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K100-2 Hiss [message #27969] Mon, 25 January 2021 15:45 Go to next message
hgibson is currently offline  hgibson
Messages: 5
Registered: July 2016
Junior Member
I know, I know, one more post about Hiss. I have an all original blue sparkle ser # 21338 with original cable, foot switch and manual with vinyl near mint except one very small, you guessed it, cig burn on top. I replaced power cord to 3 prong, kept the original for some fool who might want to put it back. All other components are the original. So I want to know what is acceptable hiss. I have heard that these amps had some hiss when new? I am looking for a baseline decibel reading for hiss that most would assume to be acceptable. Example, place iphone with decibel app mid speaker height, 2 feet in front, volume knob 1/2 way up with bass, treble centered and reverb off. Seems like this would be good information to use to determine where your amp is in terms of acceptable hiss. If you should not have any then I guess this is a mute (ho hiss) point. Looking forward to thoughts. Hugh Gibson, Vermont.

Hugh Gibson
Re: K100-2 Hiss [message #27972 is a reply to message #27969] Tue, 26 January 2021 05:57 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4738
Registered: June 2004
Location: NY
Senior Member
Due to widely veering efficiencys of different speakers , and how old the speakers Magnet may be , a test done like that would not be valid, not to mention that control pots have a 10 to 15% resistance tolerance to them!.

I will try and come up with a better way for you to test out results and get back to you.

Do you have a digital voltmeter?

[Updated on: Tue, 26 January 2021 05:58]

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Re: K100-2 Hiss [message #27973 is a reply to message #27972] Tue, 26 January 2021 09:51 Go to previous messageGo to next message
hgibson is currently offline  hgibson
Messages: 5
Registered: July 2016
Junior Member
Good Morning, Thank you for your response. First I want to know if I should have any hiss at all, if not then I need to troubleshoot. If hiss at some level was inherent in the Kustoms when built, well I dont need to do a thing. That is what I was trying to establish. For starter reference, Speakers are original Kustom G12a. I have noticeable hiss at 1/2 volume, increasing with volume and increases significantly when increasing reverb. When both are turned all the way up, hiss abundant and I pick up a local radio station. Unfortunately the music from the station is better than what i am playing. Razz You have a valid point on too many variables using decibel level as I described to troubleshoot. I was using that method to establish if hiss level needed attention at all. In any case my answer to your question is yes, I have digital mutimeter, 2 year degree in Electrical Engineering, but most of that was lecture , book learning, it would have helped to actually troubleshoot something in lab. I can read a diagram, crank calculus, solder like a pro but lost in signal tracing. Hands on failure of higher Ed. I do like a logical approach to troubleshooting and not solution by replacement. Since Hiss is so prevalent in Kustoms it begs for someone to lay out a step by step procedure for troubleshooting. Thank you for your time. Hugh, Vermont

Hugh Gibson
Re: K100-2 Hiss [message #27974 is a reply to message #27969] Wed, 27 January 2021 14:07 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4738
Registered: June 2004
Location: NY
Senior Member
While I am working on your testing procedure of the hiss issue I can tell you the old factory mod /!fix for getting rid of radio station pick up.
The fix is to add a .005 cap followed by a 47k resistor to ground off of the hot of both input jacks.
Re: K100-2 Hiss [message #27975 is a reply to message #27974] Wed, 27 January 2021 15:01 Go to previous messageGo to next message
hgibson is currently offline  hgibson
Messages: 5
Registered: July 2016
Junior Member
Thanks for the radio station squelch tip. Cool

Hugh Gibson
Re: K100-2 Hiss [message #28087 is a reply to message #27974] Sat, 06 March 2021 13:18 Go to previous messageGo to next message
hgibson is currently offline  hgibson
Messages: 5
Registered: July 2016
Junior Member
Hello Steve,
As per our previous discussion on reducing Hiss on the K100-2, I replaced Q100, Q101, Q102, C121, C124, C125, C126. I used 2n3904 and 2n3906 transistors and capped with matching tants. I did not replace any resistor in audio stream 10K or larger as you suggested, mainly because I simply missed that part of your advice since it was set aside for a time. Post work VAC readings on output were same .004VAC full vol/no treb and .020VAC (down by .006) with treble full. I tested all 3 original transistors and caps on my DVM and they seemed good. Any idea of where to go now? Can you point out the audio stream resistor numbers over 10K. more importantly what would be the logical approach to fully check everything when I have that board out again. Thanks, Hugh Gibson


Hugh Gibson
Re: K100-2 Hiss [message #28089 is a reply to message #27969] Sun, 07 March 2021 10:30 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4738
Registered: June 2004
Location: NY
Senior Member
The best way to continue on at this point is to use a oscilascope to tap into the output of the many stages of the amp to see where the added noise hash is being introduced .
If you can not do such then I would do things which continues on with a process of elimination.

Q106 and 107 comprise a boost circuit so shot gunning those would be my next thing to try.

After that I would try to cut up the audio stream into blocks to try and find out where the hash is coming from.
Unfortunately this it not too easy due to every thing being on that one board!

Anyway to do this I would lift out one end of each of these coupling caps to break open the circuit feed, as this will also separate the amp into blocks .

Starting with the input jack end of the circuit I would lift up one end, and one at a time C107,112,113,119,121 and 143.
This pain in the Ass way of trouble shooting will narrow possibility's down a bunch!

Besides transistors Noisy resistor wise width in a circuit section could be R100,102,107,119,120,129,131,133,137 and 217.

To make opening up the circuit by means of the coupling caps doable, once you lift up one end try to stand them vertical as best you can, then tin up some stripped out wire and solder into the now open hole.
Make this wire stand up vertical and long enough to solder back up to the now lifted end of the coupling cap so you can get the audio stream back to being hooked back up as easy as possible!
Re: K100-2 Hiss [message #28096 is a reply to message #28089] Wed, 10 March 2021 10:27 Go to previous messageGo to next message
hgibson is currently offline  hgibson
Messages: 5
Registered: July 2016
Junior Member
Steve, Thank you for taking a good deal of time to provide such a detailed process. I hope others who follow the forum can also benefit from your response. After reviewing I think I will acquire an oscilloscope and go at it and at the same time learn a lot more about troubleshooting in general. It has been some 25 years since I have been at the bench with a scope. I will check the forum for any other ideas or places to start with a scope. Thanks again and I will post what happens. Hugh

Hugh Gibson
Re: K100-2 Hiss [message #28097 is a reply to message #28096] Wed, 10 March 2021 11:54 Go to previous message
stevem is currently offline  stevem
Messages: 4738
Registered: June 2004
Location: NY
Senior Member
Your welcome!
Keep us posted.
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