Chasing Hum with a "new" old ST-120 - solved !

Hello!  First post here.  Unfortunately it has to be about a problem.

I recently bought a used ST-120 for quite a steal and ever since I got it home, I'm chasing hum.  At the first owners home, I checked everything out. Some dust and a few questionable solder joints, but all worked and sounded great. no hum whatsoever.  The owner was several hours from home so I stopped at my parents house along the way.  At my parents house, I plugged the amp in to a set of test speakers, plugged my phone in using an AUX cable, and still...no hum and sounds great.  I get home that night, hook everything up in my main system, and the frustration begins.  I immediately notice a strong hum/buzz coming from the speakers.

After a few days of troubleshooting, checking ground connections, and trying different setups, I managed to quiet things somewhat. I learned that I cant put any interconnects anywhere near the main power transformer (ie, passing them under the front of the chassis to the back), the amp was quieter plugged directly into the wall, and the main Mullard driver tube was a little noisy and sounded quieter when I plugged in a $10 JJ.  I was able to at least listen to my turntable this way, but it was still very annoying. In my configuration tests, I confirmed that the amp is quietest (nearly dead quiet) if I just plug an AUX cable into it and use my phone. All of the noise from above dissapears if I'm useing the AUX cable. Another discovery was that my preamp/headphone amp seems to be part of the problem. When plugged in but turned off, I get some quiet hum.  When turned on, the hum gets louder and lower in frequency.  

My solution to the noisy preamp was to just cut it out to of the chain. I ordered the step attenuator which is on its way, but I discovered what appears to be a major problem with this solution last night. When I plug my phono preamp from the turntable directly into the amp inputs, I get a MASSIVE ground hum. As soon as I touch the amp chassis or the grounding post on the preamp (or both), the hum dissapears. I tried plugging into different outlets, grounding to the ST-120 chassis, moving things arround, and nothing helps.  What has me completely baffled is that I don't have this issue when my phono preamp is plugged into my integrated amp, even at full volume.

So after troubleshooting all week, and finally feeling like I had things figured out, I'm back at square one. The common denominator seems to be the ST-120 itself.  I'm feeling defeated and need some help.

vizsla23 wrote:Hello!  First post here.  Unfortunately it has to be about a problem.

I recently bought a used ST-120 for quite a steal and ever since I got it home, I'm chasing hum.  At the first owners home, I checked everything out. Some dust and a few questionable solder joints, but all worked and sounded great. no hum whatsoever.  The owner was several hours from home so I stopped at my parents house along the way.  At my parents house, I plugged the amp in to a set of test speakers, plugged my phone in using an AUX cable, and still...no hum and sounds great.  I get home that night, hook everything up in my main system, and the frustration begins.  I immediately notice a strong hum/buzz coming from the speakers.

After a few days of troubleshooting, checking ground connections, and trying different setups, I managed to quiet things somewhat. I learned that I cant put any interconnects anywhere near the main power transformer (ie, passing them under the front of the chassis to the back), the amp was quieter plugged directly into the wall, and the main Mullard driver tube was a little noisy and sounded quieter when I plugged in a $10 JJ.  I was able to at least listen to my turntable this way, but it was still very annoying. In my configuration tests, I confirmed that the amp is quietest (nearly dead quiet) if I just plug an AUX cable into it and use my phone. All of the noise from above dissapears if I'm useing the AUX cable. Another discovery was that my preamp/headphone amp seems to be part of the problem. When plugged in but turned off, I get some quiet hum.  When turned on, the hum gets louder and lower in frequency.  

My solution to the noisy preamp was to just cut it out to of the chain. I ordered the step attenuator which is on its way, but I discovered what appears to be a major problem with this solution last night. When I plug my phono preamp from the turntable directly into the amp inputs, I get a MASSIVE ground hum. As soon as I touch the amp chassis or the grounding post on the preamp (or both), the hum dissapears. I tried plugging into different outlets, grounding to the ST-120 chassis, moving things arround, and nothing helps.  What has me completely baffled is that I don't have this issue when my phono preamp is plugged into my integrated amp, even at full volume.

