Need help with my rebuilt ST-70

OK, I think I got it narrowed down to a grounding issue. If I touch the can cap housing with the tube amp on, the problem goes away. I have redid the star ground three times and the issue remains. So, I got a grounding issue somewhere.

Awen wrote:OK, I think I got it narrowed down to a grounding issue. If I touch the can cap housing with the tube amp on, the problem goes away. I have redid the star ground three times and the issue remains. So, I got a grounding issue somewhere.

very good!...that is an excellent step!
There is a video on this forum, posted only a few days, about some grounding issues with a can cap. To cut a long story short, the guy ended up soldering one of the can caps tabs directly to the chassis which fixed his issue 100%.
If you do not have a soldering iron 'big' enough to generate the heat required to do this, I would run some heavy gauge wire from ALL the can caps tabs and solder them to your star ground. Also make sure your star ground is really well and tightly screwed to the chassis, if necessary, remove all the tags, get some steel wool or heavy gauge sand paper, and really go to town around the star ground area to really make sure that your star ground makes the best possible mechanical connection to the chassis!
If you still have some issues, maybe after doing all this and checking all your ground connections and still having some small noise issue, connect the ground wire, you'll need a 3 core power cable, to the chassis.

Awen wrote: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.

Sure you got the bias test kit hooked up correctly? You statement also seems to say you piggybacked the test kit onto the existing bias test wiring ... I can attest that can cause all sorts of grief. I ran into a similar situation where adding a meter circuit threw the bias off a bunch AND left one side of the push/pull floating. That made for some serious distortion, and a new bald spot from scratching my head trying to figure it out.

And ... bears repeating ... do a REAL close mark one eyeball check (with magnifying glass) on all the connections, not just the ones you worked on. Old solder can go intermittent or open easy. Also check for any long tails on components, as well as other misplaced solder blobs that could be the problem. Pay particular attention to the area around the driver board, as things get right close to the chassis.

sKiZo wrote:

Awen wrote: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.

Sure you got the bias test kit hooked up correctly? You statement also seems to say you piggybacked the test kit onto the existing bias test wiring ... I can attest that can cause all sorts of grief. I ran into a similar situation where adding a meter circuit threw the bias off a bunch AND left one side of the push/pull floating. That made for some serious distortion, and  a new bald spot from scratching my head trying to figure it out.

And ... bears repeating ... do a REAL close mark one eyeball check (with magnifying glass) on all the connections, not just the ones you worked on. Old solder can go intermittent or open easy. Also check for any long tails on components, as well as other misplaced solder blobs that could be the problem. Pay particular attention to the area around the driver board, as things get right close to the chassis.

excellent pointers Mr sKiZo!

MontanaWay wrote:

sKiZo wrote:

Awen wrote: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.

Sure you got the bias test kit hooked up correctly? You statement also seems to say you piggybacked the test kit onto the existing bias test wiring ... I can attest that can cause all sorts of grief. I ran into a similar situation where adding a meter circuit threw the bias off a bunch AND left one side of the push/pull floating. That made for some serious distortion, and  a new bald spot from scratching my head trying to figure it out.

And ... bears repeating ... do a REAL close mark one eyeball check (with magnifying glass) on all the connections, not just the ones you worked on. Old solder can go intermittent or open easy. Also check for any long tails on components, as well as other misplaced solder blobs that could be the problem. Pay particular attention to the area around the driver board, as things get right close to the chassis.

excellent pointers Mr sKiZo!

probably 9 times out of 10..it is a bad solder joint or a wire/s in the wrong spot...usually something really simple.....in the end!!

sKiZo wrote:

Awen wrote: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.

Sure you got the bias test kit hooked up correctly? You statement also seems to say you piggybacked the test kit onto the existing bias test wiring ... I can attest that can cause all sorts of grief. I ran into a similar situation where adding a meter circuit threw the bias off a bunch AND left one side of the push/pull floating. That made for some serious distortion, and  a new bald spot from scratching my head trying to figure it out.

