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Another SVT-CL arrived from our CenterStaging inventory. I think this came from our New York inventory, which is now here in Burbank. Older amp, even though the asset number we assigned would make you believe otherwise. This amp was in Protect Mode, which routed it to my shop. I found it had NEVER been set up as I do on all of our inventory, so I had to pull it completely apart to tackle ALL of the loose hardware thruout, repair all the fractured solder joints, etc. I finally got it mechanically back together and began the check-out of the remaining 6550 Power tubes. One had frosty white coating inside, and it's 10.0 ohm cathode resistor was open, so replaced the resistor, and now down at least one power tube.
When I powered it up with NO Power Tubes installed, I found a GRN and two RED Bias LED's lit, so I had done something wrong. Set it back down, removed the power tube cover plate, lifted the power tube PCB out, and discovered I had mis-aligned the 16-pin dual-row ribbon cable between the Power tube PCB and the main PCB. DON'T EVER DO THAT! The mismatch I made placed the two groups of 3 terminals from the 6.3VAC Heater Supply onto the Cathode Resistors and the Window Comparator circuits. All the Cathode resistors remained ok, but it took out the TL072's and TL074's IC3 thru IC6.
Leave it to Ampeg to fold ALL of the IC pins over when the PCB was stuffed at the factory, rather then just two pins at opposite corners. So, I couldn't use the Pace Desoldering iron for this, but had to carefully desolder with solder wick. Most of the solder came up, a few pins here and there required hitting the pads again. Then, I still had to go in with tiny/sharp 45 deg Flush cutters to break the left-over solder bond between the laid-over pin and the solder pad, so I could then lift them up and finally remove the failed IC's. As I began this procedure, reaching for my Surgical Loupes so I could clearly see all this, one side of the loupes ear/support arm separated from the loupes eyepiece frame!! YA GOTTA BE KIDDING ME!!
I had to stop to deal with that surgery, finding part of the rugged hinge broke free from the eyepiece frame, and was repairable, but it would take them out of service for 24hrs. Got that repaired, while getting back to the task at hand, soldering the replacement IC's into place. Used the last two TL074's I had on hand. I did have MC33079 Quad IC's, but I've never tried using those NON-BIFET Op AMP's in that circuit.
When I got the new op amps into place, the errant readings were gone. I managed to sort thru what 6550's seemed in the ballpark as far as their transconductance went, and finally got what might be a working sextet. I loaded all six tubes into place. My sorting method uses positions V3 and V4 for all the tubes. When I got the sextet into place, I was finding the tube in position V5 was reading very low, while the rest seemed to be nominal, still needing juggling for best match. I swapped V5 with V6, and still had low voltage readings on V5's Cathode resistor. I removed all of the tubes, now in Standby mode. I was seeing -0.123VDC on the cathode resistor for V5, and nothing on the rest of the tubes. Shut down, and checked DCR on it. Measured 10.15 ohms, right in the ballpark with the other five resistors. Switched back on in S/B, and I could still measure DCR on the other five cathode resistors, but not on V5's resistor.
I'm now two days into this hunt to find WHERE this errant voltage is coming from. I've also found more of the documentation errors from Ampeg on the PCB silk screen numbers vs those shown on the schematics...with the same errors on both SVT-CL AND SVT-AV schematics/PCB layout drawings. love it!!
This morning, now having my surgical loupes operational again, I found NOT having them while dealing the Ampeg's piss-poor PCB material in getting parts unsoldered had partially lifted solder pads around IC6, plus finding I had voids in the newly soldered leads on several other pins on those four IC's. Had to repair two connections with 28AWG buss wire.
I've so far unplugged the ribbon cable from the power tube PCB, taking that out of the fault equation. I've removed (using Power Amp PCB Schematic ref numbers) D50, C32, R85, D30 thus far, while still having this -0.123VDC on the Cathode Resistor for V5. I replaced D50 and C32, putting them on the foil side of the PCB so I could continue troubleshooting. Removing R85/D30 at least removed that errant voltage from appearing on D50/C32. I'm about to lift one end of R65 to see if that's where this voltage is coming from. If so, that seems to point to the non-inverting inputs of IC5A and IC5D. Brand new TL074, or at least never been soldered in.....assumed to be new/ok. Those were the lasts two I had on hand (RATS!!) I just got thru ordering more TL074's.
This errant -0.123VDC is stiff. I just looked further, this time at what is called out as R65/1k, and should have taken me to IC5. Instead following the PCB traces on the back of the PCB, it instead takes me to IC6, and the other side of that 1k resistor is at -12VDC potential. I did follow the silk screen diagram on the PCB when I installed these new IC's (TL072's, TL074's). So, there's NO WAY I should be seeing this potential, unless that IC is installed backwards. Once again, pulling this apart.
