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newbie with filter cap ? [message #19284]

Mon, 02 September 2013 09:32

smackoj is currently offline

smackoj
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hi group: I just got a K200B head and the hum is way too much. I checked the main BIG Mallory filter caps and one has juice, and the other one does not. The wiring looks like some back yd tech was moving things around. Does anyone have a schem or layout or close up pic of how the wiring for the 2 main filter caps should be?

thanks amigos, smacko jack

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Re: newbie with filter cap ? [message #19286 is a reply to message #19284]

Mon, 02 September 2013 12:06

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Welcome to the place. If you go to the Technical section of this site, you will find schematics for many of the K200B series amps. All of the power supplies are wired exactly the same, so it shouldn't be too hard to find one for your amp.

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Re: newbie with filter cap ? [message #19287 is a reply to message #19286]

Mon, 02 September 2013 13:12

smackoj is currently offline

smackoj
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thanks CB, I finally found the schem with the 2 big filter caps. took a while but click enuf buttons and shazam, there it was and she's purrin' like a kitten at idle. however, now there is some kind of distorted sound on the guitar notes when any kind of gain used? sounds a lot like a cold solder joint or a bad connection somewhere? more t-shooting in store before she plays like she did in '68.

smacko jack

[Updated on: Mon, 02 September 2013 14:04]

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Re: newbie with filter cap ? [message #19290 is a reply to message #19284]

Tue, 03 September 2013 06:38

stevem is currently offline

stevem
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This distrotion you talk about,do you hear it mainly at the end of a note being played and not on the initial attack of the note?
If so, check and see on the under side of the amp chassis that all the 4 output transistors are RCA brand, if one or more is didfferent your distortion could be whats called crossover distortion and this takes place due to mismatched output transistors.
Since you said it looked like someone has done shabby work on the amp, this could be the cause.
It will aslo help out all the filters (electrolytic types) if you leave the amp on over night.
Non use is what condems these filters.

[Updated on: Tue, 03 September 2013 06:39]

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Re: newbie with filter cap ? [message #19293 is a reply to message #19290]

Tue, 03 September 2013 09:09

smackoj is currently offline

smackoj
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thx Steve; yes, kind of a crackly breakup at the end of the note. I'll check the trans when I get home from work. I did briefly 'chopstick' the components looking for bad solder etc, and got a pretty bad howl out of the positive side Mallory filter cap. I'm going to do some more of that poking stuff too when I get back at it.

gracias, smacko jack

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Re: newbie with filter cap ? [message #19301 is a reply to message #19284]

Wed, 04 September 2013 06:33

stevem is currently offline

stevem
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I posted nothing about a transfomer, I wnated to know if the 4 output transistors where orignal RCA`s?
If you got a hum/buzz from probing around by the main power power filters than make sure that all the screw connection points are tight on them and also the the crimped on ring ends are still good.
Loosing one side of the power supply rail can harm the amp.

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Re: newbie with filter cap ? [message #19302 is a reply to message #19301]

Wed, 04 September 2013 08:49

smackoj is currently offline

smackoj
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sorry for the poor wording. i meant check the transistors not the PT. trannies are all the same name and number. i re-did the post rectifier wiring again and i have it playing right again except for still having way too much hum and it sounds like 120hz. so, when i get another hour to spare, i'll try double checking all the screw terminals etc.

thanks again amigo, jack

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Re: newbie with filter cap ? [message #19311 is a reply to message #19302]

Wed, 04 September 2013 21:24

smackoj is currently offline

smackoj
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update on the bad hum problem. I re-connected all the wires for the 2 Mallory filter caps. I ran a new ground wire from the neg. post of the cap to a different location on the chassis. These changes reduced the hum maybe 25%. So, I'm on the right path but not all the way to the finish line on this K200.

jack

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Re: newbie with filter cap ? [message #19322 is a reply to message #19284]

Thu, 05 September 2013 07:39

stevem is currently offline

stevem
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At this point after checking that the ground off of the round rectifier bridge is good, you need to face the fact that the two main filters are DONE!
Some of these hang in there way longer than 40+ years, but in a amp that did not see much use, or any use this late in its life the filters must go to get the 120 hz hum out.
Mouser electronics sells Sprague brand replacement/up grade part number 3GDY103F100CB2A, It a 10,000 mfd 100 volt filter can.
Mouser also carrys a Mallory brand with the same specs that is a 1 inch taller and works in amps with out reverb, but you may need to extend some of the wires to the filters.
At any rate, get those two orignal filters into the trash!

