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K100-2 Tremolo [message #21234]

Fri, 22 August 2014 12:33

Jerrybass1955
Messages: 45
Registered: August 2014
Location: NC

Member

Hello,
I am working on a K100-2 head tremolo. The tremolo sort of works, but with the intensity turned all the way down, I get a thumping/ticking sound.

I took a blind shot at the tremolo by changing resistor R152 to 15K (Handwritten on one of the schematics) and replaced C122, C123 and C124 with some caps I had around of the same value. Some parasitic oscillations went away, but otherwise no change in the tremolo problems. The tremolo speed seems to have increased, and I am guessing it is the 15K resistor, but the other components are also a bit different in measured values than the parts they replaced, so possibly a combination of things. It seemed like the 15K resistor might attenuate the oscillator output a bit, but it didn't seem to cure the problem and I may need to go back to 10K or a trim pot to get a nice speed range.

With the foot switch connected I can turn it completely off and get a nice quiet output. Without the footswitch there is always some tremolo and the oscillation that makes it to the output. Based on this, I think the on/off switching part of the circuit is fine. I assume that the tremolo oscillator is always supposed to be running if the footswitch is not attached (I see a nice sawtooth wave on the scope), so I am thinking that the issue is upstream of the oscillator or at the last stage and something is allowing the oscillator signal to get through and work its magic even with the intensity turned all the way down. DC voltages seem close enough.

So, I am wondering if someone can explain the theory of how this tremolo actually works. I am not quite following how the tremolo circuit does its thing by applying the output signal from Q113 to that diode network, but it seems to resemble a modulator arrangement. I am wondering if the diodes might be bad. If I measure them in-circuit, I don't seem to get much difference in resistance between forward and reverse, but that is sometimes not a valid test of diodes.

Also, is there anything special about these diodes? Will a 1N914 or a 1N4001 work if they are bad?

Any thoughts or advice greatly appreciated.

Jerry

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Re: K100-2 Tremolo [message #21237 is a reply to message #21234]

Fri, 22 August 2014 15:07

Jerrybass1955
Messages: 45
Registered: August 2014
Location: NC

Member

Just checked the diodes again and with power on, there is about .3v difference between anode and cathode on all the diodes, so it seems like they are probably ok?

Also, forgot to mention that the + and - 8VDC lines don't appear to have any of the tremolo oscillator riding on them and adding further bypassing on those lines doesn't seem to change anything.

Jerry

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Re: K100-2 Tremolo and Reverb [message #21293 is a reply to message #21237]

Wed, 27 August 2014 15:46

Jerrybass1955
Messages: 45
Registered: August 2014
Location: NC

Member

An update.

I worked on the reverb last night (the reverb was over the top even with the reverb turned all the way down). I replaced the reverb pan and that tamed it down a touch, but not near enough. So, after discovering that there are many values of resistors on this board that are different from the V4 schematic, I replaced several resistors on the board with V4 values. That helped a little more, but the reverb was still way over the top.

So at this point I decided to take matters into my own hands and did a little redesign. I replaced the 15K R170 with a 120K resistor to drop more of the recovered reverb signal before the reverb pot. The value was a guess, but it put the reverb into a pretty reasonable range. It still doesn't completely turn off with the control pot, but it is very low and tolerable amount of reverb when turned all the way down. When turned all the way up, the reverb is still rather tame by most standards, but probably more reverb than I am likely to ever need. So if I don't find some other reason why the reverb was so over the top, I may leave it as is.

Having discovered that there are significant differences between my circuit board and the V4 board, I will be going through it over the next few days/weeks to see what else might be funky. I have some Panasonic caps coming in in the next day or so and I will be ready to do a thorough once over.

Perhaps the most interesting thing that happened is that when I replaced R170, not only did the reverb get tamed down, but the tremolo ticking is almost completely gone! I had noticed some interaction between the reverb control and the ticking, but I can't put my finger on why yet.

Any thoughts as to how the two might be related to cause the ticking sound would be welcome.

