Hey Mike,
First off.......no doubt about it, Dave in Chicago is the original 9001 master! If it hadn't been for him and Larry Green there I would be among the many frustrated 9001 owners with various intermittent problems. But, with Dave's help at the beginning and after 10 years of research, experimentation and troubleshooting I was finally able to "discover" some issues in the 9001 that even Dave had never found the answer to. (Which I enjoyed sharing with him by the way
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As to your question about the electrolytic caps. They were (and are) rated at 2000 hours of use, and approx. 15 years of "storage" depending on storage temperature. The 15 years is the maximum rating at the lowest ambient temperature. So, with all 9001s (or any of our other quad gear) being at least 20 years old.......do you really think they can perform like they did when new? The answer is simply no way, and the difference is like night and day. But, as we're writing about something that can only be heard and not easily communicated without direct experience, the question I would ask is "What is your point of reference?" If you have only listened to your 9001, or another 9001 all with 20 + year old caps then that becomes your point of reference. I'm assuming you noticed a difference after Dave did the repairs and mods, so you'd agree that proper calibration and good solder connections do make a difference in the sound and performance. But, ask yourself "why did the calibration change?, and why did the solder joints go bad?"
The answer to the first part of the above question is that the caps are not holding their voltages like when they were new. So, for instance a 25vdc power supply drifts down to 18-20vdc. Some circuits can be calibrated to compensate, but many are fixed and non-adjustable.....especially the lower voltages such as 15vdc, 5vdc, etc. But, the real damage is done to more critical components than the caps. When the voltages are low......the signal current has to increase to compensate (look up Ohms Law for reference). The increased current creates more heat being generated by the resistors, overtaxes the transistors and diodes and in the final stages causes these components or the caps themselves to fail. Then you KNOW you have a problem as the circuit will not work at all.
Remember the burned out lamps you mentioned? Why do you think they burned out...........low supply voltage = increased current draw = burned filament in the bulb! The damage isn't done overnight, but it occurs slowly over time. It's interesting to me that of the over 40 restorations I've done I've never heard of a lamp or bulb blowing in a "restored" receiver! Like the other components...they won't last forever, but their life expectency if supplied with the proper voltage and current is well beyond our lifetimes!
As DC filters, the caps keep unwanted audible hums/pops/etc. out of the signal path. The audible result is that your 20yr old caps "squeeze" the signal into the midrange frequencies. So, the highs and lows go.......its all midrange and not very dynamic. Also, instead of cleanly passing a signal through....the output waveform is lower in level and not as clean and crisp as the input. Does it still make sound? Of course! Can it still sound good......only if you've never been able to A/B between a restored unit and a "perfectly working 20 + yr old unit" If you did......the difference would be quite obvious, regardless of our ever decreasing hearing ability!
So, just doing repairs to the solder joints or replacing the other components that have failed along with the obviously failing caps is strictly a band-aid approach. You've treated the symptoms.......but still haven't cured the root cause
.......its a 20yr old receiver!
Electronically speaking the electrolytic capacitor is the achilles heel of modern electronics. Unfortunately, as of yet we haven't discovered any other component that will do what they do for less $$. So, they are still used throughout our modern electronics and they are the "weak link" in everything from controlling voltages to filtering out unwanted noise or signals in everything we use that is electronic.
As to whether to do a restoration now, or wait until the problems become more obvious......is an easy question to answer. If the unit is "in use", then the weak/old caps are slowly but surely damaging all the rest of the components in the unit. Transistors will become weak themselves, resistors will change value, and lamp filaments will become brittle from the overcurrent condition. Recapping a unit like this will stop the degradation...but can't undue the damage already done to the other components.
So, the best results are obtained by restoring a unit that is "new in the box". In this case, the only components degraded are the caps from storage. The other components are in fact virtually "new" as they've never been run under an over-current condition.
You didn't mention how long you've owned your 9001s or 8001. But, I think the point is proven by those that have owned their units since new or for many, many years at the very least. Some of them have posted their thoughts here, but I've yet to hear any of them say they couldn't immediately tell the difference!
My own first two 9001s were restored circuit board by circuit board over the course of about 3 years. And it became an addiction.......as each board was completed and I listened, I was amazed at the improvement. I did the decoder boards first, the phono-preamp/CD-4 board next, then the pre-amp section and the 1st stage of the amp and finally the main power supply and ancillary pcbs. But, taking it apart and putting it back together again and again isn't very efficient and in doing units for people all over the country would make UPS stockholders rich!<img src=http://www.ezboard.com/intl/aenglish/images/emoticons/embarassed.gif ALT="
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So, the true cure is to do it once, do it right, replace all 264 electrolytic caps......then enjoy the hell out of it knowing that it can't sound any better, and when you turn it on it will work right.......everytime!
QuadBob