[Updated on: Thu, 05 September 2013 07:40]

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Re: newbie with filter cap ? [message #19324 is a reply to message #19284]

Thu, 05 September 2013 07:50

carlc is currently offline

carlc
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satevem,

I looked up that part # at Mouser and it didn't find anything. Also looked up the specs you gave and didn't find anything. Looked at Sprague

[Updated on: Thu, 05 September 2013 07:51]

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Re: newbie with filter cap ? [message #19325 is a reply to message #19284]

Thu, 05 September 2013 08:38

stevem is currently offline

stevem
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Sorry, these part numbers are from a 3 year old catalog, I would call them up and or go on there web site.
The can filters, (or Computor grade filters as they are sometimes called) you need are 2 1/2" by 3.125", in a amp with out reverb you can go to the 4.125" tall filters.
You want to step up to the 10,000 mfd at 100 volt filters.

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Re: newbie with filter cap ? [message #19328 is a reply to message #19284]

Thu, 05 September 2013 09:04

carlc is currently offline

carlc
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Looked again...
the part number is 75-36DY103F100CB2A, but yikes! $65.54 each!

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Re: newbie with filter cap ? [message #19329 is a reply to message #19284]

Thu, 05 September 2013 09:27

stevem is currently offline

stevem
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Yup, they are not cheap these days!
Another brand and or suppyer, like Newark one might be somewhat cheaper.
These are one item you do not want to surplus stock on.

[Updated on: Thu, 05 September 2013 09:27]

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Re: newbie with filter cap ? [message #19330 is a reply to message #19329]

Thu, 05 September 2013 09:47

pleat is currently offline

pleat
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I have a pair of the Sprague 4500-50V DE caps that I took out of a Kustom III monitor amp. I needed the power transformer as a donor for a K150-8 amp. PM me if anyone is interested in saving some money on new one's.
pleat

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Re: newbie with filter cap ? [message #19331 is a reply to message #19330]

Thu, 05 September 2013 11:08

smackoj is currently offline

smackoj
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thanks for the helpful info amigos. i have a couple sprague filter caps on the way so should only be a couple days until I experience the Kustom tone you all enjoy.

jack

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Re: newbie with filter cap ? [message #19333 is a reply to message #19284]

Thu, 05 September 2013 13:14

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

All of the Kustom amps have had the ground wiring routed to reduce the ground loop hums. If the wiring to the filter caps has been changed from the original factory wiring, that alone may be the cause of your problem.

The fast test for any Kustom main filter cap, is to parallel a known good cap of equal value and ratings across the original one. If the hum is reduced then the filter cap is suspect, if the hum doesn't change then changing the cap is probably not needed.

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Re: newbie with filter cap ? [message #19337 is a reply to message #19333]

Thu, 05 September 2013 17:58

smackoj is currently offline

smackoj
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I appreciate the help. and yes, the wiring was moved by a previous owner, tech or backyard mechanic. I don't have a reference source for how they came from the factory though. The amp is a 1968 K200B-4.

I have a new set of filter caps I can jump across these old Mallories and I'll give an update asap.

ps. to Steve; the 'round rectifier' is gone and a newer square one is in place. that seems to be the reason that someone moved wires around before but other things could have happened and I wouldn't know it?

