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Jet City JCA20H Choke Mod

What does a choke do one might ask, chokes help to reduce ripple and improve the stability of the DC voltage. Be careful if you perform this type of task because you are working with high voltages.

So after reading several forum post such as HC and others, I acquired the parts necessary to perform this improvement. This mod will tighten up the DC voltage and will remove any type of inherent 60 cycle noise that maybe prevelant. With the improvement. The supply voltage will be more stable. Both of my Jet City Amps will receive this mod along with a future depth mod as well (parts need to be purchased).

Before I disassembled the amp, I did take voltage reading so I have a comparison of DC voltage and AC ripple voltages using my DVM. This will be my starting point. I will remeasure the voltage again once the choke is installed. That way I have a record for future repairs if necessary.

The Parts
I bought the hardware from my local OSH

*Machine Screws - M4-.7x12MM
*Flat Washers - M4
*Nylon Lock Screws - M4-.7

Philmore Split Washers from my local Electronics Supply
The choke - Hammond 156M - from Mouser Electronics.

Total Cost - about $20-$22

PartsUsed.jpg?t=1303099081

Board Layout
The choke will replace R3 on the board

BoardLayout.jpg?t=1303099081

Parts Removed to Perform Mod

PartsRemovedforAssembly.jpg?t=1303099081

Choke Installed

ChokeInstalled.jpg?t=1303099081

Close of Installed Choke

CloseupofChokeInstalled.jpg?t=1303099081

Shot of Screws Installed on Board Side

Notice the new M7 nylon lock screws to the left of the board. One is located above the blue and black wires and the other to the left of D2. Notice resistor R3 which is Pink, this is what the choke will replace.

ChokeSupportParts.jpg?t=1303099081

I will be wiring this up tomorrow and then work on installing 1 ohm resistors for easy bias adjustment.
Responses
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Re: Jet City JCA20H Choke Mod

4/18/2011 10:12 AM

Michael Laurance (4040) wrote:

How hard is it to replace one? I have an old Fender Bassman head that blew up at a gig and it was suggested that it's likely the choke.

Thank you for sharing the mod. Looking forward to hearing more about it.

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Re: Jet City JCA20H Choke Mod

4/18/2011 12:41 PM

Randy Hano (12149) wrote:

If your choke is fried, a replacement is just to drop in the new component and solder two wires. What you may need to be cautious of though was the caps affected as well. The caps and the choke form a PI network for filtering out the AC so the DC is more pure.

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Re: Jet City JCA20H Choke Mod

4/18/2011 1:29 PM

Michael Laurance (4040) wrote:

The caps look ok, at least from what I can tell.

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Re: Jet City JCA20H Choke Mod

4/18/2011 2:58 PM

Randy Hano (12149) wrote:

Use your nose Mike and smell around the area. A bad smell potentially means bad component. Also look at the caps to see if you have no buldging on what looks like black rubber. If they are buldging, you have leakage.

Did you amp burn up the fuse as well?

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Re: Jet City JCA20H Choke Mod

4/19/2011 8:39 AM

Michael Laurance (4040) wrote:

Yeah, fuse went, and the amp smells burned. No bulging caps, fortunately.

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Re: Jet City JCA20H Choke Mod

4/19/2011 10:23 AM

Randy Hano (12149) wrote:

Use your sniffer to find where the smell is coming from. If you can't find it, remove the tubes, reload another fuse and turn the amp on. I you blow the fuse again, you got a short somewhere or bad tranny. Fuse not blown could indicate a shorted tube. You also may have some burnt resistor as well.

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Re: Jet City JCA20H Choke Mod

4/19/2011 12:06 PM

Michael Laurance (4040) wrote:

I looked at all the resistors. I can't find anything that looks obviously burned, but the smell is heavy. I'll try what you suggested.

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Re: Jet City JCA20H Choke Mod

4/19/2011 12:23 PM

Randy Hano (12149) wrote:

Yeah give it a roll. I am now saying Tranny.