And ... bears repeating ... do a REAL close mark one eyeball check (with magnifying glass) on all the connections, not just the ones you worked on. Old solder can go intermittent or open easy. Also check for any long tails on components, as well as other misplaced solder blobs that could be the problem. Pay particular attention to the area around the driver board, as things get right close to the chassis.

I am pretty sure I do....I got pin 8 going to the red test point and the black going to chassis ground. Also, if this was in fact an issue, would the amp still bias? Good tip WRT the magnifying glass.

MontanaWay wrote:

Awen wrote:OK, I think I got it narrowed down to a grounding issue. If I touch the can cap housing with the tube amp on, the problem goes away. I have redid the star ground three times and the issue remains. So, I got a grounding issue somewhere.

very good!...that is an excellent step!
There is a video on this forum, posted only a few days, about some grounding issues with a can cap. To cut a long story short, the guy ended up soldering one of the can caps tabs directly to the chassis which fixed his issue 100%.
If you do not have a soldering iron 'big' enough to generate the heat required to do this, I would run some heavy gauge wire from ALL the can caps tabs and solder them to your star ground. Also make sure your star ground is really well and tightly screwed to the chassis, if necessary, remove all the tags, get some steel wool or heavy gauge sand paper, and really go to town around the star ground area to really make sure that your star ground makes the best possible mechanical connection to the chassis!
If you still have some issues, maybe after doing all this and checking all your ground connections and still having some small noise issue, connect the ground wire, you'll need a 3 core power cable, to the chassis.

I just redid the grounds...still same issue..maybe I will try the 3 prong cord approach.

Awen wrote:

MontanaWay wrote:

Awen wrote:OK, I think I got it narrowed down to a grounding issue. If I touch the can cap housing with the tube amp on, the problem goes away. I have redid the star ground three times and the issue remains. So, I got a grounding issue somewhere.

very good!...that is an excellent step!
There is a video on this forum, posted only a few days, about some grounding issues with a can cap. To cut a long story short, the guy ended up soldering one of the can caps tabs directly to the chassis which fixed his issue 100%.
If you do not have a soldering iron 'big' enough to generate the heat required to do this, I would run some heavy gauge wire from ALL the can caps tabs and solder them to your star ground. Also make sure your star ground is really well and tightly screwed to the chassis, if necessary, remove all the tags, get some steel wool or heavy gauge sand paper, and really go to town around the star ground area to really make sure that your star ground makes the best possible mechanical connection to the chassis!
If you still have some issues, maybe after doing all this and checking all your ground connections and still having some small noise issue, connect the ground wire, you'll need a 3 core power cable, to the chassis.

I just redid the grounds...still same issue..maybe I will try the 3 prong cord approach.

it should not be necessary to have the ST70 grounded to mains, 1000's did not and most still do not. Some prefer it as a safety, some say in their instances it lowered hum/noise to an almost in audible level.
Yours by the sound of it is extremely loud, so grounding it to mains may not actually solve it all. I know you have already done so, but I would still be looking at ALL my grounding points. I assume too that the -ve of your speaker terminals are grounded to the star ground, both channels. I am also assuming that all your grounding connections go to the star ground.

MontanaWay wrote:

Awen wrote:

MontanaWay wrote:

Awen wrote:OK, I think I got it narrowed down to a grounding issue. If I touch the can cap housing with the tube amp on, the problem goes away. I have redid the star ground three times and the issue remains. So, I got a grounding issue somewhere.

very good!...that is an excellent step!
There is a video on this forum, posted only a few days, about some grounding issues with a can cap. To cut a long story short, the guy ended up soldering one of the can caps tabs directly to the chassis which fixed his issue 100%.
If you do not have a soldering iron 'big' enough to generate the heat required to do this, I would run some heavy gauge wire from ALL the can caps tabs and solder them to your star ground. Also make sure your star ground is really well and tightly screwed to the chassis, if necessary, remove all the tags, get some steel wool or heavy gauge sand paper, and really go to town around the star ground area to really make sure that your star ground makes the best possible mechanical connection to the chassis!
If you still have some issues, maybe after doing all this and checking all your ground connections and still having some small noise issue, connect the ground wire, you'll need a 3 core power cable, to the chassis.

