Amber Series 70 issues

[objective1]

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?

[objective1]

guess this kinda got lost in the shuffle....

[planet10]

this will help

dave

[planet10]

.

[objective1]

awesome thanx!....

[objective1]

Planet10....

I was printing out those schematics you so nicely found for me when i thought

"hey i should probably try and get the right channel also...while i'm at it"

so... would you mind terrribly postinig the right side also...

Thanx much :-)

[planet10]

Originally posted by objective1
so... would you mind terrribly postinig the right side also...

What i posted is the right channel.. left channel is identical (or so i assume since left channel schema were not made)

dave

[objective1]

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)

[anatech]

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

[planet10]

Originally posted by objective1
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)

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