bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
After reviewing a PC104 board for values after I suspected a prior tech may have changed some things...I see where the schematic is not accurate all the time. Some resistor values are different and some caps on the board are not even listed. I thought this was only common to Silvertone's schematics. Is is true for Kustom also?
stevem Messages: 4739 Registered: June 2004 Location: NY
Senior Member
Hello.
Kustom marks there boards with for example a REV 1 on the board after for your example the PC104 marking on the board.
If you dive into this sites techical section and then go under the schemaic listings by amp you will see a early and a latter PC104 listed.
As far as what some other person may have done repair wise, who knows!
In regards to Slivertone tube amps, I have over the eon`s worked on a good many of them and in regards to the model 1484 I know there are two schemtics for that.
If I recall right the only difference in that model lie`s in change in one resistor or one cap in the tremolo circuit.
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
I saw the two versions of the PC104 listed. My board has no markings other than PC104. When there was a rogue component I consulted both versions and found the schematics were usually in agreement with each other. I searched the internet for pictures of the PC104..but there were none to be found.
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Actually this is a result of a thumping noise in the tremolo circuit I'm trying to fix. I have already completed all of the recommendations that are mentioned on this forum. It has been recapped, cleaned, treated with Deoxit, new bridge rectifier, replaced every tantalum capacitor, all new transistors, new connections wires, all diodes checked, new three prong cord. The amp works fine (reverb and tremolo) and is now "studio" quiet, but has a thump in the tremolo that is affected only by the speed pot.
Like all manufacturers, all of the production changes at Kustom may or may not have been fully documented. And even if they were, they may not have made it to the general dealership network where most of the available on-line schematics have come from.
I usually don't worry about mismatches unless there is a known common problem that can be fixed by a circuit revision.
Regarding the tremolo thumping, when you turn down the intensity control does it go away? And of course, if you turn it off with the footswitch it stops, correct?
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Thanks Bill,
I saw different resistor values in about five different areas of both schematics so your explanation seems to fit.
The tremolo thump is on only when the tremolo footswitch is activated. The speed control speeds up the thump and the pitch. The intensity has no effect. Even if the volume is turned down, one can still hear the thump. Occurs with the reverb tank installed or removed. I probably need an O scope at this point but the one I have is broken. I'm trying to do this by feel and some good luck.
There are a few ways that the trem could cause a constant thump. One is because the intensity control doesn't really turn off the modulating signal and the another way is through the power supply lines.
Try directly grounding the wiper of the intensity pot and see if that helps. If the intensity control is working then try checking the plus and minus 8 volt lines for any signs of interference.
Beyond this, there could be interference from wiring or lead dress.
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Thanks Bill
gives me some idea of where to look. I do note that when the intensity pot is turned all the way up the thumping seems to be muffled somewhat.
It didn't do this before I changed the tantalums to electrolytics and changed most of the carbon comps to get rid of the an intermittent crackle, that is now gone! Fix one problem and get another...!
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Grounding the intensity pot did not help. I did notice that the +8 is at +10.6 and the -8 side is at -9.4. I'm assuming that it is higher due to higher line voltages since the sixties?
The higher voltages could be caused by modern line voltages, but I'm more surprised to see the big difference between the two 8 volt supplies.
The negative supply tracks the positive one, so you don't normally see that much difference between the two. Maybe that could contribute to the thumping problem.
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Yep. something is amiss with the dropping voltage near the speed and intensity pots. I see four or five volts higher in that the areas of the pots. Something is not dropping the voltage like it should. The voltage at the main caps is +40.4 and -40.2 which is about right. That's why I was confirming components to the schematic. I think a Bubba Mechanic has been in here..!
stevem Messages: 4739 Registered: June 2004 Location: NY
Senior Member
How about the caps in that circuit?
The stock ones are not rated for the 40 volts that they get hit with if a transistor fails in those two regulator circuits.
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Changed every tantalum to electrolytic and, when available, the voltages were increased. The only ones not bumped were two 6.8uf at 15v. It was all the supplier had.
I just noticed that two transistors Q124 Q125 are labeled on the schematic as 2N4249 and also RCA 38734. The first is a NTE159 part and the second is a NTE129. They both show 80 v rating and are PNP. Why would they list both?
The +8 and -8 are at -9.9 and +10.4. I'm changing that thermal diode 1N3754 just for kicks since voltages seem to be weird in that area of the board.
I'm beginning to not like solid state. Tube amps are so much simpler.
I don't own a 100 head so I can't help with a photo.
The question regarding the two PNP transistors is simple, while each one is rated for 80 volts they have different current ratings. The bigger one is used as the pass transistor for the regulator while the smaller one is just used to control the bigger one.
