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    ST70 hum when PAM1 is connected to the takeoff socket

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    antsap

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    Post by antsap on Sat Jul 27, 2019 4:50 pm

    Hey gang,

    I repaired an ST70 for a customer. It had a loud audible hum on both channels with nothing plugged in.
    I ended up replacing the cap can which helped but didn't solve the problem. I went further and found that the 470k resistor was missing on one channel, and on the other the connection from the input tip to the mono/stereo middle switch was also missing.

    Fixing all three of those problems made the ST70 dead silent when nothing was plugged in.

    Next I connected up a PAM-1 and the darn thing hummed like a son of a bitch again.
    Here's the thing, it hums badly even in the PAM1 output is not connected to the ST70 input. All I need to connect is the take off socket octal plug to the PAM1 and the hum is there. This happens on either channel.

    The PAM1 has the original CAP-can in it. It tests OK for capacitance and ESR, but I didn't check for leakage yet at full voltage.

    Anyone have any ideas? Could a shot cap can in the PAM1 cause this behavior with the ST70 even without the output connected up to the power amp input?

    Thanks everyone. I don't want to start replacing parts blindly and I really appreciate your insight.

    peterh
    peterh

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    Post by peterh on Sun Jul 28, 2019 3:07 am

    antsap wrote:Hey gang,

    I repaired an ST70 for a customer. It had a loud audible hum on both channels with nothing plugged in.
    I ended up replacing the cap can which helped but didn't solve the problem. I went further and found that the 470k resistor was missing on one channel, and on the other the connection from the input tip to the mono/stereo middle switch was also missing.

    Fixing all three of those problems made the ST70 dead silent when nothing was plugged in.

    Next I connected up a PAM-1 and the darn thing hummed like a son of a bitch again.
    Here's the thing, it hums badly even in the PAM1 output is not connected to the ST70 input. All I need to connect is the take off socket octal plug to the PAM1 and the hum is there. This happens on either channel.

    The PAM1 has the original CAP-can in it. It tests OK for capacitance and ESR, but I didn't check for leakage yet at full voltage.

    Anyone have any ideas? Could a shot cap can in the PAM1 cause this behavior with the ST70 even without the output connected up to the power amp input?

    Thanks everyone. I don't want to start replacing parts blindly and I really appreciate your insight.


    How does the st70 work with another source connected ? If it works well with a source then it has to do with the grounding of the PAM1 ( and the connections in the socket) If the user don't use
    this socket to power a preamp, then leave it .
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    antsap

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    Post by antsap on Sun Jul 28, 2019 9:35 am

    Hey Peterh,

    Using a different preAmp that doesn't use the octal takeoff socket, the ST70 sounds great and quiet. The customer uses it this way though, with a PAM1 connected to the takeoff socket so I need to get to the bottom of it.

    I did validate the power take off connections inside the PAM1, and they match up with the PAM schematic. Here is how they are connected to the socket that plugs into the ST70:


    Pin1 = white wire
    Pin2 = green wire
    Pin3 = black wire
    (pin4 = not connected)
    Pin5 = red wire
    (pin6 = not connected)
    (pin7 = not connected)
    (Pin8 = not connected)

    I also opened up the ST70 and confirmed the pinouts on the takeoff socket are wired up correctly according to the st70 schematic.
    peterh
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    Post by peterh on Sun Jul 28, 2019 11:12 am

    Note that the ground in the socket has a 10 ohm resistor to the st70 ground.
    See image from last page of manualST70 hum when PAM1 is connected to the takeoff socket Dynast12
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    antsap

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    Post by antsap on Sun Jul 28, 2019 12:32 pm

    Thank peterh. Yeah, the 10ohm resistors are in place for both sides. I remember they drifted up a bit, but I forget their exact measurements. I supposed a 3ohm drift would mean a 30% increase in resistance ... is it possible the extra resistance to ground is a source of this hum?
    peterh
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    Post by peterh on Sun Jul 28, 2019 1:32 pm

    antsap wrote:Thank peterh. Yeah, the 10ohm resistors are in place for both sides. I remember they drifted up a bit, but I forget their exact measurements. I supposed a 3ohm drift would mean a 30% increase in resistance ... is it possible the extra resistance to ground is a source of this hum?
    The resistor is there to ( i think) avoid a ground loop.

