Ampeg SVT

[bsco]

Besides the schematics g1 had pointed you to, which were on my earlier SVT-CL ht xfmr cycling thread, here's the power amp PCB parts placement guide to assist in troubleshooting.....remembering that you'll be looking at the foil side of the PCB, but it is the road map, if you will, in finding where all the parts are. I thought I had an Ampeg drawing showing the foil pattern of the power amp. I do have that for the preamp board, and that's here too.....zip files.

[ATTACH]50031[/ATTACH]
[ATTACH]50032[/ATTACH]

Ok. Thank you very much....I will see if I can get at this over the weekend but if not I will be early next week......Have to work on the convertable top and mix up some concrete this weekend......busy trying to do some stuff here around the house.....thanks to everybody for all the help so far......I do have a question nevetslab......this thing goes into fault mode as soon as the power is applied....and in standby...I re-installed those 12a tubes and removed the output tubes....and same thing......so there must be a problem with the low voltage supply which I will check out.....that front panel light is supposed to light up red when it is powered up and placed in standby mode.....right??

[nevetslab]

Ok. Thank you very much....I will see if I can get at this over the weekend but if not I will be early next week......Have to work on the convertable top and mix up some concrete this weekend......busy trying to do some stuff here around the house.....thanks to everybody for all the help so far......I do have a question nevetslab......this thing goes into fault mode as soon as the power is applied....and in standby...I re-installed those 12a tubes and removed the output tubes....and same thing......so there must be a problem with the low voltage supply which I will check out.....that front panel light is supposed to light up red when it is powered up and placed in standby mode.....right??

YES. In Standby mode, the panel light is RED. It won't go into Protect mode until attempt to come out of S/B.....assuming there is a fault condition. With the power tubes removed, and you're still getting Fault status, it remains in that mode, even after going back to S/B, until you power down, which resets the crude anlog logic circuits. So, as I'd outlined in the earlier threads and previous post, you'll have to trouble-shoot the control voltage circuits to find the fault. I gather you're not blowing mains fuses, which you did at the beginning of this pursuit. Take your time, be safe.

I've never intentionally unplugged the High Voltage Secondary connections to the power amp PCB so the high potentials aren't on the board while troubleshooting the low voltage control circuits. They are discrete connections...J13 thru J18...right-angle insulated 1/4" Fast-on terminals. I've always managed to steer clear of the high potentials. If you're nervous about it, unplug them, but make sure to document it, so you know EXACTLY where each wire/connector has to go in restoring order. Happy hunting!

[bsco]

Ok. Thank you very much nevetslab.....I will let you know how I make out.....and yes these amps do make me kind of nervous when working on them as I am not used to them.....
Cheers

[bsco]

I am back.......I did what you suggested nevetslab.....I disconnected the HV connections from the main board and taped them up.....and I removed the output tubes as well so they won't get damaged or cracked......when powered up and left in standby, the front panel light flashes......the 15V rails are down to approx. 4V each.....so I have a problem with the LV supplies.......now I can work without getting fried.......I have some schematics here from various sources so hopefully I have the right ones....the board number on this one is 2033854 REV E00.....I'll let you know how I make out......

[nevetslab]

OK....good luck on the hunt.

[bsco]

OK....good luck on the hunt.

OKKKKK.......I ended up taking that main board out completely as voltage checks were just sending me around in circles......so I started doing resistance checks and esr checks on caps....all the diodes checked ok as did the caps....I discovered that the IC for the DI was shorted pins 1,2,3,and4...pins 4 and 8 also showed a lower than normal reading....I removed it and sure enough...so I removed the other ic's and they also didn't read right on some pins.....so I am going to replace all of them.....I was thinking about mounting them in sockets and just running a very small dab of silicone on the ends of each IC where it meets with the edge of the socket....at least that way, if it ever comes back for repair it will be very easy to replace them....and the dab of silicone will keep them secured in the sockets while the amp is in use........I will have to get some TL072's Monday....I have a bunch of TL074's on hand.....I'll replace these early next week and let you know how I make out......Thank you very much for your help and all your advice on how to tackle these amps...I have to admit...I felt much safer with the HV removed from the board....
Cheers

