St-70, Left Channel, 60 Hz Hum - solved !

I've gone through many of the threads here and not sure what next to find the problem.

The amp was modded a couple of times before I got it.
I've un modded the head phone connections.
Issues with it included Lap Joint soldered that came undone.

Had the SDS board already installed. I found leaky caps,
removed board, installed all new caps.

Added back a good on/off switch.

It already had a 3 prong grounded power cord installed.

TEST: Clipping a two prong power cord to the two power conductors
of the three prong didn't make a difference.

Test: Swapping preamp tubes left to right didn't make a difference.

Test: Swapping power tubes left to right didn't make a difference.

Test: using different EL34s didn't make a difference.

Test: Using shorting plugs Increased 60 HZ hum and placed it in both channels.

Test: Pulling the left channel pre amp tube stopped the hum.

Therefore issue with the left channel presumably the old board.

Test:  Installed another used board, yellow glass type board. didn't make a difference.
Test: repeated the same tests all over again, didn't make a difference.

60Hz hum still in the left channel.

Testing voltages and measuring components from side to side, etc
everything appears to be in spec.  It holds bias, voltage is stable etc.

Used shielded cable on left channel, grounded to input ground didn't make a difference.

I put the amp on the bench and tried to find the hum with my scope couldn't find the hum.

I hooked up speakers to it on the bench didn't hear the hum,

Those speakers are not efficient.
Speaker in the main listening room are Klipsch Cornwalls, very efficient,
I hear the hum in left channel no short plugs or both channel with short plugs.

Cannot listen to it with a pre amp as too much hum in both channels.

1.  My next test will be to run a long extension cord from a different in house
power node to see if its the outlet/node is a possible issue with.


2.  Someone here, somewhere described a mathematical model for calculating
current vs voltage and the difference would point to the faulty node with in the amp.

Test 1. and see if it make a difference and report back in a bit.  It's quick and easy. 2 nodes no difference.


Test 2.  I'll need to think about, pulling wiring for measurement isn't really appealing.
 a. Trying the continuity test for ground points doesn't work.
 b. I guess then measuring their resistance wouldn't make a difference either?
 c. If b, it would have the same current also, yes?

Test 2. Looking at wiring around the
left sockets...board 5,6: pin 1,2; pin 7,2; pin 7,2:
right sockets...board 15,16; pin 1,2; pin 7,2; pin 7,2:
on either side will yield same results.

I'll re measure them and check resistance reading of output xfrmrs,
Left channel Board 12 to black output.  .121 ohm
right channel board 13 to black output.  .114 ohm

Any other way to hunt it down?

I don't feel much of a Dynaco guy....

.

Test 1:  Use different supply nodes in the home: NO DIFFERENCE


It is the amp, my system is hum free otherwise.

It's a low 60 Hz hum in left channel with inputs open.
I'll presume the second board is not the problem either.

What starts in the left channel doesn't stay in that channel
with an input or with inputs shorted, it is amplified heavily.

The ST-70 is now only connected to the speakers.

To the bench it is, pull the tubes and think.

I've noticed when it starts up there is crackling sound in left channel.
and left channel crackling upon power down.

Thinking I could desolder the Output transformer connections
and alligator clip them to opposite sides, if it moves then its transformer.

I'll Update again.

Measuring around the SDS board:

6.5 k Ohm
20.505 k Ohm
10 k Ohm
10 k Ohm

Measure in spec no damage appearing.
60 Hz is before rectifier tube.
Leaky Mains transformer?
CT ground?

Getting Late, continued....

Here's a pic in the mean time.

