Dynaco ST-70 Voltage and distortion!

'goodtimes'

Member
Hello,
My St-70 has a left channel problem. The left channel is not as loud as the right and also is distorted. I have changed tubes (one at a time-rectifier, output, and driver) with no change whatsoever. I have tried known good used tubes and NOS NIB known good tubes with no change. I have checked resistors and caps and all seem to be in spec. Another odd thing is it will bias and hold the setting on both channels.

Upon checking voltages I get the following;

GZ34-
Pin 2 to ground - 454 VDC

Pin 8 to ground - 454 VDC

Pin 4 to ground - 374 VAC

Pin 6 to ground - 375 VAC

All EL 34's are the same give or take 1V
Pin 1 to ground - 1.5

Across pins 2 and 7 - 6.0 VAC

Pin 3 to ground - 437 VDC

Pin 4 to ground - 438 VDC

Pin 5 to ground - -24 VDC
Pin 6 to ground - -24 VDC
Pin 8 to ground - 1.5 VDC

Quad Cap
Section #1 - 454 VDC Half circle symbol back side of amp - one choke lead and wire to pin 8 of the rectifier tube

Section #2 - 442 VDC square symbol left side of amp - another choke lead and the two wires from the two output transformers

Section #3 - 373 VDC full circle symbol on right side of amp

Section #4 - 287 VDC triangle symbol toward front side of amp

I have installed the new "upgraded" quad cap where
half circle = 80uf
square = 40uf
triangle = 30uf
full circle = 20uf

Any help or suggestion would be greatly appreciated.
 
It suggests your AC mains is running above 117VAC, because the input and output of the rectifier tube are a bit high, but not so high as to be a problem. Consequently, some of the other readings are a bit higher than the reference values. But, curiously, the filaments are reading a bit low, which does not seem consistent with a higher AC mains.

The EL34 grids are a bit off, per the reference values.

First and second sections of the quad cap are reading higher, as expected. Curiously, sections three and four are a bit low.

With regard to your 80/40/30/20 quad cap, if I understand what you wrote, it looks like you have the 80µF as the first section, immediately after the rectifier tube, is that correct? If so, that's too much immediately after the 5AR4. For this kind of quad cap, the sequence should be 40/80/30/20 or 40/80/20/30.

But, all the above may have nothing to do with your left channel distortion and low output. For this, I am temped to suggest some kind of change or substitution in whatever is feeding the Stereo 70. Maybe the problem is elsewhere in the system.

What does the rest of the system consist of and under what circumstances are you having the low, distorted left channel output? Phono? CD?

What is the configuration of the Stereo 70? Is it all original Dynaco circuit topology or have there been any changes to the circuit?

So, the amp is stock aside from the quad cap. I am experiencing the distortion and lower output no matter what the input or volume level. I have used two different preamps (one tube and one solid state) and simply using my iphone as a third option input with no change. I have another ST-70 that I run in the same setup with no problems. I am going to put in all new tubes (I already have them) and retest in just a moment. Thanks! and I will report back shortly.

So I put in a NEW quad of el34's and retested. It made a change of a volt here and there but not anything a change in line voltage could not be responsible for. For the -24 readings I am getting -29. I also put in a brand new 5ar4 and it also made little to no change. I suspected this would be the case as the tubes that were in there were tested as nos when I put them in and they have maybe 20 hours on them.I am not sure where to go from here.
 
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my input (used to have about 6 of these at various times).

If it were me, I'd suspect every part on the left channel. since the right channel is OK, I'd leave any
poking around the PS to absolute last.

If the board is original, then plan to replace it, there are dozens available, because these are now
50 YO and won't survive major surgery.

I would replace the coupling caps, maybe leave the non-electrolytics in, and if the problem
still exists, then replace all the resistors.

particularly if these are originals, personally, I don't trust the power/heat/stress cycling of 50+
years not to affect the operation.

if you're an expert then buy board and parts and solder/wire it up and replace. otherwise
buy a populated board but do verify the components, visually, and by what you can measure
in circuit.

good luck
 
I have the same quad cap in my dynaco and the order is 30uf, 80uf, 40uf, 20uf. At the same time I replaced the bias capacitors, did you replace these electrolytics? If these were the same age as the original can cap...they have to go, at the same time I would replace the selenium rectifier with a silicon diode.
 