So after troubleshooting all week, and finally feeling like I had things figured out, I'm back at square one. The common denominator seems to be the ST-120 itself.  I'm feeling defeated and need some help.

make sure that all amps chassies are interconnected, either by them all using grounded outlets, or with a wire connected to groundposts / screws . This prevents the
ac fields in your house to use the signal grounds that interconnects everything.
Also if cable-tv is in any way connected to the stereo, get a ground isolation contact
for the TV coax.

So all components (turntable, phono preamp, preamp, and ST-120 amplifier) are all on the same grounded recepticle. The turntable ground is curently connected to the phono preamp, exactly as it was when I was running it through my integrated amplifier. TV is not connected to this system.

Wont connecting the turntable, to the phono preamp to the ST-120 create a ground loop? (though I think I'm experiencing one anyway).

vizsla23 wrote:So all components (turntable, phono preamp, preamp, and ST-120 amplifier) are all on the same grounded recepticle. The turntable ground is curently connected to the phono preamp, exactly as it was when I was running it through my integrated amplifier.  TV is not connected to this system.

Wont connecting the turntable, to the phono preamp to the ST-120 create a ground loop?  (though I think I'm experiencing one anyway).

 

If you only use the (thin) signal ground to interconnect, and there is a ac field, then
you will have hum. By using a better cable ( thicker and better connected) the ac
will follow the better conductor. Ac fields and weak coupling to mains is in all mains connected
devices by capacitive effects.
Ideally the signal ground should be somewhat disconnected from chassies ground, but
few deviced does that.
Connect a coarse cable and see if your hum changes. Even better, make sure that protective
ground is used too.

So maybe I'm missing a key point here...

Right now, I'm throwing full, line level signal to the power amp directly from the phono preamp which is going right to the speakers at full volume (since there is no volume control on the phono stage). I just reconnected back to my integrated amp, cranked it to max volume and I am getting the same response (I guess my first post was wrong).  So clearly, I have a less than ideal ground at the amp, but I just never noticed it before as I never play the integrated past 1/4 to 1/3 volume.

The attenuator is out for delivery right now.  With any luck, I'll have it installed today.  I guess my new question is, will the attenuator solve this problem like the volume knob on my integrated or is that fact that this power amp is throwing full power at the speakers at all tims going to continue to pass my ground issue to the speakers.  I'll probably have my answer by tonight regardless.  My sense is that my frustration will continue

This is my first experience running a poweramp so this is uncharted territory for me.

vizsla23 wrote:So maybe I'm missing a key point here...

Right now, I'm throwing full, line level signal to the power amp directly from the phono preamp which is going right to the speakers at full volume (since there is no volume control on the phono stage). I just reconnected back to my integrated amp, cranked it to max volume and I am getting the same response (I guess my first post was wrong).  So clearly, I have a less than ideal ground at the amp, but I just never noticed it before as I never play the integrated past 1/4 to 1/3 volume.

The attenuator is out for delivery right now.  With any luck, I'll have it installed today.  I guess my new question is, will the attenuator solve this problem like the volume knob on my integrated or is that fact that this power amp is throwing full power at the speakers at all tims going to continue to pass my ground issue to the speakers.  I'll probably have my answer by tonight regardless.  My sense is that my frustration will continue

This is my first experience running a poweramp so this is uncharted territory for me.

UPDATE: Attenuator installed. Much to my suprise, the attenuator cuts out the ground hum to quite an acceptable level. As a perfectionist, I still need to hunt down the root cause, but I'm happy for now. In fact, I'm more than happy. I didnt realize how much my preamp was holding back this amp. I wanted to check for any balance issues caused by my novice soldering and threw on Blossom Dearie's self titled album (mono) and the speakers just dissapeared behind a holographic wall of sound. Best $55 I've spent on my setup.

