Ampeg SVT Classic won't come off standby, ever...

[vintagekiki]

I don't know if we're talking about the same thing

http://www.tangible-technology.com/schematics/Ampeg/SVT/Classic/

Have you passed the Troubleshooting procedure described on page 8

http://www.tangible-technology.com/schematics/Ampeg/SVT/Classic/SVTCL_OM.pdf


https://www.thetubestore.com/lib/thetubestore/schematics/Ampeg/SVT-Classic-Audio-Schematic.pdf

http://www.tangible-technology.com/schematics/Ampeg/SVT/Classic/svt-fault-detector.jpg

http://www.tangible-technology.com/schematics/Ampeg/SVT/Classic/SVT-CL_AC-Term_Brd.pdf

[nevetslab]

Thank you all so much for help. I really appreciate that.

I disassembled the preamp and saw the bad/burned/cut trace from J9. I soldered a jumper wire and viola! Amp turned on an everything seems to work, but it buzzes a lot. I think its a 120hz hum, it decrease a bit when it put the gray multiconnector away from high voltage wires around the grounding point of main chassis. I saw how is it wired in SVT-AV and it's kind of the same (but in AV I dont have buzz). Of course I have checked the tubes and bias and everything. It's also not the input jacks, instrument or anything simple.

Second question - I need to hook up the fan wires. The amp was US-market SVT which I rewired via AC board jumpers, to 230V. On those terminals that repair manual says to connect the fan weh wiring to 230V, there is 230V voltage and the fan is 120V. However, I have measured 120V voltage between some other pins on AC board (don't remember now which ones), so the question is can I use *whatever* I will find giving me 120V or the fan must be connected to specific jumpers?

[ATTACH=CONFIG]54973[/ATTACH]

I hadn't mentioned the solder joint issues with those 4mm spaced Molex headers that are used all over the amp. What you found on the bottom of J6 (V+FIL/V-FIL) connector is NOT uncommon over time. That's the path to the Relay board's power for Q1 that controls the relay. In fact, the sheer weight/mass of these amps from those huge xfmrs inflicts vibration damage to the PCB's over time, when these are used on tour and travel in the equipment trucks, regardless of being in foam-lined road cases. The power xfmr mtg hardware loosens, invariably have their mtg screws come loose and rattle around inside, and loose xmfrs in this amp in transit turns the amp into a shaker table. I check ALL of those 4mm square headers on the main PCB, preamp PCB & output PCB/AC Mains PCB, as well as all other connectors, as solder fractures on these amps is a constant problem. De-solder/re-solder, as well as the adding of the jumper you had to do to restore order here.

As to the buzzing.......check the main power amp PCB's metal standoff on the output xfmr side. That standoff becomes loose from the chassis screw, AND, IT is the main Grounding Connection from the Main PCB to chassis ground. If it is loose, you'll go nuts trying to find where the hum/buzz is coming from.

As a regular course of inspection when these SVT's come in, ALL of the rear panel components are closely inspected. Too often, I find solder joint fractures on the Bias Pot terminals, as well as the Preamp Out/Power Amp input jacks, Slave Out jack, XLR output connectors, as well as all the 4-mm spaced terminals at the opposite side of that PCB, which are used for the power supply connectors. Same holds true of all the preamp pot terminals on that PCB you had open to find the problem with J6.

The one drawing I've NOT come up with, or composed from all the Mains Connection drawings I have is that of the power xfmr. The default connection between the FAN connector up front on the chassis and the AC Mains relay board, which come from J14 & J15....those are directly from the AC mains input. J16-J29-J30 look to be the series connection between upper 120V Primary & lower 120V Primary, which get joined for 240V wiring. Having the fan connect between J14 and J16 would have it across one of the primaries on the main Pwr Xfmr (120V), using the primary coils as an autoformer, and should work with the 120VAC fan.

[tubeswell]

The one drawing I've NOT come up with, or composed from all the Mains Connection drawings I have is that of the power xfmr. The default connection between the FAN connector up front on the chassis and the AC Mains relay board, which come from J14 & J15....those are directly from the AC mains input. J16-J29-J30 look to be the series connection between upper 120V Primary & lower 120V Primary, which get joined for 240V wiring. Having the fan connect between J14 and J16 would have it across one of the primaries on the main Pwr Xfmr (120V), using the primary coils as an autoformer, and should work with the 120VAC fan.

All I have are these?

[boroman]

Thank you so much. That's a KNOWLEDGE...! I would check those grounds. Also my input jacks needs to be replaced. Just ordered few...

The one drawing I've NOT come up with, or composed from all the Mains Connection drawings I have is that of the power xfmr. The default connection between the FAN connector up front on the chassis and the AC Mains relay board, which come from J14 & J15....those are directly from the AC mains input. J16-J29-J30 look to be the series connection between upper 120V Primary & lower 120V Primary, which get joined for 240V wiring. Having the fan connect between J14 and J16 would have it across one of the primaries on the main Pwr Xfmr (120V), using the primary coils as an autoformer, and should work with the 120VAC fan.

