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    Need help with my rebuilt ST-70

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    Awen

    Posts : 34
    Join date : 2015-01-21

    Need help with my rebuilt ST-70

    Post by Awen on Thu Jan 22, 2015 4:32 pm

    Posted this in the photo forum by accident:

    OK, so I bought a trashed Dynaco St-70 from this guy on craigslist and rebuilt it from the ground up. I have added new tube sockets, can cap, bias supply, tubes, choke, power transformer (Tomiko toroid), new output trannies (Tomiko), and a new board I got off the bay (China) that uses the 6GH8 tubes. With no input hooked up there is only a slight hum, but as soon as I hook up an input, there is a very very loud hum that is so loud that I quickly power off the amp out of fear of something bad happening. I have already tried two different new can caps, 3 new rectifier tubes, and checked and double checked all solder connections and made sure everything is wired correctly.With no input connected, if I as much as run my hand next to the input I get a loud hum. I am at my at the end of my rope with this thing and would really love to listen to it!

    Update:
    Ok so it really is only the right channel that hums, also with both input tubes removed, the amp is dead quiet. I am going to replace all the resistors on the board just in case and maybe the coupling caps.

    GP49

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    Re: Need help with my rebuilt ST-70

    Post by GP49 on Thu Jan 22, 2015 4:37 pm

    As replied in the other thread, this sounds like a ground problem.

    The input jacks may be grounded to the chassis; however in a stock Dynaco Stereo 70, each channel's signal input ground is connected from the circuit board to the input jacks via the untrimmed leads of two 10Ω resistors on the board. I've seen stock Dynaco boards where somebody thought the factory had forgotten to trim the leads, and cut them off. If your Chinese boards follow the same design, make sure those 10Ω resistor leads weren't trimmed, and that they are properly connected to the shields of the input jacks.

    I know you say you checked the grounding, but are you sure the shields of the input jacks are connected to the signal grounds ON THE BOARDS? Just being connected to the chassis doesn't count!


    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Need help with my rebuilt ST-70

    Post by PeterCapo on Thu Jan 22, 2015 5:02 pm

    I actually eliminated my hum problem by jumping the 10-ohm resistors.


    Last edited by PeterCapo on Thu Jan 22, 2015 11:05 pm; edited 1 time in total

    MontanaWay

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    Re: Need help with my rebuilt ST-70

    Post by MontanaWay on Thu Jan 22, 2015 10:00 pm

    Awen wrote:Posted this in the photo forum by accident:

    OK, so I bought a trashed Dynaco St-70 from this guy on craigslist and rebuilt it from the ground up. I have added new tube sockets, can cap, bias supply, tubes, choke, power transformer (Tomiko toroid), new output trannies (Tomiko), and a new board I got off the bay (China) that uses the 6GH8 tubes. With no input hooked up there is only a slight hum, but as soon as I hook up an input, there is a very very loud hum that is so loud that I quickly power off the amp out of fear of something bad happening. I have already tried two different new can caps, 3 new rectifier tubes, and checked and double checked all solder connections and made sure everything is wired correctly.With no input connected, if I as much as run my hand next to the input I get a loud hum. I am at my at the end of my rope with this thing and would really love to listen to it!

    Update:
    Ok so it really is only the right channel that hums, also with both input tubes removed, the amp is dead quiet. I am going to replace all the resistors on the board just in case and maybe the coupling caps.

    before you do anything radical like that, should be a last resort, short the RCA inputs to your amp, have all the tubes plugged is, speakers connected, turn it on and see of you get hum.
    Then lets go from there.

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Thu Jan 22, 2015 11:50 pm

    GP49 wrote:As replied in the other thread, this sounds like a ground problem.

    The input jacks may be grounded to the chassis; however in a stock Dynaco Stereo 70, each channel's signal input ground is connected from the circuit board to the input jacks via the untrimmed leads of two 10Ω resistors on the board. I've seen stock Dynaco boards where somebody thought the factory had forgotten to trim the leads, and cut them off. If your Chinese boards follow the same design, make sure those 10Ω resistor leads weren't trimmed, and that they are properly connected to the shields of the input jacks.

