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Fender twin reverb (1981) cuts all sound after 10 minuttes

Fender twin reverb (1981) cuts all sound after 10 minuttes

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Old 08-11-12, 01:36 PM
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Question Fender twin reverb (1981) cuts all sound after 10 minuttes

I recently bought at 1981 Fender twin reverb. It was recently serviced by a licensed dealer and sounds amazing. But after having it for 2 months and moving to a new country, it suddenly has problems.

I play for about 10 minuttes, and then almost all the sound disappears! For the 10 minuttes everything seems to sound ok.
Does anyone know what could be the problem or know how to troubleshoot it. Im in a little cabin in Norway and dont have the possibility to go to a licensed dealer.

Any help wil be appreciated!
Kasper
 
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  #2  
Old 08-11-12, 01:55 PM
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Thumbs up Welcome to our forums!

First off we would need to know from what country was the amplifier originally from?
It is possible that the power specifications where you are now do not match what the amp was designed for.
Or, it got bumped or dropped in the move.

World power chart.
 
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Old 08-11-12, 02:15 PM
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Hi GregH, The Amp is US made but have been playing in Denmark for the last 10 years, which have the same power specs as Norway where i am located now. It have been playing fine for a month after the move, and i was quite carefull
 
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Old 08-11-12, 02:29 PM
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This is from the Norwegian Gov't site:

Q: What is the voltage of the power supply in Norway?

A: The electricity supply in Norway is 230 V (same as the UK), but appliances up to 240 V will work. Plugs have two round pins, and adapters are available.
US made devices are 120 volt 60 cycle, Norway is 230 volt 50 cycle.

Something is not right here?

This is what is shown for Norway:





Which are 220 volt 50 cycle.

If you are using an adapter then perhaps the 50 cycle power has damaged some electronic components.......are you?
 
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Old 08-11-12, 02:45 PM
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I am not using a adapter, but the amp has worked fine in Denmark for the last 10 years which im pretty sure have the same power outlets. I do think a previous owner has changed the power supply, maybe in such a way that i can work properly in Europe. Im not sure, but thrust the guy i bought the amp from.
 
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Old 08-11-12, 04:18 PM
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I wonder if these amps have thermal cutoff breakers. Maybe the amplifier circuitry is overheating (either due to a malfunction in the device or because of the power as Greg mentioned) and after about 10 minutes of use, it shuts down for safety so the circuitry doesn't fry.

I know it's not much help in how to fix it... but maybe an idea of what to look for.
 
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Old 08-11-12, 04:36 PM
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Give us the electrical specs that are printed on the rear.
They are not clear enough to read.
 
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Old 08-11-12, 04:43 PM
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Denmark supplies 230V at 50Hz at the receptacle. Norway also supplies 230V at 50Hz at the receptacle. Source: Current Solutions, Inc. Since the power at the receptacle is identical in the two countries, that shouldn't be the source of the problem.

That leaves the amp itself. Some part of it may be failing, possibly due to overheating. Given that you're now living in a cabin, and it's August, you and your amp may be playing in a higher heat environment that you were accustomed to in Denmark, and that may be contributing to the failure. This seems especially likely because you say that the amp performs perfectly for 10 minutes before losing sound. That seems like a likely amount of time for heat to build up to the point that the failure would occur.

You might try moving the amp to the coolest spot you can find, elevating it on something like a milk crate that will allow air to flow under it, and playing it early in the day. If that changes the time before shutdown, then you can be pretty certain that the problem is heat related.

The likelihood is still that you need to take it to an authorized repair shop for diagnosis.
 
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Old 08-12-12, 04:55 AM
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When were the tubes replaced? Make sure they're all lighting up, and the 6L6GC power tubes have no bright purple or pink tint. Swap all the tubes around to see if the problem changes. Make sure you're swapping the same type preamp tubes. There are two types in the amp: 12AX7/7025 and 12AT7. Don't mix those up!

A 1981 amp could also be suffering from bad capacitors and/or cold solder joints.

In any case, you can seriously damage the output transformer if you continue to operate it at less than 100%. Those transformers are not cheap.

If you decide to go messing around inside the amp, first make sure you drain the power supply capacitors! There are lethal voltages contained there that could kill you, and that voltage can be held for days after you unplug the amp.
 
