| K100-2 Hiss but no other output [message #21116] |
Mon, 11 August 2014 09:45  |
Jerrybass1955
Messages: 45
Registered: August 2014
Location: NC
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Hi Folks,
Just signed up for this forum, so please pardon me if I am covering old ground.
I recently picked up a steal on a K100-2 head with reverb and Tremolo. I have only taken a brief look at it so far. I have 40V DC at the power supply caps with no ripple measured with a VOM. The output has hiss but no signal gets through from the input. Hiss is independent of all controls. I have a signal generator coming tomorrow and I have a scope, but I am just getting started troubleshooting this guy. It hasn't run in several years at least, so I thought the Power Supply Filter Caps would be gone for sure, but they seem fine for now given that I am right on the money with the 40volts DC, (that of course may change since it may have very little load at this point).
So my questions:
1) I have a schematic, but it is very hard to read. Anyone have a relatively clean schematic they could share?
2) Anyone have suggestions on where to start troubleshooting this guy?
3) I assume the output transistors are probably OK since I get some hiss, but maybe someone knows otherwise?
4) Are there routine things some of you would replace for good measure given the age of this beast? It would be nice to keep it near original, but that is kind of useless if it doesn't work!
5) Are there any tricks to tearing this guy down to troubleshoot it that I should know about?
I appreciate any input you folks can give. This amp is like my first serious amp when I was maybe 14 years old or so. I later switched to bass and ran a K200 bass amp. Both have long since been sold, so this will be quite nostalgic for a 58 year old guy if I can get it running. I will likely be working on it at most a half hour or so a day average, so I may be posing more questions from time to time. I am looking forward to interacting with this group.
Thanks,
Jerry
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| Re: K100-2 Hiss but no other output [message #21118 is a reply to message #21116] |
Mon, 11 August 2014 11:22  |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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Jerry:
Good that you have a scope, it will come in very handy.
The schematic for the K100-2 at this site is pretty clear...better than most.
Try putting your scope probe on the base of Q124, while you have some input to the amp. The base should be the middle lead of the three. See if you have audio getting to that point at least. If not, the problem is earlier in the circuit. If you do have level there, something has gone wrong in the power amp section.
Since you hear the hiss, it's more likely the problem is earlier...but you never know. Someone may have popped the first stage transistor in the preamp, which can be a bit sensitive and fragile.
Kustom used a fair amount of tantalum caps in these amps. They were very high quality caps, but they have the habit of shorting when they go bad, which frequently shunts DC voltages to ground. They are usually orange in color. After 40+ years, they are very good candidates for replacement. I usually replace those in any older piece of equipment...even before I power things up.
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: K100-2 Hiss but no other output [message #21119 is a reply to message #21116] |
Mon, 11 August 2014 11:30 |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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Another easy place to look is to crank up the reverb, and see if you have drive to the input of the spring tank.
For that matter, if you touch the springs, or just tap on the tank with the handle of a screwdriver, do you hear that in the output? Kinda sounds like crashing of thunder. Of course you need to make sure the reverb is engaged with the foot switch, or you won't hear anything with this test.
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: K100-2 Hiss but no other output [message #21134 is a reply to message #21123] |
Tue, 12 August 2014 08:40 |
C4ster
Messages: 686
Registered: June 2001
Location: Mukwonago, WI (Milwaukee...
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Senior Member |
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I have had 2 K100's that died in the same way. The 1st input transistor is the culprit. For some reason, that transistor is very easy to pop with a little static. Just touching the tip of your guitar cord with your charged finger can pop it. (I NEVER do that)Right! I use a generic 2N3904 NPN transistor and it works fine.
Conrad
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| Re: K100-2 Hiss but no other output [message #21136 is a reply to message #21116] |
Tue, 12 August 2014 10:32 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Go to this sites technical listing for the schematic.
Forget about the three prong cord for now.
The reverb needs the foot switch only to turn it off,fix the non reverb issue once you get normal output.
The main board also needs the two wires of the pilot lamp removed from the lamp, and be very very care full with the temp sense diode in the clip that is in between the two output transistors!!
This diode has phosphor bronze leads it seems, or may be the gold plating process just makes them brittle, but they are very prone to breaking off so watch it!
What I do once I pull the board is to wash off any heat sink grease from the diode and then apply black silicone sealer to the diode where the leads enter it, and to the board where they are landed.
This will let you man handle the board and do no harm to the diode, just note that when you do the final re- assy on the head that heat sink compound is reapplied to the diode and that the clip holds it good and tight!
I also write next to each output transistor which way the slip on connector goes before I pull them off.
You do not have to use a tantalum caps, or even electrolytic type caps, and non electrolytic types certainly sound better so it depends on how deep your pockets are!
If you do use electrolytic types they need to go back in the way they came out!
