| K200B No sound just Buzzzzzzz [message #12681] |
Mon, 18 January 2010 19:15  |
ironman
Messages: 12
Registered: January 2010
Location: Chicago
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Junior Member |
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Hi All
I Picked up a K200B-5 today for $60. I couldn't help but rescue this great looking amp. At first there was nothing, but the light did come on. After thoroughly cleaning the pots and inputs I flipped the switch...HUMMMMMMMMMMMMMM. Typically I'm a tube amp guy and I've only recently begin to repair Kustom for a current music project..
• My first thought was to test the output. I'm getting 28 vdc at the output jacks. Having read my share of posts, I've come to the conclusion that that's a bad thing.
• I've tested the filter caps and they're reading 37 vdc (I think this is ok).
This part looked weird to me and seemed to have leaked in the distant past. http://farm5.static.flickr.com/4017/4286205484_7662274993_b. jpg
This looks to be a 4 channel amp and nothing looks altered or out of place. Someone has substituted one of the 4 RCA diodes on the bottom for a GE one.(thats what the small round buttons on the bottom are right?)
Where should I start to track down the problem...Could this be a transistor failure? And if so is the best thing to do to replace all of them?
[Updated on: Mon, 18 January 2010 19:56] Report message to a moderator |
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| Re: K200B No sound just Buzzzzzzz [message #12687 is a reply to message #12685] |
Tue, 19 January 2010 11:51  |
ironman
Messages: 12
Registered: January 2010
Location: Chicago
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Junior Member |
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Thanks Steve,
I located the bad power resistor (5w1ohm). Actually I noticed the tell tale burnt look before testing and confirmed it as bad.
I get cold sweats just thinking about pulling out that driver transistor. They look so fragile. But I've seen the other posts and have a good clue on the proper method to replace them.
I gues my question is will it be the T05 adjacent to the bad resistor? Is there a way to test?
Last question (for now) I'm getting some resistance across the output transistors. Is this ok. Is there a sure fire way to know if they are shot?
Thanks again Steve. Just send me the bill. ; )
Dave.
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| Re: K200B No sound just Buzzzzzzz [message #12688 is a reply to message #12681] |
Tue, 19 January 2010 12:10 |
chicagobill
Messages: 2005
Registered: April 2003
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Senior Member |
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That is one of the driver transistors. It is not original to the amp, and may or may not be good. Transistors will not leak anything physically. The spatter marks on the board are from something else.
| ironman wrote on Mon, 18 January 2010 18:15 |
This looks to be a 4 channel amp and nothing looks altered or out of place. Someone has substituted one of the 4 RCA diodes on the bottom for a GE one.(thats what the small round buttons on the bottom are right?)
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Your amp was originally intended to be a PA head. 4 independent channels with reverb. The photo and your comment about the "GE diode" tells me the amp was repaired in the past.
The four round buttons on the bottom panel are not diodes, they are the output transistors. The generic number for them are 2N3055, a very common part.
You will need to test all of the output transistors and most of the transistors on the main output board in order to fix your amp. I don't know what your skill level is and how well equipped you are, but now is the time for you to decide if you are going to tackle this yourself or take it to someone who can fix it.
Let us know what you decide to do.
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| Re: K200B No sound just Buzzzzzzz [message #12691 is a reply to message #12681] |
Tue, 19 January 2010 13:21 |
stevem
Messages: 4748
Registered: June 2004
Location: NY
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Senior Member |
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A 90% conclusive test of the common transistors used in this amp can be done with a ohm/resistance check across all of the 3 pins.reading across all of the possible two pin patterns should read in one direction of the test leads( red and black) placed as a short, and in the other test lead position will read well above a 1k resistance.
These test are done with the semiconductor out of the circuit.
If you need to replace on of the boxed heat sinked T05 drivers you will need to get a slip on star type heat sink to replace the box and make sure tha has a dab of glue on it to keep it there. You will ned to unscrew the driver board from the rear of the chassie to unsolder things and its a good practice to do a quick drawing as to how the 4 slip on two pin connectors go back on the output transistors.
Note also that the temp/bias diode mounted in the clip in the middle of the outputs can have its leads broken of all too easy, so tread lightly around there!
Bill, thank you for going over to his posted link to take a look as my computer here at work is blocked from going there, so I was stuck this morning!
