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  • Ampeg SVT-350H problem

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    I have an Ampeg SVT-350H that isn't outputting to the speaker jacks. It outputs through the power amp out jack, so the problem must be in the power section. I have signal from the power amp jack, through the IC3, and out IC4. I have signal on the left side of C31, R17, and pin 5 of IC4. I do not have signal on the right side of C31, R17, or either side of R26. I removed R26 to see if a short from that section was killing the signal and it still didn't go through C31. I checked C31 and it tested fine. I pulled the relay in case it was shorting the signal and nothing changed again. Any help would be great!

  • #2
    It appears that your power amp is not working.

    If you have signal leaving IC4- pin 7 then that is what feeds the power amp.

    " I do not have signal on the right side of C31, R17, or either side of R26" This is a red herring. It's a feedback path from the power amp. (which is not working)

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    • #3
      You see signal at the input to IC4 but not the output? I'd try a new IC4.

      They used a 5532 low noise IC, but for testing a plain old 4558 or something would work, just to demonstrate the circuit.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        I replaced IC4 with 2 different chips with no change. I get signal at pins 1 and 5. Pin 7 just produces a hum.

        Comment


        • #5
          You need to find the fault(s) in the power amp circuit. As you appear to have +/- 16VDC to run the preamp circuits IC2 thru IC4 on these schematics, we can assume there is +/- 60VDC (or so) for the High Voltage to run the power amp circuit. I assume the Output Relay is NOT closing. See what DC Voltage you have at TP11, which is the Output from the power amp to the Relay. This output amp is very similar to the SVT4-Pro, but with less output MosFET's. Check to see if the source resistors R37, R39, R41, R47, R48, R50 are all ok (0.47 ohms/5W), and not Open. Check also the gate resistors R36, R38, R40, R44, R49, R51 (47 ohm/1/2W Flame Proof). Check the MosFET's (Q14, Q15, Q16 IRFP240 & Q10, Q11, Q12 IRFP9240) to see if you can turn them on and off with the Semiconductor mode on your DMM. Those MosFET's are Matched devices, so replacing with new parts can yield imbalance in the current sharing of the output stage.

          A common fault on these amps is in the voltage gain stage that follows IC4B. Check Q4 (2N5415 PNP), Q2 (MPSA06 NPN), Q17 (MPS A56 PNP) and Q19 (2N3340 NPN) (I think it's Q19...fuzzy script). Check their associated emitter resistors R21 & R24 (33 ohms), as well as the emitter resistors for Q2 & Q17 (2.7k). Also check the 13V zeners D5, D6, D17 & D18).

          If the Current Protect circuit fired in an output failure, check those xstrs and coupling diodes Q3 MPSA06 NPN & Q8 MPS A56 PNP, D25, D29 (1N3070).
          Last edited by nevetslab; 10-13-2021, 08:15 PM.
          Logic is an organized way of going wrong with confidence

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          • #6
            Thanks for the detailed ideas. One of the first things I did was check proper voltage to the relay (it is), and then I removed it just to make sure. There was still no signal on the input side of the relay, so I traced the good signal back to the input of side B of IC4. I previously also checked a few resistors, but have since checked the rest suggested and diodes and all are good. I also previously checked the IRFPs and those seemed to be fine as well. The only things I think I haven't checked yet are Q2, Q4, Q13, and Q17. I'm confused by the schematic where the audio signal actually enters the power amp stage, and why I don't have a signal on pin 7 of IC4. I believe I still have to check the MPSs also.

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            • #7
              I looked up the schematic of the Power Amp circuit of the Ampeg SVT4-Pro to see if they call out AC voltages where you're not finding any output signal from IC4. Ampeg has a table showing AC voltages at 40VAC output. IC4 in your circuit is IC1 in the SVT4-Pro, and uses the rest of the discrete power amp in the feedback loop to function as a non-inverting amp. Note the local feedback around IC4 is NOT going back to the inverting input, but is going to the Non-Inverting input. So, you won't see AC signal there at Pin 7 on your amp. I've attached the schematics of the Power Amp circuit for the SVT4-Pro for comparison:

              Power Amp PCB Schematics (428xxh0_).pdf
              Logic is an organized way of going wrong with confidence

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              • #8
                Ohhh ok I gotcha. I'll pull the remaining transistors tomorrow and check them out. None of the resistors or diodes are coming up out of spec. I've also checked solder joints and traces and everything seems to be ok. I'm running out of possibilities besides those transistors, or something I'm missing completely.