    I know you say you checked the grounding, but are you sure the shields of the input jacks are connected to the signal grounds ON THE BOARDS?  Just being connected to the chassis doesn't count!


    Yes, I just double checked the amp is grounded from the RCA shield which is fed to the 470K resistor then to the 10 ohm resistor then to pad #9 then to star ground.

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Thu Jan 22, 2015 11:57 pm

    MontanaWay wrote:
    Awen wrote:Posted this in the photo forum by accident:

    OK, so I bought a trashed Dynaco St-70 from this guy on craigslist and rebuilt it from the ground up. I have added new tube sockets, can cap, bias supply, tubes, choke, power transformer (Tomiko toroid), new output trannies (Tomiko), and a new board I got off the bay (China) that uses the 6GH8 tubes. With no input hooked up there is only a slight hum, but as soon as I hook up an input, there is a very very loud hum that is so loud that I quickly power off the amp out of fear of something bad happening. I have already tried two different new can caps, 3 new rectifier tubes, and checked and double checked all solder connections and made sure everything is wired correctly.With no input connected, if I as much as run my hand next to the input I get a loud hum. I am at my at the end of my rope with this thing and would really love to listen to it!

    Update:
    Ok so it really is only the right channel that hums, also with both input tubes removed, the amp is dead quiet. I am going to replace all the resistors on the board just in case and maybe the coupling caps.

    before you do anything radical like that, should be a last resort, short the RCA inputs to your amp, have all the tubes plugged is, speakers connected, turn it on and see of you get hum.
    Then lets go from there.

    I will try installing shorting plugs. The right side sounds as if it has really high gain and just moving my fingers around the RCA jacks causes them to become microphonic. Also, after a few minutes of being on there is a faint burning smell coming from whichever speaker is plugged into the right side Shocked

    MontanaWay

    Posts : 767
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    Re: Need help with my rebuilt ST-70

    Post by MontanaWay on Fri Jan 23, 2015 12:01 am

    Awen wrote:
    MontanaWay wrote:
    Awen wrote:Posted this in the photo forum by accident:

    OK, so I bought a trashed Dynaco St-70 from this guy on craigslist and rebuilt it from the ground up. I have added new tube sockets, can cap, bias supply, tubes, choke, power transformer (Tomiko toroid), new output trannies (Tomiko), and a new board I got off the bay (China) that uses the 6GH8 tubes. With no input hooked up there is only a slight hum, but as soon as I hook up an input, there is a very very loud hum that is so loud that I quickly power off the amp out of fear of something bad happening. I have already tried two different new can caps, 3 new rectifier tubes, and checked and double checked all solder connections and made sure everything is wired correctly.With no input connected, if I as much as run my hand next to the input I get a loud hum. I am at my at the end of my rope with this thing and would really love to listen to it!

    Update:
    Ok so it really is only the right channel that hums, also with both input tubes removed, the amp is dead quiet. I am going to replace all the resistors on the board just in case and maybe the coupling caps.

    before you do anything radical like that, should be a last resort, short the RCA inputs to your amp, have all the tubes plugged is, speakers connected, turn it on and see of you get hum.
    Then lets go from there.

    I will try installing shorting plugs. The right side sounds as if it has really high gain and just moving my fingers around the RCA jacks causes them to become microphonic. Also, after a few minutes of being on there is a faint burning smell coming from whichever speaker is plugged into the right side Shocked

    have you checked ALL the wiring in the amp, all solder joints, if yes....check them again!...and then check them again.... Exclamation
    Interesting you chose toroids for speaker transformers though...why did you go that way?