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Old 08-13-12, 06:17 AM
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I'm thinking the tubes (one or more) is on it's way out.
Are you playing at or above the RMS value of the amp (volume above 70%)?
Odds are a tube is going and once it heats up, it kicks out.

If it is not a tube going, it'll probably be a cap that is swelled or leaking (generally easy to spot).
 
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Old 08-14-12, 04:18 AM
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Thank you all for your response. I have been without internet the last couple of days and are now eager to try some of your solutions.

1. Make sure all the tubes are lit while operating.
2. Pull all tubes to see if there is anything physically wrong with them. Things that dosent look normal.
3. Swap the tubes around(power and preamp tubes) to see if the problem changes(making sure not to swap different kind tubes)
4. Try elevating the amp so that im sure its cold enough to se if its a heating problem.

Bonus questions about tube troubleshooting:
1a. Its the any point to try swapping around my existing preamp and powertubes? Or is a case of buying new ones and try to replace them?

1b. Can i try to pull one tube at a time to see if the problem lies there?

@GregH
unfortunately what you cant see because the picture is blurred, is that the tube chart inside the amp is missing or almost gone.
 
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Old 08-14-12, 04:21 AM
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@Northern Mike


I dont normally play above 70% in volume but i have tried to "max" the amp out to hear its reaction.
 
  #13  
Old 08-14-12, 05:44 AM
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Be careful when handling the tubes. non-powdered latex gloves are suggested here. Also, make sure you match the numbers between any tubes you swap. They may look the same and fit, but their values may differ.

It should also be noted that because it takes so long for it to kick out, it may be tough to spot the actual issue. Not saying impossible, but could be tough.

One thing I didn't ask is, when it kicks out, does all the lights go out, or only loss of sound?
With the back open as it is, try playing and see if you can spot a tube kick out when it goes dead.
 
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Old 08-14-12, 08:57 AM
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awesome idea checking if all the tubes are on when it cuts the sound, i will try that first!
 
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Old 08-16-12, 05:47 AM
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Nothing works

Im sorry to say that none of my tests showed anything out of the ordinary The tubes all seem to be proper lit regardless of they are are cold, hot og the amp cuts out. Nor does temperature around the amp make a difference. Its takes 20 minuttes for the amp to cut the sound, after that you can switch it off and turn it on, and will play for maybe 5 minuttes depending on how long it was switched off.
 
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Old 08-16-12, 07:30 AM
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There is an old TV repair trick using something you may not be able to get locally. It is called Freeze Spray. It is an aerosol can that literally freezes the component it is sprayed on. After the device stops working you leave the device on and one by one cool each resistor and capacitor till you find one that when cooled cause the device to start working again. Then you replace that resistor or capacitor.

Here is one such product and a clear explanation of its use: Amazon.com: Customer Reviews: MAX Professional 7777 Blow Off Freeze Spray Electronic Component Cooler, FR-777-777 (10 oz)
...We use this to identify chips and transistor that fail when they get too hot buy letting the circuits heat up to the point that the device fails and then using this spray on individual components...
 
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Old 08-16-12, 07:42 AM
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I had a Ross silvertone amp that did that. 10-20 min after playing it would lose volume then crackle and buzz. The repair was more then the amp was worth from what I was told. It was not a tube but the pre amp something? I think it was overheating and a relay trips. It was protection so that the amp would not work until it was warmed up, then also if it overheated it would shut down.

I think it was something with the actual speaker that cause this shut down now that I think about it.

I will have to call the guy that owns it now to ask what it was exactly.


Not sure if this helps but just wanted to state that I had the same issue years ago.
 
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Old 08-16-12, 09:42 AM
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Ray,
A can of compressed air (the stuff you use to clean computers and electronics) turned upside down will do the same thing.

Be very, very careful doing this around tubes though. This can crack or blow a hot tube.
 
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Old 08-16-12, 12:47 PM
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Knowing I have been half fasting my suggestions, I thought I would sit down and go write out what I would do if it was my amp. I have the educational background in electronics (although more than 12 years ago) and have repaired a lot of my own electronics.
Keep in mind, the stuff I am describing is not a written in stone must do list.

