Okay, newbie here again with a basic question. I've been gradually restoring an ST-70. I've replaced the power supply, including a new filter can, all the caps under the chassis, and all the caps on the board. The board itself, the resistors and the tube sockets are original. All the values are the same or within an acceptable margin.
It's been running just fine, but the other day it started to oscillate a little, mostly in the right channel. I thought the bias might have something to do with it, and under biasing did make the noise go away. Obviously not a good long-term solution. Why would I be getting this if all the caps are the same values as the originals?
Here's the emabarassing question: could someone point me toward the right component to check on? I keep seeing that oscillation is caused by the cathode bypass caps, but I can't figure out which ones they are. Here's the schematic URL: http://www.drtube.com/schematics/dynaco/st70.gif
It's been running just fine, but the other day it started to oscillate a little, mostly in the right channel. I thought the bias might have something to do with it, and under biasing did make the noise go away. Obviously not a good long-term solution. Why would I be getting this if all the caps are the same values as the originals?
Here's the emabarassing question: could someone point me toward the right component to check on? I keep seeing that oscillation is caused by the cathode bypass caps, but I can't figure out which ones they are. Here's the schematic URL: http://www.drtube.com/schematics/dynaco/st70.gif
Check the negative feed-back path.
Check the 1k resistors (eyelets 13 and 12), the 47R resistors (eyelet 9), the 390 mmF capacitors (eyelets 14 and 11).
Check connections to OPTs (yellow and gray wires to eyelets and black wire to ground ).
Check the screening and ground connections, posibile positive feed-back.
Check the 1k resistors (eyelets 13 and 12), the 47R resistors (eyelet 9), the 390 mmF capacitors (eyelets 14 and 11).
Check connections to OPTs (yellow and gray wires to eyelets and black wire to ground ).
Check the screening and ground connections, posibile positive feed-back.
I'm somewhat confused by your terms. Is this motorboating (thumping at a slow rate, once or twice a second) or oscillating (squeals, noise, and harshness)? The former is almost always a function of a power supply issue, happens in both channels, and is usually pretty easy to cure. The latter will take some creativity and an oscilloscope
There aren't any cathode bypass caps in this circuit.
There aren't any cathode bypass caps in this circuit.
I'm Confused Too
Hi Sy,
Sorry, I've been reading about this phenomenon and I thought the two terms were interchangeable. It's motorboating then, not oscillating. Low, rythmic thumping.
No cathode bypass? No wonder I couldn't find them!
I just replaced the 390 mmf caps with some mica caps, but I'll check the rest of the rest of those suggestions. Maybe I miswired the OPT.
Thanks guys.
Hi Sy,
Sorry, I've been reading about this phenomenon and I thought the two terms were interchangeable. It's motorboating then, not oscillating. Low, rythmic thumping.
No cathode bypass? No wonder I couldn't find them!
I just replaced the 390 mmf caps with some mica caps, but I'll check the rest of the rest of those suggestions. Maybe I miswired the OPT.
Thanks guys.
Got a text book here. It says motorboating is an oscillation from 1 to 15 Hz.
It can be caused by an open grid circuit, or defective filter caps.
When you said you replaced the can cap did you use a NOS (new-old-stock) capacitor that is probably 20-30 years old? If so, then it probably has a lot of ESR, is drying out inside, or is simply leaking due to deterioration. NOS caps are often OK if they are given a DC voltage treatment to restore the oxide layer on the foil. Need to check them for leakage afterward to be sure.
If you didn’t get a new can that was manufactured within the last 3 years then put axials in it.
If you are a newbie at soldering you might want to check your solder joints. Impedance in the filter circuit is what causes motorboating.
It can be caused by an open grid circuit, or defective filter caps.
When you said you replaced the can cap did you use a NOS (new-old-stock) capacitor that is probably 20-30 years old? If so, then it probably has a lot of ESR, is drying out inside, or is simply leaking due to deterioration. NOS caps are often OK if they are given a DC voltage treatment to restore the oxide layer on the foil. Need to check them for leakage afterward to be sure.
If you didn’t get a new can that was manufactured within the last 3 years then put axials in it.
If you are a newbie at soldering you might want to check your solder joints. Impedance in the filter circuit is what causes motorboating.
www.quadesl.com/refurb/refurb_dynaST70.html
This is an article by the guy who came up with the Triode replacement board.
www.turneraudio.com.au/dynacost70mods.html
This Turner Audio article says that his power transformer got hot after modifications to the power supply. He did not use the Triode board.
This is an article by the guy who came up with the Triode replacement board.
www.turneraudio.com.au/dynacost70mods.html
This Turner Audio article says that his power transformer got hot after modifications to the power supply. He did not use the Triode board.
New Can Cap
I used a new CE can from Triode. I'll check my solders, but they look okay. I'm pretty good with the actual mechanical craft of electronics -- flowing solder, clean wiring -- just lousy with theory and the actual knowledge of how the circuits work.
About the recommendation from Ray_Moth to bump up the value of the cap after the choke from 20 mf to 100mf: I can just pull the lead from the choke off the 20mf tab on the multi-cap and run a separate 100mf axial cap to ground, yes? Should I use the mounting tabs of the multi-cap as the ground point? Are there any potential problems with this solution? Other than forgetting to drain the cap before I disconnect the choke I mean?
I used a new CE can from Triode. I'll check my solders, but they look okay. I'm pretty good with the actual mechanical craft of electronics -- flowing solder, clean wiring -- just lousy with theory and the actual knowledge of how the circuits work.
About the recommendation from Ray_Moth to bump up the value of the cap after the choke from 20 mf to 100mf: I can just pull the lead from the choke off the 20mf tab on the multi-cap and run a separate 100mf axial cap to ground, yes? Should I use the mounting tabs of the multi-cap as the ground point? Are there any potential problems with this solution? Other than forgetting to drain the cap before I disconnect the choke I mean?
Why disconnect the choke?
Just take some clip leads and strap the 100uf across the 20uf.
After you have checked to see if the motorboating stops and you turn the amp off, be sure and let the amp sit for awhile so the 100uf will drain down before disconnecting it. Take one of the clip leads and short the 100uf after you take it out and leave it overnight so it won't develop a charge and shock you.
Check the ground to the filter caps.
Motorboating is called this because of the putt-putt noise it makes.
Might want to check the socket pins for corrosion.
If you have an oscilloscope see if it is really the power supply causing the problem. If not, check the grids.
Just take some clip leads and strap the 100uf across the 20uf.
After you have checked to see if the motorboating stops and you turn the amp off, be sure and let the amp sit for awhile so the 100uf will drain down before disconnecting it. Take one of the clip leads and short the 100uf after you take it out and leave it overnight so it won't develop a charge and shock you.
Check the ground to the filter caps.
Motorboating is called this because of the putt-putt noise it makes.
Might want to check the socket pins for corrosion.
If you have an oscilloscope see if it is really the power supply causing the problem. If not, check the grids.
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