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Fri Jan 23, 2015 12:04 am

    MontanaWay wrote:
    Awen wrote:
    MontanaWay wrote:
    Awen wrote:Posted this in the photo forum by accident:

    OK, so I bought a trashed Dynaco St-70 from this guy on craigslist and rebuilt it from the ground up. I have added new tube sockets, can cap, bias supply, tubes, choke, power transformer (Tomiko toroid), new output trannies (Tomiko), and a new board I got off the bay (China) that uses the 6GH8 tubes. With no input hooked up there is only a slight hum, but as soon as I hook up an input, there is a very very loud hum that is so loud that I quickly power off the amp out of fear of something bad happening. I have already tried two different new can caps, 3 new rectifier tubes, and checked and double checked all solder connections and made sure everything is wired correctly.With no input connected, if I as much as run my hand next to the input I get a loud hum. I am at my at the end of my rope with this thing and would really love to listen to it!

    Update:
    Ok so it really is only the right channel that hums, also with both input tubes removed, the amp is dead quiet. I am going to replace all the resistors on the board just in case and maybe the coupling caps.

    before you do anything radical like that, should be a last resort, short the RCA inputs to your amp, have all the tubes plugged is, speakers connected, turn it on and see of you get hum.
    Then lets go from there.

    I will try installing shorting plugs. The right side sounds as if it has really high gain and just moving my fingers around the RCA jacks causes them to become microphonic. Also, after a few minutes of being on there is a faint burning smell coming from whichever speaker is plugged into the right side Shocked

    have you checked ALL the wiring in the amp, all solder joints, if yes....check them again!...and then check them again.... Exclamation
    Interesting you chose toroids for speaker transformers though...why did you go that way?

    The power transformer is toroidal, but not the output trannies. The choke and bias supply are all also brand new from Dynakit Parts.

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: Need help with my rebuilt ST-70

    Post by MontanaWay on Fri Jan 23, 2015 12:05 am

    Awen wrote:
    MontanaWay wrote:
    Awen wrote:
    MontanaWay wrote:
    Awen wrote:Posted this in the photo forum by accident:

    OK, so I bought a trashed Dynaco St-70 from this guy on craigslist and rebuilt it from the ground up. I have added new tube sockets, can cap, bias supply, tubes, choke, power transformer (Tomiko toroid), new output trannies (Tomiko), and a new board I got off the bay (China) that uses the 6GH8 tubes. With no input hooked up there is only a slight hum, but as soon as I hook up an input, there is a very very loud hum that is so loud that I quickly power off the amp out of fear of something bad happening. I have already tried two different new can caps, 3 new rectifier tubes, and checked and double checked all solder connections and made sure everything is wired correctly.With no input connected, if I as much as run my hand next to the input I get a loud hum. I am at my at the end of my rope with this thing and would really love to listen to it!

    Update:
    Ok so it really is only the right channel that hums, also with both input tubes removed, the amp is dead quiet. I am going to replace all the resistors on the board just in case and maybe the coupling caps.

    before you do anything radical like that, should be a last resort, short the RCA inputs to your amp, have all the tubes plugged is, speakers connected, turn it on and see of you get hum.
    Then lets go from there.

    I will try installing shorting plugs. The right side sounds as if it has really high gain and just moving my fingers around the RCA jacks causes them to become microphonic. Also, after a few minutes of being on there is a faint burning smell coming from whichever speaker is plugged into the right side Shocked

    have you checked ALL the wiring in the amp, all solder joints, if yes....check them again!...and then check them again.... Exclamation
    Interesting you chose toroids for speaker transformers though...why did you go that way?

    The power transformer is toroidal, but not the output trannies. The choke and bias supply are all also brand new from Dynakit Parts.

    ah....read it wrong...my mistake

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: Need help with my rebuilt ST-70

    Post by j beede on Fri Jan 23, 2015 12:26 am

    Burning tweeters? Check to see if you have any DC showing up at the speaker terminals. You may have a leaky output transformer. Have you tried putting a 1V, 400Hz sine wave across the output transformer primary (float the CT and UL leads) and measuring the amplitude at the speaker output? I don't remember what the turns ratio is for the ST-70 but it is likely somewhere in the 30:1 range.

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Fri Jan 23, 2015 8:21 am

    j beede wrote:Burning tweeters? Check to see if you have any DC showing up at the speaker terminals. You may have a leaky output transformer. Have you tried putting a 1V, 400Hz sine wave across the output transformer primary (float the CT and UL leads) and measuring the amplitude at the speaker output? I don't remember what the turns ratio is for the ST-70 but it is likely somewhere in the 30:1 range.