Time to examine output transformer :)

Not that difficult as you have one good and one suspect.
We will apply AC on the secondary and observe what voltages appears in the primary ( yes we intend to
run the transformer backwards). Happily we have 6.3V AC on hand from the filements, they should be
floating from ground if this is a genuine dynaco ST-70. On the primary we can measure the voltages
and since the tubes are removed there is nothing connected to the transformer primary.

-Remove EL34 tubes and GZ34
-connect 6.3V AC from filament winding to the 0 and 16ohm outlet Power up ( or use 6V AC from external source)
-measure AC from centertap of transformer ( B+ after the choke) and pin 3 on each EL34 ( plate ) You should have
some 100 volt here. Also pin 4 on the EL34 , all referenced to B+
- repeat with the other channel
Any significant differences indicates problem with the transformer.
 
At the moment, I am stumped - sorry. Perhaps someone else can offer some ideas...

Well, if I were facing something like this, I might start thinking about some parts substitutions, as in change-out the entire PC board and see if it helps. Then, if necessary, get a spare OPT and substitute it, that sort of thing. Trace through everything, the entire circuit, in the left channel from RCA input input to speaker output terminal strip and see what you see. Maybe there's a compromised solder joint or something.

Have you had this amp for long? IOW, was it working okay then the problem just started suddenly? Or, did the problem follow some kind of event such as the change-out of the quad cap? Some context might be helpful.

I have not had the amp long. It sat for some time. I bought it had the selenium changed for diode and brought up on a variac. At that point it checked out. I changed the quad cap once I noticed the problem to no avail.
 
Need better info how much lower output and details of noticed distortion you have eliminated the tube and your dc voltages are close so you might have a open coupling cap but without a scope it’s hard to say
The volume level is not drastically lower but low enough to hear without doubt. The distortion is similar to the sound of a dirty pot being cycled but consistent. I suppose like a signal trying to get through but struggling to do so. I have an esr meter that (with power off and unplugged) I have clamped on to each of the caps on the driver board and also on the bias caps below. They all seam to be reading within spec. Can I upload an audio track here? I could make a quick recording of each side that would show the sound of the distortion.
 
my input (used to have about 6 of these at various times).

If it were me, I'd suspect every part on the left channel. since the right channel is OK, I'd leave any
poking around the PS to absolute last.

If the board is original, then plan to replace it, there are dozens available, because these are now
50 YO and won't survive major surgery.

I would replace the coupling caps, maybe leave the non-electrolytics in, and if the problem
still exists, then replace all the resistors.

particularly if these are originals, personally, I don't trust the power/heat/stress cycling of 50+
years not to affect the operation.

if you're an expert then buy board and parts and solder/wire it up and replace. otherwise
buy a populated board but do verify the components, visually, and by what you can measure
in circuit.

good luck

I have been going blind looking through this thing! I reflowed any solder joint that looked suspect. The board is original. Caps test good but that is in circuit, not sure if that will be accurate. Every resistor I have tested has been in spec as well. I do not know the resistor values on the board but I am going to test and compare left vs. right once I finish replying to all you fine folks here! Thanks for taking the time as it is greatly appreciated.I ordered a new board from Dynakit parts. I will order all the parts and buid it up. I'm going to keep them in Stock form with 7199's. Mundorf supreme and Takman all around ;).
 
Time to examine output transformer :)

Not that difficult as you have one good and one suspect.
We will apply AC on the secondary and observe what voltages appears in the primary ( yes we intend to
run the transformer backwards). Happily we have 6.3V AC on hand from the filements, they should be
floating from ground if this is a genuine dynaco ST-70. On the primary we can measure the voltages
and since the tubes are removed there is nothing connected to the transformer primary.

