The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    Need help with my rebuilt ST-70

    Share

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Mon Feb 02, 2015 7:40 am

    MontanaWay wrote:
    Awen wrote:Well, I think I figured out the problem...well, it looks like I have a bad power transformer. The two brown wires (white wires on original Dynaco power transformer) that should be 5v are in fact 46v!

    are you REALLY sure about that?.....because it means that your rectifier tube has been getting a filament voltage of 46V!!??!!...instead of the 5V, and would NOT have lasted long!
    I think you are maybe measuring the BIAS tapping of the transformer?........

    The Bias is correct and is around 55v. Each leg of the 5V wire going to the rectifier tube is measuring 46V AC... I did blow one rectifier tube and fuse when I fist started troubleshooting this. Each time I turn the amp on, I don't leave it on very long. I think the transformer is incorrectly wired internally and has three bias taps instead of the one.....the bias wire is also brown, but it is a smaller gauge than the two other wires, so I know I didn't get them confused.

    Cubdriver

    Posts : 68
    Join date : 2014-01-21
    Age : 51
    Location : Southeastern Litchfield Co, CT

    Re: Need help with my rebuilt ST-70

    Post by Cubdriver on Mon Feb 02, 2015 10:59 pm

    Each leg of the 5V wire going to the rectifier tube? Are you measuring from each one to chassis ground, or are you measuring wire to wire (in other words, across the filament of the rectifier)? The latter is what you want to be doing - measure wire end to wire end, at the tube socket terminals, or perhaps (safer) pull the tube out and put the probes in the socket holes for the filament pins.

    That winding has no real ground reference, and if your meter is set to measure AC and you go from either wire to ground, you're likely measuring the AC ripple voltage on the rectifier's output and not the actual filament voltage. Methinks a 5V rectifier wouldn't last too long with 46V on the filament...

    -Pat

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Tue Feb 03, 2015 7:27 am

    I am measuring each leg to ground without the rectifier tube installed...so that means I have 46v on the chassis? When I measure wire to wire, I get the correct 5v. I don't know what is going on with this thing anymore. I am waiting until I get the funds available so I can get a new driver board.

    Cubdriver

    Posts : 68
    Join date : 2014-01-21
    Age : 51
    Location : Southeastern Litchfield Co, CT

    Re: Need help with my rebuilt ST-70

    Post by Cubdriver on Tue Feb 03, 2015 1:47 pm

    It means that through the high impedance of the ohmmeter, 46 volts is developed between the filament winding and chassis ground.  This is through the (probably on the order of) 10 megohm input resistance of the meter.  That's 4.6 microamps of current through 10M.  Nothing to worry about, and probably a result of capacitive coupling of the windings within the transformer.  The 5V wire to wire is the important measurement, and it's good.

    -Pat

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Wed Feb 04, 2015 7:42 am

    Cubdriver wrote:It means that through the high impedance of the ohmmeter, 46 volts is developed between the filament winding and chassis ground.  This is through the (probably on the order of) 10 megohm input resistance of the meter.  That's 4.6 microamps of current through 10M.  Nothing to worry about, and probably a result of capacitive coupling of the windings within the transformer.  The 5V wire to wire is the important measurement, and it's good.

    -Pat

    I don't know what compelled me to measure the wires to ground (Im really reaching). Well, I found something else wrong; one of the resistors on the board that was supposed to be 47K ohms, was 47R! I fixed this and now the amp is absolute dead quiet with NO input connected. With an input connected the hum is very loud, but not as loud as before. I tried playing some music through it, but did not hear anything. So, it is getting better, but still have loud hum with input connected. I also tried another pair of 6GH8 tubes and still the issue remains.

    Guest
    Guest

    Re: Need help with my rebuilt ST-70

    Post by Guest on Wed Feb 04, 2015 12:52 pm

    Awen wrote:
    Cubdriver wrote:It means that through the high impedance of the ohmmeter, 46 volts is developed between the filament winding and chassis ground.  This is through the (probably on the order of) 10 megohm input resistance of the meter.  That's 4.6 microamps of current through 10M.  Nothing to worry about, and probably a result of capacitive coupling of the windings within the transformer.  The 5V wire to wire is the important measurement, and it's good.