[nevetslab]

Leave it to the previous mfgr owner of Ampeg to have selected 16V rated 220uF caps for C2/C27 with outputs at +/- 15V. Any time I have to dig into there, they get replaced with 35V parts, as the size is still suitable, lower loss/ESR. Sounds like you found what was dragging your supplies down. You might also check the two Thermistors CB1 & CB2 (look like 220pF ceramic disc caps...that small). I had just pulled two out on an amp that were very high in resistance for the function....40 ohms and 220 ohms. 2-10 ohm NTC 1/2A thermistors are probably what they should be. 10 ohm/1/2W would suffice in lieu of using thermistors there.

Cheers,

Steven

[bsco]

Leave it to the previous mfgr owner of Ampeg to have selected 16V rated 220uF caps for C2/C27 with outputs at +/- 15V. Any time I have to dig into there, they get replaced with 35V parts, as the size is still suitable, lower loss/ESR. Sounds like you found what was dragging your supplies down. You might also check the two Thermistors CB1 & CB2 (look like 220pF ceramic disc caps...that small). I had just pulled two out on an amp that were very high in resistance for the function....40 ohms and 220 ohms. 2-10 ohm NTC 1/2A thermistors are probably what they should be. 10 ohm/1/2W would suffice in lieu of using thermistors there.

Cheers,

Steven

I will check those thermistors and replace the caps you mentioned....I have to install the sockets and chips and then install the board and fire it up without the output tubes to see if there are any more issues but I think this might be it.............won't get back to it until early next week......I will let you know.....

[bsco]

Quick question...TL072BD (original) and TL072CN (replacement)will it work ?? only difference I can find it the thermal rating......I have the CN versions here......
Cheers

[Tom Phillips]

...I was thinking about mounting them in sockets and just running a very small dab of silicone on the ends of each IC where it meets with the edge of the socket....at least that way, if it ever comes back for repair it will be very easy to replace them....and the dab of silicone will keep them secured in the sockets while the amp is in use...

If I may add my 2¢. A good quality IC socket will hold the chips sufficiently well without the silicone. I wouldn't add the silicone. It would look unprofessional and potentially cause contamination problems later.

[bsco]

If I may add my 2¢. A good quality IC socket will hold the chips sufficiently well without the silicone. I wouldn't add the silicone. It would look unprofessional and potentially cause contamination problems later.

Noted Tom...Thanks......
Cheers

[Enzo]

It is just a guitar amp, ANY TL072 will work.

[bsco]

nevertslab, I got it working......I ended up replacing all those chips....I checked all the components in those op-amp ckts and everything was fine.....I went around and re-soldered all the connections to all the heavy parts and connectors..they looked ok but I did them anyway.....those capacitors and thermistors were fine......the only thing I am waiting on now is the fuse holder......the owner broke it trying to replace the fuse.......anyway, I should have the holder later next week...and I will be able to get it back together......Thanks for all the help......
Cheers

[bsco]

nevertslab, I got it working......I ended up replacing all those chips....I checked all the components in those op-amp ckts and everything was fine.....I went around and re-soldered all the connections to all the heavy parts and connectors..they looked ok but I did them anyway.....those capacitors and thermistors were fine......the only thing I am waiting on now is the fuse holder......the owner broke it trying to replace the fuse.......anyway, I should have the holder later next week...and I will be able to get it back together......Thanks for all the help......
Cheers

nevertslab....I got the amp back together as I had a new fuse holder assy ordered for it.....The amp works fine at a low volume but if you crank it up, it will go into protect mode after a couple of minutes......I am thinking it is a problem with one or more of output tubes....trouble is how do you test for this without tearing it apart again to determine which tube or tubes would be at fault.....If I have to I will have to replace all the tubes with a matched set but that is going to be expensive...I would love to hear your thoughts on this seeing as you have lots of experience dealing with these amps.....
Cheers

[g1]

Have a read through his post about matching here: http://music-electronics-forum.com/s...l=1#post477222