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Re: Schematic Accuracy [message #20172 is a reply to message #20097]

Mon, 17 February 2014 17:14

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

Oh boy...I just saw Q112 is in wrong. Could be a real problem...eh?

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Re: Schematic Accuracy [message #20173 is a reply to message #20097]

Mon, 17 February 2014 17:36

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

Everything working now, even tremolo again, but it still has
that thumping. Rolling Eyes

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Re: Schematic Accuracy [message #20176 is a reply to message #20097]

Tue, 18 February 2014 12:24

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

Glad to hear that it's back to working again. As we were discussing before, the thumping could be caused by transfer from wiring or through the power supply lines.

If you have an oscilloscope, try viewing the 8 volt dc supply lines around Q110 to Q112 to see if there is any ticking being induced into the power supply.

Earlier you stated that a lot of the resistors were changed in this amp. Were the 1% resistors like R156-R159 changed? If they were what were they replaced with?

Were caps C133 and C134 replaced in the 8 volt power supplies? And if they were what were they replaced with?

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Re: Schematic Accuracy [message #20177 is a reply to message #20097]

Tue, 18 February 2014 13:31

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

None of the 1% resistors were changed out. Just the carbon comps with carbon film. The tantalums at Q132 Q133 were changed out to electrolytics with a higher voltage value. Last night all transistors and capacitors were checked for orientation.

I took a chop stick and adjusted lead dress (although I didn't think that in an amp with such low voltages would matter) and got no results or change in tone.

I grounded the wiper of the intensity pot with no change. I ground the wiper of the speed pot and it turns the tremolo off.

I changed the large cement 1 ohm 5 watt resistors (one of them was 260 ohm) to 1 ohm wirewound 5 watt.

I also changed the ground from the power cord from the chassis to a lug of the power transformer (just experimenting). No change. I'm going to get a friend's O scope to have a look around.

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Re: Schematic Accuracy [message #20178 is a reply to message #20097]

Tue, 18 February 2014 13:37

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

Would a change of type of capacitors cause an amplification of a stray signal?

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Re: Schematic Accuracy [message #20179 is a reply to message #20097]

Tue, 18 February 2014 13:58

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

Good catch on the ballast resistor. "Gee, I've got a resistor that's the right size to fit in there!" Unless it was meant to be R215 in the pilot lamp circuit.

Changing the tantalums for an normal electrolytic should not make any difference, at least not in these amps. As long as the correct values and voltage ratings are observed.

Interference is possible at any voltage level, frequency is probably more of a factor though.

You can try increasing the values of C133 and C134 on the output of the two 8 volt supplies. You can also try bypassing C133 and C134 with a 0.1uf cap.

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Re: Schematic Accuracy [message #20180 is a reply to message #20097]

Tue, 18 February 2014 18:25

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

Putting a .1uf across the caps did not help. I upped the 33uf caps to 100uf in the +8 supply and changed the resistor on the intensity pot back to 15k as per the specs. The thumping has decreased about 50% in volume but is still noticeable.

I checked with the DVM around the transistors you noted. All of them have pulsing voltages when the tremolo is activated. When I unground the jack, the noise stops and the voltages stabilize.

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Re: Schematic Accuracy [message #20181 is a reply to message #20097]

Tue, 18 February 2014 20:53

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

Also, I lied about the 1% resistors (not intentionally). I was just noticing that R124 a 392k 1% was changed to carbon film by someone else.

I found a 390K that actually read 392k and put it in. Don't know if that would account for the difference in the + - 8.

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Re: Schematic Accuracy [message #20182 is a reply to message #20097]

Tue, 18 February 2014 21:38

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

Was it the increase to 100uf or the replaced resistor that reduced the thumping?

The three transistors in the oscillator will/should have fluctuating voltages, as that is the function of the circuit. What I would look for is any modulation of the 8 volt supplies being caused by the oscillator circuit.

Kustom used 1% resistors not only for the value matching, but also for the fact that they are low noise metal film resistors.

Exactly how bad is the thumping?

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Re: Schematic Accuracy [message #20183 is a reply to message #20097]

Tue, 18 February 2014 23:15

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

The thumping was reduced after the increase to 100uf. The resistor had already been changed.

I see where R155 (69k) near the Intensity pot is also 1% and that has been changed to a 68k and a 1k in series using carbon film 2% from the prior mechanic. I just checked all of the 1% resistors remaining and they are all dead on with the exception of the 69k and 392k. Is not having a 1% resistor in a position that critical?

Just reassembled the amp and now there is a noise in the circuit.
Time to call it quits for today. Thanks!

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Re: Schematic Accuracy [message #20184 is a reply to message #20097]

Tue, 18 February 2014 23:17

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

To answer your question Bill...
of how bad the thumping is.... it is noticeable enough to hear
it while playing. It should be noted that the tremolo does work and you can hear the guitar coming through...just with a thump in relation to the speed of the tremolo.

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Re: Schematic Accuracy [message #20185 is a reply to message #20097]

Wed, 19 February 2014 01:44

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

The tremolo circuit is a ring/balanced modulator design centered around the four diode ring CR100-CR103. The audio signal is sent into the diode ring where it is modulated by the low frequency signal from the oscillator circuit.

The modulation driver Q113 sends a two phase signal to the diode ring. The resistor 1% values are there to insure that the two out of phase oscillator signals are as even as possible so that the diode ring will work evenly.

I'm not sure what is causing the thumping as you have checked everything that I can think of. Have you tested the four ring diodes? If you want to try finding 4 matched diodes and install them to see if that helps.

