valvestate?

[gibsonaxe]

Hi guys,i have a marshall valvestate 8080,one night we were jamming and could smell something burning,their were 5 amps going at the same time so really did check out which one it was coming from,there was'nt any smoke it was just a faint electrical burning smell.So about after the 2 or 3 jams,i had no volume well very little on the O/D channel on my valvestate.I knew it probably was'nt the pre-amp tube,but i bought a new one anyway and installed it.Looking at the board i saw the problem,on the front of the board right under 2 resistors or capacitors,not sure what they are[newb-here] was burn marks.Is this something i should attempt to try and fix myself or bring in into a shop?

I should also mention the clean channel sounds fine,the o/d channel has very low volume,at one point it was almost no volume,but after replacing the pre-amp it has gained some volume,hum?Any thoughts?Thanks guys.

[bbartilson]

Can you post a picture of the board/ parts in question?

[gibsonaxe]

Will do,thx.

[gibsonaxe]

Hope this is clear enough,figures no batteries for my digital camera sorry.

[bbartilson]

Will do,thx.

I just helped another member through one of these - I PM'ed him to take a look at your pic to identify them.

There are two pairs of higher watt resistors in that circuit. One provides the heater voltage for the tube, the other pair are dropping resistors for the +/- DC supply for the op amps.

So...

1. Go and look for the previous thread, and see if you gain any insight.
2. Like in the other thread, are any of the IC's hot? Careful with your fingers, they can get VERY hot.
3. Do you have a meter, and any ability to use it? Making voltage measurements will be necessary if you are to fix it yourself.
4. The hot resistors may or may not be a problem. They need to be measured, and we should have a look at the solder side of the board as well.

The other thread was started by GaryDean and had 8080 in the title. It was quite recent.

Best,
Bill

[gibsonaxe]

Thanks man i will check out that thread and post some findings,thx again appreciate the help.

[tortoto]

...I found this ploblem many times in valvestate
if another part is not ploblem
i change two resister from 1 watt or 2 watt
to 5 watt ceramic resister we can use it along time

[Garydean]

Hi, I'm the guy that Bill helped out a few weeks ago. The two resistors you have circled are the ones for the 12ax7. When I was finished with my repair those were still hot...normal. Please read my thread from a few weeks ago. You really need to get into the amp and check voltage readings and make sure your rails (power supply) are around -15 and +15 and no higher than 16 and change. From what I'm hearing, these amps run hot especially around the reverb section. It's not uncommon to have a few failing op-amps. Hopefully, once you read my thread from a few weeks ago, you'll have a better idea as to what is happening...if anything. You may be trying to troubleshoot something that is not a problem.

OH.. and thanks Bill for tracking me down and keeping me in the loop.

Best regards,
Gary

[bbartilson]

Thanks man i will check out that thread and post some findings,thx again appreciate the help.

No problem. That's the whole point of this place.

There are three IC's in the picture you posted. One or more of those may be getting hot. From my experience with the amp Gary was working on, it seems that IC9 and IC10 may be warm under normal circumstances.

However, if you have a dead channel, it's most likely that it's one of the IC's, or perhaps something associated with the tube. Often a failed IC will get very hot, and make the troubleshooting easy. Other times they fail in such a fashion as they don't get hot. Harder to troubleshoot, but not impossible.

The first goal in any troubleshoot is to establish that the power supply is there and correct. You'll need a meter for that. Harbor Freight sells one for $3.99. Decent meters of reasonable shop quality can be had for $50 or so. One like the Fluke I use will set you back $150 or so. Not necessary for your purposes. The Harbor Freight meter will do fine (as will a Radio Shack digital) to get you started. Let me know when you get a hold of one, and we'll talk about how to measure the supplies.

Just remember - you can always decide to back out and take it to a tech. The best scenario in that case is to do no more harm. Fixing a broken amp is one thing. Fixing an amp that someone else tried to fix usually means more than one completely unrelated problem. (The actual failure, then whatever the first guy did to screw it up further)

If the goal is to learn about amplifiers, you can end up with a repair and some knowledge. If your notion is merely to save money, that gets iffy as soon as you start sticking meter probes in the amp. One slip and it'll almost certainly cost more for a real tech to find the problems. Don't be fearful, but don't open the amp on the kitchen table with the kids running around and bumping your elbow.

-B

[bbartilson]

OH.. and thanks Bill for tracking me down and keeping me in the loop.

No problem. You just went through one of these. I expect you can help him more than I, though I'll keep an eye on the thread.

Never thought you'd be in the position of the guy with the knowledge so quickly, eh? Trust me, you're a better authority than I at this point. I haven't opened a Valvestate in ten years.

Best,
-B