    -Pat

    I don't know what compelled me to measure the wires to ground (Im really reaching). Well, I found something else wrong; one of the resistors on the board that was supposed to be 47K ohms, was 47R! I fixed this and now the amp is absolute dead quiet with NO input connected. With an input connected the hum is very loud, but not as loud as before. I tried playing some music through it, but did not hear anything. So, it is getting better, but still have loud hum with input connected. I also tried another pair of 6GH8 tubes and still the issue remains.

    when you say with no input connected, are you shorting the inputs?
    The way I see it is that with the inputs shorted and no hum, that pretty much indicates the amp is fine. Connecting an input source and getting hum tells me its an external issue now, like a ground loop in your system setup.
    Maybe other members can chime in here.

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Wed Feb 04, 2015 4:32 pm

    MontanaWay wrote:
    Awen wrote:
    Cubdriver wrote:It means that through the high impedance of the ohmmeter, 46 volts is developed between the filament winding and chassis ground.  This is through the (probably on the order of) 10 megohm input resistance of the meter.  That's 4.6 microamps of current through 10M.  Nothing to worry about, and probably a result of capacitive coupling of the windings within the transformer.  The 5V wire to wire is the important measurement, and it's good.

    -Pat

    I don't know what compelled me to measure the wires to ground (Im really reaching). Well, I found something else wrong; one of the resistors on the board that was supposed to be 47K ohms, was 47R! I fixed this and now the amp is absolute dead quiet with NO input connected. With an input connected the hum is very loud, but not as loud as before. I tried playing some music through it, but did not hear anything. So, it is getting better, but still have loud hum with input connected. I also tried another pair of 6GH8 tubes and still the issue remains.

    when you say with no input connected, are you shorting the inputs?
    The way I see it is that with the inputs shorted and no hum, that pretty much indicates the amp is fine. Connecting an input source and getting hum tells me its an external issue now, like a ground loop in your system setup.
    Maybe other members can chime in here.

    When I say no input connected, I mean nothing connected at RCA jacks. With shoring plugs connected, amp is still quiet (maybe a faint hum). But with my IPOD connected (also tried tape deck), the hum is loud and I can hear no music playing. Also when I touch my Ipod (metal back), the hum increase almost like it is an antenna.

    Cubdriver

    Posts : 68
    Join date : 2014-01-21
    Age : 51
    Location : Southeastern Litchfield Co, CT

    Re: Need help with my rebuilt ST-70

    Post by Cubdriver on Wed Feb 04, 2015 5:22 pm

    Is the outer sleeve of the input jack pair properly grounded to the amplifier chassis? What are you connecting the iPod to the amp with - a mini stereo plug to twin phono plug cable?

    -Pat

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Wed Feb 04, 2015 8:27 pm

    Cubdriver wrote:Is the outer sleeve of the input jack pair properly grounded to the amplifier chassis?  What are you connecting the iPod to the amp with - a mini stereo plug to twin phono plug cable?

    -Pat

    No, the outer sleeve is connected to the board ground which is connected to star. Yes, that is how I have the ipod connected. Maybe I should connect the inputs directly to the chassis and lift them off the board ground?

    Cubdriver

    Posts : 68
    Join date : 2014-01-21
    Age : 51
    Location : Southeastern Litchfield Co, CT

    Re: Need help with my rebuilt ST-70

    Post by Cubdriver on Thu Feb 05, 2015 2:50 pm

    If you can do so without too much difficulty, it's worth a shot for troubleshooting purposes, but first - do you have another of the mini stereo to phono plug cables? If so, try swapping that out first, as it's easier to do than reconfigure the input wiring.

    -Pat

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Fri Feb 06, 2015 7:28 am

    OK, well I tried it and there was an improvement, but I still cannot hear music. I tried other cables as well, but no change. I know there is a ground problem somewhere now. Should I run a wire from the new input grounds back to power supply ground? I also installed a three prong wire and connected the earth ground and that seemed to have helped as well. When I get the funds, I going to buy a VTA board.

    DynakitParts
    Admin

    Posts : 183
    Join date : 2008-11-30

    ST-70 problem

    Post by DynakitParts on Fri Feb 06, 2015 8:05 am

    Hi..
    Looking at the photo you posted...I'm assuming this board follows the same general layout as the original ST-70 PC-3 circuit with respect to the solder pad numbering sequence and positions on the board except for the 6GH8A conversion. I cannot cleary see all the solder pad numbers...but it just appears that the Left & Right input lead connections to the circuit board Pads 7 & 17 are possible connected incorrectly. Suggest you take a look at this.

