Dynaco Hum

[GainFactor99]

Dyanaco SCA-35: To paraphrase Frank Zappa, "Dynaco hum. Where is this Dyanco coming from?"

I'm trying to figure out the source of a very loud 60Hz hum (just about equal in both L&R channels) that only occurs when the selector is set to the phono or tape head inputs. The hum is absent when the selector is in all other positions. The phono signal is not distorted or degraded, but the hum is extremely intrusive. The hum is unchanged when the unit is connected to an ungrounded turntable. Perhaps grounding to the turntable would help, but I am unclear about how that could help given that the hum exists prior to connecting any inputs to the SCA-35.

So far, we (son and I) have replaced the power supply caps and the two 12ax7 tubes. We've tried cleaning up the selector contacts. The next step, unless someone has a better suggestion, is to replace all of the extremely oxidized RCA jacks and check their ground connections.

The heater wires are still neatly braided as are all signal wires coming in to the PC-11 preamp. This was a factory assembled unit, purchased used last month. I'm told that a previous owner by-passed the tone controls, but I have not yet had a chance to discover what mischief that potentially stirred up. The hum is still present when the unit is fully assembled (with cover on). Changing the direction of the non-polarized two prong power cord has little effect.

Any suggestions would be very welcome. I'm new to the forum and have benefitted from the collective knowledge shared here. Thanks.

 

[peterh]

Dyanaco SCA-35: To paraphrase Frank Zappa, "Dynaco hum. Where is this Dyanco coming from?"

I'm trying to figure out the source of a very loud 60Hz hum (just about equal in both L&R channels) that only occurs when the selector is set to the phono or tape head inputs. The hum is absent when the selector is in all other positions. The phono signal is not distorted or degraded, but the hum is extremely intrusive. The hum is unchanged when the unit is connected to an ungrounded turntable. Perhaps grounding to the turntable would help, but I am unclear about how that could help given that the hum exists prior to connecting any inputs to the SCA-35.

So far, we (son and I) have replaced the power supply caps and the two 12ax7 tubes. We've tried cleaning up the selector contacts. The next step, unless someone has a better suggestion, is to replace all of the extremely oxidized RCA jacks and check their ground connections.

The heater wires are still neatly braided as are all signal wires coming in to the PC-11 preamp. This was a factory assembled unit, purchased used last month. I'm told that a previous owner by-passed the tone controls, but I have not yet had a chance to discover what mischief that potentially stirred up. The hum is still present when the unit is fully assembled (with cover on). Changing the direction of the non-polarized two prong power cord has little effect.

Any suggestions would be very welcome. I'm new to the forum and have benefitted from the collective knowledge shared here. Thanks.

Could be a number of causes.
- check the hum dingers ( the 2 pots at the back) if they are correctly connected and if they alter the hum
when turned
- is hum present with shorted input plugs mounted ? Then the hum comes from the board, check groundings
are according to the manual
- what is the ripple from the power supply ? ( althought that should be 120hz if both rectifying diodes are ok)

 

[El Pee]

Have you applied rotation to the sugar plum?

 

[gadget73]

Is the turntable ground lead connected to the SCA-35 ? If not, do that. Most tables have a flying lead that needs to connect to the preamp or amp. Thorens and some others have the ground lead built in to one of the cables though.

it does not ground to the wall.

 

[Dswankey]

Nothing to add but this

The phrase "dynamo hum" appears in the plot summary from the booklet that came with the Uncle Meat album

Here is the quote itself:

"It's quiet except for a little light wind. We are traveling across the wasteland toward a huge hydro-electric dam. Dynamo hum increase as we near it."

As Frank would say "Conceptual Continuity", added bonus that's Tina Turner and the Ikette's sing background on the song

 

[THF2]

Knowing the Zappa reference that you mentioned in the 1st post, I simply cannot with any degree of seriousness respond to this post in a meaningful way without cracking up.

Sorry.

 

[GainFactor99]

Thanks for the suggestions peterh. I have adjusted the "humdingers" and they did modulate the hum a bit, but not to a useable level. I'll try your other suggestion of shorting the RCA jacks. I'll also try grounding the turntable as suggested by Gadget73. I'm still unclear about why that might solve the problem of the hum if it pre-exists any connection to the turntable, but perhaps it's similar to the hum I get if I forget to plug my guitar into my amp before turning it on. Is selecting the phono input on the Dyanaco akin to firing up a Fender Pro Reverb and tuning the volume up to 8 while the lead cord is not connected to a guitar?

And thanks to all for the Zappa references. These insights will be helpful when I return to the Utility Muffin Research Kitchen.

 

[gadget73]

its a whole lot of gain with a fairly high impedance input. Gain in a phono stage isn't too different from what a guitar pickup requires. Lots of phono stages hum when the input is open, which is why it was suggested to short the inputs and see if it stops.

 

[arts]

Have ya spent three hours yet?

 

[Dandy]

I would also suspect the previous owner's 'tone control bypass'. Have you visually inspected this area? Was it humming like this when it left his/her hands? Etc.

it sounds like a missing ground to me. As peterh says, check all the grounds according to the manual/schematic. There are lots of ground points at the switches, pots and PEC modules.

 

[GainFactor99]

Thanks everyone for the helpful tips. I'm sorry it has taken so long to reply. I wanted to try each suggestion before reporting back. Scheduling time to get over to my son's house where we're working on the project has been tricky.

I am happy to report that we kept at the Dynaco Hum, to paraphrase Frank Zappa one last time "Till dinah-moe finally did give in." The unit sounds great now when using the phono input.

