Motorboating in Dynaco ST-70

[neddyboy]

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

[d2134]

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.

[SY]

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.

[neddyboy]

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.

[SY]

If you miswired the feedback, you'd expect very high gain. That doesn't seem to be the case. Did you replace all the power supply electrolytics?

[ray_moth]

That power supply seems a$$-backward to me, with 30uF before the choke and only 20uF after it. Maybe an increase in the reservoir cap from 20uF to 100uF might give better LF stability.

[hailteflon]

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.

[DigitalJunkie]

Originally posted by hailteflon
If you didn’t get a new can that was manufactured within the last 3 years then put axials in it.

Or one of the SDS labs cap-boards.
http://triodeelectronics.com/sdss70parts.html

[hailteflon]

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.

[neddyboy]

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?