[Updated on: Thu, 05 September 2013 18:43]

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Re: newbie with filter cap ? [message #19338 is a reply to message #19337]

Thu, 05 September 2013 19:49

smackoj is currently offline

smackoj
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update on the hum problem; tried running new caps in parallel with old Mallories and got no change. then I tried 3 or 4 more wiring changes and got a 20% reduction in hum but it's still too loud and there is obviously some type of bad connection, cold solder joint, bad ground or dirty contacts in the circuit somewhere? I press on towards the goal!

thanks amigos, jack

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Re: newbie with filter cap ? [message #19344 is a reply to message #19284]

Fri, 06 September 2013 02:45

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Please describe your wiring from the power transformer secondary to the rectifier and from the rectifier to the filter caps. As I remember it the important grounds are the center tap of the secondary and the speaker output ground wires.

This weekend I will pull out one of my K200B chassis' and detail the original wiring for you. Maybe it will help.

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Re: newbie with filter cap ? [message #19345 is a reply to message #19344]

Fri, 06 September 2013 04:11

smackoj is currently offline

smackoj
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Thx CB; before I rec'd this head, someone cut the two PT black wires and spliced them back together with shrink wrap. Is this PT a replacement? I don't know for sure although a second look at the mounts on the bottom of the PT shows that there were washers and probably bolts and nuts used orig. with this PT. pop rivets were then used to install this PT into this amp. the two prong plug and wire was removed and a three prong with ground wire was added. the orig. rectifier was removed and a newer square type installed.

Wiring; the grey CT wire goes to a grnd bolted to the bottom of the chassis. the grn wire from the power cable goes to the same spot on the chassis. the blk wire from the neg side of the output jacks goes to a screw on the back wall of the chassis. the pos terminal from the 2nd filter cap to ground goes to a bolt on the back wall of the chassis.

I'll post what I have so far and then add more description about how it's wired later on today. thanks

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Re: newbie with filter cap ? [message #19346 is a reply to message #19284]

Fri, 06 September 2013 06:34

stevem is currently offline

stevem
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Pull out a volt meter and hook it across the amps speaker output with no speaker hooked up, do you read any DC voltage, or just low AC ripple?

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Re: newbie with filter cap ? [message #19347 is a reply to message #19346]

Fri, 06 September 2013 08:19

smackoj is currently offline

smackoj
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will do steve. soon as i get home from work later today.

thx, jack

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Re: newbie with filter cap ? [message #19351 is a reply to message #19284]

Fri, 06 September 2013 13:55

chicagobill
Messages: 2005
Registered: April 2003

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Original transformers were pop riveted into the chassis. This doesn't mean that your transformer hasn't been replaced.

Off the top of my head, it sounds like too many grounding points. I won't know for sure until I pull out a chassis, but I think that the CT of the secondary has to go directly to the filter caps. Same with the ground wire from the output jacks. The entire power supply grounds to the chassis at the output jacks.

Check the date code on the transformer and compare it with the date codes on the pots. That may give you some insight as to the originality of the transformer.

Steve's suggestion of checking for dc on the output is a good one.

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Re: newbie with filter cap ? [message #19353 is a reply to message #19351]

Fri, 06 September 2013 17:36

smackoj is currently offline

smackoj
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greetings amigos; the meter reading from the positive post of the output jacks is a negative .23 volts dc. -.23 vdc. when I switched the meter to AC I read plus .11 vac. +.11 vac.

I am reasonably sure that the pot dates match the transformer date which suggests 1968. The serial number also contains an 8 in it.

thanks again for all the help.

jack d

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Re: newbie with filter cap ? [message #19355 is a reply to message #19345]

Fri, 06 September 2013 19:21

pleat is currently offline

pleat
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As I always say, I'm not a tech, but the two black wires from the PT that have spliced together. Shouldn't they go to the thermister that is mounted on the power transistors heat sink?
I do have some photo's of the inside and how the wires were ran from the factory. PM me.
pleat

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Re: newbie with filter cap ? [message #19359 is a reply to message #19284]

Sat, 07 September 2013 02:29

chicagobill
Messages: 2005
Registered: April 2003

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Pleat, you may be right but we need to know what two wires have been cut and spliced.

What are the pot codes and the transformer code? As for the serial number, search the board here and find the serial number date list that Les Strickland posted. The list will give you an approximate date that the amp was produced.

I checked my K200B-1 and this is how the power supply secondary is wired. The two purple secondary wires connect directly to the AC inputs of the bridge rectifier.