DISCLAIMER:
There is a risk of electric shock with some or all of the steps I am about to describe. Be very careful, take your time, and do this when and where you can work undisturbed by distractions
.

Step 1: identify what is actually the problem, not just the symptoms

Amp stops outputting audio after 10-20 minutes of usage below 70% volume...
I will put money on it that it's a thermal related issue and is kicking out due to thermal protection (doubtful) or component failure due to heat.

Assuming this amp is like most and has a phono output (large headphone jack), grab a headset and plug it in. Play until failure or 30-40 minutes time (which ever comes first).
Most phono outputs are directly connected to the pre-amp. Some might have an independent circuit. An indicator of a separate circuit would be a volume control specifically for the phono jack.
If it does not fail during this test, pre-amp is fine and its final amp circuit or speakers (doubtful).

If it does fail, did the lights, tubes, etc turn off, or did they all stay on? If they turned off, potentially it's an issue in the power supply circuit.
If the lights and tubes stayed lit, it's probably a pre-amp circuit issues.

Step 2: checking potential problem circuit

First thing to look for, does the circuit have a fuse. Some are fast blowing, some are slow. Some just act oddly. If it has a fuse (or more), Identify the size and amperage and replace them. A fuse may not fully blow out and can work until heated. They are cheap, and in most cases I have spares kicking around.

If there are no fuses, or this did not resolve the issue, you're going to have to start looking very closely at the individual components.
The tubes would be my first target as there is a reason they are replaced in modern electronics. I'm going to however skip them for now as they can be hard to find (and I don't have a lot of experience with them specifically to tell you what to look for).
With modern electronics, the first thing I look for next to fuses is capacitors and IC chips.
For the chips, look for any that have burn marks or discoloration. Can you read the text printed on them? Likely it's not a burnt IC, but it's always possible.
The most common failure that will create the effect we are seeing is a failed capacitor (leaking or blown open). You'll need a good eye here. Look for one or more capacitors that either have a swollen top or are leaning to one side on the circuit board (looks out of place). Take your time here as sometimes they are hard to spot. (see attached picture of the two caps blown in my plasma TV).
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If and when you spot one, keep searching, there could be more than one.

if/when you spot bad capacitor(s), grab the voltage rating and the size and find replacements. In the picture above, you'll see the caps I had blown where 3300uF and 10Volts.
Once you have the replacement capacitors, you'll have to either find someone or yourself, desolder the bad caps (removing the old solder with a solder sucker) and install the new ones. Pay very close attention to the + and - markings on the old caps as you need to install the new ones in the same manner. Insert the new caps, solder, and clip the extra length off the leads (a nail clipper works good for this clipping).

I'm going to leave this write up at that and wait to hear the progress. I will suggest wearing non-powdered medical gloves (that fit, not that one size fits all crap). The reason for this is you'll not leave any skin oil on the tubes if you accidently touch them. They also can add a very light layer of protection from electric shock (very, very light, more concerned about finger prints on the tubes then protection to be honest).
One piece of advice I have is to tag everything you remove. A photo (love digital cameras) from before you remove something is also a very good practice. You can use them for write ups, to provide info to us, and if you have doubts as to how something goes back together, you have a picture of how it was installed.

Let us know how this goes.
 
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Old 08-16-12, 01:04 PM
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One tip I failed to mention is use a flashlight or really good (adjustable) desk lamp with a cooler light color.
Unless you have a well lit lab, ambiant light won't be enough.
 
  #21  
Old 08-17-12, 04:09 AM
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Older guitar amps are very basic. The 1981 Twin was part of a series that was in production from 1968 to 1982, with the schematic mostly unchanged after 1974.

Here's the schematic.
 
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Old 08-17-12, 05:13 AM
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With the schematics Rick has provided, it looks like the pre-amp test I suggested should work.

The Export version power supply schematic (bottom right) doesn't indicated if it's a slow blow fuse. The N/A power supply (bottom center) indicates a slow blow which I've seen partially blow and work until heated.

The sweet thing with this is it doesn't look to have any ICs (only looked quickly) and is very unlikely to be a multi-layer curcuit board, so it should be fairly easy to repair. I don't think multi-layer boards where used until the 80's. ICs where not commonly used, but where available back then.

I think you'll have this fixed up and running in no time (provided part availability in your area).
 
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