Ampeg SVT 4Pro equalizer circuit problem

[JazzBED]

Hello to all on the site, I did a search and didnt find a similar issue on these amps to consider. Eq on the SVT 4 pro quit passing a signal.

My SVT 4 pro developed an issue last night and I am looking for some guidance if anyone has experience with the 4Pro. I love my SVTs but this one is solid state and a new one for me.
Problem: The graphic eq selector S4 does not allow eq signal to pass when the eq selector switch is engaged during normal operation.
Indications: I do have an eq LED lamp illuminated when the switch is engaged, however no signal is heard from the speakers. With the eq switch de selected, Eq lamp is extinguished and the non eq signal sounds normal and it appears that all is operating properly albeit without eq. engage the eq... all is quiet like crickets.

So far I have determined that both power amps function normally, bridged or stereo,the pre amp delivers good signals to the power amp and to the pre out and tuner out jacks, all the tone shaping circuits function normally in addition to the bright , ultra hi, ultra lo selectors, mute functions normally. I plugged in an external pre amp to the power amp in jacks and they function normally. I don't have a foot switch to test that function on the rear panel. the rear jacks appear clean upon inspection.
After looking at the schematics found elsewhere in the site.. a big thanks to that post for the pdf files, I
I opened the cabinet and removed the cable between the equalizer board and the preamp board. The amp operated normally with the eq interface cable disconnected.

My questions are: It seems that S4 is functioning normally, Does it control the throughput? or is the switch or some part of that circuit pulling the signal to ground? ( Eq LED lamp comes on with power applied to the eq board?) any ideas about where to have a look to see where the signal is being cut off? Or does S4 supply power to the eq board ? Im wondering if the signal is diverted or is the power to the board being interrupted?

Is the issue on the eq board or more likely on the preamp board in the vicinity of S4?

any help or guidance regarding the circuits of this amp would be a big help, Im kind of rusty in the op amp arena and this schematic Im looking at is a challenge for me ..
thanks for the consideration in advance.. I certainly would appreciate the feedback.

[Jazz P Bass]

What you want to check is the operation of the Jfet switches.(Q9 & Q10)
R102 (at Q9) should always have signal.
R105 (at Q10) should have signal when the EQ switch is enabled.
If both of these conditions are true, then you have something wrong going on in the EQ board.

[JazzBED]

Thanks for the pointer! the Jfet switchers... when I looked at the print, it seemed like the EQ board was getting the power ok since the LED and all the tone shaper ICs are getting 15v. i assume the 15v bus ok at least...I was thinking the signal wasnt getting into the loop via the footswitch/panel enablers. The amp walks like nobodys business otherwise; another indication :there is an audible pop though on power up that hasnt occured before this incident; it seems like a voltage spike is occuring somewhere else on start up? Started a gig with it got about 20minutes warmup time and instantly No EQ... no output... so I changed to a spare head.

[Jazz P Bass]

".I was thinking the signal wasnt getting into the loop via the footswitch/panel enablers"
So check for a signal at R102 & R105.

[Enzo]

Don't assume the 15v supplies are OK just because the LED lights up. The fact 15 is in one place does not mean it gets to the other places. For example, are both 15v rails getting TO the ICs on the graphic board?

[JazzBED]

Enzo, thanks Indeed I am not going to assume anything with this one; havent checked the IC rails on the Eq board yet.. but I will; thanks for posting the prints too. and I certainly appreciate the help here. Here is what I found so far:
Jazz P Bass, I put the scope on R102/Q9 and the signal there is strong. Moving over to R105/Q9 no signal. Looking at the J18 header, no signal on p1, good signal on p2, and good signal on p4. good voltages on p6 and p7. these remain the same whether the switch s4 is enabled or not. Im re learning my Op amp theory with this one...

[JazzBED]

Should the PRE Signal (present on J19 pin5 of the EQ header) be found at TP23 and TP 24 regardless of the position of the panel switch?