Jerry

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Re: K100-2 Tremolo [message #21294 is a reply to message #21234]

Wed, 27 August 2014 18:17

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Here's a brief overview of the trem and reverb circuits.
Q109 is the switch that turns on and off the low freq osc. that is made up of transistors Q110, Q111 and Q112. The output of the LFO is coupled to the driver transistor Q113 through C123, C 124, R152 and the intensity pot R153. By increasing R152 you have reduced the amount of signal getting to the driver stage. The output of the driver stage is a pair of out of phase signals that feeds one side of the diode ring modulator. The audio signal from the preamp feeds the other side of the ring. When the two signal are mixed in the modulator, you get the tremolo effect.

The reverb driver is based around Q114, the return circuit from the tank is made up of Q115 and Q116. The reverb switching circuit consists of Q117 and Q118. R175 and R176 are the mixer resistors where the two switched signals merge.

When the reverb is turned on Q117 is turned on grounds out a portion of the straight signal through R174. Q118 is turned off and has no effect on the signal.

When the reverb is turned off, Q117 is turned off allowing for the full straight signal to appear at the mixer and Q118 turns on and grounds out the signal that is coming from the reverb pot.

If your reverb effect is too strong, you should look to see if the problem is that there is too much reverb signal or too little straight signal in the mix.

Please also note that Q131 which by virtue of the part number was added much later to the circuit, is there to increase only the straight signal. Does your amp have the Q131 circuitry?

[Updated on: Wed, 27 August 2014 18:18]

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Re: K100-2 Tremolo [message #21298 is a reply to message #21234]

Thu, 28 August 2014 06:29

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Jerry what is the resistance measurement you get on the input and output jacks of the reverb pan, is it between 175 and 185 ohms or so?
Does the pan say Accutrionics on it and is its metal case goldish in color?

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Re: K100-2 Tremolo [message #21305 is a reply to message #21298]

Thu, 28 August 2014 14:33

Jerrybass1955
Messages: 45
Registered: August 2014
Location: NC

Member

Thanks for the lesson ChicagoBill.

"If your reverb effect is too strong, you should look to see if the problem is that there is too much reverb signal or too little straight signal in the mix." Excellent thought - thanks and I'll see if I can figure that out! I tend to think too much reverb because it is extremely sensitive to any movement around it at all like walking or even shifting your weight. But it is an excellent perspective that hadn't occurred to me. I will check out. Seems like the dry signal is about the same with the reverb switched on as off if that is any indication, but I am going to double check that too.

There is a transistor 131 on the board.

The Original tank was about that resistance. I now have a MOD tank in there which reads around 200 on input and output. I think the original tank springs were a bit sagged but the new one is better. Both are a bit dark with a bit too much bass, so I may tweak the frequency response in the reverb circuit to see if I can make it more pleasing.

Now that my replacement caps have arrived, I am hoping to have time this weekend to work on some more serious troubleshooting. Perhaps if I have a bad cap somewhere it is throwing off some gain. And since I know the schematic is not identical to the V4 schematic, I will have to be on the watch for little oddities. Also, some of the resistors that I replaced had drifted a bit out of spec, so I am going to check the resistor values out carefully too.

Jerry

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Re: K100-2 Tremolo [message #21307 is a reply to message #21234]

Thu, 28 August 2014 17:50

Jerrybass1955
Messages: 45
Registered: August 2014
Location: NC

Member

CORRECTION - THERE IS NOT A Q131! What does the circuit look like?
Jerry

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Re: K100-2 Tremolo [message #21308 is a reply to message #21234]

Thu, 28 August 2014 21:15

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

It is a simple single NPN transistor stage with 3 resistors and a couple of caps. On the schematic it appears between Q107 and Q108.

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Re: K100-2 Tremolo [message #21310 is a reply to message #21308]

Fri, 29 August 2014 12:50

Jerrybass1955
Messages: 45
Registered: August 2014
Location: NC

Member

Sorry - should have been more specific. I meant what does the circuit look like for a board without Q131? I am trying to document the circuitry but if someone already has, a copy would be quite a nice shortcut.
Jerry

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Re: K100-2 Tremolo [message #21313 is a reply to message #21310]

Sun, 31 August 2014 20:03

Jerrybass1955
Messages: 45
Registered: August 2014
Location: NC

Member

I made some progress on the tremolo yesterday. Although the -8V line measured solid with a meter and I couldn't read any AC with the meter, When I looked on the scope I could see a very small ripple at the tremolo frequency. I replaced all of the carbon resistors and all capacitors and Q123. The problem seems to be gone! All of the parts seem to test pretty good, so I am not sure which is the culprit. But it appears cured. I still have reverb work to do so we'll see if the cure holds.