Glad that you solved the immidiate problem with hum!

Even though the AC plug may have a wider side, it plays no part in the wiring of the amp, as far as I saw in the instructions.
With my first preamp, it had a three prong connector.
When the two were wired together via interconnects, there was a hum.

I took a strong pair of side cutters, and removed the edges on the wider AC prong, then reversed it in the socket.
All hum disappeared.

Hmm, I think theres something to this.

In my continued frustration and chase of the hum I realized that the turntable, preamp and amp were all two pronged plugs so in my entire system nothing was truly connected to a dedicated ground. Getting desperate, I shoved a length of 10 guage speaker wire into the third hole of the wall socket and just started touching stuff. To my surprise, grounding the ST-120 chassis directly to the outlet ground killed the hum. To clean things up a bit, I yanked the third prong from an unused power cord, soldered it to the speaker cable, and covered the connection with heat shrink. The other end is screwed down to the ST-120.

Is there any risk continuing to run the amp like this? I know the slow blow fuse is connected to ground, so will it impair its function? I can re-wire to the phono preamp ground which does quiet things, but not as well as grounding the ST-120 chassis.

Thanks everyone for the help so far.

Running a ground to the chassis helped immensely and I found another thread where Bob recommended running a three prong cord and grounding the third to the chassis, so I think I'm safe.

I'm still getting some transformer buzz which seems to be producing some very, very quiet buzz in the speakers. Doing a little googling, I figured I'd start checking the diagnostic voltages and I'm only getting 330V off of the B+ driver board pin which is supposed to be above 400V. This amp is fitted with a Weber rectifier, so I think it should be pretty high. Cause for concern? I haven't flipped the amp yet to check the other pins but it's on my to do list for tomorrow.

If it isnt obvious yet, I'm quite a novice to electronics, so your help is much appreciated.

A few things in addition to the good advice above:

a) The Turntable 'Ground' is NOT meant to ground the system, but to remove static and hum from the cartridge - imagine this large piece of plastic spinning against another piece of plastic, and some very delicate coils and magnets. The idea is to control the bleed. Connect the 'ground' to the headshell (SOLDER) of either the left or right RCA from the TT. Back in the day, ARs came from the factory this way, and Revox and Dual would do it (via the dealer) on request.

b) Grounds, once-upon-a-time found on many receivers and integrated amplifiers, were for connection to an electrical ground. Keep in mind that the OEM versions of most Dynaco products were developed before 3-wire receptacles were common.

c) If one is connecting to an external ground, consider that a STAR ground is more desirable than a DAISY-CHAIN. Run individual ground wires - if necessary - to each piece of equipment from a single common ground.

d) And here is where it gets complicated: Not all grounds are created equal. If it should happen that the receptacle one is using for the system is at the end of the line, there may be multiple connections along that line for both the neutral and the ground wires. Further, if (once OK under the NEC), BX (now MC) cable was used and the sheathing served as the common ground, it will be an absolute certainty that the Neutral and the Ground (connected in the main panel) are not at the same potential by the time they get to the receptacle. This could be a problem and source of hum by itself, if the ground is 'above' the neutral. Ideally, the resistance between the Neutral and the Ground at the receptacle should be Zero (0, nil) - or the same as the crossed probes on the VOM.

Well made patch cords are an absolute necessity.
Tight connections, shell and signal, are an absolute necessity.
Clean jacks are an absolute necessity.

Note: I did not type "Expensive" - well made, suitable for the purpose, and clean covers it. I did not type "Cheap" either.

The purpose of a polarized plug is to reduce the chances of a hot chassis - as the on/off switch switches the HOT, and not the NEUTRAL. Keep in mind that should a power transformer winding fail, it would be entirely possible to have AC current on the chassis even when the item is off. Not common, but possible. If a device supplied with a polarized plug that has operated properly in the past begin to show bad symptoms, it ain't nohow the plug - so whatever it really is must be traced and corrected.

