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Thread: Sessionette 75 problem.

  1. #1
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    Sessionette 75 problem.

    Hi folks, don't know if anyone can help me but my problem is this:

    I've got a Sessionette 75 guitar amp that is about 25 years old. It was working fine until the other day when it started to develop a mains hum from the speaker. Not dead loud but far too loud to ignore. Then it started to get distortion and lose power. I took a guess that it might be a dried up electrolytic in the power supply so changed the two big ones. However, it made no difference.

    I had a look at the circuit board closely and noticed that the 1 watt resistor leading to the external speaker socket was burned -and so I suspect it might be one of the MOS-FET power transistors that has burned out. I can get a whole now power amp board from the guy that still services them for about 30 quid.

    However, (and this is the bit I don't understand). I read somewhere that if you plug your guitar into the faulty amp and then take a lead out of the 'send' socket (the output from the pre-amp) and send it to a different good amp rather than back in the 'return' socket it will give you an indication of whether the preamp is OK as it will sound OK through the good amp, if you see what I mean.

    I did this and although it seemed to lose most of the distortion - the mains hum was still there as loud (or louder) than ever.

    So I switched the faulty amp off at its on-board on/off switch - but the mains hum was still coming out of the good amp. Yet when I unplugged the faulty amp altogether the hum stopped.

    What's going on? Why does it still hum when the faulty amp is plugged into the good amp but switched off at the switch - yet doesn't when I unplug it from the socket? I don't want to order a new module if I've made the wrong diagnosis. Can anyone shed any light on the matter? Cheers.

    Oh and I forgot to mention I only got the problem after I connected it to a couple of 2x12 speakers. Don't know if that is relevant or not.

  2. #2
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    That second hum that you get on the good amp is very probably a ground loop hum, I'm sure the internal 230V power connections in both amps are wired differently, nothing unusual.
    Eventually you should correct one of them if you want to use both amps together on stage , but that's not the main point today.
    Please post a couple clean close up pictures of that burnt resistor.
    I guess it might be part of a "Zobel network" that often is soldered right across the output jacks, instead of in the PCB proper.
    It may burn if your amp oscillates, which after that becomes unstable. That oscillation is inaudible, but it causes the audio signal to become fuzzy, dirty, and the amp overheats, eventually burning out.
    Definitely hooking extra speakers without respecting the suggested impedance causes trouble.
    Good luck.

    If

  3. #3
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    OK thanks for the reply, will post a piccie as soon as I get the chance.

    I wired the 2x12 in series so I would have thought that would have kept the impedance up. Oddly the onboard speaker never switched off when I plugged in the external speakers. BTW I only use it for home use and was actually using it as a bass amp to practice on.
    I've read that this amp is a little bit prone to oscillation so it sounds like you are talking good sense. Cheers for the help.

  4. #4
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    Problem with sessionette 75

    Quote Originally Posted by joe-90 View Post
    Hi folks, don't know if anyone can help me but my problem is this:

    I've got a Sessionette 75 guitar amp that is about 25 years old. It was working fine until the other day when it started to develop a mains hum from the speaker. Not dead loud but far too loud to ignore. Then it started to get distortion and lose power. I took a guess that it might be a dried up electrolytic in the power supply so changed the two big ones. However, it made no difference.

    I had a look at the circuit board closely and noticed that the 1 watt resistor leading to the external speaker socket was burned -and so I suspect it might be one of the MOS-FET power transistors that has burned out. I can get a whole now power amp board from the guy that still services them for about 30 quid.

    However, (and this is the bit I don't understand). I read somewhere that if you plug your guitar into the faulty amp and then take a lead out of the 'send' socket (the output from the pre-amp) and send it to a different good amp rather than back in the 'return' socket it will give you an indication of whether the preamp is OK as it will sound OK through the good amp, if you see what I mean.

    I did this and although it seemed to lose most of the distortion - the mains hum was still there as loud (or louder) than ever.

    So I switched the faulty amp off at its on-board on/off switch - but the mains hum was still coming out of the good amp. Yet when I unplugged the faulty amp altogether the hum stopped.