I think I noted down the J14+16 but need to check tomorrow. That would make sense. What bothers me also (in AC Board) that all the schematics had later revisions than mine and my board is missing J37 & J38 pins. So when I rewired it for 230V I was a bit confused than the black wires from PT and FT were designed to go to J37 and J38 which I haven't got! But anyway, in the ampeg official diagram it shows that last three cables from PT & FT (including black as above) are marked Jxx -> N/C so "No Connection". So the question is - why put them in AC board, can I just get them all 6 hanging, unconnected?

[nevetslab]

Thank you so much. That's a KNOWLEDGE...! I would check those grounds. Also my input jacks needs to be replaced. Just ordered few...



I think I noted down the J14+16 but need to check tomorrow. That would make sense. What bothers me also (in AC Board) that all the schematics had later revisions than mine and my board is missing J37 & J38 pins. So when I rewired it for 230V I was a bit confused than the black wires from PT and FT were designed to go to J37 and J38 which I haven't got! But anyway, in the ampeg official diagram it shows that last three cables from PT & FT (including black as above) are marked Jxx -> N/C so "No Connection". So the question is - why put them in AC board, can I just get them all 6 hanging, unconnected?

[ATTACH=CONFIG]54994[/ATTACH]

Reading the chart and looking at my Relay wiring diagram, it appears the upper primary has 4 wires, while the lower primary has three. Upper coil: WHT is 0V, Blu-Wht is 100V, BLU is 110V, BLK is 120V, while the lower primary coil: BRN-WHT is 0V, GRY is 100V, BRN is 120V. Same would hold true for the Heater/Screen Supply xfmr. So, Pin 16 would be the 120V fan connection for 230/240V wiring. I have often seen a pair of wires capped off in the primary harnessing...probably because the AC mains PCB didn't have J37/J38.

[boroman]

Reading the chart and looking at my Relay wiring diagram, it appears the upper primary has 4 wires, while the lower primary has three. Upper coil: WHT is 0V, Blu-Wht is 100V, BLU is 110V, BLK is 120V, while the lower primary coil: BRN-WHT is 0V, GRY is 100V, BRN is 120V. Same would hold true for the Heater/Screen Supply xfmr. So, Pin 16 would be the 120V fan connection for 230/240V wiring. I have often seen a pair of wires capped off in the primary harnessing...probably because the AC mains PCB didn't have J37/J38.

So, would you leave 3 wires from PT and 3 wires from FT totally disconnected?
or
leave 2 wires from PT and 2 wires from FT connected (but blacks would be disconnected because I dont have J38/J37)

or it does not matter?

----

The thing that's on my mind too - what's the REAL advantage of changing 22ohm resistors to 220ohm and cutting out diodes? Ampeg says it prolongs life of tubes, but does it mean, that they get lower PV or something or they're just better protecting screens? I don't want the amp to sound even a bit weaker, if that's the point.

[nevetslab]

So, would you leave 3 wires from PT and 3 wires from FT totally disconnected?
or
leave 2 wires from PT and 2 wires from FT connected (but blacks would be disconnected because I dont have J38/J37)

or it does not matter?

----

The thing that's on my mind too - what's the REAL advantage of changing 22ohm resistors to 220ohm and cutting out diodes? Ampeg says it prolongs life of tubes, but does it mean, that they get lower PV or something or they're just better protecting screens? I don't want the amp to sound even a bit weaker, if that's the point.

Are you wiring for 230V or 240V? Either way, you do have J33 and J34 available for the unused terminals of the Power Xfmr, and J23 & J24 for the unused terminals for the Filament/Screen xfmr. So, you'd cap off the two BLK wires, not having any more spare single terminals like J37/J38.

I've never dug into the reasons for the change in the screen resistor value, nor the elimination of that parallel diode. 22 ohms would allow more screen current to flow. I can't recall any amp using a screen resistor that low before. Even 220 ohms seems low, but, I've never had issues. I've had numerous tube failures in the fleet of SVT-CL, -AV and -VR's here, often with the screen resistor burning open (both 220 ohms and 22 ohms prior to my changing them all to 220 ohms). Is there a sonic effect in that value change? Interesting question. I wouldn't think so.

[boroman]

I've never dug into the reasons for the change in the screen resistor value, nor the elimination of that parallel diode. 22 ohms would allow more screen current to flow. I can't recall any amp using a screen resistor that low before. Even 220 ohms seems low, but, I've never had issues. I've had numerous tube failures in the fleet of SVT-CL, -AV and -VR's here, often with the screen resistor burning open (both 220 ohms and 22 ohms prior to my changing them all to 220 ohms). Is there a sonic effect in that value change? Interesting question. I wouldn't think so.

Thanks. Actually all of the vintage SVT's have 22ohm screen resistors. It's also considered as a mistake and most replace them by 1k 5W (minus the diodes of course)

[DrGonz78]

Wish I knew this when I recently replaced those 22 ohm resistors on an SVT I serviced recently. If I get that amp back on the bench again I will change out those screen resistors to the higher resistance recommended. Thanks for the info.