[Updated on: Tue, 19 January 2010 13:26] Report message to a moderator |
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| Re: K200B No sound just Buzzzzzzz [message #12692 is a reply to message #12691] |
Tue, 19 January 2010 13:52 |
ironman
Messages: 12
Registered: January 2010
Location: Chicago
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Junior Member |
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Thanks Bill,
if I get in trouble I'll have to have you by the shop. I would consider myself extremely handy. I run a metal fabrication business and have essentially self taught myself everything from blacksmithing to building my own CNC plasma table. I lack a working scope but have accrss to an old one with valves that's begging to be rebuilt. That makes sense about the replacement transistor, A closer look revealed it to be foaming lithium grease or heat sink compound.
Steve/Bill- you guys have listed two part numbers for the output transistor replacements. Nte181mp and 2n3055, did I get this correct? I found the star type heat sinks on ebay for $7-8 for a handful.
Looks like I've got my work cut out for me (good fun!) I'll keep in touch to be sure.
Dave
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| Re: K200B No sound just Buzzzzzzz [message #12695 is a reply to message #12694] |
Tue, 19 January 2010 15:52 |
ironman
Messages: 12
Registered: January 2010
Location: Chicago
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Junior Member |
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Good info- I was just doing my research into transistors. Therea a good Wickipedia expanation for the 2n3055. When I checked on mouser I couldn't believe the price difference between the 2n3055($2.10ea) and the matched pair of nte181 ($16.60/pair).
What is the proper replacement for the driver transistors (the ones on the square heat sinks.) These are the T05 type I understand but what are they rated? Oh boy looks like I need a schematic. ; )
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| Re: K200B No sound just Buzzzzzzz [message #12697 is a reply to message #12681] |
Tue, 19 January 2010 17:30 |
chicagobill
Messages: 2005
Registered: April 2003
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Senior Member |
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Yes, those NTE prices will kill you, that's why I rarely if ever use them.
The 2N3055 is a very common and often used part, but there are many newer transistors that can be used in these amps as well, like the MJ150XX series.
I usually buy outputs 10 or 12 at a time. I will then gain match them with my own meter. In most cases 7 out of 10 will fall into the same gain range, with the rest being slightly higher or lower. So in my experience, if you were to just randomly purchase two 2N3055 that came from the same manufacturing batch you'd probably find them to be reasonably well matched.
I just recently purchased a number of transistors that I want to test as driver replacements for the 40409 and 40410s. I should dig them out and run some tests.
I will need to talk with you about the metal fabrication work that you do. There are times when I wish I had access to someone who could make a metal bracket or some such thing-a-ma-bob.
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| Re: K200B No sound just Buzzzzzzz [message #12719 is a reply to message #12698] |
Fri, 22 January 2010 16:00 |
ironman
Messages: 12
Registered: January 2010
Location: Chicago
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Junior Member |
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Steve/Bill,
I'm about to service the amp and am looking through my replacement parts. Is there a surefire way to replace the transistors and get the pin configuation correct? I would imagine that there is a correct way to do this but I thought I'd ask before I began unsoldering connections.
Dave
ps- Bill I'll pm you later with contact info for my shop.
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| Re: K200B No sound just Buzzzzzzz [message #12725 is a reply to message #12720] |
Sat, 23 January 2010 00:42 |
ironman
Messages: 12
Registered: January 2010
Location: Chicago
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Junior Member |
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Well it took some time and careful removal of the board. I've replaced all of the output transistors. I decided to focus next on the bad 1ohm resistor and the two adjacent driver transistors. I replaced the driver transistors with a matched set of nte128/nte129 parts. In replacing them it's clear that there is a small tab that helps orient their location. My fear is that because there was a previous repair the orientation of the nte736 transistor may be wrong. It was the one adjacent to the bad 1ohm resistor (you can see the new resistor in the pics below). In looking at the oposite side of the board the orientation of the left nte128 transistor is in reverse! Should I match this orientation? I don't have a schematic. I'm in no hurry to fire the amp up. Just want to get it correct. Should either side if the Driver Transistors match in orientation? See pics below...
Dave
Here are the new transistors on the right side of the PC board (you can see the new 1ohm resistor to the left). Please note the small tabs and how they are configured.
http://farm5.static.flickr.com/4048/4297894772_325eed7286_o. jpg
Here are the original transistors on the left side of the PC board. Please note the tab configuration. Should the replacement transistors match this configuration? Perhaps this is why the amp repair failed?
http://farm5.static.flickr.com/4034/4297894744_998ff97e5a.jp g
Thanks for sticking with me I'm almost there!!!
[Updated on: Sat, 23 January 2010 10:33] Report message to a moderator |
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