1969 Kustom K200A-1 (Restoration) Three Questions (Power Section)

[Rick Redrock]

Thanks for allowing me to join this forum ! I am currently on a labor of love restoration of a 1969 Kustom K200A-1 Bass Amp Head. I am going about this as logically as I can, by starting with the power section, 120vac in, thru transformer to rectifier and then off to the giant caps ! Here are my three questions....

Question #1 - By looking at the schematic (and a test doc from Don) - I should see -40vdc to center tap and +40vdc to center tap via the three (3) output leads (2 Purple/ 1 Gray CT) of the secondary winding of the transformer.... Is this correct ?

Question #2 - The rectifier was replaced sometime in the past, and is shorted... The replacement used was a S6458-2 (2H).... Can anyone suggest a good choice on a new rectifier ?

Question #3 - If for some strange reason my transformer is bad, are replacements available ? or am I just SOL ? :-)




Thanks to all.... Much appreciated !

Rick

[nosaj]

Schematic links.
1 Please post a link to what your working off of . Schems I have don't show a voltage.
2 if rectifier has been replaced in the past and is again shorted It's possible a filter cap has failed. What do you have in regards to test equipment
3. Not sure on replacements but the power transformer is easy to test before sinking money into it. )Will follow up on that after you answer number 2.

You should consider a DimBulb tester (here is a video explaining it ) https://www.youtube.com/watch?v=k5oNQ1etN2c

How to build one https://antiqueradio.org/dimbulb.htm

nosaj

[Enzo]

Not correct. Transformers make AC not DC, so if the transformer has two 40v sides, they measure AC volts not DC.

Interior photos would be way more useful that exterior shots. I mean yes, it looks like an amp from here.

If your rectifier bridge is shorted, if it were in my shop I would grab one of those 35A square block bridges at 200v or more and bolt it to the chassis and wire it in. DOn't really care what 50 year old parts were there originally.

[J M Fahey]

Schematic links.
.......
nosaj

Dear Nosaj, can you please post/link/upload/attach here a downloadable .pdf?
Your google docs link is a "preview" and trying to download it only brings an .html
Thanks.

[Rick Redrock]

Hey Guys.... The schematic Nosaj is referencing is the model with Reverb and Tremolo, which my model does not have..... Here is my model https://docs.google.com/file/d/0B1Fx...?pli=1&rm=demo

[Jon Snell]

1/ Use C2 & C3 ground point for reference voltage readings.
2/ Any current "square block" type, here is an example; https://uk.rs-online.com/web/p/bridg...ifiers/6875734
3/ If the transformer is 'bad' there will be either no output at all or heat turning to smoke unless the fuse blows first.

[vintagekiki]

Schematic links.

post #19
https://music-electronics-forum.com/filedata/fetch?id=841374
Kustom K200

[Rick Redrock]

Thanks everyone for all your assistance..... I wanted to reply to Nosaj's question and let him know that right now I only have a multimeter and a crude tone-out rig via some software on a PC. I am going to build a "Dim Bulb" current limiter today (thanks for the link).
I want to order a Variac as well.... Can anyone suggest a decent (beginner level) Variac ? I would also like to purchase a Isolation Transformer, so any thoughts on that subject would be greatly appreciated as well.
Lastly, I want to get a oscilloscope, as I do intend to continue restoring amps, tube radios, etc....

Enzo, I will order a 35a-200v square block rectifier as you suggested....

The K200A-1 model I have uses two 2500uF-50v filter caps.... I did measure them (out of circuit) and it appears they are both bad. Looking at the models/schematics that included Reverb/Tremolo, it appears they used 4500uF-50v caps... I'm wondering if I should beef mine up to that as well, just to be on the safe side ?

If I can get to the point where I have -/+ 39.5vdc off the rectifier, I will purchase new filter caps and then attempt to tackle the circuit board entities...... https://docs.google.com/file/d/0B1Fx...?pli=1&rm=demo

Bringing this amp back from the dead may have a higher than desired cost to it, but as I said... this is a labor of love AND learning project..... If I can increase my overall knowledge, then it will be worth it :-)

Thanks again everyone ! I'll keep you posted on my progress ! and thanks for helping a newbie ! :-)

Rick

[vintagekiki]

https://irationaudio.com/2015/03/26/193/
Kustom K100-1

https://irationaudio.files.wordpress.com/2015/03/full-k100-1-schematic.jpg
Kustom K100-1 Schematic

[vintagekiki]

Question #1 - Is this correct ?