It may help to isolate the trem oscillator power lines from the rest of the power supply.

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Re: Schematic Accuracy [message #20186 is a reply to message #20097]

Wed, 19 February 2014 10:32

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

Definitely will give that a try. This all started after I changed out carbon comps to carbon film on the board in the area of the effects pots trying to eliminate an occasional crackle.

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Re: Schematic Accuracy [message #20187 is a reply to message #20097]

Wed, 19 February 2014 11:13

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

So the thumping only happened after the resistor changes?

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Re: Schematic Accuracy [message #20188 is a reply to message #20097]

Wed, 19 February 2014 12:21

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

If I remember correctly, yes. I changed the last twenty or so carbon comps in the area of the Intensity, Speed, and Reverb pots all the way to the end of the board.
None of them 1% resistors, however!

Of interest in the schematic (which does not follow my board) is in the sections that handle the +8. On both ends the resistors at R180 R181 are 1k and the other end is R190 R191 is 1.5k. All of my resistors here are 2.2k.

I also wonder what the addition of Q131 and that whole part of the circuit is supposed to address?

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Re: Schematic Accuracy [message #20189 is a reply to message #20097]

Wed, 19 February 2014 13:20

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

Resistors R180/R181 and R190/R191 are dropping resistors used to reduce the 40 volts down to a level that will make it easier for the pass transistors to regulate the 8 volts needed by the preamp circuits.

Rather than use a larger wattage resistor they opted to use two smaller resistors in parallel. The value really shouldn't be that critical as the regulator circuitry ultimately controls the final output voltage.

The added circuitry around Q131 is a non-inverting buffer that amplifies the straight signal before it is mixed in with the reverb signal at the reverb depth control. The depth control is actually more like a balance control: CCW straight signal only, CW reverb signal only. This is why when the reverb tank is bad and the reverb signal is not present, the depth control acts like a volume control.

It may be that originally there was not enough straight signal to mix with the reverb signal and keep the overall volume up to spec.

As for the earlier resistor replacement, it's not just the 1% resistors that can change things. Is it possible that you put a wrong value in there? Or swapped the position of two resistor values?

If it was working before the resistor swap, then there is reason to believe that the problem was created by the change. Go back and check what you can, and see if there is anything that seems wrong.

Did you replace a lot of transistors as well? If you did, there may have been a change in the gain of a stage or something that has caused the new problem.

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Re: Schematic Accuracy [message #20190 is a reply to message #20097]

Wed, 19 February 2014 14:02

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

I found a couple of pictures on my camera of the circuit. I used this to confirm resistor values after I changed a few going by the schematic that later turned out to be the wrong revision.

I was pretty careful and marked the positive position of every capacitor. I can see all of the resistor values with the exception of three or four. I need to review some of the values since some of these resistors is hard to tell the yellow and orange. Brown looks like purple sometime.

I just had some loud noise in the circuit with the volume all the way down. When I checked I had no voltage at the main caps.
I changed the rectifier and now everything is back. May have a solder joint going to the pilot light where the +40 comes in. Another problem to resolve. Everything working right now. Tremolo still has a bump in it. I completely rebuilt the diode circle you mentioned earlier.

Maybe I can find another one of these and use this one for parts..... Twisted Evil

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Re: Schematic Accuracy [message #20191 is a reply to message #20097]

Wed, 19 February 2014 14:05

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

As for the transistors...yes I changed them all...I guess I could test the old ones and start replacing the ones in the tremolo circuit. Just a thought.

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Re: Schematic Accuracy [message #20192 is a reply to message #20097]

Wed, 19 February 2014 17:06

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

Try and resist the temptation to change more things without at least honing in on the circuit that is causing the problem. If you are not careful, you may end up with even more problems.

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Re: Schematic Accuracy [message #20196 is a reply to message #20097]

Wed, 19 February 2014 23:21

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

Lesson well learned....thank you! Rolling Eyes

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Re: Schematic Accuracy [message #20201 is a reply to message #20097]

Thu, 20 February 2014 06:49

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Just to make you feel better, I have a K100-2 with the same thumping tremolo issue ( thats why I got it for 20 bucks) and since I have two of them the one with this problem has been side linged for a good while.
When you put up your post about your amps trem issue I pulled out my problem K100-2 and dove back into the circuit 2 nights ago.
Since it seemed the problem was in more than likly to be in the osc the first thing I did was replace / shot gun the 4 transisors and all the electro caps from the foot switch jack on back thru the circuit and got no change, then it was O scope time.
With my scope I could see the + 8 volt rail being modulated by what ever is producing the voltage pump, and at that point my 40 year old scope gave uo the Ghost, so now its electrolyltic replacement time for that first before I get back to sniffin out my K100 more!

[Updated on: Thu, 20 February 2014 07:01]

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Re: Schematic Accuracy [message #20202 is a reply to message #20097]

Thu, 20 February 2014 12:03

bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA

Member

Stevem,
You make me feel so much better knowing I'm not the only one with an amp with this problem. This amp did not have a thump until I shotgun approached the components to get rid of an occasional crackle. I suspected a carbon comp and proceeded to change them all until the crackle disappeared..(which it did). In my shotgun install, I also changed every tantalum to aluminum electrolytic and new transistors after I read somewhere that the newer transistors were much quieter than the old.
I don't know what version you have, but my board does not have the elusive Q131 and associated mod on the board. I have a transistor at the extreme corner of the board where the grey (reverb) wire is attached.
Amp plays so well, reverb works, tremolo works with a thump in the background..... Confused

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