ANYONE run into this problem before?? I see I'll need to sit down and revise the schematics and layout drawings to get all that correct.
When I powered it up with NO Power Tubes installed, I found a GRN and two RED Bias LED's lit, so I had done something wrong. Set it back down, removed the power tube cover plate, lifted the power tube PCB out, and discovered I had mis-aligned the 16-pin dual-row ribbon cable between the Power tube PCB and the main PCB. DON'T EVER DO THAT! The mismatch I made placed the two groups of 3 terminals from the 6.3VAC Heater Supply onto the Cathode Resistors and the Window Comparator circuits. All the Cathode resistors remained ok, but it took out the TL072's and TL074's IC3 thru IC6.
Leave it to Ampeg to fold ALL of the IC pins over when the PCB was stuffed at the factory, rather then just two pins at opposite corners. So, I couldn't use the Pace Desoldering iron for this, but had to carefully desolder with solder wick. Most of the solder came up, a few pins here and there required hitting the pads again. Then, I still had to go in with tiny/sharp 45 deg Flush cutters to break the left-over solder bond between the laid-over pin and the solder pad, so I could then lift them up and finally remove the failed IC's. As I began this procedure, reaching for my Surgical Loupes so I could clearly see all this, one side of the loupes ear/support arm separated from the loupes eyepiece frame!! YA GOTTA BE KIDDING ME!!
I had to stop to deal with that surgery, finding part of the rugged hinge broke free from the eyepiece frame, and was repairable, but it would take them out of service for 24hrs. Got that repaired, while getting back to the task at hand, soldering the replacement IC's into place. Used the last two TL074's I had on hand. I did have MC33079 Quad IC's, but I've never tried using those NON-BIFET Op AMP's in that circuit.
When I got the new op amps into place, the errant readings were gone. I managed to sort thru what 6550's seemed in the ballpark as far as their transconductance went, and finally got what might be a working sextet. I loaded all six tubes into place. My sorting method uses positions V3 and V4 for all the tubes. When I got the sextet into place, I was finding the tube in position V5 was reading very low, while the rest seemed to be nominal, still needing juggling for best match. I swapped V5 with V6, and still had low voltage readings on V5's Cathode resistor. I removed all of the tubes, now in Standby mode. I was seeing -0.123VDC on the cathode resistor for V5, and nothing on the rest of the tubes. Shut down, and checked DCR on it. Measured 10.15 ohms, right in the ballpark with the other five resistors. Switched back on in S/B, and I could still measure DCR on the other five cathode resistors, but not on V5's resistor.
I'm now two days into this hunt to find WHERE this errant voltage is coming from. I've also found more of the documentation errors from Ampeg on the PCB silk screen numbers vs those shown on the schematics...with the same errors on both SVT-CL AND SVT-AV schematics/PCB layout drawings. love it!!
This morning, now having my surgical loupes operational again, I found NOT having them while dealing the Ampeg's piss-poor PCB material in getting parts unsoldered had partially lifted solder pads around IC6, plus finding I had voids in the newly soldered leads on several other pins on those four IC's. Had to repair two connections with 28AWG buss wire.
I've so far unplugged the ribbon cable from the power tube PCB, taking that out of the fault equation. I've removed (using Power Amp PCB Schematic ref numbers) D50, C32, R85, D30 thus far, while still having this -0.123VDC on the Cathode Resistor for V5. I replaced D50 and C32, putting them on the foil side of the PCB so I could continue troubleshooting. Removing R85/D30 at least removed that errant voltage from appearing on D50/C32. I'm about to lift one end of R65 to see if that's where this voltage is coming from. If so, that seems to point to the non-inverting inputs of IC5A and IC5D. Brand new TL074, or at least never been soldered in.....assumed to be new/ok. Those were the lasts two I had on hand (RATS!!) I just got thru ordering more TL074's.
This errant -0.123VDC is stiff. I just looked further, this time at what is called out as R65/1k, and should have taken me to IC5. Instead following the PCB traces on the back of the PCB, it instead takes me to IC6, and the other side of that 1k resistor is at -12VDC potential. I did follow the silk screen diagram on the PCB when I installed these new IC's (TL072's, TL074's). So, there's NO WAY I should be seeing this potential, unless that IC is installed backwards. Once again, pulling this apart.
ANYONE run into this problem before?? I see I'll need to sit down and revise the schematics and layout drawings to get all that correct.
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