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                • #9
                  Regarding seeing signal from the output of IC4, look differentially across the output resistor R43 (1.5k) that feeds the center of the emitter resistors of Q2 & Q17. It's the output current driving the Voltage Gain Stage. You can also look for voltage signal at the collectors of Q4 and Q19, those being the gate drive busses of the output stage.
                  Logic is an organized way of going wrong with confidence

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                  • #10
                    Well I got closer to solving the issue(s). I finally pulled the 2n5415 at Q4 and checked it out. It was bad, but in a really weird way. It was acting like an NPN rather than a PNP.... oooooooooook. So I swapped it with a good one and finally have sound through the speaker outs. The new problem is the sound is distorted and highly compressed, almost having an old 8-bit popping kind of sound. The limit switch also seems to do nothing and the LED doesn't illuminate when kicked in, but the LED is tested good. Did something go bad in the limiter section that is messing with the signal chain even when not engaged?

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                    • #11
                      You need to check the DC Voltages thru the Power Amp circuit. Do you have DC offset on the output (ahead of the relay)? I looked at the Test Point chart on your SVT350H schematic, but don't see all of the pertinent DC voltages that would help you sort out what's happening and causing this distortion. Do you have a scope and generator to work with?

                      I wonder if the current limiter circuit is turning on. It normally will only conduct, stealing gate drive signal if one or more of the MosFET's aren't healthy, and pulling current under drive. I've never seen your amp physically, so I don't know how 'service-friendly' it is to work on. I own an SVT-4 Pro, and work on all but the SVT-7 & SVT8-Pro amps often. Tedious to work on, NOT being service-friendly.

                      There is a useful Test Point chart in the SVT4-Pro Power Amp schematic I posted earlier that would provide DC Voltages at the circuits in the Voltage Gan Stage that then drives the N-Ch and P-Ch MosFET's. If Q4 was bad, you may want to replace it's partner in crime Q19 2N3440 NPN, as well as the cascode xstrs MPS A06 & MPSA56. I can't really read the schematic designation numbers....all fuzzy. If there's any bad resistors, diodes or xstrs in this voltage gain stage ahead of the MosFET stage and bias circuit, you'll have problems. The gate voltages, found at the collectors of Q4 and Q19 should be around +/- 3.6VDC at idle. If there's suspect behavior on the current limiter, you can lift the coupling diodes that would shunt the gate busses at D25 & D28. Your amp only senses current thru the first pair of MosFET's, so doesn't sense current thru the other two pairs.
                      Logic is an organized way of going wrong with confidence

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                      • #12
                        It's actually quite an easy unit to work on, other than the pads are extremely easily to cause to lift. So I started my run through the test point voltages. TP7 & 8 are spot on, as I would've expected since I seem to be getting appropriate +/-16V everywhere else. Now on the new Q4, I'm getting 64ishV on E and B, but the collector (TP9) is showing nothing when it should be 5.5V. The same goes for TP10 off of Q6. Do the IRFPs supply the voltage for these lines? I apologize for all the help needed as amps are not my wheelhouse.

                        Comment


                        • #13
                          Working thru the voltages, seeing as you have +/- 5.5VDC on the bases of the cascode stage parts MPSA06, MPSA56, that puts +/- 4.9V at their emitters, and hence with 2.7k emitter resistors, you have 1.23mA of current flowing thru those xstrs, and thru the base-emitter resistors for the Voltage Gain xstrs Q4 and Q19 (2N5415 & 2N3440). 1.23mA thru 680 ohms yields 1.23V drop, and subtracting the B-E junction, you have 0.63V across the emitter resistors (33 ohms), which yields 19.2mA current thru Q4 & Q19 at idle. But, you aren't seeing that, as your voltages at TP9 & TP10 are at 0V? The MosFET's are NOT supplying that current to set the buss voltages at the defined TP voltages in the chart. So, Q4 and Q19 aren't conducting. You need to find out why.