    Kevin @ Dynakit

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Fri Feb 06, 2015 8:24 am

    DynakitParts wrote:Hi..
        Looking at the photo you posted...I'm assuming this board follows the same general layout as the original ST-70 PC-3 circuit with respect to the solder pad numbering sequence and positions on the board except for the 6GH8A conversion. I cannot cleary see all the solder pad numbers...but it just appears that the Left & Right input lead connections to the circuit board Pads 7 & 17 are possible connected incorrectly. Suggest you take a look at this.

    Kevin @ Dynakit

    I got it to play music!! In the Dynaco 70 layout, it has the 470K resistor across the input center conductor to shielding and then to ground via the 10 ohm resistor leads, well I was looking at the inputs on a LM3886 chip amp that I built and the center conductor is not joined to the shield, so I went ahead and cut that connection and I can now hear music, but the gain isn't very much as it isn't very loud and there is a really high pitched sound that I can still hear above the music on both channels Sad The pad numbers as asked above are 7 and 17, I originally thought that may be the issue as well, but the input goes to pin 2 on a 6GH8 instead of 3. OK, so at least I can hear music and the amp has no bias issue, I am running it at 1.45V and it is holding. I just need to figure out what is up with this high pitched sound. It is like a high pitched whistle that can be heard above the music. My next question is should I connect the input ground to the power ground and should I run a wire from the earth safety ground to the power supply star ground?

    Awen

    Posts : 34
    Join date : 2015-01-21

    Figured it out!!

    Post by Awen on Mon Feb 09, 2015 10:02 am

    Well, I got it working! I appreciate all the help I have received from everyone here, this is a very helpful forum. The issue was that my dumb ass had the 470K input resistor connected in series with the ground instead of having it parallel across the center conductor and shielding. I feel like such an idiot! Well, at least I got it working and it sounds amazing!! I cant imagine how good this would sound with the VTA board. Honestly, I think this sounds better than all my other amps combined! The amp is dead quiet, with the exception of a slight mechanical buzz coming from the toroidal power transformer, but the sound is faint and doesn't make it through the speakers. I am using my Bottlehead Foreplay II as the preamp, which seems to be a good combo with the ST-70.

    Cubdriver

    Posts : 68
    Join date : 2014-01-21
    Age : 51
    Location : Southeastern Litchfield Co, CT

    Re: Need help with my rebuilt ST-70

    Post by Cubdriver on Mon Feb 09, 2015 3:22 pm

    Glad to hear that all is well!

    -Pat

    GP49

    Posts : 735
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Need help with my rebuilt ST-70

    Post by GP49 on Mon Feb 09, 2015 3:38 pm

    Awen wrote:Well, I got it working! I appreciate all the help I have received from everyone here, this is a very helpful forum. The issue was that my dumb ass had the 470K input resistor connected in series with the ground instead of having it parallel across the center conductor and shielding.


    Which sounds like it was a grounding issue all along, that miswired resistor holding the input ground at 470kΩ above chassis ground.  NO WONDER IT HUMMED.

    If that's what it was, it would have been spotted in about two seconds with a multimeter.

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Mon Feb 09, 2015 9:15 pm

    GP49 wrote:
    Awen wrote:Well, I got it working! I appreciate all the help I have received from everyone here, this is a very helpful forum. The issue was that my dumb ass had the 470K input resistor connected in series with the ground instead of having it parallel across the center conductor and shielding.


    Which sounds like it was a grounding issue all along, that miswired resistor holding the input ground at 470kΩ above chassis ground.  NO WONDER IT HUMMED.

    If that's what it was, it would have been spotted in about two seconds with a multimeter.

    I was originally measuring from center conductor to ground and getting 470K ohms when I should have measured from center conductor to outer shield....I don't know why I measured it like that Mad

    GP49

    Posts : 735
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Need help with my rebuilt ST-70

    Post by GP49 on Mon Feb 09, 2015 9:21 pm

    Actually you still will!

    But checking for proper ground at the input jacks, you would measure from the shield of the RCA jack to chassis ground, and (in a properly wired stock Dynaco Stereo 70) get 10Ω which is the value of the resistor used to "break" a possible ground loop.

    Sponsored content

    Re: Need help with my rebuilt ST-70

    Post by Sponsored content Today at 6:14 pm


      Current date/time is Wed Dec 07, 2016 6:14 pm