I've lost the left channel on my ST-120. It first the left channel had a diminished volume then silent. I swapped the tubes around, reversed inputs and speaker wires just to make sure it wasn't something external. I can't see anything visual. Nothing looks burnt or smells burnt. Voltages are normal. So maybe a bad solder, component, not sure yet. I had an idea, Can I swap the pin 6 leads off the driver board left to right? That would tell me if the problem is in the driver board, right?
+7
DavidR
ImFritz
corndog71
Bob Latino
peterh
cci1492
Mr C
11 posters
ST-120 no left channel - fixed !
Mr C- Posts : 99
Join date : 2019-07-11
Age : 69
Location : Visalia CA.
cci1492- Posts : 331
Join date : 2016-05-09
Age : 63
Location : NJ
Check driver board tubes.
Mr C- Posts : 99
Join date : 2019-07-11
Age : 69
Location : Visalia CA.
cci1492 wrote:Check driver board tubes.
I've moved the driver tubes around in different positions and no matter what sockets they occupy the left channel is out. That's when I thought about swapping the pin 6 leads.
peterh- Posts : 1833
Join date : 2012-12-25
Location : gothenburg, sweden
I would advice not to ( if you do you must also move the NFB connection ) and you haveMr C wrote:cci1492 wrote:Check driver board tubes.
I've moved the driver tubes around in different positions and no matter what sockets they occupy the left channel is out. That's when I thought about swapping the pin 6 leads.
one working channel, don't screw that one up !
Isolate fault the usual way with signal on input and a scope
Mr C- Posts : 99
Join date : 2019-07-11
Age : 69
Location : Visalia CA.
peterh wrote:I would advice not to ( if you do you must also move the NFB connection ) and you haveMr C wrote:cci1492 wrote:Check driver board tubes.
I've moved the driver tubes around in different positions and no matter what sockets they occupy the left channel is out. That's when I thought about swapping the pin 6 leads.
one working channel, don't screw that one up !
Isolate fault the usual way with signal on input and a scope
Thanks, I'll scrap that idea. I thought it may be a way to determine if it's something on the driver board or if it was elsewhere.
Mr C- Posts : 99
Join date : 2019-07-11
Age : 69
Location : Visalia CA.
Is the driver board the same between the ST-70 and the ST-120?
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
Mr C wrote:Is the driver board the same between the ST-70 and the ST-120?
The board is the same but the components on the board are slightly different.
Bob
corndog71- Posts : 840
Join date : 2013-03-19
Location : It can get windy here
Have you checked all of your cables?
Mr C- Posts : 99
Join date : 2019-07-11
Age : 69
Location : Visalia CA.
corndog71 wrote:Have you checked all of your cables?
That was one of the first things I did. Switched left and right input interconnects and switched left and right speaker wires. Swapping the interconnects the left stayed out. swapping speaker wires there was no sound on the right.
ImFritz- Posts : 49
Join date : 2017-01-01
Age : 64
Location : Redford, Michigan
I would do a simple continuity check and test for shorts on the interconnects and the speaker wire.
DavidR- Posts : 156
Join date : 2017-08-10
Location : MetroWest, MA
Mr C wrote:...................................................................... swapping speaker wires there was no sound on the right.corndog71 wrote:Have you checked all of your cables?
Seems as if you've isolated the problem.
rjpjnk- Posts : 262
Join date : 2018-07-18
Mr C, do you have any test equipment? Scope or multimeter? I forget.
Mr C- Posts : 99
Join date : 2019-07-11
Age : 69
Location : Visalia CA.
DavidR wrote:Mr C wrote:...................................................................... swapping speaker wires there was no sound on the right.corndog71 wrote:Have you checked all of your cables?
Seems as if you've isolated the problem.
What I should of said I swapped the wires at the terminals on the amp. Not the wires themselves. so the right speaker and wire was connected to left terminal and the left speaker and wire was connected to the right terminal. I got sound from the left speaker and wire, connected to the right terminal. And none on the right.
Mr C- Posts : 99
Join date : 2019-07-11
Age : 69
Location : Visalia CA.
rjpjnk wrote:Mr C, do you have any test equipment? Scope or multimeter? I forget.
All I have is a Fluke 29 Multimeter. I been poking around on the driver board with it. I haven't found any differences between left and right side. It does do capacitance and I get the the same numbers off the left and right side of the board. All the resistors off the power tubes measure fine. So is there a way to check the output transformer? I went over a few solder connections but, to me anyway, they all look okay.
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
Mr C wrote:rjpjnk wrote:Mr C, do you have any test equipment? Scope or multimeter? I forget.
All I have is a Fluke 29 Multimeter. I been poking around on the driver board with it. I haven't found any differences between left and right side. It does do capacitance and I get the the same numbers off the left and right side of the board. All the resistors off the power tubes measure fine. So is there a way to check the output transformer? I went over a few solder connections but, to me anyway, they all look okay.
If you have the stepped attenuator, resolder those connections for the left channel. If you don't have the attenuator, check and resolder the left channel input connections and the left channel driver board input connections.
Bob
Mr C- Posts : 99
Join date : 2019-07-11
Age : 69
Location : Visalia CA.
I have no attenuator. Tonight I resoldered the connections on the input jack and the driver board input connections from the jack. Still no left channel. I get continuity between the jack and the board.Bob Latino wrote:Mr C wrote:rjpjnk wrote:Mr C, do you have any test equipment? Scope or multimeter? I forget.