    I just checked and there is no DC at the speaker terminals. Also of note is that with the original transformers hooked up, the amp has the same issue. I am a little scared at this point to hook up the shorting plugs out of fear of blowing my speakers. Is seems like the amp has some kind of infinite gain on the right side. I am going to check and recheck everything non the board this evening and while I am at it go ahead and replace all the board resistors; hopefully I can figure this out and enjoy some music!

    MontanaWay

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    Re: Need help with my rebuilt ST-70

    Post by MontanaWay on Fri Jan 23, 2015 11:35 am

    Awen wrote:
    j beede wrote:Burning tweeters? Check to see if you have any DC showing up at the speaker terminals. You may have a leaky output transformer. Have you tried putting a 1V, 400Hz sine wave across the output transformer primary (float the CT and UL leads) and measuring the amplitude at the speaker output? I don't remember what the turns ratio is for the ST-70 but it is likely somewhere in the 30:1 range.

    I just checked and there is no DC at the speaker terminals. Also of note is that with the original transformers hooked up, the amp has the same issue. I am a little scared at this point to hook up the shorting plugs out of fear of blowing my speakers. Is seems like the amp has some kind of infinite gain on the right side. I am going to check and recheck everything non the board this evening and while I am at it go ahead and replace all the board resistors; hopefully I can figure this out and enjoy some music!

    re checking everything is good, swapping all the resistors is probably not the solution. If I were to swap components, I would start with caps,as they are in general, more prone to failure or large drifts.

    Awen

    Posts : 34
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    Re: Need help with my rebuilt ST-70

    Post by Awen on Sat Jan 24, 2015 9:54 am

    I replaced the four .1 uf caps with .22 Russian PIO and the same issue remains. I will replace the two .05 caps today. One thing I forgot to mention was that I removed the power takeoff sockets and installed a bias test point kit in its place. I removed the power wires and hooked them directly to the board, but other than that I removed all the other wires with the exception of pin 8 which is my bias test point; not sure if this makes a difference or not. Also, do I need to ground the body of the output transformer to the chassis? I do not see it in the ST-70 instruction manual. My ST-70s chassis was painted by the previous owner.

    turbotoy

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    Re: Need help with my rebuilt ST-70

    Post by turbotoy on Sat Jan 24, 2015 10:21 am

    If you thermally failed a tweeter, the amplifier is probably oscillating at a high (ultrasonic) frequency. Check everything in the feedback circuit of that channel.

    Bob Latino
    Admin

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    Re: Need help with my rebuilt ST-70

    Post by Bob Latino on Sat Jan 24, 2015 11:39 am

    Awen wrote:Also, do I need to ground the body of the output transformer to the chassis? I do not see it in the ST-70 instruction manual. My ST-70s chassis was painted by the previous owner.

    You don't need to ground the body of the output transformer. Just make sure that the BLACK secondary wire that connects to the "G" terminal on the speaker strip is grounded to one of the ground lugs next to the quad cap.

    Bob

    MontanaWay

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    Re: Need help with my rebuilt ST-70

    Post by MontanaWay on Sat Jan 24, 2015 12:04 pm

    Awen wrote:I replaced the four .1 uf caps with .22 Russian PIO and the same issue remains. I will replace the two .05 caps today. One thing I forgot to mention was that I removed the power takeoff sockets and installed a bias test point kit in its place. I removed the power wires and hooked them directly to the board, but other than that I removed all the other wires with the exception of pin 8 which is my bias test point; not sure if this makes a difference or not. Also, do I need to ground the body of the output transformer to the chassis? I do not see it in the ST-70 instruction manual. My ST-70s chassis was painted by the previous owner.

    since your driver board is relatively new, I would not worry about replacing caps. My reference to consider replacing caps before resistors was a general reference only.
    Did you ever check your BIAS setting?, I cannot see too many earlier posts in this reply window I am currently in. Same goes with checking tube sockets, making sure the contacts are tight AND clean.