I also discovered that one side of power stage isn't working. So I have to figure that out too. Not sure if it is the driver or the output transistor or both or neither. 1 Ohm resistors are good and 100 ohm resistors were replaced since they had drifted high by about 15%. So the fun continues!

Thanks for the suggestions.
Jerry

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Re: K100-2 Tremolo [message #21324 is a reply to message #21234]

Tue, 02 September 2014 16:40

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Good catch on the power supply issue. Reading with a DMM is a good basic test, but the scope will let you see what is really happening.

By one side of the power stage is not working, do you mean one half of the push/pull signal is missing at the speaker output?

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Re: K100-2 Tremolo [message #21326 is a reply to message #21234]

Tue, 02 September 2014 17:23

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Since you only have two output transistors I guess you are missing one half of the output wave, but I do not think you would be missing the whole thing at low volume.
I would think that at low volume the amp would stay in class A mode at least for a bit, no? unless the phase splitter section is dead!

[Updated on: Tue, 02 September 2014 17:25]

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Re: K100-2 Tremolo [message #21367 is a reply to message #21326]

Fri, 05 September 2014 10:01

Jerrybass1955
Messages: 45
Registered: August 2014
Location: NC

Member

Sorry for the slow response. Yes one half of the power amp seems to not work. If I drive it hard I can see a little output from it that looks close to a square wave of low peak to peak value (didn't measure it - obviously not what it is supposed to be). I took a bit of a time out to find appropriate replacement parts for the power amp and drivers, so I should be able to start trying to figure out what is going on next week. Interestingly, it still puts out close to a watt before it starts distorting. I am testing it through a 16ohm Epiphone Valve Jr. speaker cab that is handy and even with less than one watt it is surprisingly loud and clean!

In the meantime, I replaced all of the tantalum caps (in the name of therapeutic soldering and in recognition of 40+ years of loyal service) and about a dozen or so resistors that either could be read in-circuit as significantly out of spec (sometimes because of removing a cap or another resistor) or which was part of a pair that looked like they should be better matched. In the process, something went haywire with the preamp. Signal is only getting through the first transistor cleanly. Since I didn't do too much new in the preamp area, I suspect something in the regulated + or -8V supply got fouled up when I 'fixed' it. Not too worried about it, and should be back on it some time fairly early next week.

BTW, I used Panasonic FM and FC series 50v or 35v caps as suggested where I could get correct values and Nichicon PM series for the others (I don't remember which). The tantalum caps were all quite close in value and I couldn't read any leakage or shorts. Some of the 33uF bypass caps were high (37-41) but not sure if those had high upper tolerance and were actually in spec. Resistors had generally drifted high when they were out of spec and the ones I measured ranged from almost dead on value to about 22% high. I replaced them with 1% metal film or 5% carbon film 1/2 watt resistors.

The power supply issue was a bit surprising - logical, but surprising. I would have thought I would be able to measure a little AC on a good meter, but not so. I am using a Sainsmart DDS120 USB digital scope (hooks to your PC for display). It is not as good as my real scope (Tectronix), but the real scope isn't here at my house right now. Kind of surprised that I could see the ripple on this scope and not read it on a good voltmeter - but I could. BTW, the Sainsmart scope is only about $60 and comes with two good probes. It is only 8 bit vertical resolution, so you see lots of stairsteps, but it is quite usable for the money. I may pop for the DDS140 which is still only 8 bits, but still only about $100. You can also add a logic analyzer for about $20 and a signal generator for about $30 for the DDS140 (or cheaper in a package deal). Very usable product for short dollars. If you don't have a scope or want a small portable cheap backup, I am not sure this can be beat for the price.

Jerry

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Re: K100-2 Tremolo [message #21370 is a reply to message #21234]

Fri, 05 September 2014 13:27

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

It may be that the frequency of the ac was too low for your meter to read it correctly.

I and my test gear are so old that I wouldn't know what to do with a computer based scope. I guess the shop of the future will have almost all of the test gear computer based with add-on apps for each specialized function. If I could get a 30 inch monitor and have one window open for the schematic, another for the voltmeter and another for the scope and another for the signal generator and another for the online forum... I guess I'm gonna need a 50 incher.

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