Note that the 5 watt dropping resistor for the pilot lamp gets very hot, so steer clear if the amp has been on for two or three minutes!
The factory mounting of this resistor so close to the board always burns it to one degree or another so I pull it up off the board as far as it will go with out breaking the leads off.
[Updated on: Tue, 12 August 2014 12:17] Report message to a moderator |
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| Re: K100-2 Hiss but no other output [message #21137 is a reply to message #21116] |
Tue, 12 August 2014 10:55 |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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Jerry:
You might wanna try the "Panasonic FM" series aluminum electrolytic caps. Mouser (and others) carries them. I've had very good results with them in hundreds of pieces of equipment. They are rated for the higher 105 degree C temps, and have pretty low ESR. Panasonic also makes their own electrolyte paste (as opposed to buying it from a second party in China), and hasn't been subject to the "bursting cap syndrome" common in computer devices.
As was suggested, ya might wanna start with the first stage transistor, and work your way out. I haven't had any of the newer replacements fail in mine, but then again, the originals were still working too. The newer transistors are MUCH quieter however. After replacing most of the preamp transistors, and the summing board transistors in my K200, you can hardly tell that it's even on. My noise is almost 80db down from the max output!
For the schematic, go to vintagekustom.com, click on the "Technical" amp head on the left, and then to "schematics by amp model". You will see your K100-2 on the list.
Sorry about the miss-lead on the reverb foot switch. I had my switch logic confused. I don't have a foot switch for the reverb on my K200 either, but I did wire in a toggle switch on the back of the amp to turn it on and off. Since I'm a bass player, I didn't really need it on, and when it's off, the amp is considerably quieter. Always go with whatever SteveM, ChicgoBill, or C4ster say here. They have much more experience with Kustom amps than anyone else here...including me. 
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: K100-2 Hiss but no other output [message #21193 is a reply to message #21190] |
Sat, 16 August 2014 03:07 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Check the tank transducers with an ohmmeter. Both input and output coils will normally have around 180-200 ohms dc resistance. Sometimes you just can't see the damage. If there is no noise from shaking the tank, then the output transducer or return circuit or wiring is suspect. If you pull out the RCA plug from the output side of the tank, you can touch the tip of the plug and see if there is a buzz.
Almost all Kustom amps use switching transistor circuits to turn on and off the FX, so there is no audio signal carried on any of the footswitch cables. It could be possible that the trem circuit is putting a pulse into power supply. Maybe a bad cap.
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| Re: K100-2 Hiss but no other output [message #21220 is a reply to message #21116] |
Tue, 19 August 2014 12:49 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Try and isolate the turn off oscillation to the power amp or the preamp, that will at least divide the search into smaller sections. You can ground out the audio at the input of the power amp and see if the sound is coming from through the audio path. Most likely a failing tantalum cap somewhere.
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| Re: K100-2 Hiss but no other output [message #21224 is a reply to message #21116] |
Wed, 20 August 2014 09:58 |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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Jerry:
Two things I always try to get changed first on any older piece of gear are the aluminum electrolytic caps, and especially any tantalum electrolytic caps.
The aluminum caps usually just dry out, the ESR goes up, and they usually just gradually stop working. Like your power supply caps getting weak. They can sometimes short, but that's far less common.
Tantalum caps are a different story. They can dry out, and just stop working, but their failure mode most often is to SHORT. If the cap is across the power supply as a filter, you can imagine the mess that can cause. If they are just in there to block DC, they suddenly start passing the DC they are supposed to be blocking, and can damage the following stages.
Tantalum caps are BAD NEWS! For the longevity of your amp, you really need to get rid of them all before they cause serious problems for you down the road.
You may be asking, why would they be used then.?. Well, they do very accurately hold their value over the long term...usually till they short. They are very good for timing circuits...like setting the repeat rate of your tremolo / vibrato. They were quite expensive back in the days these Kustom amps were built. It's not that Kustom was cutting corners, they thought they were putting in the best possible parts money could buy. We didn't know at the time that they would become so likely to short after many years of use.
BTW, tantalum caps are most likely to short when the amp has been sitting for a long time...like years, and then powered up. In daily service, they are not nearly as likely to short. I see this a LOT in older broadcast gear that has been sitting for years, then someone plugs it in, and the thing starts smoking, or catches fire!
Getting rid of all the tantalum and aluminum electrolytic caps in your amp would still be advisable. It will sound better, be quieter, be less likely to oscillate, and last a lot longer before the next failure.
I have very good results with the Panasonic FM series caps. They are rated for the higher 105 degree temps, are low ESR, and last a long time. I test every cap before installing it for value and ESR, and these FM series are consistently right on their rated value too...generally not more than +5%. That makes them pretty good replacements for timing circuits where tantalum caps were originally used.
Just my 2 cents worth,
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: K100-2 Hiss but no other output [message #21226 is a reply to message #21116] |
Wed, 20 August 2014 13:39 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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I'd recommend the Panasonic FC series caps.