The positive out from the bridge goes to the positive terminal of one filter cap. The negative out goes to the negative terminal of the second filter cap.

The gray secondary center tap wire goes to the negative terminal of the positive side filter cap. There is a black wire from the same negative terminal that grounds to the chassis at the speaker output jack. Finally there is a short black wire from that same negative terminal that connects to the positive terminal of the negative side filter cap.

So the entire power supply single point grounds to the positive filter cap's negative terminal and that terminal point grounds to the chassis at the speaker output jack.

A technical point here, the green ground wire from the ac cord should be (by code) connected directly to the chassis to its own screw terminal. Not a common point or to a mounting screw for something else. The green wire should also be longer than either the hot or neutral wires (black and white), so if the cord should be pulled out of the amp, the green wire will stay connected to the chassis after either the black or white wire has been pulled loose.

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Re: newbie with filter cap ? [message #19364 is a reply to message #19359]

Sat, 07 September 2013 08:18

smackoj is currently offline

smackoj
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ok, thanks mucho hermanos. I have a picture of the wiring that pleat so kindly sent and it all matches with CB's good description of the layout.

I'll do the changes and report back to everyone.

hasta la vista, smacko

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Re: newbie with filter cap ? [message #19366 is a reply to message #19364]

Sat, 07 September 2013 09:24

smackoj is currently offline

smackoj
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ok; changes made to wiring to copy factory, NO chg in hum. I am now looking at two possibles;
1. the pwr switch is bad?

2. having the reverb pan disconnected creates a hum?

I took the reverb pan off first thing and have done all my tests without it hooked to the amp.

thanks, smacko

ps. OOPs 3 poss.

3. the pilot light does not have a bulb in it? waiting for the 28 volt ones to get here via mail.

[Updated on: Sat, 07 September 2013 09:27]

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Re: newbie with filter cap ? [message #19367 is a reply to message #19366]

Sat, 07 September 2013 10:21

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further tests: with no input signal and spkr connected, humming gets much louder by raising vol on the right side channel aka pc403. adding vol. to other channel, pc303 does not add any hum.

power tran code 1005836

pot code 304813

ser. num. 27851

gracias amigos

[Updated on: Sat, 07 September 2013 10:27]

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Re: newbie with filter cap ? [message #19368 is a reply to message #19367]

Sat, 07 September 2013 11:49

pleat is currently offline

pleat
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Location: Belding, Mi

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PC403 is the Selective Boost/Clipper circuit board, so I am assuming you have the K200B-4 amp.
I don't think the power switch would cause the hum. They either work or don't work in general.
I have a K200B-2 head that had excessive hum in the right channel with no instrument plugged in. The hum followed the volume control, turn up the volume the hum increased.
Chicago Bill suggested that the input jacks might need cleaning of the shorting tab of the plug and to make sure that the jacks were making a good ground to the chassis. Cleaned the contacts and rotated the input jacks a little and tightened them back down. Hum went away.

I did have a K50 amp that took on a lot of hum with the wrong pilot lamp. I had used a 12V I had on hand and got some hum. I know the circuits are different but just tossing out some suggestions.
pleat

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Re: newbie with filter cap ? [message #19369 is a reply to message #19368]

Sat, 07 September 2013 12:07

smackoj is currently offline

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thanks pleat; I did try pulling the output jacks and cleaning those 2 contacts with no change. I will now try the 4 input jacks and see if I can get some relief? I did try moving them around but haven't tried pulling them off. that's next.

correct it's a 200B-4

adios for now.

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Re: newbie with filter cap ? [message #19370 is a reply to message #19284]

Sat, 07 September 2013 12:25

chicagobill
Messages: 2005
Registered: April 2003

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Pleat is right, the switch can not cause your hum.

Now you tell us that the hum is changed by the controls on the right channel. If that's the case, then there may be a problem with the grounding connection or with the board itself. Maybe a bad cap or transistor. The quick test is to unhook the blue output wire that connects the board to the power amp board. If the hum goes away then you have found your problem.

If you turn down the reverb control or turn off the reverb with the foot switch there should not be any hum from the reverb circuit.