I just redid the grounds...still same issue..maybe I will try the 3 prong cord approach.

it should not be necessary to have the ST70 grounded to mains, 1000's did not and most still do not. Some prefer it as a safety, some say in their instances it lowered hum/noise to an almost in audible level.
Yours by the sound of it is extremely loud, so grounding it to mains may not actually solve it all. I know you have already done so, but I would still be looking at ALL my grounding points. I assume too that the -ve of your speaker terminals are grounded to the star ground, both channels. I am also assuming that all your grounding connections go to the star ground.

Yes, all connections go to star ground. One thing I though was strange on this amp is how the 470k ohm resistor at the input connects from the ground to the center conductor. Also, I have the bias resistor soldered to a terminal and that I have going to star ground as well.

Awen wrote:

MontanaWay wrote:

Awen wrote:

MontanaWay wrote:

Awen wrote:OK, I think I got it narrowed down to a grounding issue. If I touch the can cap housing with the tube amp on, the problem goes away. I have redid the star ground three times and the issue remains. So, I got a grounding issue somewhere.

very good!...that is an excellent step!
There is a video on this forum, posted only a few days, about some grounding issues with a can cap. To cut a long story short, the guy ended up soldering one of the can caps tabs directly to the chassis which fixed his issue 100%.
If you do not have a soldering iron 'big' enough to generate the heat required to do this, I would run some heavy gauge wire from ALL the can caps tabs and solder them to your star ground. Also make sure your star ground is really well and tightly screwed to the chassis, if necessary, remove all the tags, get some steel wool or heavy gauge sand paper, and really go to town around the star ground area to really make sure that your star ground makes the best possible mechanical connection to the chassis!
If you still have some issues, maybe after doing all this and checking all your ground connections and still having some small noise issue, connect the ground wire, you'll need a 3 core power cable, to the chassis.

I just redid the grounds...still same issue..maybe I will try the 3 prong cord approach.

it should not be necessary to have the ST70 grounded to mains, 1000's did not and most still do not. Some prefer it as a safety, some say in their instances it lowered hum/noise to an almost in audible level.
Yours by the sound of it is extremely loud, so grounding it to mains may not actually solve it all. I know you have already done so, but I would still be looking at ALL my grounding points. I assume too that the -ve of your speaker terminals are grounded to the star ground, both channels. I am also assuming that all your grounding connections go to the star ground.

Yes, all connections go to star ground. One thing I though was strange on this amp is how the 470k ohm resistor at the input connects from the ground to the center conductor. Also, I have the bias resistor soldered to a terminal and that I have going to star ground as well.

does the Chinese driver pcb manufacturer actually specify that 470K to be there? The VTA does not. And as you say, both inputs have that 470K, so in theory if that was a problem your left channel ought to miss behave as well.
My feeling is that you are still having a ground issue somewhere in the right channel circuit..........
Bias resistor should be good, since your BIAS as you've mentioned is OK. A lot of times the bias resistor is soldered to a lug which in turn is held to ground by one of the tube socket screws.

MontanaWay wrote:

Awen wrote:

MontanaWay wrote:

Awen wrote:

MontanaWay wrote:

Awen wrote:OK, I think I got it narrowed down to a grounding issue. If I touch the can cap housing with the tube amp on, the problem goes away. I have redid the star ground three times and the issue remains. So, I got a grounding issue somewhere.

very good!...that is an excellent step!
There is a video on this forum, posted only a few days, about some grounding issues with a can cap. To cut a long story short, the guy ended up soldering one of the can caps tabs directly to the chassis which fixed his issue 100%.
If you do not have a soldering iron 'big' enough to generate the heat required to do this, I would run some heavy gauge wire from ALL the can caps tabs and solder them to your star ground. Also make sure your star ground is really well and tightly screwed to the chassis, if necessary, remove all the tags, get some steel wool or heavy gauge sand paper, and really go to town around the star ground area to really make sure that your star ground makes the best possible mechanical connection to the chassis!
If you still have some issues, maybe after doing all this and checking all your ground connections and still having some small noise issue, connect the ground wire, you'll need a 3 core power cable, to the chassis.

