blown on the left channel.
When i swapped that fuse with the good right channel fuse, the left channel has a hum without any input signal. With an input signal the hum stays the same, and there is no audio at any volume level (beyond the hum).
One interesting thing that i noticed when i cracked open the amp...was that the circuit boards for the left and right are not only different, but the circuits differ also.
This is a unit i purchased in 1980...and has been in a box for most of the last 20 years.
So any thoughts?
i had read that it uses an lm391 op amp. Do those suckers go bad...and if so, are they still available anywhere?
maybe ultimately that is my problem... :-(
the circuit boards are different...and the circuits on them are also different
i'll have to see if i can document the differences...
but there are different values on some resistors, and there are at least 3 more components on the left side (caps and chokes)
the circuit boards are different...and the circuits on them are also different
i'll have to see if i can document the differences...
but there are different values on some resistors, and there are at least 3 more components on the left side (caps and chokes)
Hi objective1,
The L & R channels should be the same. The extra components you see may simply be some "housekeeping" stuff.
Why not look at National's site and see what they have on the LM391-100. I think that might be the only one in that series left. Anyway, the application notes may give you some good hints. I have often seen commercial equipment be almost identical to the application suggested circuit.
Look here for details, it seems to be discontinued, but distributors may still have stock. Buy a couple while you are at it. I see that both Newark and Digikey indicate 0 stock on these. Mouser doesn't carry National.
-Chris
The L & R channels should be the same. The extra components you see may simply be some "housekeeping" stuff.
Why not look at National's site and see what they have on the LM391-100. I think that might be the only one in that series left. Anyway, the application notes may give you some good hints. I have often seen commercial equipment be almost identical to the application suggested circuit.
Look here for details, it seems to be discontinued, but distributors may still have stock. Buy a couple while you are at it. I see that both Newark and Digikey indicate 0 stock on these. Mouser doesn't carry National.
-Chris
objective1 said:
Maybe you have a Frankenstien (ie someone "fixed" the amp by putting a 2nd gen board in)... there were 2 sets of schema, i picked the cleanest ones, but looking at the 2nd set now it is different...
Here it is.. the one i 1st posted said rev 2, and is dated 11-6-84.
This one is not clear enuff to pull a date.
Both the ones i owned would have pre-dated 1984
dave
Attachments
Thought i'd post a picture.. you can see on the right hand side board how someone has drilled a hole to mount 2 new caps...and in the bottom laft corner...looks like a different value resistor..
What's frustrating is that i bought this amp new, and i honestly don't remember ever getting it serviced...
Whatever... old news now.. time for some new adventure...
any thoughts on how to proceed?...
think maybe i should attempt to recreate the right channel circuit back on the left side board...
btw
i've never mentioned my skillset.
i'm relatively handy with a soldering iron and a multi meter..
but i really don't know anything about circuit design or trouble shooting.. figured i would just try to verify the components on the schematics to try and find the problem..
What's frustrating is that i bought this amp new, and i honestly don't remember ever getting it serviced...
Whatever... old news now.. time for some new adventure...
any thoughts on how to proceed?...
think maybe i should attempt to recreate the right channel circuit back on the left side board...
btw
i've never mentioned my skillset.
i'm relatively handy with a soldering iron and a multi meter..
but i really don't know anything about circuit design or trouble shooting.. figured i would just try to verify the components on the schematics to try and find the problem..
An externally hosted image should be here but it was not working when we last tested it.
Hi objective1,
After that you can approach this in a methodical manner.
-Chris
It isn't uncommon for repairs to be made, and the product resold. This is called "open stock" or "B stock" or "Demo Stock". It should not be sold as a new piece. It could even be called "show stock". Basically all these names alert you that the unit is not in absolute new condition. The actual condition is only limited by how honest the distributor or dealer is. Some dealers really push that line. I have seen one service manager that would basically lie to your face on these matters.
I can't see the colors clearly. Remember that heat can easily change the way the colour bands appear. Measure them with your meter to be sure.
That's hard to say. The first thing you should do is trace the changes on a copy off the schematic first and post it please. There may be an answer in there. You may find that the board was attempted to be modified to the newer standard, or the technician couldn't fix it, so he "made it go". Either way, find out what is going on before you start changing things.
After that you can approach this in a methodical manner.
-Chris
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