    What's going on? Why does it still hum when the faulty amp is plugged into the good amp but switched off at the switch - yet doesn't when I unplug it from the socket? I don't want to order a new module if I've made the wrong diagnosis. Can anyone shed any light on the matter? Cheers.

    Oh and I forgot to mention I only got the problem after I connected it to a couple of 2x12 speakers. Don't know if that is relevant or not.
    Dude i just had a right mare with my sessionette! A week ago it started humming Loud and the signal from the input stopped working, just an annoying hum. The trebles/bass/channel swithes etc made no difference.. same annoying sound.. I phoned my mate Brett 'the vision' Robertson who got me to plug my guitar into the effects return at the back and the guitar came through. this showed the output was still ok, which is good because apparently the outputs for these bad boys are quite hard to come by. Next i took it to Blues 'The Spark' Mike who said the 'TL 071 CP Op Amp' had probably blown. online from RS components i ordered a 1 x TL 071 CP and 3 x TL 072 CP (actually i ordered double that amount so i had a spare set), replaced them with the ones ones the board and BAM, Hows yer Father!! Up and Running this bad boy is sweet and once again rippin it up nicely!
    Not sure if that helps but helped me.
    Oh one thing to be aware of, if you get the Op Amps (TL071&72) i was told not to touch the pins with you hands as they can screw them up, something to do with static?!?!!
    Good Luck!
    Chops

  5. #5
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    Hi
    If you ever need the amp to be serviced, I have had a few thru my workshop. Great little amps and fantastic sound.
    I can also supply and/or install a new replacement power amp for the Sessionette 75 and other session models and matches the exact input and output specs of the old albeit now more reliable.

  6. #6
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    Quote Originally Posted by Chops View Post
    Dude i just had a right mare with my sessionette! A week ago it started humming Loud and the signal from the input stopped working, just an annoying hum. The trebles/bass/channel swithes etc made no difference.. same annoying sound.. I phoned my mate Brett 'the vision' Robertson who got me to plug my guitar into the effects return at the back and the guitar came through. this showed the output was still ok, which is good because apparently the outputs for these bad boys are quite hard to come by. Next i took it to Blues 'The Spark' Mike who said the 'TL 071 CP Op Amp' had probably blown. online from RS components i ordered a 1 x TL 071 CP and 3 x TL 072 CP (actually i ordered double that amount so i had a spare set), replaced them with the ones ones the board and BAM, Hows yer Father!! Up and Running this bad boy is sweet and once again rippin it up nicely!
    Not sure if that helps but helped me.
    Oh one thing to be aware of, if you get the Op Amps (TL071&72) i was told not to touch the pins with you hands as they can screw them up, something to do with static?!?!!
    Good Luck!
    Chops
    Was this relatively straight forward?

  7. #7
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    Please consider this was some 5 years ago.

    Besides, I guess "chops" was here only about that problem and was never seen again.

    Now, what is your actual question/problem?
    Juan Manuel Fahey

  8. #8
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    Welcome to the forum.

    The Sessionette 75 has sockets for the op-amps, so they're straightforward to replace. Just make sure the orientation is correct. Undo the four screws in the rear lower panel and remove it. Unplug the mains transformer, then remove the four screws holding the amp chassis in place and it pulls right out to gain access to the circuit board.

    Edit; I answered your question, but this may not be the answer to your problem.
    pdf64 and TheUnWiseOne like this.

  9. #9
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    Quote Originally Posted by Mick Bailey View Post
    Welcome to the forum.

    The Sessionette 75 has sockets for the op-amps, so they're straightforward to replace. Just make sure the orientation is correct. Undo the four screws in the rear lower panel and remove it. Unplug the mains transformer, then remove the four screws holding the amp chassis in place and it pulls right out to gain access to the circuit board.

    Edit; I answered your question, but this may not be the answer to your problem.
    Thank You for replying

    If it's relatively easy I may as well give it a try? Guitar aid have offered to fix it, but if I can just try swapping these relatively cheap op amps first and not make things any worse?

  10. #10
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    Randomly swapping parts isn't a good way to fix an amp. It can sometimes work, but you rely on luck rather than judgement. Similar symptoms can have wildly different causes, like going to the doctor and complaining about bright red urine. Could be bladder cancer, or could be that you've eaten too much beetroot.

