Not sure what I am going to do with the receiver at this point. It really sounds good and works flawlessly. I understand that the fuse resistors go out of spec and become unreliable over time, but if I understand it right the worse that happens is one fails causing an open circuit and likely putting the receiver into protection?
I'm not clear what the VD1212 problem is. I do have the replacements on hand, but what problem does a failure of one of these cause and why are they prone to failure?
Replacing e-caps is a whole other issue. I understand that they can go out of spec over a long period of time. In my experience (which is limited) e-caps below 10 µF are more likely to be out of spec than larger caps. If an e-cap tests out of spec, it should be replaced. However, I have changed my mind on wholesale re-caps. So many of the caps I have pulled out tested in spec, making me question why I was pulling them out in the first place? I was doing it because the majority opinion is they should be replaced. There is no harm in replacing all e-caps (unless you make a mistake or cause another problem). However, is there really a need? I think this is subjective. My experience completely recapping a Pioneer SA-8100 showed very little change in sound quality after the recap. I learned a lot, but will not do that again.
So, unless I am at risk of potential damage to the 9090 by not replacing anything right away, I am taking a cautious approach. The main issue is I really want to know why I am doing something. Not just doing something because someone says it should be done. I have learned a lot from the great people on AK and I want to keep on learning and be smarter about stereo repair. I have done enough of following advice blindly on my Pioneer gear. Don't want to keep doing that.
Thanks for the tip on the fuses. Agree on the driver board for the 9090, which does not have the "pins" that the later models did. The "pins" are printed on the PCB and insert into a slot, so no way to mess that up.
VD1212 become unstable and unreliable with age and can, and will fail to cause problems in other areas.
Yes E caps go out of spec over a period of time, this is to expected and normal. They have a service life, when that service life has expired, generally about 20 years, then they can no longer be guaranteed to be reliable, depending on many different situations they may last a lot longer but I would not count on that.
There is a need to replace them, your basic meter cannot predict the life of a cap, and whilst it may test okay now, maybe next year, or maybe sooner, you might have some serious problems.
Experience tells me that from the Mid 70's, these caps are getting very close to the end of their life, as units I am working on from the early 70's late 60's are definitely done and should be all changed out.
Example, I am on my second of two AU777's and in both instances all the caps were reading more than a 100% over their rated capacitance, they are also showing a lot of ESR, there were some which the legs were corroded through....now these caps are only about 5 or so years older than the caps in your 9090DB there.
As far as sound quality changing after a recap, it really depends on the design of the amplifier and what caps you use to do the recap with. Some amplifiers show little change, the G series Sansui's show little change as they are mostly direct coupled with little or no Ecaps in the signal path. Something like a AU777 shows a huge difference.....
You are talking a risk not recapping something as old as a 9090DB, they are over 40 years old, twice the service life of the caps, you are on borrowed time.
And lastly, yes well I have just told you why it should be done, but you can do as you wish.....
I have just finished doing two G6700's which both had caps that had Ecap legs eaten by glue, and several Ecaps which were low value/leaky....Now these units were both from 1980....
So go figure....
A 1976 Pioneer SX1250 I did recently had the main 22,000µf caps ALL bulging around the vent, this is the same year as your 9090DB.
The amp is a time bomb, if you're in there, you might as well spend a few dollars on caps and get 'em done.......
But its up to you.....Personally I would be doing it.
Change the VD1212 and the fusible resistors, do not be foolish and think because they are fine now that they will be fine for any great length of time.