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Re: Jet City JCA20H Choke Mod

4/19/2011 1:46 PM

Michael Laurance (4040) wrote:

Yeah, but if it was the tranny, it wouldn't power up. I get power, just no sound. Tubes light up properly and everything.

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Re: Jet City JCA20H Choke Mod

4/19/2011 3:49 PM

Randy Hano (12149) wrote:

I didn't know it powered up. You said that the fuse blew in an earlier post. Do you have DVM? If so you will need to start measuring voltages. Becareful when you do as the DC voltages are 300 and up.

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Re: Jet City JCA20H Choke Mod

4/19/2011 4:05 PM

Michael Laurance (4040) wrote:

Yeah, I'm a little afraid of that. These things can kill you if you're not careful.

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Re: Jet City JCA20H Choke Mod

4/19/2011 7:35 PM

Randy Hano (12149) wrote:

Rule one when you measure voltage, use your right arm for measurements. I will find a good ground with the black wire and then probe with the red using my right hand. I have been zapped before so safety must be taken. I could also be called Sparky when I was zapped those several times.

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Re: Jet City JCA20H Choke Mod

4/20/2011 9:35 AM

Michael Laurance (4040) wrote:

OK, that scares me a bit. Im' not a sissy or anything, just not into getting zapped hard. Is there an effective way to drain the energy first to prevent that?

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Re: Jet City JCA20H Choke Mod

4/20/2011 11:02 AM

Ken Richardson (9051) wrote:

Be careful, getting zapped the right way, or wrong way could get you killed!!



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Re: Jet City JCA20H Choke Mod

4/20/2011 11:06 AM

Michael Laurance (4040) wrote:

See? That's what I'm saying. There must be a way to prevent it, though, or tube amps would be killing people every day. There's a procedure that Weber goes into, but I can't recall what it is, and I don't know if it would prevent anything like this happening.



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Re: Jet City JCA20H Choke Mod

4/20/2011 12:30 PM

Randy Hano (12149) wrote:

Always keep your left hand behind your back. Use your right hand to measure. Your heart is on your left side of your body and would be in a direct path with your left arm, that is why you use your right.

I have been zapped by 30-40K when we used to work with monitors. This was by accident but the power was disconnected. The CRT's anodes are where the high voltage is stored.



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Re: Jet City JCA20H Choke Mod

4/20/2011 12:33 PM

Ken Richardson (9051) wrote:

A friend of mine told me something similar to that in regards to working with breaker boxes - just use one hand, the other behind your back. That way you don't have a path from right hand to left hand for the electricity to follow.



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Re: Jet City JCA20H Choke Mod

4/20/2011 12:44 PM

Randy Hano (12149) wrote:

That is correct. Using both hands you have created a direct path from right through the heart. Use only your right as the right side does not have the heart in any of the paths. It then has to be a freak incident.



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Re: Jet City JCA20H Choke Mod

4/20/2011 2:08 PM

Michael Laurance (4040) wrote:

I guess it makes sense. I can get that if a cap was holing energy and discharged it could zap you, but could a choke or any other sort of transformer store energy like that?



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Re: Jet City JCA20H Choke Mod

4/20/2011 2:43 PM

Randy Hano (12149) wrote:

Chokes or Tranny's only store energy when they are energized - AC. No AC, no energy. Caps are like batteries but the storage capability is dependant on the cap size (measured in FARAD's)



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Re: Jet City JCA20H Choke Mod

4/20/2011 9:43 PM

Warren Hunt (5730) wrote:

My choke should be here tomorrow and installed by Wednesday next week. Delay due to Easter Break this weekend.



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Re: Jet City JCA20H Choke Mod

4/21/2011 9:55 AM

Randy Hano (12149) wrote:

Cool. Look for the follow up thread sometime today. I worked on it so more last night. Just need to bias and jam now.

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Re: Jet City JCA20H Choke Mod

4/20/2011 12:25 PM

Randy Hano (12149) wrote:

For changing caps? Sure. Take a 10K resistor and hookup leads to each side of the resistor. Clip one side to ground, and attach the otherside to the positive side of the cap. In a second or two it is discharged.