Home » VintageKustom.com » Replacement Part Suggestions » A4 Hiss and Hum (Help!)
A4 Hiss and Hum [message #12555] Mon, 28 December 2009 18:47 Go to next message
Shaun_Musings is currently offline  Shaun_Musings
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Registered: April 2009
Location: Reading, MA
Senior Member

Okay, the hum is noticable and annoying. The hiss is loud when the trem/vibe is activated. Could this just be that it needs a better cleaning? The Reverb tank hisses a little until it is turned up...

Shaun


Born on the Baaaaayou (Ok, in Massachusetts)
Re: A4 Hiss and Hum [message #12558 is a reply to message #12555] Tue, 29 December 2009 06:48 Go to previous messageGo to next message
stevem is currently offline  stevem
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Registered: June 2004
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These amps circuit baords are loaded with white plastic cased electrolytic caps made by Mallory, and as these componets age they drift in rated value by a massive amount which changes the gain structure and hence noise level of these amps in a big way! In fact I did not realize how far these caps could drift untill I got my cap testing meter back from my friend, heres the story.
Just last weekend I was checking out a 1966 Frank style amp that I picked up( which I may put up for sale here in a couple of weeks)which share similar circuits and componets to the latter 200A serise amps.
In reguards to the main preamp board for both channels it has some 16 of these caps on it, and the worst offenders where the larger 90mfd rated caps.
All of these white cased caps have a 20% tolerance. which means that the 90mfd stamped rating on the cap body could in actuality be any where from 72mfd to 108mfd.
In this amp all of the 90mfd marked caps checked in at any where from 142mfd to 156mfd. This is a minimum of 58% which is a huge difference!
As it turned out this amp had a shorted 10uf cap on the second channel that was causing the botttom of the test sign wave to clip very early and cut off.

In short, to get these frank type models and the A models to perform as intended, all of these white Mallory made caps should be replaced. Also the paper cased 500mfd caps on the voltage regulator board and the reverb board if your model has reverb need to be replaced.
For greater life and stability all of these replacement caps should have their volatge rating doubled!
Also as far as the noise problem goes, replacing all of the transistors with modern low noise ones will tame alot of hiss from the amp, or at least the first two to three transistors for each channel, at lees than 10 cents each these days its a no brainer money wise.
Also of interest on the frank amps I noticed that back than for these models they used 75 volt rated main power supply cans when the latter ones used 50 volt cans.
This explains why these model amps so rarly need these replaced when compared to the latter amps that use the 50 volt cans that have their power supply running them at only 10 volts less than their rating.
Re: A4 Hiss and Hum [message #12559 is a reply to message #12555] Tue, 29 December 2009 08:18 Go to previous messageGo to next message
Shaun_Musings is currently offline  Shaun_Musings
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Registered: April 2009
Location: Reading, MA
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YTyThanks soooo much for the info. First the ungrounded, two-prong plug HAS to go. Now, with regards to the caps and transistors, I need makes and exact models. I'll be grabbing them tonight if I have time...

Born on the Baaaaayou (Ok, in Massachusetts)
Re: A4 Hiss and Hum [message #12561 is a reply to message #12555] Tue, 29 December 2009 12:38 Go to previous messageGo to next message
stevem is currently offline  stevem
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Registered: June 2004
Location: NY
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As far as transistors go , I would just replace the 5 you have on each or your amps PC-102 boards and see first how much that drops the noise level.
All of the 10 transistors can be replaced with NTE 123AP transistors.
In regards to the all the non main power supply caps you will just have to take a count of now many and the values needed in your amp.
Re: A4 Hiss and Hum [message #12588 is a reply to message #12561] Wed, 30 December 2009 21:11 Go to previous messageGo to next message
Shaun_Musings is currently offline  Shaun_Musings
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Registered: April 2009
Location: Reading, MA
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Alright, we'll start with the trannys... thanks!!

