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The problem is the volume cuts out then slowly fades back in and then fades back out and then in and back out and stays there. I turn standby off then on and it works for a bit and happens again. It has to be power amp section because it a happens whether in clean or lead and I can hear the preamp when volume is cut and it sounds like it should just way low. All my voltages are proper for the preamp and PI. The screen and plate voltages are right on for the power tubes. I switched power tubes and same thing happens. I havent input a sine wave and chased it down the line yet. I thought I would get some ideas from the heads around here.
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These amps have a history of problems with the Molex-type connectors used between the various circuit boards.The filament supply connections are particularly suspect.One can sometimes see the scorched connectors.
I don't know if this is the problem with your amp, but it was so common that Lee Jackson (the designer) has posted a series of servicing videos for this amp on YouTube that deal with this and other servicing aspects for this amp.You can find them here:
ampeg vl-1002 - YouTube
Check them out........Good luck
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Yep. I know all about the heater problem with the molex connectors. The previous owner or one of his friends tried to do the heater mod and well.....when he was playing at his church he said the amp popped and then filled the place with smoke. When I got the amp there were torched resistors everywhere. After rehab it fired up and sounded good then the volume problems showed up.Comment
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Well you can add me to the list of Molex repairers on several of these and I do the mod Lee Jackson provided in the video and it works but be real careful firing the amp up if you don't have all of the screws in that power board that hold the sockets on with the spacers because those are grounds and you can cause serious arching if they aren't all grounded.
I would patch a cable from the send to return to see if the loop jacks could be faulty and would do it with the attenuator turned off.KBComment
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Yea, I can finally get back into this site!!!
Alright, the fx send return are not the culprits. Any other ideas. I can run the signal out of the fx and into a small combo and it is fine.Comment
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Yes but did you patch the cable from the send jack to the return jack and leave it in there and play the amp ? What happens is if these jacks get corroded or loose that signal that passes normally across them gets interrupted and it causes the signal to go up and down. The other source of volume swell is bad or weak power tubes.KBComment
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Ok so we know the problem is from the return in to the power tubes. I would pull two tubes and see if it still does it or you can even try one tube at a time as it will work with only one power tube then see if you can isolate it to a particular socket. May also want to monitor the plate voltage and bias while this is happening.KBComment
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Alright Amp Kat I have it narrowed down to the two inner tubes. I think I can feel confident about the outer two tubes. It has been 15mins and they are rock solid. They actually sound really clear and well defined as the two inner tubes had alot more breakup before they went out. Also the volume cut out alot quicker. Bias stayed solid even after the volume cut. I forgot to check the plate voltage. What about the two inner sockets should I check?
Thank you for helping me with this.
The 1.5k and 1k resistors check out ok.Last edited by chunkitup; 09-18-2012, 02:26 AM.Comment
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No problem fwiw I only use two tubes in my personal one and it works good.Originally posted by chunkitup View Post
If you look on that power board right next to the tube sockets you will see some resistors and they are small 1/2 watt 1.5k bias feed resistors. (R5-8) Make sure they are reading 1.5 kohms. The screen resistors I believe are on the underside of the board so you can't see them but you can see the traces from pin 4 of the socket to the other side of the connection but be careful there could be high voltage there so those should ohm out at 1 kohms also. I would also solder touch up the socket pins which are pretty easy as they are right there. Another thing to check is when you get it all back together and go to test it see if the tube heaters are dropping out (orange glow going out) when the volume swells. If that is the case then it brings us back to that Molex connector that is just waiting any minute to fry because of over currect on the heater supply.
Did you do the mods yet in the Lee Jackson Video ?KBComment
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The 1k and 1.5k resistors check out. I took the reading with the amp off so there was no high voltage threat. Now tube socket #3 looks as if someone put a new socket in. The solder joints on the pins are all new and shiny. I will reflow those joints. I will put the tubes back in and see what happens. If it swells then goes out then I will check the heaters.
As far as mods I have not seen the Lee Jackson video. I did put in a fixed dampner of 1M//470pf and a 2.2M//.001uf paralled with the preamp control. Also a .001uf bright cap on that same control. Do you have a copy of the Lee Jackson video?Comment
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That's a special tube socket that is longer than the regular size so make sure it is the right one as if it's the shorter one the board won't fit right and would have to bend the board to make it fit. If the board fits good it's probably the right one.KBComment
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Well I took out the power board and replaced the v2 and v3 screen resistors and reflowed all the solder joints and so far so good. It hasnt cut out yet and its been 15mins which is longer than it has ever lasted.
Now I have to check out Mr Jacksons video.
Thanks alot Amp Kat. Hopefully we can put this issue to bed.Comment
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