Replacing can capacitor on Fisher K-10 spring reverb

[Lee_M]

Hey guys, I'm trying to source replacement electrolytic caps for my Fisher K-10.

Most of them are easy to find, but the 4-part chassis-mounted can capacitor (C5-A, C5-B, C5-C and C5-D on the schematic) is being a bit of a pain.
It's a 40+40+10+10uf...Which seems basically impossible to find an exact replacement for.

I've found a 40+40+20+20uf cap, But I'm not sure if increasing the cap values for C5-C and C5-D (20uf instead of the original 10uf) could cause any trouble for the power transformer or selenium rectifier?

I'm considering replacing the selenium rectifier with a silicon rectifier bridge while I'm in there, But might just leave it "as-is" if increasing the cap values won't cause any issues with it.

If it was mounted in a larger enclosure (anything remotely bigger than a VHS case) I'd consider disconnecting the can, use individual caps on a bit of tag board inside the box and be done with it....But I don't think there's anywhere close to the amount of free internal space that I'd need to pull that off.

For reference (or cloning, if you're inclined...parts list is near the back page of the PDF), I've attached the manual with a full schematic at the rear.

Here's the cap I've got in mind as a replacement...
https://www.amplifiedparts.com/products/C-EC4020X2-525

 

[mjrippe]

Did one recently and I removed the can and mounted the replacements caps on the outside of the chassis.  Put the positive legs in the middle of the hole and run the negatives through the mounting holes for the can.  Solder all the negs together and to the chassis.  Use the pos terminals just like the old ones.  Hot glue or silicone to keep them all in place.

 

[Lee_M]

How neat was the finished result? Do you have any pics?
I've been thinking of putting this unit up for sale once I've recapped it (and taken care of anything else that needs attention), So a neat cosmetic finish is fairly important.

Just out of interest, What was the reasoning for using individual caps rather than a can?
If it was mainly due to the price difference between standard axial caps versus can caps, I'd rather spend the extra bit on the canned type and keep it looking as close-to-original as possible...Providing the increased capacitance for  C5-C and C5-D won't cause issues.

 

[beatnik]

i have one coming soon, was thinking to use a can cap  for the 40+40 section and mounting the smaller caps inside the chassis. i don't see any reason for using a 40$ cap when you can spend 1/4 of that and get the job done

 

[Lee_M]

i have one coming soon, was thinking to use a can cap  for the 40+40 section and mounting the smaller caps inside the chassis. i don't see any reason for using a 40$ cap when you can spend 1/4 of that and get the job done

Which caps are you planning on using for the can and your internal 10uf caps?

Most of the 10uf/350v caps I can find are pretty big and often expensive....Sprague Atoms (Vishay TVA series) especially!
Also, the only current production 40+40uf can cap I've seen was the same price (and brand) as the 40+40+20+20uf that I linked to.

If I can keep the external appearance of the unit original (or as close to original as possible) I'll be happy.

 

[beatnik]

i have been happy with this supplier

http://www.tube-town.net/ttstore/Capacitors/F-T-Capacitors/F-T-32-32-F-500-V-Multisection::404.html

http://www.tube-town.net/ttstore/Capacitors/F-T-Capacitors/F-T-10-F-%40-450-Volt-axial::400.html

 

[Lee_M]

Thanks for the link, Those F&T axials are very reasonably priced! Nice and compact too

I'd actually been planning on using the F&T 50+50uf can caps for replacing some of the various cans in my hammond L100, That thing is full of them!!

Do you think dropping the C5-A and C5-B caps from 40uf to 32uf could cause any audible increase in PSU ripple and noise?

I've been doing a bit of reading around various forums and from what I've read, it seems that increasing capacitance value (within reasonable limits!) is only really a concern with tube rectifiers...SS rectifiers don't seem to care (again, within reason) about increased capacitance.
Am I on the right track here?

 

[beatnik]

you are right - some rectifier tubes have limits for filter capacitors

no problems with solid state rectifiers although in the case of guitar amplifiers excessive filtering can compromise the tone, especially in the low end

in the case of the k-10 i don't think there will be a difference between 32uF and 40uF given also the fact that the tolerances are quite high (-10/+30% on the F&T)

tube town is a nice supplier they have a good selection reasonably priced and shipping to the uk is not too expensive

 

[mjrippe]

How neat was the finished result? Do you have any pics?
I've been thinking of putting this unit up for sale once I've recapped it (and taken care of anything else that needs attention), So a neat cosmetic finish is fairly important.

