Princeton Chorus overdrive shuts off??

[turk05]

For my first post here, I have a stumper.  I just picked up a Princeton Chorus red knob amp.  Great cosmetic condition, the clean channel is great and the overdrive channel works, for a while.  After having the amp on for a few minutes and using input 1, the overdrive channel will just stop working.  You can push the amp switch or the footswitch, and the overdrive led lights up on the amp and footswitch, but the clean channel is actually working, the clean volume is working, and no overdrive.  If you turn the amp power off and back on, the overdrive is immediately back.  It does this with or without the footswitch connected.   I've cleaned pots and cleaned jacks.  I'm not a tech.... but I do own a soldering iron!  I'm just not sure where to start on this one.  I got the amp at guitar center, so my 40 days to return is running.  I love the amp for what it is, and I want to keep it. 

Anyone ever seen this before?  Any ideas?  Thank you all.

[Jazz P Bass]

There may be something going on with the switch IC, CD4053.
http://www.ti.com/lit/ds/symlink/cd4053b.pdf

You will have to check the inputs and see if the outputs are responding.

This can be accomplished with a volt meter.

[g1]

Agree with JPB, and from quick glance it seems the 4053 is the only thing running off the +/-8V supply, so you might want to check that those voltages are holding up.

[turk05]

Thank you for the responses!   I can see two 4053's on the schematic, U12B and U12C, but one only U12 on the diagram, which I think means U12 is the 16 pin chip and the U12B and U12C are the two half's of U12?  If so, the chip is a Motorola MC14053BCP, Date Code? FFMY9009.  Is this the same as the TI chip?  Now is where I get lost.  Any advice on which pins would give me the info I need?  I'm attaching photos of the front and back of the board in case that's helpful.  If you need more info I'll provide all I can. 

What is the basic process for testing?  Power up and measure what pin or pins?  voltage +/-when overdrive switch is pushed on or off?  Something else?  Sorry I'm not more competent on this stuff....

[Enzo]

a 4053 is a three section analog switch.  A, B, and C are sections of it.  SO yes, there is ONE 16 pin IC on the board, the a,b,c sections are referred to in the data sheet as well as on the schematic.

Various companies make 4053, the TI version is CD4053, the Moto version is MC14053.  They are all 4053s.

U12C for example, switches pin 4 to either pin 3 or pin 5, per the drawing.  Pin 9 is shown as the control pin for that section, and controlled by signal SELA.  (I read that as select A)

Look on the schematic above the power amp IC, and you will see the third section U12A.

The testing is straightforward.  Use the footswitch or panel switches.  SELA is also TP16, so when you click the overdrive select button, see if the SELA pin on the IC changes voltage.  If so, then we know the control signal is getting to the IC.  Then use an ohm meter to check that pin 4 is connecting to the two other pins as you switch back and forth.

Look at U12B over to the right, see that control signal SELA also controls that section, again, use a meter to see if pin 15 connects to pins 1 or 2.

To me it is sufficient to look at the ICs and see if they switch back and forth.  But if you just have to know which is which, then look closer at the schematic.  Inside the little box see it has "0" and "1" next to pins 5 and 3?  Those refer to the SELA signal.  If it is at a logic 1, which means a large positive voltage.  (Anything over about 11 volts.)  Then pin 4 should connect to pin 3.  If SELA is a logic 0 (anything from zero volts up to about 4v), then pin 4 connects to pin 5.

[turk05]

I see that now on the schematic.  If any of those tests fail, replace the chip, or do I try something else?  Thank you again for all this insight.  I really appreciate it. 

[Enzo]

I don't know what else I would do, check to see if the IC is receiving a control signal.  And then if it does, then does the IC react to it properly.  That is pretty much what we do to anything - in, out, do its thing?

Those are parts I stock anyway, so if I had reason to suspect the part, I;d swap it out.  Parts are cheap, labor is not.  At least in the world of commercial repair shops.

[g1]

And the power supply pins are shown at U12A section.
+8V at pin 16, -8V at pins 7&8.  If those voltages disappear the chip will not function.
The +/-8V supply is shown in the lower left area of the schematic, TP7 & TP8.

[turk05]

Got the meter out and Voltage at pin 16, 7 and 8 was steady and present, SELA voltage changed as the switch was turned on and off, but no switching volt or ohm changes when I put the meter on the switching pins of U12B and C.  My local electronics store had MC14053BCP chips for .49 apiece.  Cheapest thing I have bought in a long time.  Swapped the chip,and the amp is behaving as it should for a short session this evening.  I'll get some more time on it later this week, but it does appear the amp is fixed.  Enzo and G1, I am eternally grateful for your assistance.  If I can help either of you with any Quadrajet problems or V-Twin issues, please let me know!  I'm better with a wrench than I am with a soldering iron or a multimeter....  Many Thanks.

[Enzo]

If the IC fixed it. great.


But the testing procedure is valuable elsewhere.    You found SELA was working. Good. You do realize we needed resistance readings in ohms from pin 4 to 5 and pin 4 to 3 depending.  Not voltage, and not resistance to ground.

[turk05]

I did measure ohms between the pins for B and C when the overdrive switch was turned off and on.  One meter probe on each pin.  For instance I put red on pin 4 (or15) and got no changes when black was placed on 3 or 5 (or 1 or 2)  and the overdrive switch was turned on or off with the footswitch or  the switch on the amp.  I hope that was what you instructed me to do??   I'll put some time on the amp tonight.  But the few minutes I managed last evening showed all good.  I'll know more tonight.

One other thing I noticed was less noise on startup and shut down of the amp.  Is that in my head or could this chip effect that as well?

[Enzo]

Yes, you did it correctly, I was just unsure from your first response.

I can imagine power switching making noise if the inner stuff of the IC was faulty. Sure.

[turk05]

Unfortunately, after about 20 minutes of playing, the condition has returned, just as I described it in the original post.  The overdrive will not turn on even though the switch is activated and the LED on the amp is lit.  I must have misinterpreted something in the meter readings or the problem is elsewhere.  Anything else I should try?

[g1]

You are going to have to check the same things but while the problem is occurring.
Wait til the unit is in the fault mode, then do your checks.
The light runs off the push switch, so all it tells you is that the push switch is good.
The reason I mentioned the +/-8V supply is that the 4053 is the only thing running off it, so it is a good suspect.
Beyond that, go back through all the checks Enzo mentioned.  Check Sel A at TP16, then check the pins of the 4053.

[turk05]

I decided to start simple this morning.  I pulled the board again and went through all the solder joints on the board with a magnifier.  I found 4 or 5 suspect solder joints, including one pin on U11 and one pin on the overdrive switch on the amp, and re-soldered them with new solder after removing the old stuff.    I also cleaned off a bunch of old brown crud from around a bunch of the solder joints, probably old flux?  You can eat off that board now....

Put the amp back together, and after several hours of playing today, I cannot get the amp to fail to switch between clean and overdrive as it is supposed to, including using both the footswitch and the amp switch.  The problem is gone.  I guess the old car stereo troubleshooting trick worked on this amp (check and resolder, check and resolder....).   

I think it's fixed.  If the problem does come back, I'll re-post.  Again, thank you all very much for your help.