Question #2 - Can anyone suggest a good choice on a new rectifier ?

Question #3 - If for some strange reason my transformer is bad...

Answer #1 - Yes
3 secondary winding from transformer go to bridge rectifier. ¹⁾
2 Purple wires connected to ~ pin of bridge rectifier, and Gray CT connected to cap GND center tap +/- ¹⁾

Answer #2 - https://www.galco.com/buy/ST-Semicon/S6458-2
Or any bridge rectifier for voltage of 100V and current > 5A

Answer #3 - Before thoughts about changing transformer, check whether voltage of 40V drops on C2 and C3 under full load.
If +/- 40V voltage drop occurs, replace C2 and C3. (2500u/ 50V)

2)
For Kustom (K100) and for all old-timers older than 10 years, rule applies, if the sound "muddy" or not defined, replace all coupling caps that goes to base of transistors (10u)
To avoid transients (motorboating, whistling, etc ...), replace caps in preamp power supply.

[52 Bill]

Just a few things here. What is wrong with the amp? Was it working when you got it?

All K200 series amps that I have ever seen originally came with 4500uF Computer Grade filter caps. Are you sure that yours are original?

Depending upon the version, the A-Series amps will either have two internal fuses or two circuit breakers located on the front panel. The main internal power fuse is usually a 3A pigtail fuse.

The power transformer can be replaced if it is bad with any number of units available, but they were very well built and rarely go bad.

[J M Fahey]

Hi Friends, can you straight upload/attach .PDF files here?

Or anywhere else BUT Google Docs.

That so called "service" hijacks files and only allows you to "see" them inside their site, but not rotate, print, edit, etc. and even less download.

At the very bottom of the answer window you have the "Upload Attachments" button.

Practical example:



EDIT:

Answer #3 - Before thoughts about changing transformer, check whether voltage of 40V drops on C2 and C3 under full load.
If +/- 40V voltage drop occurs, replace C2 and C3. (2500u/ 50V)

It WILL drop under load, between 15% and 20%, thatīs normal on any "raw" unregulated supply.
Now if it drops 30-40-50% under load, yes, thatīs catastrophic.

[vintagekiki]

It WILL drop under load, between 15% and 20%, thatīs normal on any "raw" unregulated supply.

Yes it is "soft" rectifier (transformer wound with thinner wire or rectifier caps with lower capacity)
Drop under load 20% it's 64% power (W).

[g1]

Hi Friends, can you straight upload/attach .PDF files here?

Or anywhere else BUT Google Docs.

That so called "service" hijacks files and only allows you to "see" them inside their site, but not rotate, print, edit, etc. and even less download.

I tried to find that source file, but it comes from the 'vintagekustom' website and that is how they post all their schematics (on google docs with no download option).
They do mention they are trying to get them printable but want to put a watermark on them:
http://www.vintagekustom.com/literat...h/AmpList.html

[Rick Redrock]

Greetings All.... Please excuse my delayed response..... I wanted to share a photo of what I first saw when I pulled out the chassis of the beast, but the system says I'm over my upload quota .... Well, it was nice burnt red wire coming of the rectifier to one of the filter caps. I thought DOH ! the whole thing is gonna be fried.... But after a lot of testing of all four boards (Pre-amp x 2, Regulator, Power Amp). Both 4500uF filter caps are testing good, as are the windings of the transformer... it appears the only thing that fried was the rectifier itself (I'm fricken hoping !).... Somebody had put a 10amp fuse in the damn thing as well (should be 3 amp)... I guess I won't really know for sure what the true status is until I power it up.... I am happy to say that I completed building my Dim Bulb Tester, so that's a good thing huh ? Don't have a Variac yet.... I have a new rectifier ready to be installed tonight, and I just ordered all new caps..... So things are moving forward.....

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Thanks again for everyone help, suggestions and input... It is much appreciated ! Rick