Re: Repair advise [message #19143 is a reply to message #19139] |
Thu, 15 August 2013 08:59 |
Jonomega3
Messages: 81 Registered: November 2012
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I was thinking this Baynesville electronics http://www.baynesvilleelectronics.com/index2.ivnu
would have a better selection of parts. Also, I was thinking, would there be a benefit to getting some NOS parts online? I guess its hard cause there are so many values but I was just wondering your thoughts. I see there are 7 metal film resistors in just this one preamp board and all but 2 are different values in ohms.
Same with the carbon comps, speaking of values, what value transistors should I get or are there different values would be better question? I couldn't tell from the schematics and is there a specific make (name) would be best for my use? Update...I'm looking for numbers SE4002 and SN4249 with the transistors? As far as caps, electrolytic or axial or other?
metal film resistors seem still available. What type are the black rectangular caps?
[Updated on: Thu, 15 August 2013 10:11] Report message to a moderator
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Re: Repair advise [message #19144 is a reply to message #19064] |
Thu, 15 August 2013 13:26 |
chicagobill
Messages: 2006 Registered: April 2003
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That place looks like a lot of fun. They most definitely will have a better selection of parts than RS. There used to be a lot of those types of place around here, but sadly all are long gone.
For the preamp circuit there are two types of transistors used. One is the SE4002 NPN and the other is the 2N4249 PNP. These are both very basic transistors. Almost any modern transistor will replace them with no problems. The easiest types to find will be the 2N3904 and 2N3906. These are used in so many items they have become a sort of industry standard part. That's why you can find them anywhere electronic parts are sold.
In most cases finding NOS parts is really not worth the time and trouble as the parts will be expensive and their performance will not be any better than that of the modern parts. In fact sometimes the old parts will be worse than their modern equivalents. So unless there is no modern replacement, I tend to stay away from NOS parts, except the ones that I bought when they were new and have stored for 40 years.
If you plan on repairing amps as a hobby, then yes buying a stock of parts to have on hand will be a good thing to have. What I usually do is when I need to order a part, I will order a few more than I need. That way I will build up a stock of parts for future use. Of course I only do this when ordering parts that are generic enough to be used in other things. I just needed to order a memory backup battery for a Jupiter 6 synth. The last time I needed one was probably 10 years ago. I only ordered one.
I have never seen a preamp resistor burn out in a Kustom amp before. I have seen them get noisy, go off value or break into pieces, but never burn out. Under normal circumstances there is never enough current being drawn in the preamp circuits to cause one to burn out.
Personally, I am a firm believer in only replacing parts that need to be replaced. Generally the only times I will break this rule is when there are certain needs for reliability or safety and when there are known high failure rates of certain components.
Just my 2 cents.
[Updated on: Thu, 15 August 2013 13:27] Report message to a moderator
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Re: Repair advise [message #19146 is a reply to message #19064] |
Thu, 15 August 2013 15:17 |
chicagobill
Messages: 2006 Registered: April 2003
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Senior Member |
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The white Mallory caps are electrolytic capacitors. Mallory has been out of the cap business for about 20 years now. Almost all of the modern caps are made in the far East. Sprauge, Nichicon, Panasonic are a few names of current manufacturers.
Electrolytic caps are one of the common parts that are known to go bad. A lot of it depends upon how the amp was stored and used, but they can dry out and go open circuit or worse short circuit.
Any modern cap you find will probably be half the size of the old ones. Just make sure that you get the correct values for capacitance in microfarads (uF), and voltage ratings. There are two styles of cases/mounting, Axial where the leads come off of the ends of the casing for horizontal mounting and Radial where the leads come off of one end of the cap for vertical mounting.
There are 8 electrolytic capacitors on the preamp board: 4-10uF, 3-25uF and 1-100uF. The only value that is different from modern values is the 25uF, you will more likely find 22uF caps. Use the 22uF value if that is what you find.
The voltage ratings should be a minimum of 25 volts. The one 100uF cap is a filter cap, the rest are used in the audio circuits.
When you replace the electrolytics, be sure to install them in the correct orientation, as they are polarized. They will all be marked with either a plus or a minus sign pointing to one of the legs. Make note of how the original ones were installed, and install the new one the same way.
As far as the smaller value caps, these are all mylar film types and should not need replacement.
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Re: Repair advise [message #19148 is a reply to message #19064] |
Thu, 15 August 2013 16:15 |
chicagobill
Messages: 2006 Registered: April 2003
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Senior Member |
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You can leave the board in the amp, but be sure to unplug the amp from the wall.
The black caps are probably mylar film caps, what are the values marked on them 0.22uF or 0.022uF maybe?
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Re: Repair advise [message #19152 is a reply to message #19064] |
Fri, 16 August 2013 12:16 |
chicagobill
Messages: 2006 Registered: April 2003
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Senior Member |
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I don't know if you have started testing things yet, but something that I though of was to connect the output of the working preamp board to the input of the clipper/boost circuit and see what happens.
Just a thought.
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Re: Repair advise [message #19155 is a reply to message #19154] |
Fri, 16 August 2013 13:39 |
Jonomega3
Messages: 81 Registered: November 2012
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Do you mean take the 4249 out of the board to test? update, I tested the 4249 on the straight non fx channel board and it read 6-7 v just like you described prior.
Update..took 4249 out of circuit still read less than 1/2 v on all leads
[Updated on: Fri, 16 August 2013 14:54] Report message to a moderator
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Re: Repair advise [message #19163 is a reply to message #19162] |
Fri, 16 August 2013 20:44 |
Jonomega3
Messages: 81 Registered: November 2012
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Yes, and good advise too. I've moved on to playing this thing instead of standing over it scratching my head, haha. Oh, and brilliant idea testing the fx through the straight channel! That worked perfectly.
I mainly use the (RCA plug) preamp out of the A-4 in front of my 65 Fender DR, much better sounding than any digital fx I've heard and at a reasonable playing level.
Anyway, I haven't played it through my Kustom 2x15 cab in a while so I thought I'd try it out and, the power section does sound a bit noisy (slightly distorted)The preamp is very clean with very little noise. So maybe down the road I'll go through the power section. Hopefully you wouldn't mind giving me some more pointers then? I'm going to print out this thread and keep it for future reference. What a great experience, I learned so much and really enjoyed it, thanks again!
[Updated on: Fri, 16 August 2013 20:49] Report message to a moderator
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