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Repair [message #28494]

Wed, 06 April 2022 10:12

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

These amps always have a certain level of noise / gain hiss to them at around 2khz.
Does the volume control in either channel make a difference in the level of hiss being heard?

In terms of the reverb hum that is most likely due to the pan having been taken out of the amp and reinstalled the wrong way .
The end of the tank with the connection marked output can not be the end closest to the power transformer which is on the right side when viewing the open chassis from the front of the amp.

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Re: Repair [message #28495 is a reply to message #28494]

Wed, 06 April 2022 17:29

NLKustom
Messages: 50
Registered: March 2022

Member

stevem wrote on Wed, 06 April 2022 10:12

These amps always have a certain level of noise / gain hiss to them at around 2khz.
Does the volume control in either channel make a difference in the level of hiss being heard?

In terms of the reverb hum that is most likely due to the pan having been taken out of the amp and reinstalled the wrong way .
The end of the tank with the connection marked output can not be the end closest to the power transformer which is on the right side when viewing the open chassis from the front of the amp.

Stevem, is this a reply to my question?. if so, the volume control doesn't make a difference to the power hiss you hear, only if you turn them to max it increases a bit.
As for the reverb, I changed one RCA chord because it failed the ohm test on the shield side. The tank is in the right way with the output plug farthest away from the transformer. Actually it wont fit the other way, the rca outputs would hit the back of the amp making it impossible to install. Still think it's a ground issue though. When you turn up the reverb knob, you hear a loud hum, and the volume decreases. The volume also decreases a bit when you turn on the intensity knob.

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Re: Repair [message #28497 is a reply to message #28494]

Thu, 07 April 2022 07:27

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

In this model amp( and many other Kustom's ) only the rca cable used on the output side of the tank should have its outer cable shield making a connection from end to end.

If the cable used on the input side is the same then you will have a problem making ground loop taking place.
The input cable should not show a connection being made from the outside of the preamp board mounted rca jack to the pan's rca jack.

If you look in this sites schematic section at PC1000 you will see how it needs to be wired.

The condition your having of the volume decreasing leads me to think that your tank has a problem, or it's a replacement tank of the wrong type .

Is there a date code on the tank?

With the output cable plugged in and the reverb turned up if you bang on the tank hard do you hear the reverb springs crash?

Let's get this issue ironed out before we go back and look at what may be a hiss problem.

[Updated on: Thu, 07 April 2022 07:31]

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Re: Repair [message #28499 is a reply to message #28494]

Thu, 07 April 2022 16:25

NLKustom
Messages: 50
Registered: March 2022

Member

Thanks for the help Steve.
So I checked the tank again, the RCA chord going to the "input" jack is not showing 0 on the ohm meter on the shield connectors.
The RCA chord going to the "output" jack is showing 0 on the ohm meter, so I'm assuming it's shielded.
The reverb tank has the "input" jack farthest away from the power transformers, looking at it from the front of the amp., and the "output" jack is closest to the transformers, or in the middle of the amp.
The tank is an Accutronics, these are the numbers stamped on it. 031-005-00. 25-75-301. 23572-0. 4FB2A1A. It looks like someone has peeled back the foam on both ends of the tank for some reason, to look inside I suppose. Everything looks fine in there, the foam is still in place.
When I shake the amp I can hear the reverb springs through the speakers.

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Re: Repair [message #28500 is a reply to message #28494]

Thu, 07 April 2022 18:35

NLKustom
Messages: 50
Registered: March 2022

Member

Update:
So it turns out the reverb tank was put in backwards with the output by the power transformer. Switched it around all the reverb hum disappeared. With no loss of volume. Thanks for the valuable info Steve.
But I am getting a bit of loss in volume and a bit less clearer tone when I turn on the intensity effect.

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Re: Repair [message #28504 is a reply to message #28494]

Thu, 07 April 2022 19:12

NLKustom
Messages: 50
Registered: March 2022

Member

Steve, another question.
I installed a 3 prong power chord to this amp, so should both RCA tank reverb cables be shielded?

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Re: Repair [message #28506 is a reply to message #28494]

Fri, 08 April 2022 11:27

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

The up grade to a 3 prong power cable has no impact on the reverb circuit .

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Re: Repair [message #28508 is a reply to message #28506]

Fri, 08 April 2022 18:45

NLKustom
Messages: 50
Registered: March 2022

Member

Where can you get those short RCA cables with a 90 degree on one end?
From the numbers I gave you from the reverb tank, Is it the right one?
Thanks again.