All I have is a Fluke 29 Multimeter. I been poking around on the driver board with it. I haven't found any differences between left and right side. It does do capacitance and I get the the same numbers off the left and right side of the board. All the resistors off the power tubes measure fine. So is there a way to check the output transformer? I went over a few solder connections but, to me anyway, they all look okay.
If you have the stepped attenuator, resolder those connections for the left channel. If you don't have the attenuator, check and resolder the left channel input connections and the left channel driver board input connections.
Bob
rjpjnk- Posts : 262
Join date : 2018-07-18
Didn't you have some other problem a while back? What was it I forget. Wondering if it is related.
Mr C- Posts : 99
Join date : 2019-07-11
Age : 69
Location : Visalia CA.
rjpjnk wrote:Didn't you have some other problem a while back? What was it I forget. Wondering if it is related.
I had lost bias on V7, had to replaced the 10 ohm resistor. But that's on the right side, and it was a while ago, so I don't know. When I first put it together I lost a couple of 5ar4s and a 6550 red plated but since I got a veriac that hasn't been an issue. I have 80 to 100 hours on it now before this happened.
MechEngVic- Posts : 106
Join date : 2019-01-16
I would take my multimeter, set it to DC voltage, connect my negative probe to a ground point and start probing every resistor on that channel; amp on the bench, all tubes, cap, and rectifier connected, and dummy loads connected to speaker outs of both channels. If you get voltage on one side of a resistor and 0 V on the other side of the same resistor, that resistor is most likely blown. If you get 0 V on both sides, check the upstream resistor, or it may be a 0 V leg of a pre-stage tube. I'm guessing it's one of the high value resistors going to a pre-stage tube grid judging by the low volume then no volume.
Dale Stevens- Posts : 201
Join date : 2014-07-06
Age : 75
Location : Loris, SC
Mr C, u said u " swapped" your tubes around; Did you replace with NEW driver tubes? That's what I would do, most certain the center one. Just a thought, Dale
Mr C- Posts : 99
Join date : 2019-07-11
Age : 69
Location : Visalia CA.
Thanks Mechengvic, great guidance there. Here is a diagram of my findings. You can see I have B+ voltage at both ends of R15, R19 and R21. Compare it to the right side. I'm not sure what this means, hopefully you guys can guide me to whats next. I been looking for a solder bridge on the left side but nothing obvious. Maybe Bob will look at and say Ahh Ha it's this!
MechEngVic- Posts : 106
Join date : 2019-01-16
Whoa, didn't expect that! Looks like you have B+ running rampant. The ST-120 schematic isn't widely available so maybe an owner or Bob will chime in on what this means. But it looks like B+ is running backwards through something.
rjpjnk- Posts : 262
Join date : 2018-07-18
This helps a lot. I don't have a VTA120 schematic, but assuming it is similar to the VTA70 you readings clearly show there is no bias current flowing in the phase inverter tube on the left. You can tell this because there is no voltage drop across R19 and R21.
On the right side you see you are dropping 114 volts across R20 and R22, which are 27K indicating 4.2ma bias current which matches the number specified on my schematic. Also, the 394v at the top of R20 and R22 is the correct voltage assuming your B+ is 430.
So yes, clearly the left side is not working.
As for why it's not working. It might be a bad voltage regulator (The little black transistor like thingy connected to the cathodes of that tube)
EDIT: I found this snippet of a VTA ST70 schematic on Google images that shows the phase inverter circuit for reference. It may be old and yours may have slightly different value components and voltages, but same basic idea.
On the right side you see you are dropping 114 volts across R20 and R22, which are 27K indicating 4.2ma bias current which matches the number specified on my schematic. Also, the 394v at the top of R20 and R22 is the correct voltage assuming your B+ is 430.
So yes, clearly the left side is not working.
As for why it's not working. It might be a bad voltage regulator (The little black transistor like thingy connected to the cathodes of that tube)
EDIT: I found this snippet of a VTA ST70 schematic on Google images that shows the phase inverter circuit for reference. It may be old and yours may have slightly different value components and voltages, but same basic idea.
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
My question to Mr. C is > "Did you at any time ever use any driver tube OTHER THAN a 12AU7/12BH7/5963/5814/6189 on this board ?" If you did, it is possible that you have taken out the LM334 IC current regulator for that channel. This will cause a dead channel. You can't use 12AT7, 12AX7, 12AV7, 12AY7, 5751 tubes on the VTA driver board. Although these tubes are plug compatible with a 12AU7 type tube, they have other characteristics that are incompatible with the board. I have had a couple of customers plug the wrong driver tube into the board and as a consequence found themselves with a dead channel. Both times we found that the LM334 IC for than channel was now bad. I can't say for sure that this is your problem but it could be ?
Bob
Bob
anbitet66- Posts : 143
Join date : 2009-12-23
Location : Valley Stream, NY
Mr. C,
A quick way to provide the (possibly) missing bias voltage to your 12AU7 tubes would be to use a 2.2k to 2.7k resistor and bridge from the V+ terminal to chassis ground (the V- terminal) across the LM334 IC to provide an approximation of the bias current. This is just a quick test to see if sound is restored. If you can try this, you'll either have your answer, or need to look elsewhere. If Bob is right, this would be the quick way to prove it.
Tony
A quick way to provide the (possibly) missing bias voltage to your 12AU7 tubes would be to use a 2.2k to 2.7k resistor and bridge from the V+ terminal to chassis ground (the V- terminal) across the LM334 IC to provide an approximation of the bias current. This is just a quick test to see if sound is restored. If you can try this, you'll either have your answer, or need to look elsewhere. If Bob is right, this would be the quick way to prove it.
Tony
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