-Remove EL34 tubes and GZ34
-connect 6.3V AC from filament winding to the 0 and 16ohm outlet Power up ( or use 6V AC from external source)
-measure AC from centertap of transformer ( B+ after the choke) and pin 3 on each EL34 ( plate ) You should have
some 100 volt here. Also pin 4 on the EL34 , all referenced to B+
- repeat with the other channel
Any significant differences indicates problem with the transformer.

So, I did a power transformer test according to the Curcio restoration guide and it checked out to be in spec as well?! I did not do the same procedure on the Good Working ST-70 as I was listening to it while I was working :)
 
replacing the can cap is a good move to extend life - but if the right channel is Ok then it's
not in the PS.

one thing to check is swap the input left and right channels. if the problem moves
then the input is not balanced left-to-right and is before the amp. this happened to
me once, the preamp had a balance problem and I blamed the amp which was
coincidentally a ST70.

I suspect a cap. I had similar random hiss/noise through one channel of a different
preamp and it turned out to be a cap. removed caps were tested for value and ESR
and were within acceptable margins.

since you are building a new driver board, you can focus your time, attention, and energy
on the replacement.
 
So I tested resistance on output transformers and they are pretty identical on every measurement. I also checked all resistors in circuit left against right and found no discrepancy. All caps test in spec in circuit. I will clean and tension sockets now.
 
resistors and caps that test OK may not be. I still suspect cap(s) that test OK
but don't function well in audio especially after 50+ years.

I can send you a set, free, for substitution to isolate the problem component.

if all tests well, and everything is soldered/re-flowed, and the voltages are Ok,
then there's not much else you can do. it would be a repeat of earlier actions
and if nothing measured changes to account for the problem then it will
continue to exist.

you could start swapping components from right channel into the left -
like tubes to id where/when the issues change.

PM me with address if you are interested in parts - nothing boutique but
different enough to reveal.
 
I cleaned and tensioned all the contacts and its no better...Boo hoo hoo. It is a good thing anyhow. Last ditch before new board and components is a reflow of remaining solder contacts.

Thanks everyone! New board and such should be here this week.
 
resistors and caps that test OK may not be. I still suspect cap(s) that test OK
but don't function well in audio especially after 50+ years.

I can send you a set, free, for substitution to isolate the problem component.

if all tests well, and everything is soldered/re-flowed, and the voltages are Ok,
then there's not much else you can do. it would be a repeat of earlier actions
and if nothing measured changes to account for the problem then it will
continue to exist.

you could start swapping components from right channel into the left -
like tubes to id where/when the issues change.

PM me with address if you are interested in parts - nothing boutique but
different enough to reveal.
Bob, Thanks

I'll PM you
 
So I tested resistance on output transformers and they are pretty identical on every measurement. I also checked all resistors in circuit left against right and found no discrepancy. All caps test in spec in circuit. I will clean and tension sockets now.
Resistance testing does not tell anything about shorted turns.
Do the test i wrote about before messing up the amp totally ! And compare the two sides.
 
Resistance testing does not tell anything about shorted turns.
Do the test i wrote about before messing up the amp totally ! And compare the two sides.
My apologies,
Time to examine output transformer :)

Not that difficult as you have one good and one suspect.
We will apply AC on the secondary and observe what voltages appears in the primary ( yes we intend to
run the transformer backwards). Happily we have 6.3V AC on hand from the filements, they should be
floating from ground if this is a genuine dynaco ST-70. On the primary we can measure the voltages
and since the tubes are removed there is nothing connected to the transformer primary.

-Remove EL34 tubes and GZ34
-connect 6.3V AC from filament winding to the 0 and 16ohm outlet Power up ( or use 6V AC from external source)
-measure AC from centertap of transformer ( B+ after the choke) and pin 3 on each EL34 ( plate ) You should have
some 100 volt here. Also pin 4 on the EL34 , all referenced to B+
- repeat with the other channel
Any significant differences indicates problem with the transformer.

I am not clear on how to connect the 6 VAC to the taps? Plug in a cord to my variac (set to 6.3VAC) and connect the two bare wire ends to the 0 (C) and 16 Ohm (16) speaker terminals? Then with my meter across center tap of output transformer and pin 3 of each EL34 socket I should 100 VAC? Pin 4 should also read 100 VAC?
 
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