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Tuner Trimmer Capacitor Value?

Brivan

Brivan

Well-Known Member
I didn't post this in any particular forum since I'm thinking the values may be relatively close from one receiver brand to another. I have an Onkyo TX-4500 that I suspect as having a defective tuner trimmer capacitor. The trimmer cap I'm referring to, specifically, is T6. This is the trimmer cap that will give you dial position adjustment for stations on the higher end of the dial (an inductor usually handles this role for stations on the lower end).

The tuner was working normally, but the dial position was off slightly on the high end (low end and AM positions were fine), so I attempted to move the trimmer cap, but it felt stuck and took a little persuasion to move but, when I did that, I was never able to regain proper adjustability - I could locate my station a couple megahertz away but the signal was weak and I couldn't get it moved back anywhere near where it was supposed to be. Non-residue contact cleaner and compressed air didn't help.

This is the second TX-4500 I've had this happen to. I didn't realize until this last failure that this appeared to be a common failure point, otherwise I would have left well-enough alone. I have a spare TX-4500 and was able to very carefully remove the trimmer cap from it and install it in one of my other "bad" receivers and get the tuner running normally, again. But I'm short one trimmer cap and am looking for a replacement.

I'm unable to measure the value of the good trimmer, likely because the value is so low. I've got an ide that they are somewhere in the 1pF to 5pF area but am not sure about it. Would anyone have any idea of the value of one of these trimmer caps and, even better, where a close replacement might be sourced? Sprague-Goodman has some in their GKU-series trimmers but they're obsolete. Thanks!
 
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Had them go wonky before. not sure about the earlier trimmer caps used in tuned circuits but it's value could be color coded such as these.
varicaps.jpg
 
Interesting. All three of the trimmer caps I've worked with have a red-dyed screw head. I've attached a photo of one of them. I wonder if they're 0-20pF? Thanks for the input!

Onkyo TX-4500 Tuner Trimmer Capacitor T6.jpg
 
Hyperion said:
What does the Service Manual say it should be?
Click to expand...
That's part of the problem - I haven't been able to find any reference to it in the service manual. I also looked in the service manuals for two other receivers (hoping for at least a ball park estimate) but didn't find a part number or value reference for those , either. The OEM trimmers must have a fairly sizeable range since they're all (regardless of brand) very touchy to adjust. I'd like to get as close as possible - the front end seems to be very particular about how these are set and once the value of these trimmers are changed, the main tuning capacitor trimmers need to be reset, as well.

With the first TX-4500 I had tuning problems with, I ended up trying to retune everything else, not knowing this trimmer cap (T6) was likely bad. That's the one I still need to fix and hopefully, once I get a replacement for it, I can fix all the other dumb tuning adjustments I made trying to work around the problem.
 
You will need to be careful choosing a replacement, don't get caught in the trap where the trimmer's characteristic minimum capacitance is larger than the minimum required. ;)
 
eizner23 said:
A lot of them don't. not many choices available. you may have to experiment. i'd try 0-20pf
Click to expand...
I looked into capacitance substitution boxes, but they don't have low-enough values. It's understandable - when dealing with picofarads, I can't imagine using test leads and not changing the capacitance value. I saw that Amazon sells a kit (like your previous photo) but there were a fair number of negative reviews. I'd like to see if I can find a kit with some better-quality trimmers in it. And, yeah, the 20pF looks as good a place to start as any.
 
Hyperion said:
You will need to be careful choosing a replacement, don't get caught in the trap where the trimmer's characteristic minimum capacitance is larger than the minimum required. ;)
Click to expand...
Going to try to fit a decent quality trimmer kit with some different values I can try.
 
Hi, I'm resurrecting this older thread as I am in the exact same position at the original poster. The tuner was off and as I usually do, just adjust the trim cap on the tuner and as the original poster, it broke. So I can't get it tuned in now and I'm only getting very strong stations and WAY off from there correct position. So, to the original poster, did you ever find a good trim cap that worked? I currently have a 3 - 10pf trim cap. But as you noted, don't know what the correct value that should be used. If you were able to find the correct one, can you please post that value? thanks.
 
Brivan said:
I looked into capacitance substitution boxes, but they don't have low-enough values. It's understandable - when dealing with picofarads, I can't imagine using test leads and not changing the capacitance value. I saw that Amazon sells a kit (like your previous photo) but there were a fair number of negative reviews. I'd like to see if I can find a kit with some better-quality trimmers in it. And, yeah, the 20pF looks as good a place to start as any.
Click to expand...
The stray capacitance added by trying to use a cap sub box in that setting would pretty much throw a wrench into the works, even if the box has such low C values
 
quattroGuy said:
Hi, I'm resurrecting this older thread as I am in the exact same position at the original poster. The tuner was off and as I usually do, just adjust the trim cap on the tuner and as the original poster, it broke. So I can't get it tuned in now and I'm only getting very strong stations and WAY off from there correct position. So, to the original poster, did you ever find a good trim cap that worked? I currently have a 3 - 10pf trim cap. But as you noted, don't know what the correct value that should be used. If you were able to find the correct one, can you please post that value? thanks.
Click to expand...
I think I ended up sourcing another donor unit for the trimmer capacitor. I don't have anything in my notes in regards to a good substitute, so a direct replacement is likely what I used.
 
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