Glad you figured it out!

Enjoy!

Thanks Peter!

I may try to rig up a star ground this week just to test it out. I think I had ground problems long before this amp came into the house, I was just able to mask them enough to get by. This amp brought everything back out in the open. It started when I switched from a low output MC cartridge to a MM cartridge. The first issue was that if the turntable and nearby plasma TV were running, I got TONS of EMI. The solution was to shift the table away and not have the TV on when listening to records. I suspect tonearm wires/connections are part of the culprit.

Back to the Amp, could a bad quad cap be a reason for low B+ voltage? I swapped the Weber SS rectifier with a spare tube rectifier and I was getting the same low B+ voltage off the driver board. That rules out the rectifier. I suppose the power transformer could be bad too...

My plan is to walk through the power supply connections and reflow any questionable joints. I'll pull some voltage readings while I'm there.

Back to the Amp, could a bad quad cap be a reason for low B+ voltage? I swapped the Weber SS rectifier with a spare tube rectifier and I was getting the same low B+ voltage off the driver board. That rules out the rectifier. I suppose the power transformer could be bad too...

Sure, a bad quad cap could lead to low B+. And, more typically, to additional hum. This would also result in increased power draw, likely overheating of the cap, and other external phenomenon.

But, to go over the 'math' - a full-wave rectifier with capacitor filtering should deliver 1.414 x the AC input voltage, less the voltage drop of the tube.

A half-wave rectifier as above will deliver 0.707 x the AC input voltage, less the drop.

So, it could also be a failing rectifier. Which, again, would also explain the buzz/hum.

On the Webers, they are not created equal - the WS 1 has a low voltage drop. The WZ34 a rather high voltage drop. As noted in another thread:

WS1 DC Voltage Drop: 1 VDC
WZ34 DC Voltage Drop: 17 VDC
5AR4/GZ34 Voltage Drop: 10 VDC

So the differential may also be caused by the difference in voltage drop..

Just a thought.

Great info. I now need to reach back to college physics to remember the fundamentals of electronics to breakdown what this means.

I'm running the Weber WZ68 which has the same 17V drop. The only reason I think its the cap is because I have two rectifiers giving me nearly the same reading ( +/- 3-4 V) unless the spare that's in there is also bad. They guy gave me a shoebox full of spare tubes.

vizsla23 wrote:Thanks Peter!

I may try to rig up a star ground this week just to test it out. I think I had ground problems long before this amp came into the house, I was just able to mask them enough to get by.  This amp brought everything back out in the open.  It started when I switched from a low output MC cartridge to a MM cartridge.  The first issue was that if the turntable and nearby plasma TV were running, I got TONS of EMI.  The solution was to shift the table away and not have the TV on when listening to records. I suspect tonearm wires/connections are part of the culprit.

Back to the Amp, could a bad quad cap be a reason for low B+ voltage?  I swapped the Weber SS rectifier with a spare tube rectifier and I was getting the same low B+ voltage off the driver board.  That rules out the rectifier. I suppose the power transformer could be bad too...

My plan is to walk through the power supply connections and reflow any questionable joints.  I'll pull some voltage readings while I'm there.

No, i doubt that a bad can cap can absorb that amount of power without getting hot.
I think you should examine the measuring method, either the meter is strange
or you are measuring at the wrong point.
The preamp board is late in the B+ chain, thus chances are that the missing voltage is
absorbed in resistors before the point you measure. Check with the schematics!

Thanks! I thought the same, but I was going off of the diagnostic sheet that was provided in the instructions. Perhaps I am measuring off the wrong point. The multimeter should be good. I just bought it. I know its reading VAC ok.

I believe that the "specified" B+ Voltages on the 120 are based on 120 VAC at the receptacle. VTA/Bob: please correct me if that is not correct.