    But you mean that the additional load of MAP1 will cause hum ? Is this for either octal connector ?
    Will it hum even if tubes are removed from the pam1 ?
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    antsap

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    Post by antsap on Sun Jul 28, 2019 4:22 pm

    That's right. The hum happens when you connect the octal connecter. It happens on either side, and it doesn't matter if the preAmp output is connected to the ST70 input or not, just the octal connecter causes the hum.

    I tried removing the two 12ax7's from the PAM-1 and it made no difference, the hum was still there.

    I'm about to make sure the other sides of the 10ohm resistors in the ST70 have a good connection to ground
    peterh
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    Post by peterh on Sun Jul 28, 2019 5:00 pm

    antsap wrote:That's right. The hum happens when you connect the octal connecter. It happens on either side, and it doesn't matter if the preAmp output is connected to the ST70 input or not, just the octal connecter causes the hum.

    I tried removing the two 12ax7's from the PAM-1 and it made no difference, the hum was still there.

    I'm about to make sure the other sides of the 10ohm resistors in the ST70 have a good connection to ground
    It might be your pam1 that draws a lot of current. When tubes are removed only a pair of 10uF caps are loading the B+. They might be faulty. What is the voltage on pin 5 of the octal connector with and without pam1 connected ?

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    antsap

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    Post by antsap on Sun Jul 28, 2019 5:08 pm

    400vdc unloaded, and down to 326vdc with the PAM connected.

    Thinking this confirms the quad cap can in the PAM has one foot in the grave?
    peterh
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    Post by peterh on Sun Jul 28, 2019 5:53 pm

    antsap wrote:400vdc unloaded, and down to 326vdc with the PAM connected.

    Thinking this confirms the quad cap can in the PAM has one foot in the grave?
    Looks like it. Just to be sure; is it with tubes removed in the pam1 ?


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    antsap

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    Post by antsap on Sun Jul 28, 2019 5:54 pm

    No, those measurements were taken with the tubes in the PAM1.
    The hum behavior was the same with the tubes in or out. Should I re-measure again without?

    peterh
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    Post by peterh on Mon Jul 29, 2019 2:38 am

    antsap wrote:No, those measurements were taken with the tubes in the PAM1.
    The hum behavior was the same with the tubes in or out. Should I re-measure again without?

    No i do not think its necessary . Replace the cap in the pam1, but i do think you can ship the st70 to it's owner.
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    antsap

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    Post by antsap on Mon Jul 29, 2019 11:08 pm

    Peterh, thanks so much for your time and expertise on this. I went ahead and ordered a replacement cap can. I'll keep you posted once I get the PAM fixed up.

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    antsap

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    Post by antsap on Fri Aug 02, 2019 12:51 pm

    Parts came in last night. This morning, I swapped out the cap can. Unfortunately, it didn't really make any difference. The darn hum is still there like before Sad

    I tried pulling the tubes again out of the PAM, but that didn't make a difference either.

    Oiy! Any other ideas what might be causing this? Thanks!
    peterh
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    Post by peterh on Fri Aug 02, 2019 4:45 pm

    antsap wrote:Parts came in last night. This morning, I swapped out the cap can. Unfortunately, it didn't really make any difference. The darn hum is still there like before :(

    I tried pulling the tubes again out of the PAM, but that didn't make a difference either.

    Oiy! Any other ideas what might be causing this? Thanks!
    Try to check how much current it draws, both DC on B+ and AC on the filament.
    I suppose you tested this with another ST70 ? An excellent "cheat way" to pinpoint a problem is
    to have another unit to test with. In your case another ST70 ( or another pam1)
    PeterCapo
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    Post by PeterCapo on Fri Aug 02, 2019 5:05 pm

    Just a couple of thoughts...

