| Getting a K200 up to snuff. [message #22535] |
Sat, 28 February 2015 14:26  |
ARKustom
Messages: 37
Registered: January 2014
Location: United States
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I've got a K200 Frankenstein head with reverb and vibrato I'd like to get back to top condition. I'm pretty proficient with a soldering iron. It sounds pretty good, but the vibrato isn't working and the reverb is hummy. I want to get it sounding like the day it left the factory. What do I need to do to accomplish this? It's also got a layer of gunk(smoky bar residue?) on the inside. Is there anything safe to clean the boards and components with? Thanks.
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| Re: Getting a K200 up to snuff. [message #22536 is a reply to message #22535] |
Sat, 28 February 2015 14:50  |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Hello,and welcome!
I good stiff paint brush can remove a lot of the dust while you hold a vacuum cleaner to suck up the dust, I have also made good use of a can of spray on flux remover to get the boards washed off but you need to do such out side due to the vapors most times.
Your vibrato may not be working due to the Opto couplers lamp being dead, which is 1301 on the schematic you can find on this site.
You will find this lamp and its solar cell in black shrink tubing on that 301 board and if you have the effect on and look at that Opto coupler in a dark area you should see that lamp flashing on and off if the bulb is good.
It's a common bulb, and can be replaced but I forget the industry number for it right now, you can also still by the whole Opto coupler new.
You reverb issue could be due to the pan itself, or someone may have had it out and remounted it back in the wrong way which will make for a hum issue.
Does sound like it had a normal amount of reverb going on?
If your going thru the amp top to bottom I would also go about replacing the resistors behind the 4 output transistors as it does not take much of a drift in there resistance value to knock near 40 watts of power off of the amp as I had happen to one of my two Frank heads, yea you gotta get the can filters out of the way which can be a bit of a pain, but your only taking about 4 bucks in parts!
The new resistors should be within 5% of each other also!
[Updated on: Sat, 28 February 2015 15:03] Report message to a moderator |
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| Re: Getting a K200 up to snuff. [message #22539 is a reply to message #22535] |
Sat, 28 February 2015 15:30 |
ARKustom
Messages: 37
Registered: January 2014
Location: United States
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I didn't see any light flashing from the opto coupler. Is it possible that the shrink wrap obscures all light, or should I see it through the shrink wrap? The vibrato used to work sometimes if you turned the amp off and then back on a couple of times.
[Updated on: Sat, 28 February 2015 15:47] Report message to a moderator |
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| Re: Getting a K200 up to snuff. [message #22547 is a reply to message #22535] |
Sat, 28 February 2015 18:04 |
ARKustom
Messages: 37
Registered: January 2014
Location: United States
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Member |
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Also, upon visual inspection, one of the wires from the diodes(?) (4 of them on a metal plate on the right of the can caps)is not connected. It appears to go from the left(nut) side of the 2nd one from the front around the back of the plate to the right side of the one closest to the rear of the chassis. I don't think I have knocked it loose during the time I've had the chassis out. Would the amp still have been working like this, or did it probably happen recently? Should the wire be connected to the right side of the most rearward diode?
[Updated on: Sat, 28 February 2015 18:09] Report message to a moderator |
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| Re: Getting a K200 up to snuff. [message #22551 is a reply to message #22535] |
Sun, 01 March 2015 02:08 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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It would depend upon which wire from the diode was unhooked. It would either make the amp not work at all, or it could cause the amp to have issues.
I'm not sure that I understand which wire and which diodes you are talking about. There are four diodes that are connected to form a basic bridge. If you look at the schematic you should be able to figure out where the loose wire goes.
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| Re: Getting a K200 up to snuff. [message #22575 is a reply to message #22535] |
Mon, 02 March 2015 12:33 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Sad about Radio Shack. I just went into one of the closing stores and bought a can of DeoxIt for half price, which made it a fair price. I used to buy stuff from there all the time. If I needed a simple part, I could get one there and not have to make a big order someplace. As society changed, they slowly stopped carrying parts in the stores and started to do nothing but sell phones.
If the overall power is low, the first thing I check is the driver transistor emitter bypass cap C203. If it dries up and goes open, the amp will lose all power.
As for Steve's suggestions, none of those changes will hurt anything, but I don't think that they will really do anything to help in case of a major output fault.
The fuse is really there to protect the speakers from being hit with any voltage when an output transistor shorts and puts dc on the speaker line. The output transistors are rated for 15 amps, so a 4 or a 5 amp fuse will not make much of a difference.
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| Re: Getting a K200 up to snuff. [message #22594 is a reply to message #22535] |
Wed, 04 March 2015 21:57 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Both of the effects are on one board, so if the amp is working and just the FX are dead then look at the side board. The reverb control is a balance control, so if the reverb is dead, the reverb control will work like a volume control in reverse. If you shake the tank do you hear the springs through the speaker output?
The vibrato circuit is always passing signal. The effect is caused by the light bulb adding in the out of phase signal through the photocell. I find that the quality of the photocell will make a big difference in the depth of the vibrato effect.
What brought the amp back to life?
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| Re: Getting a K200 up to snuff. [message #22598 is a reply to message #22597] |
Thu, 05 March 2015 00:44 |
ARKustom
Messages: 37
Registered: January 2014
Location: United States
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I can hear the reverb springs through the amp with movement, but the amp's volumhe stays the same when I turn up the reverb level. I'm not hearing a change in tone from the vibrato controls like I did before. I also replaced the rca cables on the reverb.
[Updated on: Thu, 05 March 2015 00:45] Report message to a moderator |
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| Re: Getting a K200 up to snuff. [message #22599 is a reply to message #22535] |
Thu, 05 March 2015 02:19 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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If you look at the schematic the signal from the FX preamp goes directly to the input of the vibrato circuit at Q305. The signal comes out of the vibrato circuit at the emitter of Q307 through C314. From there it goes to two places, the input to the reverb driver circuit at Q301 and to the CCW lug of the reverb depth control.
If you hear the springs, then you know that the reverb return circuit is all working. The output from the return circuit comes from Q304 and connects to the CW lug of the reverb control. The output to the power amp comes from the reverb control wiper (center lug). If you don't hear a change in the volume of the straight signal when you turn up the reverb control, then check the wiring to the pot and be sure that it follows the schematic.
If you don't hear any vibrato, check to see that the bulb is still flashing. The two controls will control how fast or slow the flashing rate is and the depth control will change how bright the bulb flashes. If all is right with the bulb, then check to see that the photocell is connected to the circuit. With the amp turned off, you can read the resistance of the photocell and see if it changes when it is exposed to light. Use a small flashlight to shine into the heat shrink tubing. It should read very high resistance when totally dark and low when exposed to light.
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