Filter Cap Replacement - 1984 Mark II C+

Pre Recto days. Boogie modified Fenders, Mark I-IV, Dual Calibers, etc

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DrewV
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Post by DrewV » Tue Jul 25, 2006 4:02 pm

Thanks Stokes.. There is a large 1 or 2 watt 680 ohm resistor that was almost touching the cap. Also the cap is darker brown on the end closest to the resistor. The boards are all cleaned up, I've checked most of the resistors, cailubed the pots, deoxited the jacks....Now it's time to relax a little and wait for delivery! Boogie On.........dv

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DrewV
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Post by DrewV » Tue Jul 25, 2006 4:08 pm

:D side note: 2 of the 3- 30uf/500 caps on the psb are seriously bulging on the + end... Boogie On... dv

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Post by stokes » Tue Jul 25, 2006 4:57 pm

Mine doesnt have that cap.I see the 680ohm resistor but I dont have a cap near mine except for the 470mf electro.Sounds like those 30mf's are about to blow.

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Post by DrewV » Tue Jul 25, 2006 6:09 pm

Let 'em blow, I pulled them out of the amp already! I'm not sure about a couple resistors: (1/4 0r 1/2 watt) on the psb. one is supposed to read 22k (red/red/orange). It's reads 5.17k. Also the 6.8k (blue/silver/red) resistor right next to it ( they are connected on one end) is reading the same 5.17k They look good and don't test to be shorted. Any ideas? ..Boogie...dv

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Post by stokes » Tue Jul 25, 2006 6:40 pm

Those are your bias supply resistors.Mine are different because I rewired mine to make it adjustable.As far as the values,when you try to read resistors "in circuit"your readings are not always right since they are connected to caps.If you want to read values you must lift one end of the component in question,or your results will not be accurate.I wouldnt worry about the bias resistors,chances of them goin south is very slim.I happen to have my amp on the variac right now cooking the new caps.A couple of hours more and I'll be done for another ten years-with this one.Between me and my 2 sons we have 14 amps in the basement,keeps me busy!

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Post by DrewV » Tue Jul 25, 2006 7:26 pm

I had an idea that they were in circuit. I'm going to leave them alone..I buy my 6L6's from Boogie anyway. 14 amps! that"s gotta be close to being in HEAVEN!!! The FedX man just dropped off the caps I ordered from Mouser YESTERDAY! Time to heat up the iron. thanks...Boogie On...dv

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Post by DrewV » Tue Jul 25, 2006 10:14 pm

UhOh...The 2- 47uf63V caps on the psb are high temp capsrated at 105*. the replacements I got are rated at 85*. should I stick with what I've got or order the high temp caps? thanks ... dv

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Post by stokes » Tue Jul 25, 2006 11:34 pm

Dont sweat it the 85 deg.-its 85deg celsius-like 200deg farenheit.They'll never get that hot anyway,theres no appreciable current in the bias supply.You do have a variac to form the new caps,right?If not I can tell you another way to do it.Email me if you need the info.

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Post by Tuna141 » Wed Jul 26, 2006 9:15 am

Stokes,

Sorry to jump in...I have a variac, but have never "formed" electrolytic caps - can you explain how this is done? (for future reference)

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Post by stokes » Wed Jul 26, 2006 10:57 am

Let me start by saying this is the way I learned to do it.There are others who do it differently and I am sure they will jump in with their techniques,which I am sure work fine also.I take all the tubes out first, if you have a tube rectifier you will have to sub a SS rectifier here.If you dont have a plug in sub,simply wire in two 1N4007 diodes across the tube socket.Plug the amp with the new caps installed into the variac,start with the variac at 40.Leave it for an hour or more and slowly turn the variac up to 50.Raise the variac by 10 every hour or or so till you reach the point where the DC voltage on your caps get close to the rated voltage. If the caps are rated for 450vdc stop at about 400v and leave it for a couple of hours.Keep a close watch on the volts going to the caps because with no load the voltage could pass the dc rating of the cap.Should take 8-10 hours to complete but you will have nicely formed caps.I use the same technique before firing up old amps that havent been used in a long time to reform the old caps before putting full voltage on them.Always drain the old caps before trying to remove them.I made that mistake many years ago and will never forget again-it hurts!

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Post by DrewV » Wed Jul 26, 2006 12:34 pm

Stokes, please don't think that I am dissing you,.You are totally knowledgeable on the subject of guitar amps and have way more knowledge than I do. I truly appreciate the info you have presented in this forum. Regarding the question of forming the filter caps, here is what Jon from thetubestore.com had to say when I emailed him about forming the caps I was buying from him: "They are ready to go. Simply install" . After doing some net research, it seems that the idea of forming caps is valid in 2 circumstances: 1) to paraphrase Stokes, ' when the amp is old and hasn't been fired up for a while' and 2) when the caps you are installing are VOS caps. That's my 2 cents!! Anyone else want to chime in? and thanks again Stokes for the latest info re high temp caps....Boogie On...dv

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Post by stokes » Wed Jul 26, 2006 1:34 pm

By VOS I assume you mean very old stock.Do you know when the caps you are buying were manufactured?How long they were sitting on the sellers shelf?I dont take chances,having seen what they do when they explode,if you are lucky they only make a mess of the inside of the chassis which is no fun to clean up,if you can clean it all.Before they explode they draw major current,which is no good for every component in the amp.Some suppliers do sell pre-formed caps,Kendrick is one I know of.Dont know if the tubestore does,I would ask before assuming.As for VOS caps,if I know they are old I dont use them,I dont buy caps from ebay or flea markets if I have any doubt about a cap I would throw it out,before I even think about putting it in my or someone elses amp.Your amp is worth over $2,000-if you want to take that chance... I am sure 9 out of 10 times you can get away with not forming the caps,odds may even be better but I dont want to be the one who fries someones equipment.The "damage" may not be imediate,but caps that are properly formed will outlast ones that are put in and fired up cold.I am sure there are alot of people who will disagree,and I aint saying I am 100% right,but it is the way I was taught and it is a good insurance policy.Like I said before, I change filter caps after 10 yrs.,usually sooner,thats just me.I dont like when they pop.Even if the caps seem okay I still notice an improvement after a cap job.

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Post by DrewV » Wed Jul 26, 2006 2:13 pm

Yeah, once again your points, "Do you know when the caps you are buying were manufactured?How long they were sitting on the sellers shelf?" are TOTALLY VALID. Your method is failsafe. I don't have a vari-ac, tho I could probably take the amp into one of several music/repair shops here in Austin and have them do it. Does the other method you mentioned involve putting a 100watt light bulb in series with the ac? Boogie on..dv

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Post by DrewV » Wed Jul 26, 2006 3:26 pm

Here's a link that supports Stokes' position: http://www.harpamps.com/micKcaps/Formin ... itors.html
Boogie On...dv

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Post by stokes » Wed Jul 26, 2006 4:01 pm

Drew,
Yes the other method involves the light bulb in series,but you have to leave it on this way for a day or two.I have never done it this way but there are some who swear by it.I would think it is better than not doing anything.Even fresh,brand new caps have to be formed to function efficiently.I read about the light bulb/series procedure somewhere.I believe you have to monitor either the current across one of the power resistors in the supply rail or the voltage as it rises.I am not sure offhand but I can dig out some books and look it up for you.The light bulb in series apperatus is easy and cheap to make,and is very handy if you ever have a problem with fuses blowing.If there is a short in the amp the bulb will glow bright but not blow the fuse so you can pull power tubes and such till the bulb dims.....getting off topic here.Let me know if you need the info on the bulb/series procedure.

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