                          Check if C32 47uF/35V cap across the bias circuit & across TP9 & TP10 is shorted. If the Bias circuit is turned on hard, that would prevent the MosFET's from conducting. It's rare to find the two Bias xstrs (Q5, Q6)....MPS A06's dead short, but that too would pull TP9 & TP10 to 0V. Normal bias will set what the TP9 & TP10 voltages are. The Current Limiter circuit, if turned on, will pull the gate busses down towards the output buss potential (TP9, TP10). Across the base/collector of each current limiter xstr (Q3, Q8?) are 22nF/50V caps. If those were shorted, then there's a conductance path between the output buss and the Gate buss thru D25, shorted cap, R45 & R47, and similarly thru their negative side parts. If you have Gate/Source shorts on any of the MosFET's, that too will pull the Gate Voltage down to Output Buss level.

                          The only other possibilities are solder joint fractures or open resistors, or open diodes, or non-conducting xstrs MPS A06, MPS A56 cascode stage in the primary voltage string....R20 to D32 to Q2 to R22 to R23 to Q17 to D34 to R25. That string sets up everything.
                          Logic is an organized way of going wrong with confidence

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                          • #14
                            I had to take a break from the amp for a bit as it was starting to annoy me. I checked c12 (believed to be c32) in the bias circuit for a short, and it was fine. I'm also not getting a short from Q5 or Q6, and c14 and c15 read fine. I double-checked the 1N3070s around Q3 and Q8, and all are OK. I've gone through all the resistors and diodes and I can't find a faulty one. I have voltage all the way around from R20 to R25. All of the 0.47ohm ceramic resistors are all measuring 1ohm. I just figure that's from age and tolerances as they're all the exact same. I did notice that R35 and R45 are not original, and seem to have a char mark under them. These replacements are testing fine, but what would have surged them out, or what else could've gotten damaged down the line if that's what occurred? I guess I'll order some MPSs and replace all of them just to try and narrow something down.

                            Comment


                            • #15
                              Are you still getting NO voltage for TP9 & TP10 (relative to GND)? What's the voltage across emitter resistors R20 & R25 (680 ohms)? What is the voltage across the 33 ohm emitter resistors for Q4 & Q19? What's the Voltage on the Output Buss? Not having voltage at TP9 & TP10 tells me Q4 and Q19 aren't conducting. I'm at home, and don't have the schematic that I can enlarge to make out what R35 and R45 are, so I'll have to look when I get into the shop later this morning.

                              I'm now in the shop. I see R35 and R45 are 270 ohms. Only way those two resistors could have been overheated/burnt would be for the MosFET Output Stage to be latched up to the Positive Rail or the Negative Rail, and the output buss be equally high or low relative to GND. Then there's a path between R35 or R45 to conduct to ground thru D26/R46 or thru D27/R34.

                              No where in this dialog has the subject of adjusting the bias pot and having the nominal 25mV across the MosFET Source Resistors. If the bias isn't adjusted, (such as set so the two bias xstrs Q5 and Q6 are turned on hard, there would be NO Voltage between TP9 and TP10). Try adjusting the bias pot, monitoring the voltage across all six MosFET Source resistors R47, R48, R50 & R37, R39, R41 for a nominal 25mV. Be careful, as you could misadjust it so as to turn the MosFET's on hard, and they can fail under that misadjustment. Assuming all six MosFET's are relatively matched, and the Source Resistors are all close to 0.47 ohms, 25mV across them (both N-Ch side AND P-Ch side), you'd have the nominal TP9/TP10 Voltage that isn't there.
                              Last edited by nevetslab; 10-29-2021, 06:11 PM.
                              Logic is an organized way of going wrong with confidence

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