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Sun Jan 25, 2015 12:00 pm

    OK, well I got this partially figured out, there was some solder that had leaked through the hole in the input on the right side and was making contact with one of the resistors and the resistor had some paint chipped off just enough to become conductive to the solder that had leaked through. Well this fixed the most of the hum and the amp is pretty quiet now and I can only hear a little hum on the right side if I put my ear up to the speaker, the left side is dead quiet.

    Now the above is with no input connected.....when I connect my ipod the right side starts making a roaring (possibly motor boating) sound as soon as B+ starts coming up. I am at my wits end with this....any help or suggestions would be greatly appreciated.

    I should also mention that the bias is perfect and all internal parts to include the tube sockets are new.

    MontanaWay

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    Re: Need help with my rebuilt ST-70

    Post by MontanaWay on Sun Jan 25, 2015 12:09 pm

    Awen wrote:OK, well I got this partially figured out, there was some solder that had leaked through the hole in the input on the right side and was making contact with one of the resistors and the resistor had some paint chipped off just enough to become conductive to the solder that had leaked through. Well this fixed the most of the hum and the amp is pretty quiet now and I can only hear a little hum on the right side if I put my ear up to the speaker, the left side is dead quiet.

    Now the above is with no input connected.....when I connect my ipod the right side starts making a roaring (possibly motor boating) sound as soon as B+ starts coming up. I am at my wits end with this....any help or suggestions would be greatly appreciated.

    I should also mention that the bias is perfect and all internal parts to include the tube sockets are new.

    well that is a good start!
    I am assuming you had the inputs shorted when you did you test before plugging your iPod in, not that this is super critical, but its a sure bet way to test for 'internally' generated noise.
    A roaring noise, with my experience, is sometimes attributed to having the output tube/s connected to the wrong output of the driver pcb, ie. 'phase inverted' so to say, but that would cause a really loud noise even with no input.
    Do you have a Pentode/Triode circuit in your amp?, if yes, switch the right channel to Triode and see if that noise is still present.

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Sun Jan 25, 2015 12:17 pm

    MontanaWay wrote:
    Awen wrote:OK, well I got this partially figured out, there was some solder that had leaked through the hole in the input on the right side and was making contact with one of the resistors and the resistor had some paint chipped off just enough to become conductive to the solder that had leaked through. Well this fixed the most of the hum and the amp is pretty quiet now and I can only hear a little hum on the right side if I put my ear up to the speaker, the left side is dead quiet.

    Now the above is with no input connected.....when I connect my ipod the right side starts making a roaring (possibly motor boating) sound as soon as B+ starts coming up. I am at my wits end with this....any help or suggestions would be greatly appreciated.

    I should also mention that the bias is perfect and all internal parts to include the tube sockets are new.

    well that is a good start!
    I am assuming you had the inputs shorted when you did you test before plugging your iPod in, not that this is super critical, but its a sure bet way to test for 'internally' generated noise.
    A roaring noise, with my experience, is sometimes attributed to having the output tube/s connected to the wrong output of the driver pcb, ie. 'phase inverted' so to say, but that would cause a really loud noise even with no input.
    Do you have a Pentode/Triode circuit in your amp?, if yes, switch the right channel to Triode and see if that noise is still present.

    No I did not have the inputs shorted I have a set of shorting plugs, but they are connected together (right and left) and will not fit the small spacing of the ST-70s RCA jacks. The output tubes are connected correctly and they mirror the left side, which is dead quiet. No I do not have a pentode/triode switch in my amp circuit. If I had to describe the sound, it would be sort of like a honking sound.

    MontanaWay

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    Re: Need help with my rebuilt ST-70

    Post by MontanaWay on Sun Jan 25, 2015 12:28 pm

    interesting......without looking at the amp, I would just do another very careful visual inspection of the driver pcb, even the smallest of a solder drop, as you've found out, will cause a problem.
    We can negate a 'common' problem, such as your power supply, otherwise the left channel would also miss behave.
    I know you've done it, but go over ALL of your right channel wiring, making sure nothing is 'reversed', all soldering is good, go over the driver pcb again, making sure no shorts, dry solder joints...you know the routine...check, check and then check again....thats the best I can suggest right now.
    Any chance of you posting a good sharp photo/s of the inside of your amp, thanks.

    ps you'd be amazed what I have found, when I had problems and thought I checked it all, after checking it all again, a few times, I usually find a small mistake!