    Both the Stereo 70 manual and the PAM-1 manual have charts with reference voltages taken at indicated nodes in the circuit.  I'd go through all of them to see how your Stereo 70 and PAM-1 compare:

    https://dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf

    http://akdatabase.com/AKview/albums/userpics/10004/Dynaco%20Preamp%201%20Assembly_Owner%26%2339%3Bs.pdf

    If that all checks out okay, consider temporarily shorting the four 10-ohm resistors that are supposed to avoid ground loops.  There are the two that go to the sockets and two on the Stereo 70 PCB.  Actually, just do two of them first and see what happens.  There have been reports that shorting them actually decreases hum as it did with mine, though I wasn't using the takeoff sockets.  It's worth a try.
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    rjpjnk

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    Post by rjpjnk on Tue Aug 06, 2019 2:49 pm

    antsap wrote:400vdc unloaded, and down to 326vdc with the PAM connected.

    Thinking this confirms the quad cap can in the PAM has one foot in the grave?

    I am not familiar with the PAM1 circuit, but it sounds to me like the B+ is dropping more than I would have expected here when you plug it in. I would repeat this test without the PAM1 tubes installed. I would guess virtually no current should flow in this case, but to know for sure I'd need to see a schematic.

    The suggestion above to get the test voltages specified in the manual is a good idea. These measurements would of course be with all tubes in place.

    If this does not reveal a problem then you will have to dig deeper. Since the hum is just the same whether the PAM1 tubes are installed or not I would leave them out. They will only complicate debugging. Them perhaps you could connect one wire at a time between the ST70 and PAM to see when the hum starts?

    Post a schematic if you have one.

    EDIT: Nevermind, I see the manual posted above has the PAM schematic. I will take a look and see if anything jumps out...
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    rjpjnk

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    Post by rjpjnk on Tue Aug 06, 2019 3:16 pm

    After a quick look at the PAM1 schematic I see there is very little connected when the tubes in the preamp are removed, so it should be simple to get to the bottom of this.

    In reading back through your thread I think the most important clue is you hear the hum even when the audio connection from preamp to power amp is not connected. This means something the PAM1 is doing to the power supply itself is causing the problem.

    With the tubes removed there is not much on the B+ side except the quad cap and associated resistors. You already replaced this. On the filament side there are two diodes and a couple large caps. My guess is something is wrong here. As Peter said above, if those diodes are Selenium that may be the issue. Also one of the 1000uf caps could be bad.

    In any event, if you try connecting the filament supply and B+ supply separately you can then see which one is overloading the ST-70. My guess is filament.

    EDIT: there are also some .05uf coupling caps on the B+ side. If one of these are shorted that could also be a problem. But with the tubes out it is not likely.
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    antsap

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    Post by antsap on Sun Sep 22, 2019 10:15 am

    Hi all,

    Sorry for the delay in responding to this thread. I ended up fixing a bunch of other amps and I'm just getting back to the dynaco now.

    So I spoke with the owner, and he dropped off a second PAM1 he has which we know is good as it works just fine with his other ST70. I took this so I can isolate if the problem is really in the PAM1 or in the ST70.

    With the known good PAM1, the current ST70 I have here behaves exactly the same. Silent as a door mouse without the PAM connected (or with another non power supplied preAmp driving it), and hums loudly with the power of the PAM connected independent of the audio input from the PAM.

    So I think its something in the ST70. Anyone have any ideas?

    Thanks!!
    peterh
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    Post by peterh on Mon Sep 23, 2019 2:39 am

    antsap wrote:Hi all,

    Sorry for the delay in responding to this thread. I ended up fixing a bunch of other amps and I'm just getting back to the dynaco now.

    So I spoke with the owner, and he dropped off a second PAM1 he has which we know is good as it works just fine with his other ST70. I took this so I can isolate if the problem is really in the PAM1 or in the ST70.

    With the known good PAM1, the current ST70 I have here behaves exactly the same. Silent as a door mouse without the PAM connected (or with another non power supplied preAmp driving it), and hums loudly with the power of the PAM connected independent of the audio input from the PAM.