    There are certain instances where there's a prescriptive fix - a specific and common failure that occurs frequently with a particular amp - such as solder joint failures on Fender Deville tube sockets. It's better to diagnose a fault, or at least narrow it down to improve the outcome. I frequently get equipment that's previously had a large number of components replaced but still got a problem, and often the actual fault is really simple to diagnose and fix.

    I don't want to discourage you from tackling the amp yourself, though if you let us know the specific symptoms we may be able to give you better guidance.
    52 Bill likes this.

  11. #11
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    Quote Originally Posted by Mick Bailey View Post
    Randomly swapping parts isn't a good way to fix an amp. It can sometimes work, but you rely on luck rather than judgement. Similar symptoms can have wildly different causes, like going to the doctor and complaining about bright red urine. Could be bladder cancer, or could be that you've eaten too much beetroot.

    There are certain instances where there's a prescriptive fix - a specific and common failure that occurs frequently with a particular amp - such as solder joint failures on Fender Deville tube sockets. It's better to diagnose a fault, or at least narrow it down to improve the outcome. I frequently get equipment that's previously had a large number of components replaced but still got a problem, and often the actual fault is really simple to diagnose and fix.

    I don't want to discourage you from tackling the amp yourself, though if you let us know the specific symptoms we may be able to give you better guidance.
    Thanks _ I know you're right, I'd be swapping it with hope rather than any expectation!

    Just as I got it sounding the way I liked - My little session Amp was crackling in and out of volume like it was switching between the channels for a couple of days, turning it off and on temporarily cured for a bit - then it did it and the temp cure didn't work. Now there is a hum and nothing from the input

    The only way I get sound now is through the FX return.

    http://www.uploadarchief.net/files/d...-schematic.gif

    Thanks for any help you can give a novice.

  12. #12
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    Quote Originally Posted by TheUnWiseOne View Post
    The only way I get sound now is through the FX return.

    Thanks for any help you can give a novice.
    Did you actually try plugging in a spare cord from the FX send to the FX return?

    Have you tried to physically bump the amp and cause the problem to start or stop?

    Have you considered starting a new thread that will just deal with your amp's problems?

  13. #13
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    Question

    Quote Originally Posted by 52 Bill View Post
    Did you actually try plugging in a spare cord from the FX send to the FX return?

    Have you tried to physically bump the amp and cause the problem to start or stop?

    Have you considered starting a new thread that will just deal with your amp's problems?
    Plugging a spare cord doesn't fix it -I have tried it, there is just a mild hum from the speaker. I get some sound from plugging guitar into FX return - but the eq and channel do nothing :-/

    The problem initially started about a week ago, when the amp sudden gave a loud crackling tone, turning the amp off fixed it temporarily, then it seemingly went completely.

    I'm new here so didn't know to post a new thread - thank you for your advice. .

  14. #14
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    To start a new thread go to the index page of the section that you want to post in and look at the top of the listing section. There is a button for starting a new thread on the left hand side.

    Have you tried the physical stimulation of the amp chassis?

  15. #15
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    Quote Originally Posted by 52 Bill View Post
    To start a new thread go to the index page of the section that you want to post in and look at the top of the listing section. There is a button for starting a new thread on the left hand side.

    Have you tried the physical stimulation of the amp chassis?
    Thanks - I'l start a new thread

    I'm not sure I know what you mean about physical stimulation of the chassis...

  16. #16
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    Hi Folks,

    Just come across this forum, I was searching for a solution to my Sessionette 75.
    This is the problem that i'm experience now: After playing guitar for about 3 to 4 minutes the sound becomes fainted, I mean gets lower.

    I did some work on it, I replaced the power caps and the Zener diodes on the power rail but no avail.

    Any body come across such problem, and if so can you share some light?

    Its a nice all round amp, and not to heavy to take to gigs, I would like to repair and keep it, like it.

    Thank you

  17. #17
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    Welcome to the forum.

    You say you're experiencing the problem now. So, did it do it before you replaced the components?

    Favourite for volume dropping would be dirty switching contacts in the FX loop sockets. Try linking the FX send and FX return with a patch cord - does that clear the problem?
    52 Bill likes this.

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