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Reverb Hiss Kustom Lead I [message #16309]

Sat, 22 October 2011 06:08

blindmouse
Messages: 25
Registered: February 2011
Location: The Netherlands

Junior Member

I fixed and restored my recently purchased Kustom Lead I. The only thing I still need to do is replacing the big filter caps. (I did put in some new F&T's but one was open and blew two rectifiers) There still is a small issue: turning up the reverb from zero an increasing amount of hum is added to the signal. I cleaned all reverb connectors. Now the MC1458 dual opamp provides the reverb signal. Could it be this opamp that causes the hiss?

http://i839.photobucket.com/albums/zz318/jooptraynor/1976%20Kustom%20Lead%20I/PICT00221536x1152.jpg

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Re: Reverb Hiss Kustom Lead I [message #16312 is a reply to message #16309]

Sat, 22 October 2011 10:32

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Are you getting hiss and hum, or just hum, as you are not clear on this.
Hum is due to the return/output side of the reverb pan being too close to the power transformer and kustom always tryed to mount the pans in a way to keep the output side of the pan on the oposte side of the amp, Fender want so far with the first head with built in reverb as to add 6 inches to the height of the cabinet to back the pan away from the PT.
With any amp regardless of make turning up the reverb will always add some degree of hum unless the amp has digital effects.

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Re: Reverb Hiss Kustom Lead I [message #16313 is a reply to message #16312]

Sat, 22 October 2011 12:10

blindmouse
Messages: 25
Registered: February 2011
Location: The Netherlands

Junior Member

I'm so sorry. I wrote the word 'hum', I meant the word 'hiss'. There is no hum on the reverb. There is a hiss that troubles me. Embarassed

Some updating:
- There is also that so called 'frying eggs popping' sound;
- The hiss and frying eggs sound stay when drive and volume are down to zero.

[Updated on: Sat, 22 October 2011 14:04]

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Re: Reverb Hiss Kustom Lead I [message #16314 is a reply to message #16309]

Sat, 22 October 2011 18:15

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

That noise sticking around when those two controls are down means trouble in the driver/output stage. my moneys on the drive
.
The hiss means there is alot of gain going on for one or more reasons in the reverb send or recovery end of the circuit. It could be due to a electrolytic bypass cap shifting way up in value, or it could be that IC going south.
If you pull the input jack from the pan and the hiss goes away then its coming from the input/drive side of the circuit.

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Re: Reverb Hiss Kustom Lead I [message #16321 is a reply to message #16314]

Sun, 23 October 2011 10:37

blindmouse
Messages: 25
Registered: February 2011
Location: The Netherlands

Junior Member

Well, some work to do.
I just ordered some 2200uF/63V Vishay filter caps, some 3A 1000V rectifier diodes and some 4558 dual op amps. When they come in I'll start working on the amp again. Played it through a reissue Jensen C12R. I really did like that sound.

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Re: Reverb Hiss Kustom Lead I [message #16339 is a reply to message #16321]

Sat, 29 October 2011 06:33

blindmouse
Messages: 25
Registered: February 2011
Location: The Netherlands

Junior Member

I got the stuff in and first started to replace the 1900uF/40V filtercaps with the new 2200uF/63V ones. Fired it up with a '100W lightbulb in series' and it all looked good. Then gave it the 230V and played the amp. Sound was good, a bit more bassy, but I adjusted that with the bass-knob. The I tried to put in the 3A 1000V rectifier diodes. The leads were too thick and I replaced the two original ones I hadn't replaced before with 1A 1000V ones. Now I noticed these four rectifiers came in two pairs: the blown ones are 500V and the ones that did not blow are 300V ones! The rectifiers done I fired up the amp and everything was ok. Now I put in a new dual op-amp for the reverb/tremolo circuit. The original one, a 1458 I replaced with an upgrade, a 4558, I also installed an op-amp adapter. When I fired the amp up I noticed a difference in sound. It sounded cooler, flat, the emotion had gone. I put back the original 1458 and that good old familiar sound was back again. Are my ears fooling me???

Changing out the op-amp didn't take away the 'frying eggs' sounds. Cleaning the reverb pot didn't either. When I disconnect the reverb pan completey (both input and output) the unwanted sound is still there. Any ideas????

Thanks and kind regards.

[Updated on: Sat, 29 October 2011 06:48]

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Re: Reverb Hiss Kustom Lead I [message #16341 is a reply to message #16309]

Sat, 29 October 2011 09:59

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Just because the new chip will drop in and work is no garranty its bias to the ideal level, also even chips with the same part number from different production runs and or makers will sound somewhat different.
If you still cared to play around with the new chip you could measure the old chips idle current, install the new chip, take measuremnts and then change out the circuits bias resistors to match the current as seen with the old chip.
If you unplug the harness from the preamp board(s) to the driver board/output board and the hiss remains then the noise is in the driver board.
If thats the case I would just shot gun it will all new transistors, as in the end it saves time and cost less than 10 bucks.
Also replace any electrolytic caps on that board with ones with a higher voltage rating.

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Re: Reverb Hiss Kustom Lead I [message #16346 is a reply to message #16341]

Sat, 29 October 2011 14:48

blindmouse
Messages: 25
Registered: February 2011
Location: The Netherlands

Junior Member

No hiss, no 'frying eggs' sound with the pre amp board disconnected! So it is the pre amp board I guess. Could it be the 80848 IC (I3 on the PC5129 board) that's causing trouble?
Does solid state amps have 'plate resistors' like tube amps? Bad tube amp plate resistors can cause the weird frying eggs sounds.

Thanks

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Re: Reverb Hiss Kustom Lead I [message #16347 is a reply to message #16309]

Sat, 29 October 2011 21:27

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Yes, the other chip can also be the cause of the noise. These solid state amps do not have plate resistors, but some of the resistors have a similar function and can also contribute to the noise.

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Re: Reverb Hiss Kustom Lead I [message #16351 is a reply to message #16347]

Sun, 30 October 2011 14:16

blindmouse
Messages: 25
Registered: February 2011
Location: The Netherlands

Junior Member

Well, I think it's about time I go to an old friend of mine. The guy has a scope and is a lot more into the transistor thing than I am.

Thanks

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