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Re: Repair [message #28509 is a reply to message #28494]

Fri, 08 April 2022 21:19

rodak
Messages: 516
Registered: October 2001
Location: Georgia

Senior Member

I checked my two favorite cable sources, but neither had right-angle plug cables.
One did have a right-angle adapter:
https://www.mycablemart.com/store/cart.php?m=product_detail& amp;p=6001

They're cheap, anyway.

www.combo-organ.com

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Re: Repair [message #28510 is a reply to message #28494]

Sat, 09 April 2022 10:11

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

If I had to guess at the volume drop off issue your having it would be the two electrolytic caps at each end of the intensity control .
One is a 10 uf and if I recall right the other is a 33 Uf.
This circuit board is a good bit of a pain in the Ass to get out, so if you go thru the work to do that then replace all off the electrolytic type caps and get it done in one shot.

Is your reverb working right now?

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Re: Repair [message #28511 is a reply to message #28510]

Sat, 09 April 2022 11:14

NLKustom
Messages: 50
Registered: March 2022

Member

Steve, the reverb is working with no volume drop, but it doesn't seem overly strong. I can live with it.
Turns out the volume drop is only when I used the effects, when I turn on the intensity and use trem-vib.
I thought it was the reverb but I had both effects on at the same time.
I have pedals that do trem and vib so I won't be tearing it down for that either I guess.

[Updated on: Sat, 09 April 2022 11:17]

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Re: Repair [message #28512 is a reply to message #28494]

Sat, 09 April 2022 16:57

NLKustom
Messages: 50
Registered: March 2022

Member

Should the reverb tank have a case ground to the amp chassis?
I've seen pictures of tanks in Kustom amps with a case ground. Just wondering.

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Re: Repair [message #28513 is a reply to message #28494]

Sat, 09 April 2022 17:14

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

That ground wire was only in the early amps.

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Re: Repair [message #28514 is a reply to message #28494]

Mon, 11 April 2022 12:57

NLKustom
Messages: 50
Registered: March 2022

Member

Steve, this is info I found on the reverb tank.
Type 4FB2A1A, Accutronics, Geneva Ill.
4 = Long 2 springs
F = 1475 ohms input impedance
B = 2250 ohms output impedance
2 = 1.75-3 sec decay rate(medium)
A = grounded input, grounded output
1 = no lock
A = open side up.

Is this the right tank for my K150-8?
Concerned about the first "A" part, grounded input grounded output.

Other numbers on the tank are:031-0005-00, 25-75-301 & 23572-0.

[Updated on: Mon, 11 April 2022 12:58]

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Re: Repair [message #28516 is a reply to message #28494]

Mon, 11 April 2022 15:37

NLKustom
Messages: 50
Registered: March 2022

Member

Here's more info from amprepairparts.com
There are 3 different replacement reverb tanks listed for Kustom amps.
4FB2A1A replacement for K200-A (that's the one in my amp)
4FB2C1A replacement for K100-2,K100-5,K150-2,K200B5,K250-2,K250-4,K400-B5
4FB3D1A replacement for most Kustom amps from the 60's & 70's.

Which tank is the right one for the K150-8?

Thanks,
Greg.

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Re: Repair [message #28518 is a reply to message #28494]

Mon, 11 April 2022 19:52

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

The impeadence of your each end of your tank is right I can tell that.
Tomorrow evening I will take a look in my k100-8 to see how input is set up and I will let you know.

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Re: Repair [message #28519 is a reply to message #28518]

Mon, 11 April 2022 20:52

NLKustom
Messages: 50
Registered: March 2022

Member

stevem wrote on Mon, 11 April 2022 19:52

The impeadence of your each end of your tank is right I can tell that.
Tomorrow evening I will take a look in my k100-8 to see how input is set up and I will let you know.

Steve, the main difference in those three tanks is the 5th letter "A, C or D", which is:
A = input grounded, output grounded
B = input grounded, output insulated
C = input insulated, output insulated

Just need to figure which one goes with the K150-8

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Re: Repair [message #28521 is a reply to message #28494]

Tue, 12 April 2022 10:30

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

Most Kustom amps used a tank with the input and output grounded. The correct way to test to see which tank should be used in your amp, would be to test the ground connections on the RCA jacks mounted to the pc board.

There are two jacks, one for send and one for the return. Normally the return jack has a connected ground circuit, meaning that the shell part of the jack is directly connected to the ground trace on the pc board. Usually the send jack is not directly connected to the ground circuit and therefore needs to be connected to the circuit by the ground connection inside the reverb tank.

Unplug the tank from the PC board. Take your ohmmeter and read the resistance from both RCA jacks shell connections and see if one or both are connected to the ground (chassis) circuit. If both are connected to ground, then your tank should be insulated input grounded output. If only the return jack is connected to ground, then both jacks should be be grounded.

All of this information is only for the older plexifront amps. The slant metal front amps of the 70's used an IC chip to drive the tank as part of its feedback loop, and therefore need an isolated input jack.

So long story short, in my opinion you have the correct tank.