I just redid the grounds...still same issue..maybe I will try the 3 prong cord approach.

it should not be necessary to have the ST70 grounded to mains, 1000's did not and most still do not. Some prefer it as a safety, some say in their instances it lowered hum/noise to an almost in audible level.
Yours by the sound of it is extremely loud, so grounding it to mains may not actually solve it all. I know you have already done so, but I would still be looking at ALL my grounding points. I assume too that the -ve of your speaker terminals are grounded to the star ground, both channels. I am also assuming that all your grounding connections go to the star ground.

Yes, all connections go to star ground. One thing I though was strange on this amp is how the 470k ohm resistor at the input connects from the ground to the center conductor. Also, I have the bias resistor soldered to a terminal and that I have going to star ground as well.

does the Chinese driver pcb manufacturer actually specify that 470K to be there? The VTA does not. And as you say, both inputs have that 470K, so in theory if that was a problem your left channel ought to miss behave as well.
My feeling is that you are still having a ground issue somewhere in the right channel circuit..........
Bias resistor should be good, since your BIAS as you've mentioned is OK. A lot of times the bias resistor is soldered to a lug which in turn is held to ground by one of the tube socket screws.

No, the PCB does not have the 470K on it. I Did some more testing and it appears the issue is in both sides, but more so on the right. When testing the amp I have it connected to my variac which is on the right side, so I am hearing more from the right. I am throwing in the towel for now...

Awen wrote:

MontanaWay wrote:

Awen wrote:

MontanaWay wrote:

Awen wrote:

MontanaWay wrote:

Awen wrote:OK, I think I got it narrowed down to a grounding issue. If I touch the can cap housing with the tube amp on, the problem goes away. I have redid the star ground three times and the issue remains. So, I got a grounding issue somewhere.

very good!...that is an excellent step!
There is a video on this forum, posted only a few days, about some grounding issues with a can cap. To cut a long story short, the guy ended up soldering one of the can caps tabs directly to the chassis which fixed his issue 100%.
If you do not have a soldering iron 'big' enough to generate the heat required to do this, I would run some heavy gauge wire from ALL the can caps tabs and solder them to your star ground. Also make sure your star ground is really well and tightly screwed to the chassis, if necessary, remove all the tags, get some steel wool or heavy gauge sand paper, and really go to town around the star ground area to really make sure that your star ground makes the best possible mechanical connection to the chassis!
If you still have some issues, maybe after doing all this and checking all your ground connections and still having some small noise issue, connect the ground wire, you'll need a 3 core power cable, to the chassis.

I just redid the grounds...still same issue..maybe I will try the 3 prong cord approach.

it should not be necessary to have the ST70 grounded to mains, 1000's did not and most still do not. Some prefer it as a safety, some say in their instances it lowered hum/noise to an almost in audible level.
Yours by the sound of it is extremely loud, so grounding it to mains may not actually solve it all. I know you have already done so, but I would still be looking at ALL my grounding points. I assume too that the -ve of your speaker terminals are grounded to the star ground, both channels. I am also assuming that all your grounding connections go to the star ground.

Yes, all connections go to star ground. One thing I though was strange on this amp is how the 470k ohm resistor at the input connects from the ground to the center conductor. Also, I have the bias resistor soldered to a terminal and that I have going to star ground as well.

does the Chinese driver pcb manufacturer actually specify that 470K to be there? The VTA does not. And as you say, both inputs have that 470K, so in theory if that was a problem your left channel ought to miss behave as well.
My feeling is that you are still having a ground issue somewhere in the right channel circuit..........
Bias resistor should be good, since your BIAS as you've mentioned is OK. A lot of times the bias resistor is soldered to a lug which in turn is held to ground by one of the tube socket screws.