The 1N3754 diode is part of the bias string for the two output transistors. The only thing that it will change is the voltages at the bases of the drivers Q127 and Q128.
As for the caps in the regulators, unless there is a major circuit meltdown, they will never see the 40 volts from the main power supply.
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Hi Bill
Thanks for your input.
Maybe I didn't explain it well enough about the transistors on the schematic as Q124 Q125 They are both labeled with BOTH numbers as 2N4249 and RCA 38734. Is this suggesting that both are acceptable?
I'm looking for a picture of the board since there are three components on this board NOT listed on the schematic. ie..Q131 is listed on the schematic but not listed in the transistor location diagram and not found on the board. I have a .1 100volt ceramic cap that is not listed but is on the board....strangely found clipped on one side.
I'm still trying to identify why the +8 is uneven. The schematic shows different resistor values in that section of the circuit. At last measurement it is +10.7 and -9.9. I installed the old rectifier and old wires and the mains are still +40.5 and -40.4
Okay, I didn't understand and I also didn't look at the schematic. The 2N4249 is the generic number, the 38734 is the Kustom part number for the equivalent transistor.
What is it about the main 40 volt supply that you are trying to fix?
Q131 is shown on the transistor layout between Q115 and Q107 just above the speed control R148.
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Ill have to look again for Q131. I'm having schematic neuro bloack right now. The copy I have would not print fine and is slightly blurry. I have my laptop nearby to confirm things, but it's all becoming a blur of symbols lately. I've been at this for a week now....!
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Q115 and Q116 are near the speed control. Q131, all alone according to the schematic, is no where to be found in that area of the board. I really need a pic....
I have access to two versions of the schematic Rev.3 and Rev.4, the Q131 circuit mod is shown on both and the transistor is drawn in on the location diagram directly under the "sis" in word transistor.
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Well, I found the elusive Q131...and it is NOT in the location
the diagram has it in. It is mounted at the extreme corner near the reverb pot. Looks like the transistor I have in there is wrong... Could that be the problem?
If your board is early enough, there may not be a Q131. I haven't been able to find any earlier K100-2 schematics than Rev.3.
On the Rev.3 version the entire section that contains Q131 is in bold, which usually means that it has been revised from the original version. So maybe it was added in later to correct some problem.
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Thanks Stevem.
I just changed the effects jack with a brand new Switchcraft double shorting stereo jack...and now I can't turn the reverb off. Used a Dremel tool with brush and brushed the metal face
plate clean. Put a touch of Deoxit on washers and tightened
the jack good. I believe in good grounds!
I have two footswitches and both won't turn it off so I test both footswitches on the meter and the reverb switch works perfectly just not on the amp...!
stevem Messages: 4739 Registered: June 2004 Location: NY
Senior Member
Transistors Q117 and 118 are the controls to turn off the reverb, do they check good?
Also since you recaped the whole amp did you reinstall C130 the right way with its negative end coming off the bace lead of Q117.
These two transistors are in line with each other at the three O clock position of the speed control.
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Hi Stevem
I got the reverb issue fixed. It now turns off and on...but now the tremolo no longer works...I guess it fixes the thumping problem...lol
Also, another schematic thing...I don't have a Q117 Q118 in the position shown on the Rev 3 or Rev 4 schematic. I do have a transistor about to fall of the corner of the board where the grey effects wire comes into the board. That's not shown on the diagram either. That's part of reverb circuit so I'm good on that one.
After changing a few value resistors and checking all of the transistors my +8 is now at +10 and -10.4. That's closer than what it was. Coming out of the transformer I read +41 and -39.9. I don't know if it is supposed to be perfectly equal or I have a transformer problem. The amp sounds great.
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
The tremolo schematic seems to be following my board very close. The only questionable component seems to be R152 which according to the revision was penciled in as a 15k from a 10k. My board has a 47k (which appears to be original). I don't know whether to stay original or follow schematic.
The 40 volt power supplies will rarely be exactly equal. There are far too many variables that can cause a volt or two difference.
The 8 volt supplies are normally fairly close in value, as the negative supply tracks or follows the positive supply.
As far as R152 is concerned, it is there to limit the amount of signal from the low frequency oscillator going to the tremolo modulator. So it is sort of like a pre-depth control resistor that will limit how deep the effect will go.
If you take a voltage reading at R152, you should find a rising and falling voltage that changes rate with the speed control. If you don't find this voltage, then the oscillator may not be running.
bluezebra Messages: 89 Registered: February 2014 Location: Lafayette, LA
Member
Only thing screwy is voltage at Q109 is -1.8 instead of -.44. When I engage the footswitch, I see voltages go from 0 to a value, so I'm guessing that's working.