| Author |
Message |
    
Cm3885
Intermediate Member
Username: Cm3885
Post Number: 400
Registered: 12-2003
| | Posted on Saturday, May 08, 2004 - 11:49 pm: | 
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Hey guys.
I found and bought a mav 250 at a CB shop that the guy was selling at a very reasonable price(pretty much parts value price) because it has an issue.
I think it might have a week driver tube/s. On low the amp keys up 10 watts and swings to about 50. On high it does 80 watts key and swings to around 200 or so. I had a 250 once (well, ive had 2 or 3 over the past 12 years) and it did about 550 to 600 swing on high and about 200 swing on low so i kinda am familiar with them. He had it working and it made a flash and then it started to slip in power.. But i want a second opion before i drop some $$$ into it. Im not tech at all but i think the driver tubes are either blown or low. This sound like the problem? Te gave me 4 6LQ6 tubes in the deal as well.. The amp has 4 6JT6 tubes in the driver section and they look to be original... |
Cm3885
Intermediate Member
Username: Cm3885
Post Number: 405
Registered: 12-2003
| | Posted on Wednesday, May 12, 2004 - 1:50 am: |
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Hello? |
Tech808
Moderator
Username: Tech808
Post Number: 2365
Registered: 8-2002
| | Posted on Wednesday, May 12, 2004 - 6:46 am: |
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Cm3885,
Sorry, but I can't offer you any help on this one.
Hopefully one of the other Tech's or member's can help.
Lon
Tech808 |
Jp1116
Member
Username: Jp1116
Post Number: 51
Registered: 5-2002
| | Posted on Wednesday, May 12, 2004 - 8:00 am: |
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Probably the only way to know for sure would be to test the tubes. Probably not the help you wanted but one of the most logical places to start. You might consider picking a tester up, it beats buying tubes and finding out that wasn't the problem. |
Cm3885
Intermediate Member
Username: Cm3885
Post Number: 406
Registered: 12-2003
| | Posted on Wednesday, May 12, 2004 - 1:48 pm: |
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Wow.. Most of my technical posts ive posted have ended up with a "sorry i cant help you answer".. LOL!! I thought you guys were techs and knew eveything!! J/K! LOL
Im gonna test the tubes out at a local electronics shop that still has an old tube tester. It doesnt test under a load but it will help me find which tubes are gassy or shorted. |
Dindin
Intermediate Member
Username: Dindin
Post Number: 109
Registered: 2-2004
| | Posted on Wednesday, May 12, 2004 - 8:10 pm: |
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CM3885,another problem cropping up in the D&A's are the capacitors breaking down due to heat and age.if they are still original you might want to replace them anyways.probly close to 30 years old.Also never discharge them directly to ground with a screwdriver as I have seen so many do.that sudden discharge in a cap that size can cause it to short internally rendering it useless.they should only be drained through a resistor.I just checked the schematics posted in C.B.Tricks for the D&A line,the one 4 the maverick shows a 400 volt b+,I have owned 3 drifferent mavericks and they all had a tripler on the high voltage supply giving a 1200 volt B+.makes me wonder how many different versions of the Maverick they built.I have never owned any of the brown or black and white D&A's,all mine were the old grey hammertone. |
Cm3885
Intermediate Member
Username: Cm3885
Post Number: 407
Registered: 12-2003
| | Posted on Thursday, May 13, 2004 - 1:55 am: |
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Whay do you mean drained though a resitor? ive always seen most people (yes me included) use a screwdriver to drain the voltage. This resistor deal is totaly new to me because until your reply ive always known most techs around here to use a screwdriver.. |
Jp1116
Member
Username: Jp1116
Post Number: 52
Registered: 5-2002
| | Posted on Thursday, May 13, 2004 - 6:53 pm: |
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Dindin, look at the schematic again and you will see that on the top transformer there is a center tap going to the 6AQ5 tube. That is where the 400 volts is going. On the original schematic it shows that there is 800 volts going to the tubes from the bridge rectifier. I've never seen a doubler or a tripler in these amps but there were so many variations who knows.
CM3885 the bleeder resistor drains the charge out of the filter capacitors so you don't have to do the screwdriver trick. That is if you don't forget and discharge through you fingers arms, ect. It is preferable to take a 470k ohm resistor and attach one from end to end on each capacitor. Next best is to put the resistor after the bridge rectifier and the capacitors. Look at the Palomar 300a schematic on CB Tricks for more detail. |
Cm3885
Intermediate Member
Username: Cm3885
Post Number: 409
Registered: 12-2003
| | Posted on Friday, May 14, 2004 - 1:26 pm: |
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Well i didnt get charged today when i took the tubes out to check them.
Most of the 6LQ6's checked at 70% or above. I had 4 that were bad and 4 that were good and the driver tubes were all pretty much gone. So now i need to find some good used 6JT6 tubes and run the old tubes until they are pretty much dead then later on get some new ones. I know a few guys here locally that can get me pretty good deals on NOS tubes if i need them. |
Nobody077
Junior Member
Username: Nobody077
Post Number: 22
Registered: 4-2004
| | Posted on Friday, May 14, 2004 - 4:23 pm: |
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The fact that you said the amp was working until it made a flash and the power started to drop, would keep me from putting fresh tubes in the amp untill you realy look it over good.
I have had a few Phantoms, Mavrics, and raiders over the years to fix for friends and have personaly owned 2 phantoms myself, Other than the powersupply caps going bad due to age many of the amps have had bad tube chokes. Check the choke that feeds the tubes on the top side of the rf deck, if it looks discolored replace it.
As for useing a screwdrive to drain the amp I alwase do, even when I know that they have a good bleeder resistor on the caps. I aint taken no chances, realy not on the 2500V supplies Im dealing with for my 6m GI6B tubed amp, Im building. |
Cm3885
Intermediate Member
Username: Cm3885
Post Number: 410
Registered: 12-2003
| | Posted on Friday, May 14, 2004 - 11:55 pm: |
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Ive decided to get new tubes but im gonna save them for later after i have my tech go through it. Im going to use my old ones and find some good used 6JT6 tubes to replace the ones that were bad. That way if the amp fries the old tubes i will notbe out much and i wont waste the new tubes im gonna get.
As a sidenote, whats the diff between 6LQ6, 6JE6, and 6MJ6 tubes? Ive seen the 6MJ6 tubes that have all 3 numbers on the same tube.. |
Cm3885
Intermediate Member
Username: Cm3885
Post Number: 411
Registered: 12-2003
| | Posted on Saturday, May 15, 2004 - 12:03 am: |
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BTW, the flash we saw was in fact one of the driver tubes. I checked them all and 2 were almost flat dead and one of the two showed a dead short.. So i assume the one that showed a short was the one that flashed.
Nothing like playing detective to solve a problem in my amp... LOL! |
