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Dynaco ST 70 noise in one channel

Crystalboy

Crystalboy

Member
I required a pair of Dynaco ST 70 as well as a PAS 3 some months ago.This is my first experience with tube gear. I wanted them checked up and didn't try them out before handing them over to a tech in my area. Got them back a couple of days ago and my first impression was nice, I really like the tube sound. I've been doing some closer inspections in the days that followed, and I can definitely hear a low level hum if I press my ear close to the woofer, this without the PAS connected. This I can live with and is coming from both channels. But I also hear some low level static noise coming from tweeter and mid range at the right channel. It basically sounds as a FM channel not tuned in properly. Non of these sounds are volume dependent and is always the same, with or without the PAS connected. I usually work on my own SS stuff (recap and whatnot), I'm a newbie but learn from a lot of you folks here and other forums. I really didn't want to delve in to working on tube stuff myself, but now I really don't now which way to turn. Return it to the tech that didn't notice this, hand it over to someone else or are there any checkups that I can do? I do not own a oscilloscope but now my way around with a DMM. The tech mentioned to me that he set the bias slightly lower than 1.56, I checked it and it was around 1.45 on both channels.

As far as I know he did this on the ST 70:
-Replaced power cord
-Replaced PSU capacitors
-Replaced the can capacitor
-Replaced all EL34 tubes
 
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You might also have a weak tube situation. Try swapping the 7199 tubes right to left and see if the noise migrates. Same with the EL34 tubes.
 
These are often very old units, to expect like new operation is wishful thinking, though getting it is achievable. To really guarantee an old ST70 to operate quietly with an ear near the speaker, especially high sensitivity ones, you are going to need to ensure the wiring is run exactly as the assembly book shows, replace every carbon comp resister, replace the old circuit board, and install new tube sockets. Ask me how I know. Any one of these things could also cause noise in a single channel as well. A cap refresh on these is typically required to ensure safe operation and adequate power supply filtering. For some owners and units that is sufficient for their listening pleasure. I measured a 5 dB reduction in hiss at the front of the speaker by replacing every old carbon comp resister. Someone with low sensitivity speakers might not notice it at all. My unit had random sizzling, fizzing and pops, only eliminated by replacing the output tube sockets. Cleaning and tightening them was not sufficient to correct it long term.

When people say a modern kit is the real value equation, I pretty much agree now. If you already own one or can get a real good deal, and can do the restoration yourself an older unit can be cost effective too. I probably broke even on mine.

Bill
 
Schmidlapper said:
These are often very old units, to expect like new operation is wishful thinking, though getting it is achievable. To really guarantee an old ST70 to operate quietly with an ear near the speaker, especially high sensitivity ones, you are going to need to ensure the wiring is run exactly as the assembly book shows, replace every carbon comp resister, replace the old circuit board, and install new tube sockets. Ask me how I know. Any one of these things could also cause noise in a single channel as well. A cap refresh on these is typically required to ensure safe operation and adequate power supply filtering. For some owners and units that is sufficient for their listening pleasure. I measured a 5 dB reduction in hiss at the front of the speaker by replacing every old carbon comp resister. Someone with low sensitivity speakers might not notice it at all. My unit had random sizzling, fizzing and pops, only eliminated by replacing the output tube sockets. Cleaning and tightening them was not sufficient to correct it long term.

When people say a modern kit is the real value equation, I pretty much agree now. If you already own one or can get a real good deal, and can do the restoration yourself an older unit can be cost effective too. I probably broke even on mine.

Bill
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There are high quality replacement populated boards available that deal with these problems, and the 7199 issues.
 
Yes, and there remains a few point to point wired items that can also be updated. It boils down to what level of restoration toward reliability and performance suits an informed person. If a tech says it just needs recapped, the average owner is going to say, okay, and it will operate as a unit with sixty year old carbon comp resisters and corroded tube sockets does. I want better.
 
This is a before and after RTA measurement taken about a inch in front of the left 101dB sensitive speaker, amp has no input signal and volume is set at normal listening level, ~ 78dB at listening position about 8’ further back. The green dashes are the pre work measurement and the red bars are post work. The amp at time of pre measurement had Solid state rectification, a new DynaKit CCA with metal film resisters, new bias diode, bias pots, bias caps, metal film PS filter resisters, PS can cap (which the first cap was failing already), power switch, fuse holder, chord and tubes. Post work measurements reflect only the following additional changes. New 550V working /600V surge can cap (120hz spike), new metal film resisters in the bias circuit and all output tube locations, new output tube sockets and several wire dress adjustments to match assembly diagrams. I misspoke previously, there were many - 20dB improvements in the noise floor made in just those last upgrades. That is why I mention them.
D2124B84-059E-4395-8A95-BA2AF56AC048.jpeg
 
In my experience with ST70s and MKIIIs a Can capacitor(s) that’s starting to fail will produce a low level hum and static….just went back an replaced a Can on one of my MKIIIs for that very reason. It checked good during the build but noticed the hum not long after it was in play.