    So I think its something in the ST70. Anyone have any ideas?

    Thanks!!
    You got me clueless this time. But the good news is that the problem is isolated to the ST-70, which means that all digging is concentrated here.

    As i have no PAM a question that could help me understand :
    Is the audio output from the PAM on it's own rca cable connected to the ST-70?
    If so, is the hum present even if this cable is disconnected ?
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    antsap

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    Post by antsap on Mon Sep 23, 2019 9:10 am

    peterh wrote:
    You got me clueless this time. But the good news is that the problem is isolated to the ST-70, which means that all digging is concentrated here.

    As i have no PAM a question that could help me understand :
    Is the audio output from the PAM on it's own rca cable connected to the ST-70?
    If so, is the hum present even if this cable is disconnected ?


    Yeah, I'm rather confused myself. The audio output is on its own cable yes. The hum is present without the audio cable connected. It starts and stops with the octal power connector from the PAM being plugged into the ST70, so no audio input is involved at all.
    This behaves the same on both sides of the ST70. I swapped the rectifier tube with a new one just to try and eliminate common components between the two sides but this had no effect. It also has been re-capped (new cap can, and new bias caps), and its running a diode in place of the selenium job.

    Thanks for the help, this one has me stymied!
    peterh
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    Post by peterh on Mon Sep 23, 2019 1:39 pm

    antsap wrote:
    peterh wrote:
    You got me clueless this time. But the good news is that the problem is isolated to the ST-70, which means that all digging is concentrated here.

    As i have no PAM a question that could help me understand :
    Is the audio output from the PAM on it's own rca cable connected to the ST-70?
    If so, is the hum present even if this cable is disconnected ?


    Yeah, I'm rather confused myself. The audio output is on its own cable yes.  The hum is present without the audio cable connected. It starts and stops with the octal power connector from the PAM being plugged into the ST70, so no audio input is involved at all.
    This behaves the same on both sides of the ST70. I swapped the rectifier tube with a new one just to try and eliminate common components between the two sides but this had no effect. It also has been re-capped (new cap can, and new bias caps), and its running a diode in place of the selenium job.

    Thanks for the help, this one has me stymied!
    As the problem is in the st-70 one must seek the solution there.
    Myself would start with examining the st-70, comparing with the schematics
    to find whats different.
    A clue could be to / as others suggest) disconnect all wires in the octal, and reconnect them one by one until hum begins. But my guess is that something is wired differently in the st-70
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    Post by antsap on Wed Oct 02, 2019 10:28 pm

    Hmm. I've checked the wiring against the schematic several times but I certainly may have missed something. I'll go over it again to see if I can spot something.

    I tried another experiment which may be telling?
    If I run the amp in stereo mode, and only connect one side to a speaker (other to a dummy load), I only get the hum if I connect the octal PAM1 power connecter to the same side that is routed to the speaker. So if I connect the power take-off to the opposite side connected to the dummy load, the hum is not present on the other side.

    So I believe this exonerates any of the common circuitry to both sides of the amp. I also measured the hum, it's definitely 120hz
    peterh
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    Post by peterh on Thu Oct 03, 2019 12:32 am

    antsap wrote:Hmm. I've checked the wiring against the schematic several times but I certainly may have missed something. I'll go over it again to see if I can spot something.

    I tried another experiment which may be telling?
    If I run the amp in stereo mode, and only connect one side to a speaker (other to a dummy load), I only get the hum if I connect the octal PAM1 power connecter to the same side that is routed to the speaker. So if I connect the power take-off to the opposite side connected to the dummy load, the hum is not present on the other side.

    So I believe this exonerates any of the common circuitry to both sides of the amp. I also measured the hum, it's definitely 120hz
    THis must be this years Great Mystery !
    I do think you find the cause, and i hope you will let us know !
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    Post by Dale Stevens on Tue Nov 12, 2019 3:08 pm

    Antsap, is this years great mystery solved? Dale

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