Not having the pilot lamp installed will not cause any hum.

The pot code and transformer code dates the amp to a probable 1968 construction.

The preamp boards make their ground connection through the input jack wiring. The power supply grounds through the speaker jacks, and the power amp grounds with a small wire that gets screwed down by one of the pc board mounting screws. If any of these connections get loose, dirty, oxidized, etc. there can be problems with the amp.

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Re: newbie with filter cap ? [message #19371 is a reply to message #19370]

Sat, 07 September 2013 12:31

smackoj is currently offline

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thanks CB; I'm on the trail of this hum monster!

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Re: newbie with filter cap ? [message #19373 is a reply to message #19371]

Sat, 07 September 2013 13:47

smackoj is currently offline

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all inputs pulled off chassis and checked/filed shorting pin with no imprv. the blue feed wire from the pwr amp disconnected. at last, 50% imprv. but still a lot of hum. I think I need to re-visit the pwr filter caps? I continue my quest to slay the monster....wish me luck.

jack

[Updated on: Sat, 07 September 2013 14:36]

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Re: newbie with filter cap ? [message #19374 is a reply to message #19373]

Sat, 07 September 2013 19:00

smackoj is currently offline

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Stardate Log; 2296.5; the K200 is alive 60% less hum. I believe the remaining hum is a combination of power filter dry rot and poor grounding along the front panel (14 pots and switches)maybe still an issue with the four input jacks also? thanks to all my new friends for your eager input.

jacko

[Updated on: Sat, 07 September 2013 23:31]

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Re: newbie with filter cap ? [message #19375 is a reply to message #19284]

Sun, 08 September 2013 07:21

stevem is currently offline

stevem
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I tested 3 of my perfectly performing K200b`s last night and with no speakers hooked up, here`s is the average of my voltage measurements that I had you make on yours.
My DC voltage was .003 volts.
My AC ripple was .011 volts.
After checking all that you just did that I see from your last posting, I would say the the new filters will be the last item of noise reduction.
You posted its 69% better right now, so what have your voltage numbers changed to on your amp now?

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Re: newbie with filter cap ? [message #19376 is a reply to message #19375]

Sun, 08 September 2013 08:51

smackoj is currently offline

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hi steve; I worked another couple hrs on her this morning in hopes I can use it in about an hour with my worship group. the reduced amt. of hum was still too high. I re-burnished all the pot contact pts on the left channel (rt channel is un hooked). I did the same for the output jacks. I finally decided to pull the newer silicon rectifier that someone had replaced. I put in another new one I had about the shop and then ran it with my current limiter. It sounded almost hum free! I took it off the current lim. and it started humming again although not as much as before. I decided to try something and I soldered a 200 ohm pwr resistor after the power fuse and bingo! hum down to almost nothing again.

I have it all buttoned up for travel in a little bit so I can't meter the dc volts on the output now. I'll do it later today and see what she reads?

adios for now, jack

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Re: newbie with filter cap ? [message #19377 is a reply to message #19284]

Sun, 08 September 2013 14:46

chicagobill
Messages: 2005
Registered: April 2003

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Well, I'm glad that you got it working to your satisfaction. I would like to know what your reasoning for adding the resistor to what I assume is the 120 volt ac primary is?

I personally would not add that resistor. You are treating the symptom, not curing the disease.

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Re: newbie with filter cap ? [message #19378 is a reply to message #19377]

Sun, 08 September 2013 18:02

smackoj is currently offline

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I wanted to see what the amp would do in a 'real life' situation, so I added the resistor to reduce current which helps reduce hum (on this amp at least). then I played the amp at church for apprx 3 hours and it did very well. loved the tone, the sustain and the way the notes decay very much like a tube amp. I wanted to make sure I was somewhat on the trail of getting this amp functional and I knew I had enough safety between the resistor and a 2 amp fuse to keep it from melting down. I appreciate your point about masking the symptom instead of finding the cure. this has been quite an adventure in noise finding which all vintage amp owners know is a must if we really want to play them instead of just look at them. Cool

Cool

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