    Awen

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    Re: Need help with my rebuilt ST-70

    Post by Awen on Sun Jan 25, 2015 1:59 pm

    MontanaWay wrote:interesting......without looking at the amp, I would just do another very careful visual inspection of the driver pcb, even the smallest of a solder drop, as you've found out, will cause a problem.
    We can negate a 'common' problem, such as your power supply, otherwise the left channel would also miss behave.
    I know you've done it, but go over ALL of your right channel wiring, making sure nothing is 'reversed', all soldering is good, go over the driver pcb again, making sure no shorts, dry solder joints...you know the routine...check, check and then check again....thats the best I can suggest right now.
    Any chance of you posting a good sharp photo/s of the inside of your amp, thanks.

    ps  you'd be amazed what I have found, when I had problems and thought I checked it all, after checking it all again, a few times, I usually find a small mistake!

    I am going go over everything again as suggested. One thing I just notices is a very slight buzz/hum coming from the power transformer that sounds like an aquarium pump, is this normal? It is a brand new Tomiko toroidal, all my voltages are good.

    Awen

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    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Sun Jan 25, 2015 3:06 pm

    I looked over everything again, but I am at a loss...tried to hook up an ipod again.....amp started going crazy on the right side. Super loud hum that build in volume until I turn off amp out of fear of something bad happening. I am tempted to let it go to see if it settles.

    MontanaWay

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    Re: Need help with my rebuilt ST-70

    Post by MontanaWay on Sun Jan 25, 2015 3:30 pm

    Awen wrote:
    MontanaWay wrote:interesting......without looking at the amp, I would just do another very careful visual inspection of the driver pcb, even the smallest of a solder drop, as you've found out, will cause a problem.
    We can negate a 'common' problem, such as your power supply, otherwise the left channel would also miss behave.
    I know you've done it, but go over ALL of your right channel wiring, making sure nothing is 'reversed', all soldering is good, go over the driver pcb again, making sure no shorts, dry solder joints...you know the routine...check, check and then check again....thats the best I can suggest right now.
    Any chance of you posting a good sharp photo/s of the inside of your amp, thanks.

    ps  you'd be amazed what I have found, when I had problems and thought I checked it all, after checking it all again, a few times, I usually find a small mistake!

    I am going go over everything again as suggested. One thing I just notices is a very slight buzz/hum coming from the power transformer that sounds like an aquarium pump, is this normal? It is a brand new Tomiko toroidal, all my voltages are good.

    buzzing of mains transformers is OK, nothing wrong with that, although it should be fairly low in volume.

    MontanaWay

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    Re: Need help with my rebuilt ST-70

    Post by MontanaWay on Sun Jan 25, 2015 3:33 pm

    Awen wrote:I looked over everything again, but I am at a loss...tried to hook up an ipod again.....amp started going crazy on the right side. Super loud hum that build in volume until I turn off amp out of fear of something bad happening. I am tempted to let it go to see if it settles.

    that kind of distortion/problem is not going to 'settle'.......unfortunately, you do have an issue somewhere and it needs to be fixed.
    Right now, I am at somewhat of a loss. I would need to see the amp, listen to that distortion and go from there. Maybe other members here can chime in.
    Are you able to post a good quality photo of the inside, some close ups and one overall shot, this might just help as here, thanks.

    PeterCapo

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    Re: Need help with my rebuilt ST-70

    Post by PeterCapo on Sun Jan 25, 2015 4:07 pm

    Try some other source, other than the iPod, like a regular, full size CD player and check the continuity of whatever interconnects you use.  

    If the problem persists, it's got to be some kind of wiring issue, as discussed possibly a solder bridge, miswire or something along those lines.

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