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Re: Repair [message #28643 is a reply to message #28494]

Sat, 25 June 2022 21:22

NLKustom
Messages: 50
Registered: March 2022

Member

On this amp, Can the "monitor output" jack, be used to plug in a external speaker or is it just a feed to the mixing board?

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Re: Repair [message #28644 is a reply to message #28643]

Sun, 26 June 2022 14:51

rodak
Messages: 516
Registered: October 2001
Location: Georgia

Senior Member

I'm sure it's line-level, meant for input into a mixing board or power amp. You wouldn't get anything plugged into a speaker - way too small a signal there to drive a speaker.

www.combo-organ.com

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Re: Repair [message #28645 is a reply to message #28494]

Mon, 27 June 2022 05:38

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

This monitor jack is set up to feed another power amp.
While it has close to 4 volts of output it does not have the needed current to drive a speaker.

This level of voltage outout is dependent on how loud you play the amp and with 3 to 4 volts of potential outout you will need to pad it down if you pump it into a mixing board.

Even 3 volts is hotter then zero DB line level input.

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Re: Repair [message #28684 is a reply to message #28494]

Thu, 21 July 2022 22:32

NLKustom
Messages: 50
Registered: March 2022

Member

Steve
After playing this amp for a bit, the reverb starts to make a buzzing noise. If you turn the reverb off it goes away, but if you give the amp a slight tap or hit, it starts to make an awful rumble noise and the buzzing goes away also. Then after 5 minutes or so the buzzing is back again.
Any ideas? It seems to be a reverb issue, the amp is pretty quiet otherwise.
Also when you turn on the trem-vib effect, the overall volume decreases and gets a little muffled.

[Updated on: Thu, 21 July 2022 22:34]

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Re: Repair [message #28692 is a reply to message #28494]

Fri, 22 July 2022 16:25

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

Have you peeled back the cover on the reverb tank to look in it ?

Your saying that your reverb is weak a long with this issue now makes me think that you have one busted spring of the two in the tank.

If so I can tell you how to fix it.

Your tremolo volume drop issue I will look into on Sunday or Monday when I get back from a short getaway

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Re: Repair [message #28774 is a reply to message #28692]

Sun, 25 September 2022 20:46

NLKustom
Messages: 50
Registered: March 2022

Member

Sorry for the long delay in a reply. Been busy with work.
Finally got a look inside the amp tonight. The springs in the tank are fine. Replaced the RCA chord to the input side with a new one and the buzzing went away.

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Re: Repair [message #28775 is a reply to message #28494]

Mon, 26 September 2022 07:11

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

Good news to hear!

Just keep in mind that the connection points on the tanks RCA connectors and on the amps boards are Aluminum.
While these do not rust, they do tarnish up to a dull gray and May times make for connection issue that mimic a cable problem.

It's very easy to atleast clean up the outer ring with just steel whool,

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Re: Repair [message #28776 is a reply to message #28775]

Tue, 27 September 2022 17:15

NLKustom
Messages: 50
Registered: March 2022

Member

Steve:
Update. Had the amp on today and the roaring came back. different than the buzz. But a roar. And if you hit the amp it roars more then stops.

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Re: Repair [message #28777 is a reply to message #28494]

Wed, 28 September 2022 06:52

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

If you turn the reverb off / down does this noise stop?
If not then does unplugging both connections from the tank stop this noise?

Your discribing of this sound as a roar kinda really does not help.

Does what your hearing sound like a AM radio tuned in between stations?

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Re: Repair [message #28778 is a reply to message #28777]

Thu, 29 September 2022 22:03

NLKustom
Messages: 50
Registered: March 2022

Member

Yes, turning off the reverb stops the noise/roar.
The sound is not the normal hiss you hear from the power being on. It's hard to describe. It's like the reverb starts acting up and begins to roar. If you shake or hit the amp you hear the springs in the tank rattle and make a loud noise, then the "roar/echo hiss" stops. The reverb goes back to normal, then it will happen again. Almost like some electrical interference brings it on. I really don't know how else to explain it.
I never tried unplugging both connections from the tank, because the unit is all put together when it happened.
And, no the sound is not like an AM radio in between stations. The AM sound is there as soon as you turn on the power. When this noise is happening you can't hear the reverb effect while playing guitar. It's like a thunder and lightning storm. Give the amp a tap so you shake the reverb springs and the noise goes away and you can hear the reverb effect with the guitar again.

[Updated on: Thu, 29 September 2022 22:15]

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Re: Repair [message #28779 is a reply to message #28494]

Fri, 30 September 2022 11:27

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

I would first suggest that you peal back the foam covering the top of the tank and see if both springs in there are still connected on each end to the transducer.

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Re: Repair [message #28780 is a reply to message #28779]

Fri, 30 September 2022 16:00

NLKustom
Messages: 50
Registered: March 2022

Member

I've looked inside the tank. the sponge was been peeled back before I bought it. The springs are attached and everything looks fine inside.
Obviously something is weak somewhere in the reverb system.

