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Re: Schematic Accuracy [message #20176 is a reply to message #20097] |
Tue, 18 February 2014 12:24 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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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 #20179 is a reply to message #20097] |
Tue, 18 February 2014 13:58 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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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 #20182 is a reply to message #20097] |
Tue, 18 February 2014 21:38 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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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 #20185 is a reply to message #20097] |
Wed, 19 February 2014 01:44 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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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 #20189 is a reply to message #20097] |
Wed, 19 February 2014 13:20 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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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 #20192 is a reply to message #20097] |
Wed, 19 February 2014 17:06 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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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 #20201 is a reply to message #20097] |
Thu, 20 February 2014 06:49 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4772 Registered: June 2004 Location: NY
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Senior Member |
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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] Report message to a moderator
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