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| Re: Reverb not working [message #20659 is a reply to message #20657] |
Tue, 27 May 2014 13:01   |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The first thing to check is the reverb tank. If you gently shake the cabinet with the amp running do you hear the reverb springs rattle through the speakers?
If you do hear the springs, then the return circuit and the output of the tank are okay and the problem is either in the drive circuit or the tank input. If you do not hear the springs, then the problem could be anywhere.
What sort of skills do you have regarding working on amps?
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| Re: Reverb not working [message #20661 is a reply to message #20660] |
Tue, 27 May 2014 18:30 |
smackoj
Messages: 67
Registered: September 2013
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Member |
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hi again and thanks for the help. I believe this is a 200A model due to dating the PT to 1968. with no 1/4" plug in the ft sw jack I get a horrible, loud buzz. with a 1/4" plug in the jack I get nothing. turning the controls makes no diff just normal sound of amp with no effect engaged. slapping it does not produce any sound thru the spkr.
should I be peeling the black covering on the pan next?
I have a good amount of time working on amps but not a lot of that has been tracing faults on pcbs.
gracias, jack
[Updated on: Tue, 27 May 2014 18:31] Report message to a moderator |
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| Re: Reverb not working [message #20667 is a reply to message #20666] |
Wed, 28 May 2014 12:49 |
smackoj
Messages: 67
Registered: September 2013
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So sorry, early Alsheimer more than likely. It is not a PA head. It has two channels. Channel 1 has reverb, speed, intensity, vol. bass, treble and a ft switch jack on the lower left (PC303). Channel 2 has I'm pretty sure Select Boost and Harmonic Clipping effects (PC403) with a ft. switch jack on the lower right. This model did use a ground wire from the reverb tank to the chassis also. I hope this helps get us all on the same page.
thanks jack
[Updated on: Wed, 28 May 2014 13:15] Report message to a moderator |
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| Re: Reverb not working [message #20671 is a reply to message #20657] |
Wed, 28 May 2014 22:12 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Having the reverb spring sounds prove that the return circuit and the tank output coil is okay. So the problem is either in the drive circuit or in the reverb tank input coil.
Do you have an ohmmeter? If you do, read across the input jack of the tank and see what resistance you get there. I'm going to guess that there should be 50-200 ohms there.
With a K200B you can turn on and off the FX with the front panel controls so for the time being leave the footswitch jack empty.
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| Re: Reverb not working [message #20675 is a reply to message #20657] |
Thu, 29 May 2014 12:31 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Either the drive circuit or the cable connecting to the tank.
Check the cable for continuity and shorts. Make sure the RCA connectors are clean on both ends. The jack on the pc board side will not be grounded (to avoid a ground loop).
The drive circuit is centered around Q317. Test +&-8vdc voltages, test the transistor and the two tantalum caps C332 and C333. If it's not the transistor it usually is the caps. There are a total of 7 parts in the circuit, test them all.
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| Re: Reverb not working [message #20677 is a reply to message #20657] |
Thu, 29 May 2014 13:22 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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If you look at the schematic, there is a small layout drawing that shows the positions of the transistors.
Q317 is located directly above the reverb depth control. It is a generic NPN transistor and will be either a SE4002 or a PET8002, which are the two basic transistors that Kustom used in all of their amps.
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| Re: Reverb not working [message #20678 is a reply to message #20657] |
Fri, 30 May 2014 06:26 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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If I might add my 2 cents worth here it would be to get out to a local radio shack if you have one and just buy for all of 6 bucks the 3 new parts and just do the shot gun thing on it!
If the Radio shack near you still has parts section than buy a pack of 2N3904 transistors and two 10uf 16 volt or better caps, be they polarized or not,If they are marked as in polarized you will need to reinstall the new caps with the negative end strip in the same board hole as in the way the original cap came out.
I think as Bill here does that the cap 333 is shorted and pumping DC voltage thru the pan and into the recovery side amp of the reverb circuit which makes for the loud 120 hz buzz problem that you have with the amp.
Note that with the ago of these amps now that once Tanalum caps start to fail in the amp, be ready for more to go south!
[Updated on: Fri, 30 May 2014 06:36] Report message to a moderator |
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| Re: Reverb not working [message #20680 is a reply to message #20657] |
Fri, 30 May 2014 08:07 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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You can use non polar types and caps in the 25 to 30 range will last longer also.
If that c333 cap is bad you will read some amount of dc voltage across the RCA jack if unplugged at the input of the pan.
[Updated on: Fri, 30 May 2014 08:11] Report message to a moderator |
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| Re: Reverb not working [message #20682 is a reply to message #20657] |
Fri, 30 May 2014 12:23 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The tantalum caps are just a different form of a capacitor. Like any cap, they can fail by shorting or by losing value and going open circuit. In your case, either failure results is no signal getting to the reverb tank.
Caps like mylar or polys can be used as replacements, but to get equal values of capacitance, they will need to be huge by comparison to the equivalent polarized cap.
If C333 was to short, as Steve noted, there would be dc on the RCA jack. It would also cause the dc supply voltages to off from their normal + and - 8 volt values.
I hate going to Radio Shack, because I'm always frustrated at the small selection of components that they carry now. At one time you could get nearly all of the common parts that a typical amp might need. Now I pretty much only go there when I'm desperate to find a fuse. That being said, they should have electrolytic caps that are equivalent to the two tantalums that you need and probably an NPN transistor as well.
[Updated on: Fri, 30 May 2014 12:23] Report message to a moderator |
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| Re: Reverb not working [message #20685 is a reply to message #20657] |
Fri, 30 May 2014 18:03 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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I'm gonna say yes, but you need to check the actual part that you have. The typical SE4002 type transistor is a plastic TO-39 case (I think) and the leads will be arranged in a triangular pattern E-B-C. There usually is a flat spot on the rim of the case that corresponds with the Emitter lead.
