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SVT-CL fault green/red blinking.

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  • #16
    Have you checked to see if both 12AU7 driver tubes are ok? I've never seen them responsible for the GRN Bias LED not working. I just found the brief Theory of Operation I had written on the SVT-CL's LED Bias Indicator light circuit:

    All six comparators in each half are toggled LOW if there is no bias. The GRN LED comparator circuit will only work when all six working tubes are installed (3 upper, 3 lower). With less than 3 working tubes installed (per half), the GRN LED's won't light. The Range for GRN LED's lighting is 147mV thru 294mV (14.7mA to 29.4mA) in the window comparator. Above 294mV, GRN remains lit, and RED lights up too. It's a diode AND circuit on the output of the comparators for the GRN LED, such that all three (in each half) have to toggle HIGH so the GRN LED's will light. The GRN LED's conduct from Ground thru the 2.2k resistors (R70, R79) to the +15V rail. With all three comparators toggled HIGH, that second 2.2k (R69, R78) that ties to the diode AND circuit goes high and the GRN LED conducts. Otherwise, it stays LOW (goes to -15V), and the LED won't light. The RED LED comparator circuit is a DIODE OR circuit, where any of the comparators exceeding 294mV at their input will cause the comparator to change state and the RED LED will conduct. So the RED LED comparator circuit will spot a defective tube, or one that's pulling greater than 30mA Plate Current.

    My mention of the Preamp is regarding general maintenance, including source of hum. Usually, dead-patching the Power Amp input on the rear panel will tell you if the hum is from the Preamp.

    I've never read about a technique using a 9V battery to troubleshoot the LED Comparator Circuit with regards to the GRN LED circuit working properly.
    Last edited by nevetslab; 08-27-2020, 08:45 PM.
    Logic is an organized way of going wrong with confidence

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    • #17
      Originally posted by nevetslab View Post
      Have you checked to see if both 12AU7 driver tubes are ok? I've never seen them responsible for the GRN Bias LED not working. I just found the brief Theory of Operation I had written on the SVT-CL's LED Bias Indicator light circuit:
      Yes, whole set was changed from another (good working) Classic head.

      Many thanks for the tips about the bias circuit - It seems that left side, no matter which tubes I put, can't get into green and looks like you described - one tube might not getting HV at all
      I exchanged power tube board from another CL and it's the same, so the fault is not on the tube board.
      I did the same with the preamp and again, the same conclusion, so need to investigate (again) the main PCB.

      I've never read about a technique using a 9V battery to troubleshoot the LED Comparator Circuit with regards to the GRN LED circuit working properly.
      It wasn't fro troubleshooting LED CC, but only to "lit LED" - giving it a bit of voltage when amp is off of course. That will tell us if the diode is OK or broken

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      • #18
        While it is rare that I find Solder Joint fractures on the power tube sockets, I have had them, which did interfere with proper operation of all six power tubes. Complex system and can be tedious to find the faults. Having managed a fleet of Rental SVT-CL's, SVT-VR's and SVT-AV's over the past 11 years, I have gotten to know the amp quite well. And still can be hoodwinked by peculiar faults. Good hunting!
        Logic is an organized way of going wrong with confidence

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        • #19
          I was editing this last post yesterday but I it didn't saved (?)
          Anyway. I found issues:

          - I hooked preamp from another CL and amp sounds great and bias is fine on both channels (ignore my last posts about that!)
          - Found that in the amps main preamp midrange pot is shot as well as the master vol. pot too - trash
          - Green LED from left channel does not work at all. I know the part number but it isn't available anywhere... might try something else

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          • #20
            On one of my adventures repairing the Bias LED's, thinking Ampeg had used a RED & GRN LED module. But, all it was was the black plastic holder, which cradled stock T1 size RED and GRN LED's. Replacement req'd unsoldering both the RED and GRN LED assembly in question, unbending the leads of the GRN LED, then replacing it, minding polarity, of course.

            Have you pried off the POT covers of the two pots....Midrange and Master Volume pots? They're CTS pots, with the wiper terminal integral to a conductive contact ring that can be slipped out of the fiber resistance module holding the resistance track & end terminals. I've used metal cleaner on that middle ring, to get back to clean metal surface. The Rotor assembly is part of the plastic shaft, and the two pair of contacts can easily be cleaned. Assuming the resistance track hasn't been worn thru, that can be cleaned, then when reassembled, red bearing grease actually does a great job, applied lightly the tracks & ring. Then, carefully reassemble and fold the cover flaps back down. Been pulling pots apart for full cleaning for years....though not always successful.
            Logic is an organized way of going wrong with confidence

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            • #21
              Pots are shot, do doubt. No cleaning would reapir them. They are mechanically shot.
              I've already ordered those 4 pcs op-amps, as they cost pennies. Maybe it would repair the green light. If not, I'll unsolder them all and change.
              nevetslab - thanks for all the tips!