No, the PCB does not have the 470K on it. I Did some more testing and it appears the issue is in both sides, but more so on the right. When testing the amp I have it connected to my variac which is on the right side, so I am hearing more from the right. I am throwing in the towel for now...

what do you mean by the variac connected on the ride side?.....you are using the variac to keep mains at about 115VAC, yes?

The right speaker is close to where I have the variac set up. I only have it connected to the variac to enable me to quickly power it down if there is an issue. I will try to get some pics when I can and post them, but the wiring is really messy now from rewiring it over and over.

Awen wrote:The right speaker is close to where I have the variac set up. I only have it connected to the variac to enable me to quickly power it down if there is an issue. I will try to get some pics when I can and post them, but the wiring is really messy now from rewiring it over and over.

OK I see.
You know, although a right royal pain in the backside, it may pay you to completely strip the amp, pull out all the wiring, and start over again. A 'rats' nest is your worst enemy. If you do go down this track, twist every wire (you may need to extend some, if so, use heat shrink at the joints), make sure each solder joint is nice and neat.

The photo is one example

I think I am going to do some experimenting and see what happens when I remove the NFB.

Awen wrote:I think I am going to do some experimenting and see what happens when I remove the NFB.

Nope, that didn't work, all it did was cause a high pitch howling noise. I may have to bite the bullet and rebuild the entire amp.

Awen wrote:

Awen wrote:I think I am going to do some experimenting and see what happens when I remove the NFB.

Nope, that didn't work, all it did was cause a high pitch howling noise. I may have to bite the bullet and rebuild the entire amp.

not having seen the amp, but by the sounds of your description, so much wiring, mods, 'repairs' have been done to it, that yes, it is probably best to bite the bullet and start from scratch.
Not only will the end product be nice, neat and clean, it is also a really good way to get to know your amp and tube amps in general!
Sorry that we could not identify your problem! If you do decide to pull it apart and start again, take your time, as you may just find the problem area during diss assembly!

When I connected the bias test points and removed the old power take off sockets, I did not connect the wire that goes from pin #5 (on the power take off socket) to the pad via the 10 ohm resistor. I notice that wire connects to the pcb ground. Is it possible that wire not connected can cause my issue? It seems as though it would increase the ground potential and by having is disconnected I don't have as good of a ground area?

Got a camera?  A picture would help a lot. I think those 10 ohm resistors to the front sockets were for the PAM connections and not really needed in your rebuild.

I will upload a pic as soon as I can. I wonder if the two 6GH8 tubes could be bad? It would be weird if they were both bad at the same time. It is just so weird because the amp is very quiet now until I hook anything up to the input. I tried a couple different sources and there is a loud honking noise the increases with intensity.

I got two more 6GH8 tubes on the way...I gotta rule them out before I rebuild this thing, plus they are cheap so I don't have much to lose.

Here is a pic of my mess, it was much neater before I start" alt=""/>ed rewiring it. Also some of the traces had lifted on the board, which I remedied with jumpers.

Awen wrote:Here is a pic of my mess, it was much neater before I started rewiring it. Also some of the traces had lifted on the board, which I remedied with jumpers.

well I've seen much worse!
I would like to see far more 'twisting' on all AC wiring...and NO CRIMPS on ANY wiring...I am assuming that's what I see on the start ground!
All the wiring too should be neatly routed.
I know you've done a lot of 'repair' wiring work, but if you do decide to re build it, then really pay attention to twisting all wires, even if it means having to extend them, and neatly route them around the outside edges of the chassis, then when its all done, use small zip ties to hold it all together nice and neat.
I am not sure how much you have got invested in this amp, but it may pay you to 'suck it up' and get yourself the VTA driver board and maybe salvage as many parts as you can/want from that Chinese driver board!

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!

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?........