I uploaded a disappearing pic.
Tried to post it in here.

https://servimg.com/view/20284507/2

https://servimg.com/view/20284507/2

What a nightmare !
A few observations : Shorting inputs should never increase any hum. This seems to indicate a
groundloop somewhere

Removing the 7199 tubes : if hum remains will localize the area to the EL34 / bias / B+
( unbalanced powertubes might cause hum )

How/where is the SDS board connected to ground ? That could be your source, and a source that
will disappear if you rebuild with a cancap ( restoring the original config)

As for diagnosing, a scope should be able to find and measure the hum, just turn sensitivity up.
Having a value of the hum will make it much easier then to resort to listen with the amp
moved from the workbench.

Bob Latino wrote:Post a photo of the inside wiring of your amp. Someone may be able to pick up on something in the wiring that could cause your left channel hum. Check the stickys on how to post a photo on this forum.

Bob

Bob

I thought I did, in post 4.

I'll read the sticky.

Thx

Pic is posted over in the Photo area, same name as this thread.

I posted an updated pic with callouts over in the Photo area.

I'll replied there also.

I measured the Heater windings on each side of amp.
Started with each half from the center tap (CT) to pin2 then pin7.
from the close sockets (rear),
far sockets (front),
then to the board eyelets 5,4 on the hum channel,
then the other side ending with eyelets 15,16.

The hum channel (left) Heater Green, in OHMS

Close Socket (rear), Ohms
CT to p7 .029
CT to p2 .025
P2 to p7 .052

Far Socket (front) Ohms
CT to p7 .032
CT to p2 .036
P2 to p7 .075

Board eyelets, Ohms
CT to 5 .045
CT to 4 .042
eyelets 5-4 .086

The non-hum channel (right) Brn Heater, OHMs
close socket (rear)
CT to p2 .024
CT to p7 .026
P2 to p7 .061

Far socket (front), Ohms
CT to p2 .029
CT to p7 .029
P2 to p7 .068

Board eyelets, Ohms
CT to 15 .039
CT to 16 .042
Eyelets15-16 .088

UPDATE

Amp only, no shorting plug.
Left Channel
Left
0.013 Vac Hum

Right
0.007 Vac Hum

Hum in both channels with shorting plugs.
Left
0.058Vac Hum

Right
0.056 Vac Hum

Increased hum with shorting plugs and finger touching the shorting plug
Left
0.250 Vac Hum

Right
0.283 Vac Hum

Hi Bob,

Thanks for stopping by.  Nice pic of your ST-70, easy to figure out where
everything is going.

Could never get any information other then at some point someone made it a
headphone amp, or with head phone jack removed, a stereo amp through that
switching jack.  

I was also trying to figure out why every socket has it's own terminal strip.

Also, I couldn't tell if it was soot marks or if it was smoke/bar residue?
I tried swabbing the chassis with iso, hard to get in some areas without ripping everything out.

I think the wires packed in so tightly is one of the main ways to add the SDS cap board.
It sure doesn't make it easy....

I'm guessing this has had numerous issues, even from the customizeres...
lap solder joins that failed.
a 470K resistor on input never soldered the ground connection.
Or maybe they didn't do that on purpose as that was the hum channel also.

Maybe something happened where the cap board was needed, but that
never solved the underlying problem... It is what it is.

Measurements of Output Primary windings
taken with HP34401A, with tubes removed, unplugged:

Left
Blue pair = 40.020 Ohms (Blue & Green)
Wht-stripe = 48.560 Ohms

Right
blue pair = .114,63 KOhms or 114.63 Ohms (Blue & Green)
wht-stripe = 48.329 ohms


40 ohms on the bad, hum, left side,
114 Ohms on the presumably good right side.

Any one have ST-70s outputs they don't mind measuring
and posting the result so I can verify?

Or maybe Blue & White-Blue
then Green & White-green

Colors are so faded blue and green almost look the same.

Advice > I would take that amp apart and just reuse the 3 transformers, the chassis, the driver board and maybe the SDS cap board and completely rebuild the amp especially if you intend to use the amp as a "daily driver" in your music system. The Dynaco ST-70 came out in 1959 and some of these amps are now over 60 years old. Amp parts don't last forever .

Let me emphatically second Bob's advice!

