Fellow Earthlings,
I recently acquired a very nice Threshold S/500 Series II to drive my Magnepan 3.7s, and as far as I know (and the seller knew) it is still all original. Here is my tuneup list:
1. Replace power supply filter caps
2. Replace 3/4 turn bias pots with Bournes 3299, 5K ohm, 15 turn vertical screw
3. Replace bridge rectifiers with KBPC3504 (3506)
4. Upgrade binding posts (just for fun)
5. Replace feet with something more substantial (one broken)
Here's the questions:
1. I've searched google and the DIY site for this but come up empty. There is a large green metallized polyester capacitor on the input board, ERO MKT1813, 10uf, 100v. This is listed as a "DC Film Capacitor, MKT Axial Type"
I haven't seen any reference to anybody replacing this as part of a tuneup. Replace it? Or leave it?
2. Is there anything else that would be worthwhile to attend to while I'm in here? Always looking to learn from other's experience.
Thanks for your time
I recently acquired a very nice Threshold S/500 Series II to drive my Magnepan 3.7s, and as far as I know (and the seller knew) it is still all original. Here is my tuneup list:
1. Replace power supply filter caps
2. Replace 3/4 turn bias pots with Bournes 3299, 5K ohm, 15 turn vertical screw
3. Replace bridge rectifiers with KBPC3504 (3506)
4. Upgrade binding posts (just for fun)
5. Replace feet with something more substantial (one broken)
Here's the questions:
1. I've searched google and the DIY site for this but come up empty. There is a large green metallized polyester capacitor on the input board, ERO MKT1813, 10uf, 100v. This is listed as a "DC Film Capacitor, MKT Axial Type"
I haven't seen any reference to anybody replacing this as part of a tuneup. Replace it? Or leave it?
2. Is there anything else that would be worthwhile to attend to while I'm in here? Always looking to learn from other's experience.
Thanks for your time
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Polyester is known to have higher distortion, better to use polypropylene instead if it fits the board.
You may have trouble finding one that does in axial. Such a heavy amp needs wider feet, too.
You may have trouble finding one that does in axial. Such a heavy amp needs wider feet, too.
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Polyester caps do not get bad with age.
Not like electrolytics do.
Leave that good 10uF in place, this is my advice.
The same goes of course for tantals. Let it be. Don't overdo your repair.
Replacing older electrolyts is the way to go. They have liquid inside. And can dry out.
Not like electrolytics do.
Leave that good 10uF in place, this is my advice.
The same goes of course for tantals. Let it be. Don't overdo your repair.
Replacing older electrolyts is the way to go. They have liquid inside. And can dry out.
With the hugs cost of screw terminal caps, it might be worthwhile investing in a Peak ESR meter to determine condition of those caps. The huge ones do not deteriorate as fast as smaller ones, perhaps because of a smaller ratio of seal area to volume. I have had huge caps go bad with high ESR, but in the 40-50 year timeframe, not 20 years. Uoe of these 50 year old organs was about 2-3 hours a week. Brands were Sangamo and CDE.
I agree, those narrow feet seem a little out of place on an 80 pound amp. I bought some 1 1/2" diameter rubber feet, which I think will distribute the mass a little better on the shelf as well.
Thank you, that is perfect. I'm also coming from a place where I think Nelson knew (and knows!) a whole lot more than me when he designed this, and I don't want to start re-engineering it with different components unless there's a specific reason to (like 15 turn bias pots)
Right, no point in wasting money for no reason. Luckily, I actually own those 4 CDE caps, which I bought for another project less than two years ago. Considering we have reason to believe the Mallorys are in the area of 35 years old, I think it might be more cost efficient for me to install the caps than buy a new meter.
Although... it might be interesting to learn if Threshold used Mallory caps in the power supply, it's possible these were changed out somewhere along the line. I also don't know if we can tell the age of those Mallorys using a date code hidden in the number strings printed on them. (?)
Always something new to learn, hm?
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When I do a google search for "Threshold S/500" and look at Images, there are an awful lot of them that show the same light blue, same height filter capacitors. Of course, I can't read the lettering on the side, so it doesn't prove anything - but I would say there's more reason to believe that the Mallorys that came in the amp are the 35 year old originals.
The organ caps that wouldn't produce 100 watts into 4 ohms were labeled 14000 uf, read ~15000. Old ones .14-16 ESR, new ones .06-.08 ESR. Organs probably get 5600-6000 hours in 30 years. A dedicated user that listens to radio 14 hours a day can put that on an amp in 3 years.
If you have turned the amp on since purchase, the caps are past reforming. If has been sitting 5 years, a little time at 50-80 vac from a variac may prevent an explosion. I've worked on power supplies that sat idle 15 years since organ was salvaged. At turn on caps did not explode.
If you have turned the amp on since purchase, the caps are past reforming. If has been sitting 5 years, a little time at 50-80 vac from a variac may prevent an explosion. I've worked on power supplies that sat idle 15 years since organ was salvaged. At turn on caps did not explode.
Briefly, you charge them through a resistor from a lab supply, and let cook for a while.
There are various preferences for detailed methods. A simple one:
Charge each cap from a power supply equal to their voltage rating through separate 10k @ 5W resistors.
After 15 to 30 minutes, the drop across the resistors should be less than 10 VDC.
(Otherwise it may be bad.) Then allow the cap to cook for an hour or so, and then slowly discharge it.
Do you have a lab supply with at least the same output as the caps' voltage rating?
If not, a Variac would be another option.
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Thanks, Rayma,
I do have a variac, but no lab supply - not sure what that is. (?) But I do appreciate you taking time to explain this!
I do have a variac, but no lab supply - not sure what that is. (?) But I do appreciate you taking time to explain this!
Okay, just did a google search, no need to take up your time. I will peruse that and come back smarter. 🤓
Then the amp itself can be the power supply, if you remove the audio board connections.
Just power the amp sans audio boards from the Variac, with the rectifiers, 10k resistors, and capacitors installed.
Be sure your Variac is fused properly.
Slowly turn the Variac up to 120 VAC in say 10V increments, while checking the voltage drops across the 10k resistors.
Once the resistor drops are only a few volts, turn it up some more.
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Perfect, thank you. Also learned a lot here:
It seems that reforming is a good option for capacitors that have little use, but have been in storage for a while. Not so much for old capacitors that have been used to the point that the electrolyte is depleted. This looks like a great option for me to try with the new CDE capacitors to bring them up to like-new spec before installing them in the amp. They have been sitting for a while since I bought them, and who knows how long before that.
This is really useful information, I never knew I could recondition a capacitor (or that a capacitor even needed to be reconditioned!) before.
It seems that reforming is a good option for capacitors that have little use, but have been in storage for a while. Not so much for old capacitors that have been used to the point that the electrolyte is depleted. This looks like a great option for me to try with the new CDE capacitors to bring them up to like-new spec before installing them in the amp. They have been sitting for a while since I bought them, and who knows how long before that.
This is really useful information, I never knew I could recondition a capacitor (or that a capacitor even needed to be reconditioned!) before.
Yes, any electrolytic cap unused for more than a year should be considered to be a candidate for at least mild reforming.
Many "brand new" ones will qualify.
Many "brand new" ones will qualify.