Just out of interest, What was the reasoning for using individual caps rather than a can?
If it was mainly due to the price difference between standard axial caps versus can caps, I'd rather spend the extra bit on the canned type and keep it looking as close-to-original as possible...Providing the increased capacitance for  C5-C and C5-D won't cause issues.

She was not the prettiest girl at the prom, but she worked just fine    Sorry, no pix.  Did it because cosmetics were not important and single caps are cheaper.  Client loves the sound of it BTW.

 

[Lee_M]

in the case of the k-10 i don't think there will be a difference between 32uF and 40uF given also the fact that the tolerances are quite high (-10/+30% on the F&T)

tube town is a nice supplier they have a good selection reasonably priced and shipping to the uk is not too expensive

Good point.

Thinking about it, If I were to use the F&T caps I'd probably go for the 50+50uf cap instead of the 32+32uf.
More filtering probably isn't a bad thing, given that the rectifier is solid state (so should be able to handle a bit of extra capacitance) and 50uf is well within the sort of tolerances (-10% +50%) that I've seen on a lot of can capacitors.

Thanks again for those links, That tubetown site looks very helpful.
They sell the F&T caps for much cheaper than the other sources I've seen.

Client loves the sound of it BTW.

Yeah, I love mine too! They sound awesome.

I'm thinking of selling my K-10 after the recap, Which is why I'm being so picky about cosmetics.
Then using the cash from the sale to build a tweaked clone of it from scratch, Based on the "Dr. Zee" mods; http://www.mzentertainment.com/studio_workshop_fisher_k10_spacexpander_spring_reverb.html

I'd probably ditch the meter circuit and add a meatier output stage (plagiarised from the LA2A circuit, or something like that) with transformer balanced i/o.

 

[abbey road d enfer]

I've found a 40+40+20+20uf cap, But I'm not sure if increasing the cap values for C5-C and C5-D (20uf instead of the original 10uf) could cause any trouble for the power transformer or selenium rectifier?

Changing the value of the 1st cap could create problems, but increasing the others doesn't present any risk at all.
Replacing the rectifier with modern silcon rectifiers will probably increase B+ by a few percents, which is not an issues, at least not more than the fact that these units have been designed for 115V and now mains are 120V.

 

[Lee_M]

Changing the value of the 1st cap could create problems, but increasing the others doesn't present any risk at all.
[/quote]

Sweet! In that case, I think I'll go for the can cap.

Would the potential risk of increasing the first cap be due to higher inrush current damaging the power transformer on startup?

 

[abbey road d enfer]

Changing the value of the 1st cap could create problems, but increasing the others doesn't present any risk at all.

Sweet! In that case, I think I'll go for the can cap.

Would the potential risk of increasing the first cap be due to higher inrush current damaging the power transformer on startup?
[/quote] I think the rectifier would be the more at risk. Your intention to replace it with silicon is wise. In fact, if you do that, you may have to insert a resistor that would drop some voltage, as the selenium bridge does.

 

[Lee_M]

Thanks for the pointers 
What kind of "ballpark" resistance and wattage would you recommend for the dropping resistor if I install a silicon rectifier?

 

[pucho812]

Replacing can caps is easy, skip the can and just get the four values you need. A can cap is basically x amount of electrolytic caps in one package. They all wire so that you have individual positive connections but the can is the negative and they all tie to it. If it really matters can gut the original can cap and use it as housing for the caps.

 

[abbey road d enfer]

Thanks for the pointers 
What kind of "ballpark" resistance and wattage would you recommend for the dropping resistor if I install a silicon rectifier?

It must be done on the bench. there is no formula (well, there is, but there are too many unknowns (transformer DCR and inductance, rectifier I/V curve...).
First measure the rectified voltage with the selenium rectifier, then after having replaced with the silicon, measure the voltage and add a resistor until the new voltage matches the old one. Then you'll know what voltage is dropped into the resistor and you can calculate the dissipated power. If you experiment with 1/4W resistors, you may blow a few of them. I would think 1W is a minimum.

 

[Lee_M]

Ok, Thanks for the help. It's much appreciated