Shaun


Born on the Baaaaayou (Ok, in Massachusetts)
Re: A4 Hiss and Hum [message #12589 is a reply to message #12588] Wed, 30 December 2009 21:35 Go to previous messageGo to next message
Shaun_Musings is currently offline  Shaun_Musings
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Registered: April 2009
Location: Reading, MA
Senior Member

Well, I've ordered the transistors. Sigh. Now I just need someone (glances in Don's direction) to send me a schemantic for this beauty. Also, how do I replace them? I'm guessing it's not too bad...

Shaun


Born on the Baaaaayou (Ok, in Massachusetts)
Re: A4 Hiss and Hum [message #12594 is a reply to message #12589] Thu, 31 December 2009 07:22 Go to previous messageGo to next message
pleat is currently offline  pleat
Messages: 1452
Registered: June 2004
Location: Belding, Mi
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Shaun, I'm a bit confused? What schematics are you looking for? Reading some of the other posts, I see stevenm mentioned a PC102.
I have some kustom lit. that shows all the circuit boards used in each model amp in the K100, K200, SC amps, Combo amps and Kasino, and I don't see a PC102 listed in their information. I checked my schematics and I don't have a copy of it.
Don
Re: A4 Hiss and Hum [message #12595 is a reply to message #12594] Thu, 31 December 2009 07:51 Go to previous messageGo to next message
Shaun_Musings is currently offline  Shaun_Musings
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Registered: April 2009
Location: Reading, MA
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Sorry for the confusion, Don. I mean Schemantics for thew A4 and B5. I ordered the 123AP transistors. I hope I didn't get the wrong ones... I also need the model for a better Mallory Cap. Those don't change the original set-up, do they?

Born on the Baaaaayou (Ok, in Massachusetts)
Re: A4 Hiss and Hum [message #12596 is a reply to message #12595] Thu, 31 December 2009 08:37 Go to previous messageGo to next message
pleat is currently offline  pleat
Messages: 1452
Registered: June 2004
Location: Belding, Mi
Senior Member
Shaun,
I don't have many of the schematics you need, I have PC105 and PC803. I did have at one time 5 full Kustom notebooks with all their schematics. I sold them last year to a collector in NV.
One huge problem Kustom had with the A series amps with the white mallory caps. The gals soldering the boards would solder the cap and then try and wiggle it to make sure it was secure on the board. Problem was, the leads were solid on the board, but from the heat in soldering, the leads would pull loose inside the cap, and kustom had a lot of failures in the field with bad caps. Once Kustom saw what was happening, they instructed the girls not to try and move the caps until it had time to cool down inside the cap. Problem solved, and when the B series amps came out, kustom stopped using the white caps.
Don
Re: A4 Hiss and Hum [message #12597 is a reply to message #12596] Thu, 31 December 2009 09:00 Go to previous messageGo to next message
Shaun_Musings is currently offline  Shaun_Musings
Messages: 323
Registered: April 2009
Location: Reading, MA
Senior Member

How can I get the schems from you?

Well, the caps aren't broken... I was acting on the advice of someone who suggested these should be replaced. I'll leave them alone. The transistors, NTE 123AP, are these hard to get in or out? Anything I should watch out for?


Born on the Baaaaayou (Ok, in Massachusetts)
Re: A4 Hiss and Hum [message #12608 is a reply to message #12597] Thu, 31 December 2009 17:10 Go to previous message
pleat is currently offline  pleat
Messages: 1452
Registered: June 2004
Location: Belding, Mi
Senior Member
Shaun, you can send me a email at djt@chartermi.net and I'll send you what I have. Given the history of the white mallory caps, I'd replace them as long as your changing transistors. I'm not a tech, but I do know you should use a heat sink when replacing transistors. Too much heat soldering them in can cook them. I assume that your trying to get excessive hiss and hum out. I'd take the advice of stevenm and replace the caps as well.
I know I've always had reservations on buying any of the A series heads, knowing that I don't want to replace all those mallory caps, knowing the early problems kustom had with them, and adding 40+ years in the mix, I stick with with the B series heads.
Don
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