Check your wallplate voltages, it may be related to that - more-so, check the unloaded and loaded wallplate voltages. If the (presumably) dedicated circuit feeding your system is too small, or if not dedicated, overloaded with other 'stuff', that could result in significant voltage drop from the panel.

I have a 10-gauge line from the panel fused at 20A serving the main system, which will draw up to 18 A if I decide to serenade the next county. But the others are on regular receptacles, as none but the test-bed system will ever draw more than 5 A or so. We have a pretty steady 118 VAC at the wallplate, summer and winter.

Main voltage is close to 123 VAC

vizsla23 wrote:Main voltage is close to 123 VAC

Oh MY!!

If you are suffering from low B+ at 123 VAC at the wallplate, then something is definitely amiss. A cap failing open would give you a fair amount of hum, but not substantially lower B+. A cap failing short would probably get very hot. If your rectifier, tube or solid-state was starting to fail, that could affect B+ directly. Are either/both of your chokes heating up? If they are asked to stop an excessive amount of AC, that would both lower B+ and get them pretty hot.

Do you have the ability to measure AC at the quad-cap (AC-over DC)? A good multi-meter has that capacity, and it can be quite revealing. Substantial AC = bad rectifier.
Do you have the ability to measure actual current draw? It is my direct experience that a tube amplifier will run at about 80% - 85% of nameplate in the quiescent state, inputs shorted. And if you are running at/over nameplate, that excess is showing up as heat, somewhere.

Please let us know. Ultimately, you will have to throttle back to less-than-120 VAC to the amp. Either via a true-sine-wave power-conditioner (ideal) or a variable autotransformer (less than ideal).

Thanks for digging in.

I'm hesitant to say it's a bad rectifier only because both rectifiers (tube and SS) are giving me basically the same reading, and I don't think that both are fried.

When I get home, I first want to make sure the B+ driver board connection is actually correct and check for any failed joints. I'll also check true wattage. I've been a little hesitant to run the amp on its side to get readings while running, but I guess I have to now. Nothing seems to be running excessively hot (that I can tell).

vizsla23 wrote:Thanks for digging in.  

I'm hesitant to say it's a bad rectifier only because both rectifiers (tube and SS) are giving me basically the same reading, and I don't think that both are fried.

When I get home, I first want to make sure the B+ driver board connection is actually correct and check for any failed joints. I'll also check true wattage.  I've been a little hesitant to run the amp on its side to get readings while running, but I guess I have to now.  Nothing seems to be running excessively hot (that I can tell).

I can attest to the fact that a bad joint on a capacitor can lead to low B+ voltage.

I'm guessing it would most likely be at the cap can if I'm getting even stereo signal from my speakers. I need to look at the wiring diagram again, but I think most of the remaining caps are separated by channel, right?

vizsla23 wrote:I'm guessing it would most likely be at the cap can if I'm getting even stereo signal from my speakers.  I need to look at the wiring diagram again, but I think most of the remaining caps are separated by channel, right?

I'm not a jeenyous by any means.
I just know I had a low B+ on one side before the sound test, and found a joint with some corrosion on a lead, cleaned it up, and B+ behaved.

I'm curious if my crappy reflows are to blame. My new connections for the attenuator were solid but any attempts to fix old joints turned out to be pretty ugly. Not really worse but definitely not better.

vizsla23 wrote:I'm curious if my crappy reflows are to blame.  My new connections for the attenuator were solid but any attempts to fix old joints turned out to be pretty ugly.  Not really worse but definitely not better.

FWIW, I'll take my solder sucker, and or some braid, and remove the old solder.
I'm a big fan of liquid flux. Personally I find using it flows joints much better than what's provided in the core of most solder.
Sometimes after removing the old solder, I might take an exacto to the exposed leads, and give them a bit of a scrape before reflowing.
If you don't feel good about a joint, that's enough to do something about it it until you do.