And yes I’ve gotten a bad “new” Can…I’d check the voltage(s)

VR
Andy
 
Thanks for the suggestions everyone! I've tried re-seating the tubes, switching positions of the driver tubes (7199) and fault was still present on the right side. The EL34's are all new and matched, according to him.The noise is similar to a problem I had with a Quad 303, where the carbon resistors had not drifted more than 5% but had become noisy on one channel. Replaced all of them with metal film and the problem was solved. I also took a look inside the ST 70, he had only replaced the PSU capacitors and not the can.. I'm going to order a can and replace it myself. I'm not really in the mood to rip out all the carbon resistors seeing how fragile these boards seem to be. I'm thinking of going for one of these boards and be done with it. Anyone here who has any experience with them?
 
6DZ7 said:
So you are using each amp in mono mode with the outputs bridged going to one speaker for each amp, correct?
Click to expand...

Thanks for correcting me lol, that's what happens when you write in a hurry before you're going out. It's only one ST 70, the whole sentence was poorly worded on my part.
 
Crystalboy said:
Thanks for the suggestions everyone! I've tried re-seating the tubes, switching positions of the driver tubes (7199) and fault was still present on the right side. The EL34's are all new and matched, according to him.The noise is similar to a problem I had with a Quad 303, where the carbon resistors had not drifted more than 5% but had become noisy on one channel. Replaced all of them with metal film and the problem was solved. I also took a look inside the ST 70, he had only replaced the PSU capacitors and not the can.. I'm going to order a can and replace it myself. I'm not really in the mood to rip out all the carbon resistors seeing how fragile these boards seem to be. I'm thinking of going for one of these boards and be done with it. Anyone here who has any experience with them?
Click to expand...

PSU capacitors and the can should be ubiquitous, the Can is the PSU capacitors. If your tech says they replaced the PSU caps, they either replaced the can, re stuffed the can with new caps or bypassed the can caps with external caps. There should be no reason to replace it again without evidence it is failing. Are you confusing the bias caps with the PSU caps? They too are usually replaced during a recap.

The board you link to looks like a legit product. I have no experience with that seller myself. One thing to watch out for if you do replace the circuit board,, the original solid wires can be difficult to get a good solder joints with. I had several that looked good, but I found later were actually loose in the solder joint. To be honest I did not re cut and re strip the ends after desoldering them, so that may explain it, They may have been corroded enough that the solder flux couldn't completely clean them off. Some of the wires can look a little short but re stripping the ends would likely give the best result. Just be aware of it.
 
I used the Vintage Electron 6CG8 board to ditch the 7199 availability problem, I liked the parts choice and drop-in quality vs price. I also used the larger capacity quad can replacement because it made sense to do so.
 
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Probably just did the bias supply caps underneath. Thats all thats external to the can.

If you use the larger can, just make sure you don't connect the 80uf section directly off the rectifier. The new one should be 80/40/?/?, use the 40uf section as the first cap.

if moving tubes around doesn't do anything, agree that its quite possibly a noisy CC resistor. Those old boards aren't the most fun to rebuild but it is possible. As cheap as new ones are though its sort of hard to justify.
 
Best of luck, there are a multitude of posts on here and the Internet covering the various specifics. I usually just like to emphasize the last mile stuff I have learned. In a vintage amp there is no way to predict what enough might be.
 
BuzzK said:
I had a faulty silver mica cap make all kinds of static. Never suspected it.
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I've only run across two of those ime so far, one in a BC-348 receiver AGC, another in an OS-8 oscilloscope.
One of many reasons I chose a new replacement populated board to rebuild a Dyna Stereo 70.
 
After starting this thread I noticed that the amp after warming up made some crackling pop sounds coming from the speakers from time to time. I decided to start off with replacing the quad capacitor, which I did earlier today. The hum is now gone and have reset the bias. The noise in one channel is still there but slightly lower, had no real hopes that that issue would’ve been cured with a fresh can. Next step is to replace the driver board, I’ll give you all an update when that is done. Thanks for all the helpful input!

1FB2F397-FE6D-4760-84BC-B47494B4AFF4.jpeg
 
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you may want to seriously consider replacing the black caps. Chances are they are leaking and allowing DC to flow through. Whilst you're at it, check the resistor values as well.
Did you clean and re tighten the power tube sockets as well? Also check all solder joints, anything that looks even a little dry, re touch it.
 
Hello again! I wanted to give a small update. Have replaced all capacitors now as well as all CC resistors. I decided to use the old instead of a replacement board. It sounds great now without any hiss. It was a noisy resistor causing the noise in one channel. What surprised me though is that most resistors was well within specs. But some of them had drifted upwards of 30%. The worst offender was one of the dropping resistors on the power tubes which measured over 2k, though it should be 1k. I’m probably gonna return with questions regarding the upcoming refurb of my PAS 3.

5AF2E329-B7D6-4306-A07B-046DA9C078B7.jpeg
 
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