[Updated on: Fri, 30 September 2022 16:02]

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Re: Repair [message #28781 is a reply to message #28494]

Fri, 30 September 2022 20:05

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

Poor connections ( metal to metal ) with the jacks and plugs can make for a high impedance with can make the pan go nuts and feedback.

With one end of each rca cable unplugged spin the other end while it's plugged in a good number of times.
This will hopefully untarnish the connection points where there very had to
Get at.

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Re: Repair [message #28782 is a reply to message #28781]

Sat, 01 October 2022 11:29

NLKustom
Messages: 50
Registered: March 2022

Member

Steve, it seems it also acts up with the least little amount of external vibration. And the amp picks up other electrical appliances, like the de-humidifier cutting in and out makes the amp pop. Also picks up the dimmer on my pot lights, especially if I'm using my 2015 Gibson Traditional with the 50's wiring.

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Re: Repair [message #28783 is a reply to message #28494]

Sat, 01 October 2022 15:11

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

So then if you totally unplug the tank does the issue go away?

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Re: Repair [message #28790 is a reply to message #28783]

Fri, 07 October 2022 08:57

NLKustom
Messages: 50
Registered: March 2022

Member

Steve:
I haven't tried it with the tank unplugged, but with the reverb turned off, most of the noise isn't there.
With just the amp powered on, you can hear a low frequency "hiss", the thing is not quiet by no means. and every now and then you hear what I can only describe as low distant rolling thunder, and it still pops when the de-humidifier cuts in, but not as loud. But that might be a grounding issue with my electrical system.
I want to take it to my local dealer for a check up , but don't know if those young guys know much about these old amps and their inner workings. I don't want to be replacing things un-necessarily.
Also sometimes the reverb won't work at all until you give the amp a tap, then it makes a little roar and kicks in.
There is a loose connection there somewhere. Taking it to my music shop tomorrow, gotta leave it. Will let you know what I find out.

[Updated on: Fri, 07 October 2022 18:33]

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Re: Repair [message #28791 is a reply to message #28494]

Sat, 08 October 2022 12:05

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

Sounds like a bad solder connection to me and it may be at the rca jack on the circuit board where the output of the tank plugs in.

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Re: Repair [message #28961 is a reply to message #28494]

Sat, 04 March 2023 17:18

NLKustom
Messages: 50
Registered: March 2022

Member

Hi Steve,
Update on the amp.
Took it back from the shop after 4 months. The guy said there was no input going to the reverb tank, he replaced two resisters or something, now there's input going to the tank, but nothing coming out. He's saying a transducer is gone in the tank and I need a new tank. So I took the amp, everything works but no reverb.
So I need to get a new tank, the one in there is 4FB2A1A, which is "input grounded, output grounded", don't know if that's the right one, or should it be 4FB2C1A, which is "input insulated, output grounded".?
Which tank is right for the K150-8?
I just bought the schematics online and sent the pdf to him, but there is no mention of what is the proper tank for that unit.
Help please!!!

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Re: Repair [message #28962 is a reply to message #28961]

Mon, 06 March 2023 06:48

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

So the question is now which transducer or its wiring is open.
You may just have one off the very small wires the go from the transducer to the rca jack that have broken off at the jack.

Look very close for this because many times when they brake off they do not move far enough away for them too look like they are disconnected..

These can easily be soldered back on and then with a dab of bath tub caulk be reinforced so that vibration will not undue them ever again.

Please report back on what you find.

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Re: Repair [message #28968 is a reply to message #28962]

Wed, 08 March 2023 10:05

NLKustom
Messages: 50
Registered: March 2022

Member

Can anyone here tell me which reverb tank goes in that K150-8?
I have the schematics but I can't read them.
Is it 4FB2A1A. 4FB2C1A or 4FB3D1A?
4FB2A1A is in it now, but I don't know if it was swapped out at some point.
It all depends on the grounding vs insulated connections.
Would like to know before I order one.

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Re: Repair [message #28969 is a reply to message #28968]

Wed, 08 March 2023 18:01

NLKustom
Messages: 50
Registered: March 2022

Member

So I found some info on here that the Plexi K150-8 does indeed use the reverb tank 4FB3A1A.
So I ordered one from Tubes and More part # P-RMOD-4FB3A1A and a new angled rca wire connector set.

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Re: Repair [message #28970 is a reply to message #28969]

Thu, 09 March 2023 14:35

rodak
Messages: 516
Registered: October 2001
Location: Georgia

Senior Member

Where did you buy the schematics at? Was it a PDF or an actual physical copy? I have a PDF, but would love to have a nice, full size foldout copy.

Always happy to support anyone selling these old schematics (as long as it's not horribly over-priced)

www.combo-organ.com

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