[Jazz P Bass]

Have you checked for a signal at the jfet switches?
My post #2 above was in error.
The signal should be on R102 (PRE) & R105 (EQ) at all times.

And yes, the PRE signal will be present at J19 pin 5 at all times.
The signal at TP 23 is a buffered PRE signal.
TP 24 will not be the PRE signal as it has passed through the EQ circuit. It is now EQ signal.
What you must understand is the fact that the EQ circuit is always on.
It takes the PRE signal & makes the EQ signal.

What is getting switched in & out are the jfets (Q9 & Q10) (schematic page 2/quadrant A-3.5)
One or the other jfet will pass a signal on to U2.
Q9 will pass the PRE signal & Q10 will pass the EQ signal.

The schematic can be found here: AMPEG SVT-4PRO SCH Service Manual free download, schematics, eeprom, repair info for electronics

[JazzBED]

I do not have any noticable signal at R105 . I also was thinking that the Eq circuit was always on ... I am thinking something on the Eq board has opened up at this point. I should be able to pick off some sort of signals as they pass through the IC sections of U9 thru U11 between TP 23 and TP 24?

no worries on the #2 post, it was good to get me thinking about how JFET switch action works. (not that I know now.. ) i noticed that particular circuit is used in several places on this amp... the only ampeg guru down here retired, so i am sort of left to my own devices to fix this thing... many humble thanks for the help here.

[Jazz P Bass]

Walk the signal through the EQ.
The best place to look is at the output pins of the TL074 ic's. (pins 1, 7, 8 & 14)

At first, with no signal, look for any opamps that have a Vdc reading on the output pins.
If none, then inject the signal & trace it through.
It will be a Vac reading now.
Put all of the sliders in the middle (no cut or boost).

Of coarse please verify that the opamps have the correct power voltages.
Pin 4 is +Vdc & pin 11 is -Vdc.
Link: http://circuits.datasheetdir.com/37/...074-pinout.jpg

[drewl]

Working on one now with a noisy J176 FET on the input of the first tube.

I also replaced the plate leads to that tube, and I think that fixed it.
I mis-read V1b's plate as 47.5K and was shocked to see it's actually a 475K, so Ihave to swap it out for the correct value.

[JazzBED]

It has been a while since I looked at the issue with the Eq board. Update: the voltages present on all the IC of the board are proper at 15vdc on pins 4 & 11 respectively of polarity. U9 output of pin 1 is absent. If I inject a signal at the junction of C69/R161 , the eq comes back to life and the output EQ sig. is present.
since I have to pull that TL074 IC off the board anyway. Would installation of a PDIP socket in that slot work ok for ease of future replacements (IC plugin) or would installing the socket cause the connection to become noisy? Have you had experience with IC Socket installation in this area of board??

Walk the signal through the EQ.
The best place to look is at the output pins of the TL074 ic's. (pins 1, 7, 8 & 14)

At first, with no signal, look for any opamps that have a Vdc reading on the output pins.
If none, then inject the signal & trace it through.
It will be a Vac reading now.
Put all of the sliders in the middle (no cut or boost).

Of coarse please verify that the opamps have the correct power voltages.
Pin 4 is +Vdc & pin 11 is -Vdc.
Link: http://circuits.datasheetdir.com/37/...074-pinout.jpg

[diydidi]

Socket the darn thing. Get a good quality socket.

[JazzBED]

socket(s) on the way ;yep.. Mouser carries a Harwin part (D2814-42) which looks like it will work ok;

Socket the darn thing. Get a good quality socket.

[JazzBED]

Desoldered and Removed U9 IC (TL704CN) (no thruput from Pin 1), Installed a new Harwin Socket (D2814-42); installed Texas Instruments TL704CN IC. the Eq board returned to proper function. Thanks to all who helped me with the finer points on this one!!

Socket the darn thing. Get a good quality socket.