I think we reduced the hum incrementally by cleaning contacts, replacing the 12AX7's, and checking grounds, BUT the real improvement came when we realized that we were misinterpreting the meaning of the Low and High phono inputs. We mistakenly assumed that the Audio Technica LP120 turntable would need the "Low" input. Our reasoning was that the turntable has both a line level option and a phono out, which we thought would be a low level signal. I think early on we may have tested both the low and high inputs, but that was before we got into the back and cleaned the corrosion, so it is possible that we did initially try the high level phono input. Before cleaning, it provided a very weak signal path.

After all our efforts we were discouraged to find that the hum still muddied the sound when we were using the phono Low input. But, when on a whim we finally tried the turntable using the "High" level input, we were rewarded with a beautiful, clear sound with ample volume. In fact, when enjoying a beer and listening to our accidental success, we never went beyond 10 o'clock on the volume control as we sampled good pressings of Bill Evans, Jimmy Smith, and Jimmy Hendrix. Very satisfying.

So, here are a few results of the various experiments:

*ground the turntable to the amp: Unfortunately, the Audio Technica LP120 doesn't seem to offer an option to do that. The manual suggests that everything on the turntable is isolated and that the ring of the integrated RCA connectors handles any necessary grounding.

*short the inputs: Shorting the inputs did not reduce the volume dependent hum in any of the phono inputs. But, one interesting finding was that the "Tape Head" input produced a haunting oscillation that sounded like a seal barking when the volume was at about 1. The interval between the "barks" shortened as the volume level was increased (this sound freaked out Ozzie, the family puppy). The sound became a constant howl at about 8 o'clock on the volume control. We never figured out what was causing this, but perhaps when we rebuild the preamp we'll find the culprit.

*check the grounds: We did this in the process of preparing to replace the input board with DynaKit's fancy gold plated replacement kit. Unfortunately, we were discouraged to find that the original input panel was secured with 16 (!) rivets. The prospect of drilling those out gave us pause and we eventually decided to clean everything but to leave it as found.

In the end, the hum that we found objectionable at the amplification level required to enjoy phono signals from the Low input receded to be virtually inaudible when listening at all reasonable volume levels using the High phono input.

Thanks for all the suggestions. This was a fun project and there's more to be done.

 

[Lavane]

I hate chasing hum problems. Good to hear you solved it though. I didn't know the LP-120 doesn't have a ground wire. I have the LP-1240 and it does have one. You could always add one I guess. I have a U-Turn Orbit Custom that doesn't have a ground wire. Adding one temporarily did help with one preamp I was using.

 

[gadget73]

*short the inputs: Shorting the inputs did not reduce the volume dependent hum in any of the phono inputs. But, one interesting finding was that the "Tape Head" input produced a haunting oscillation that sounded like a seal barking when the volume was at about 1. The interval between the "barks" shortened as the volume level was increased (this sound freaked out Ozzie, the family puppy). The sound became a constant howl at about 8 o'clock on the volume control. We never figured out what was causing this, but perhaps when we rebuild the preamp we'll find the culprit.

this sounds like poor decoupling between the stages, which is almost always bad power supply filter caps.

lousy caps also tend to make things hum. It will be 120hz though. Most other hum sources will be 60, so its a useful way of determining one from the other.

 

[dcgillespie]

Gain -- If the "Hi Level" phono inputs are displaying greater gain (requiring a lower volume control setting for a given volume level) than the "Lo Level" inputs do, then something is clearly wrong with the way the low level input jacks or the PECs associated with them are wired. The "Hi" and the "Lo" level designations refer to the output level of the moving magnet cartridge being used. The Hi Level phono inputs then are only for use with magnetic cartridges that have a notably greater output than normal -- usually on the order of 20 mV or more. To handle the greater output, the Hi Level input jacks are padded down so that high output cartridges cannot overload the input stage of the phono preamp.

But such high output cartridges make up only a very small portion of magnetic cartridges manufactured. The vast majority of moving magnet phono cartridges have a more typical output of no more than 10 mV and as such, should be plugged into the Lo Level phono input jacks, which have no padding and therefore display a higher gain than the Hi Level phono input jacks do. If your amplifier is not acting in this fashion, then the jacks, wiring, or PECs associated with them are compromised or miswired in some way. If it takes a much higher volume setting when using the Lo Level phono jacks to achieve the same sound level as as when using the Hi Level phono jacks, then something is clearly wrong in the low level jack area, and needs to be sussed out.

As for the squeal at elevated volume controls settings when set to the Tape Head setting, this is a quirk of the SCA-35 that I can virtually say: "they all do that" -- and they do. If Dynaco knew about it, they likely figured "who would run the volume control up that high when a tape head is supplying a signal anyway", and so disregarded it. The fix for the squeal is to install appropriate -- grounded -- shields over the phono preamp tubes, which will prevent the feedback that is taking place between the nearby selector switch wiring, and the internal plate structures of the 12AX7 tubes used on that board. With the shields installed and grounded, then the antisocial behavior otherwise displayed will disappear.

I hope this helps!

Dave

 

[GainFactor99]

Thanks Dave. I'm embarrassed to say that in my post I got it wrong. In trying to reconstruct what we did this weekend and provide an account here, I switched "Hi" and "Low." In fact, our delayed discovery was that we should have been using the "Lo" input and when we did, everything sounded great. Thanks for straightening that out.

That is interesting information about the Tape Head setting. I doubt it will ever be used, so we'll leave it as is.

Your input has been very helpful.