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              • #22
                Before I'd solder new OP amps, I have finally managed to test LEDs.
                Each pair of green/red LEDs have 4 small pins (solder joints) near the edge of main PCB. It's divided for 2 vertical pins for the red LED and 2 vertical pins for green LED.
                You can test the light with 9V battery. Four of them are fine so the problem really might be those OP amps.

                nevetslab do you know by any chance which OP amps belong s to which bias side? Is IC3 & IC4 left bias and IC5 & IC6 right bias? Can't be read from the schematics...
                Last edited by boroman; 09-08-2020, 08:07 AM.

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                • #23
                  YES. IC3A, IC3B and IC4A control the GRN LED for upper tubes V1, V2 & V3; IC3D, IC3C & IC4B control the RED LED for upper tubes V1, V2 & V3. IC6A, IC5a and IC5B control the GRN LED for the lower tubes V4, V5 & V6; IC6B, IC5D and IC5C control the RED LED for the lower tubes V4, V5 & V6.

                  This past weekend, to beat the heat in LA County, I spent those days in the shop to take advantage of the air conditioning, and compiled a complete volume of all the service records I've written up in my daily technical journal on the Ampeg SVT-CL, -AV, -VR, -SVT2-Pro & SVT II. I'm just now going thru editing those 347 pages of notes, and will come across the notes made when I had removed the dual LED assembly for the Bias Lights, discovering it was just a unique holder for T1 size LED's, as that particular repair also had an open GRN LED. Though, it could have been a RED LED, but I'll know when I finally find that particular record. I was hoping I had taken photos, but never found those in my files....so probably not.

                  Logic is an organized way of going wrong with confidence

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                  • #24
                    nevetslab I quite dont't get that naming... Schematic says IC3, IC4, IC5 and IC6 (not six IC's with A/B names - just regularly numbered ones), so IC3 and IC4 corresponds to Bias point #1 and first (left triangle) tube side, and IC5 & IC6 for Bias point #2, right?

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                    • #25
                      Originally posted by boroman View Post
                      nevetslab I quite dont't get that naming... Schematic says IC3, IC4, IC5 and IC6 (not six IC's with A/B names - just regularly numbered ones), so IC3 and IC4 corresponds to Bias point #1 and first (left triangle) tube side, and IC5 & IC6 for Bias point #2, right?
                      IC4 & IC6 are TL072 Dual Op amps (hence IC4A pins are 3, 2 & 1, and IC4B are pins 5, 6 & 7). IC3 & IC5 are TL074 Quad Op Amps, (hence IC3A pin s are 3, 2 & 1, IC3B pins are 5, 6 & 7, IC3C pins are 10, 9 & 8, and IC3D pins are 12, 13 & 14...using the sequence for Non-inverting, Inverting and Output for the pin-outs, not counting for the supply pins.

                      My SVT-CL schematic shows each of the op amp cells with the letters as I had called out. The silk screen drawing, used for a guide in locating where all the parts are, of course only label the IC numbers.
                      Logic is an organized way of going wrong with confidence

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                      • #26
                        We may have different schematics then! My CL layout has only IC3-6 assigned to bias lights. TL072 & TL074 - each pair for each side
                        And never mind - I have replaced them all (what a pain to unsolder them!) and you were right! It solved the problem and now it runs perfectly fine. It's good that I tested the light with 9V battery before so I know this is not the LED that has failed. Love it! Thanks again!

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                        • #27
                          My PWA Power Amp Parts Layout dwgs are 07-419-04 (superceeds 07-419-03/11), & 07-419-12, and schematic drwg is 07S419-03 Rev 4 & 07S419-04. All call out IC3, IC4, IC5 & IC6, with the letter used with the IC number inside the triangle. I haven't seen any that don't. I have the same drawings in my SVT-AV and SVT2-PRO files. Nice to hear replacing them solved the problem........glad to have been of service!
                          Logic is an organized way of going wrong with confidence

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