While I specialize in different amps than Bob does, they have one thing in common - the amps (unless worked on since) were built 50 to 60-ish years ago. That is well beyond the lifespan of almost every component inside the amp. If you intend to use the amp - not just have it as a museum piece - then a total rebuild is an investment I strongly recommend you make. Properly redone these amps can give trouble free service for many more years to come. Just fixing what broke pretty much assures you'll have plenty of opportunities to practice your repair skills again in the future!

Jim McShane wrote:Advice > I would take that amp apart and just reuse the 3 transformers, the chassis, the driver board and maybe the SDS cap board and completely rebuild the amp especially if you intend to use the amp as a "daily driver" in your music system. The Dynaco ST-70 came out in 1959 and some of these amps are now over 60 years old. Amp parts don't last forever .

Let me emphatically second Bob's advice!

While I specialize in different amps than Bob does, they have one thing in common - the amps (unless worked on since) were built 50 to 60-ish years ago. That is well beyond the lifespan of almost every component inside the amp. If you intend to use the amp - not just have it as a museum piece - then a total rebuild is an investment I strongly recommend you make. Properly redone these amps can give trouble free service for many more years to come. Just fixing what broke pretty much assures you'll have plenty of opportunities to practice your repair skills again in the future!

That sounds nice, but before doing any of that, I should solve the hum problem first.
I don't want to do all that and have the same issue.

DynacoGuy wrote:
That sounds nice, but before doing any of that, I should solve the hum problem first.
I don't want to do all that and have the same issue.

There are a few reasons for stripping it down and rebuilding.

You learn the amp circuit. What’s original and what’s not. I gained a lot of confidence in myself by doing this.

You can check each part and replace as needed. Many parts can’t be properly measured until they’re out of the circuit.

You can ensure every solder joint is new and solid. This eliminates a lot of mystery problems.

Looking at your picture the first thing that caught my eye was the position of the SDS Cap Board...it's right on top of the AC mains fuse. Not sure if it matters that the SDS board is on the left channel side of your amp, but I have to wonder about that too.

I'm unfamiliar with the SDS Cap board (the component/copper trace layout), and I wonder if one of the left channel DC supplies on the SDS board has picked AC radiation from the 120vac Fuse wires. This would be a "layout" problem, and the stock Dynaco ST-70 is a textbook example of proper wire routing and component layout.

Quick test: Remove the wires from the fuse holder and move the mains supply away from the SDS board.

If that solves your problem, I'd remove the SDS board and use a Multi Section Can Cap. If it doesn't solve your problem, I'd take a closer look at the SDS cap board for ground issues.

Morgan Jones wrote a book "Building Valve Amplifiers" that discusses proper orientation and layout. I read that book when I rebuilt my ST70. It was like taking a College Level Course with Lab. The stock ST70 is an example of proper orientation and layout. Chapter by chapter, the book describes every aspect of that amp.

Things I learned:
If AC wires have to cross DC wires they should do so at 90 degrees to minimize AC induction into DC supplies. Never run AC and DC lines (DC bias supply) parallel to each other. The AC fuse on top of the SDS board bothers me. I don't know if it's interacting with DC on the board or perturbing the SDS board's ground plane. (Personally, I'd remove the SDS board entirely and simply use a multi section can cap).
The ST70 also uses a "Star Ground" scheme with "smaller local (island) star grounds". Make sure the ground layouts and connections are proper. I'm not sure about the SDS ground scheme or the capacitor schematic and layout.

Good luck.

Problem Solved.

Hum eliminated!

Missing chassis ground on the common output terminals.

Added a grounding wire from each common output terminal
to the central ground buss. The buss uses the same ground
as the original amp.

Jim McShane used a similar setup in his Citation II grounding scheme
I believe.

Issue wasn't the SDS board--as many have been installed. If issue,
there would have been many posts in many forums about it.

Thanks everyone for your suggestions.

DG