Mine don't have the mod, yet! Many of these wonderful receivers have survived from the late 60s and quite a few were brought home from SEA in the late 60s & early 70s. I attribute the survival of mine to not trying to blare music to the neighborhood. Some have said that the long survival of some of these 1040 bds indicates that not all of the components had the same problems but it is better to be safe than . . . . .
Skippy124's excellent instructions for this mod can be found at post #431 of this sticky:
http://www.audiokarma.org/forums/in...ly-asked-and-more.144580/page-22#post-4347793
Don't procrastinate as long as I have.
Ghazzer, we have restored quite a few 5000/A/X. I was quite curious about the problem, ie. catching on fire, overheating, etc. I tend to agree with you about this. If it hasn't caught on fire in 45 years, I don't think it is too likely that it will now.
I have two 5000's personally. I was quite curious about this problem and have experimented a bit. On two units with the original 1040 boards, recapped and gone over but not modified, I tried various scenarios.
1. Just playing in normal use, a lot. No problems
2. Playing loud, bass heavy music. Bias set when warm.
3. Playing loud, bass heavy music. Bias set when cold.
First of all, by later standards, the heat sink area on the 5000 series is small. This means they can get quite hot.
I found that there was a condition, when the bias was set cold, where the unit would basically go into thermal runaway, or what would almost be thermal runaway. Run really hard the heatsinks would get quite hot, and when the loudness was backed down, they would not cool off. The bias current was then quite high and stayed there, because it had been set cold. I would imagine that If I played very loud for a long time with the bias set like this, thermal runaway could occur, and perhaps a burning component at some point.
With the bias set by first setting the bias at almost zero, then running the unit loud, until hot, then setting the bias to factory spec, no quasi thermal runaway condition would ever occur. Now perhaps with some of the original units, and certain components out of spec, or even close to out of spec, this problem occurred. I think all those units have weeded themselves out. I just don't think it is a realistic concern with surviving 1040 board 5000's.
We do the mod if customers want us to, no problem. I haven't done it for two units that I have. One of them plays in a warehouse business that I own a part of, and it gets played really loud for long periods of time. No problems after almost 6 mos. of running.
Just my 2 cents on the subject.