Most of the 2N3904 are plastic TO-72 cases with leads in a straight line E-B-C.
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| Re: Reverb not working [message #20688 is a reply to message #20657] |
Sat, 31 May 2014 12:07 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The footswitch does exactly the same thing that the front panel switch does.
There are two transistors that turn on and off the reverb signal in the amp, Q320 and Q321. They are normal bipolar transistors being used as switches. One is an PNP and the other is an NPN. The bases of both transistors are connected together and are connected to the low voltage supplies via the resistor divider network of R388, R389 and R390. Turning on and off the reverb will cause the voltage at the bases of the transistors to change from a positive voltage (reverb on) to a negative voltage (reverb off). Because the two transistors are of reverse polarity, when one turns on the other turns off.
Because the noise stops when you turn off the reverb (is this correct?) you know that the switch is changing the control voltage at the bases of Q320 and Q321, and that Q321 is working, shunting the reverb signal to ground (no hum). Therefore the hum is being generated in the reverb return circuit, or the reverb tank output, or the cable that connects the two.
If you pull the RCA cable from the tank output does the hum change? If you ground the tip of the RCA cable, thereby grounding the input of the return circuit does the hum change?
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| Re: Reverb not working [message #20690 is a reply to message #20657] |
Sat, 31 May 2014 21:17 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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If grounding the input of the return circuit quiets the amp, then the problem is either the tank output or the cable itself. Is there a wire grounding the tank chassis?
Check the tank, are both RCA jacks grounded to the case? Read the resistance across the output RCA jack. There should be 180-200 ohms there.
Test the cable for continuity of the hot and shield conductors. Clean and tighten the RCA connectors. You may need to use some DeoxIt to clean the metal parts.
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| Re: Reverb not working [message #20700 is a reply to message #20693] |
Mon, 02 June 2014 11:50 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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If the hum does not respond to the reverb control, then I don't understand why the hum stops when the input of the return circuit is grounded.
As Steve mentioned, the output side of the tank must be as far away from the power transformer as possible.
Additionally, the tank grounding must be done carefully to avoid a ground loop. The original tank had both input and output jack grounds connected to the case and only the return side cable shield was connected to circuit ground. The drive side shield was left floating. If all of this is true in your amp, then there should not be a separate wire grounding the case.
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| Re: Reverb not working [message #20703 is a reply to message #20700] |
Mon, 02 June 2014 20:21 |
smackoj
Messages: 67
Registered: September 2013
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Member |
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I checked the rev. again. The hum does get louder as the rev. control is turned up. I don't have the tank even close to the amp. I have 2 tanks from other amps I am using for testing. I have them on the bench 3 ft from the amp with stereo cables connecting to the in and out on the preamp pcb.
The ground wire may be an add on? It is a wire soldered to the tank side and was on the orig. tank when I got the amp.
thanks amigos for all your help.
jack
[Updated on: Tue, 03 June 2014 06:45] Report message to a moderator |
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| Re: Reverb not working [message #20705 is a reply to message #20657] |
Tue, 03 June 2014 06:44 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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As Bill posted your stereo rca cables have a full shield end to end, and the send cable you use should not have the shield connected on the rca end that plugs into the circuit board send female rca.
The shield / ground should only be picked up at the pan.
Also it is normal for some hum to be added as the reverb is turned up so keep that in mind.
[Updated on: Tue, 03 June 2014 06:46] Report message to a moderator |
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| Re: Reverb not working [message #20706 is a reply to message #20705] |
Tue, 03 June 2014 06:47 |
smackoj
Messages: 67
Registered: September 2013
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Member |
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thanks, I'll modify the cable end and try it again tonite when I get back home.
p.s. on my post before this one. the orig. tank is bad and the only way I get the rev. effect is with a different tank I had laying about the shop.
jack d
[Updated on: Tue, 03 June 2014 06:48] Report message to a moderator |
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| Re: Reverb not working [message #20707 is a reply to message #20657] |
Tue, 03 June 2014 09:56 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The cable should not need to be modified. If I remember correctly the RCA jack on the pc board does not have the shell connection hooked up to circuit ground on the drive side.
Edit: What tank are you using to test with? It needs to have a higher impedance input coil to work with this amp, as well as both RCA jacks grounded.
[Updated on: Tue, 03 June 2014 13:03] Report message to a moderator |
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| Re: Reverb not working [message #20713 is a reply to message #20657] |
Tue, 03 June 2014 23:34 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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I have one of the folded line tanks that sounds great with Kustom heads too. Must be the beautiful girls that made it. Any reverb tank with a med or high impedance input coil will work with a Kustom amp, so any of your tanks will be good for testing.
I can't stress enough the ground wiring of this and all amps for that matter. Most Fender amps fix the problem by having the input jack isolated from the case. Mesa runs a non shielded send wire. Kustom lifted the ground of the drive signal socket. If somebody "fixed" the amp by running a separate ground wire to the tank case or by grounding the RCA drive jack on the pc board, you may need to revert the circuit to the original configuration to get the buzz to clear up.
Woodson amps were built just like Kustom amps, I think the story was that it was a relative of Bud Ross.
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| Re: Reverb not working [message #20714 is a reply to message #20713] |
Wed, 04 June 2014 06:19 |
smackoj
Messages: 67
Registered: September 2013
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Member |
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thanks for the excellent info CB. I'll do some A-B testing later today when I get a chance to hit the shop.
this Woodson head looks identical inside. like one or more of the orig. factory guys/gals put it together.
jack d
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