As for the tantalum caps, I'm not ready to write them off as much as Dave, but I haven't had them burst into flames before.
Kustom was about the only company using them back in the day for guitar amps. I have not seen any great failure rate in any of the Kustoms that I've seen. I see more failures in regular electrolytics of the same age than of the tantalums.
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| Re: K100-2 Hiss but no other output [message #21228 is a reply to message #21227] |
Thu, 21 August 2014 13:52 |
Jerrybass1955
Messages: 45
Registered: August 2014
Location: NC
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Member |
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Here is an interesting paper about tantalum caps. http://www.avx.com/docs/techinfo/rmtant.pdf Interestingly, it appears that some caps are starting to incorporate fuses to assure that they fail as an open circuit.
I think I am convinced that at least in areas where there is significant voltage (with respect to voltage rating) on the cap, I will ditch the tantalums and get some aluminum electrolytic caps for that application. I may just get some 50V caps for all applications to give a margin of safety. I have never had a tantalum do the flash-bang, but I have opened up a TV or two to find ribbons of foil everywhere from an aluminum electrolytic failure.
While waiting for some other parts to arrive, I decided to go ahead and do the three prong AC conversion and install a panel mount fuse holder in case the surge from the bigger PS caps starts popping fuses (or in case I make other goofball errors while doing the repair). In the process, I broke my only fuse holder! So, looks like I have a little setback, but I'll be back on it as soon as I can.
Since reverb tanks are pretty cheap, I went ahead and ordered a replacement so I can use it for comparison and figure out if the problem is with the circuit or tank. I'll go with whatever sounds best.
BTW, do any of you have experience with Nichicon electrolytic caps? I only ask because there appears to be a wide selection with good specs (and my next door neighbor is a Nichicon rep.!).
I'll be back!
Jerry
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| Re: K100-2 Hiss but no other output [message #21229 is a reply to message #21116] |
Thu, 21 August 2014 16:45 |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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Jerry:
Nichicon caps would be my close second choice. Very good quality for the most part.
They did have a few types that suffered the expanding cap syndrome a few years back (bad electrolyte paste), but I think they have corrected all those problems now. Just make sure you get fresh stock. Your friend would probably know more about all that than I do. Perhaps you can update us after speaking with him about which type would be best for service in our Kustom amps.?. I do usually try to go with 105 degree C rated caps, just because they seem to last longer, and be of better quality.
I was worried about the turn-on, in-rush current blowing fuses on my K200 head too after I more than tripled the original factory filter cap values, but so far, the original fuse has not blown...much to my surprise. I would rather have an easily replaceable fuse socket on the rear of the amp myself, and may still add one.
I'd also like to add an MOV after the fuse to protect the amp from surges and over-voltage. Some may think that's silly, but way back in the 70's my band had an outdoor gig, and the generator we were getting power from went crazy, and started pumping out about 200+ volts. Our Kustom amps and PA were OK, only suffering blown fuses. The guy with the Hammond B3 and a different brand of solid-state amp was not so fortunate.
I've also had stage hands inadvertently put 240 up to the 120 volt sockets on the stage. Trust me, it happens.
Anyway, a heavy duty MOV after a replaceable fuse should protect my amp.
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: K100-2 Hiss but no other output [message #21233 is a reply to message #21232] |
Fri, 22 August 2014 11:44 |
Jerrybass1955
Messages: 45
Registered: August 2014
Location: NC
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Member |
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Hi Folks,
While I am waiting for capacitors and some other misc. parts (Aluminum electrolytic), I polished off replacement of the fuse holder and power cord, etc. I put a clamp-on ferrite on the cord and now only have to replace the bridge rectifier (which should be a no brainer).
While I had the soldering iron hot, I took a shot at the tremolo and changed the resistor R152 to 15K and replaced C122, C123 and C124 with some caps I had around. Some parasitic oscillations went away, but otherwise no change in the tremolo problems. The tremolo speed seems to have increased, and I am guessing it is the 15K resistor, but the other components are also a bit different in measured values than the parts they replaced, so possibly a combination of things.
Since I now no longer have "hiss but no other output" I will probably start a new thread or two dealing with the Tremolo and Reverb problems so they might be easier to spot in the unlikely event I do something useful to others.
Oh yea, I have some MOVs and will likely put one in this beast too. I do that in the tube amps I have build.
Thanks again to everyone for your help.
Jerry
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| Re: K100-2 Hiss but no other output [message #21238 is a reply to message #21116] |
Fri, 22 August 2014 15:35 |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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Well Jerry, you are certainly headed in the right direction. I'd say getting rid of the oscillations is a pretty important, major step.
Good idea to start up a new topic for reverb / tremolo.
Your reward will be a really great amp which will run for many years.
And just look at all the fun you've had!
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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