Motorboating in Dynaco ST-70 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 30th June 2007, 11:52 PM   #1
diyAudio Member
 
Join Date: Sep 2006
Default Motorboating in Dynaco ST-70

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
  Reply With Quote
Old 1st July 2007, 12:18 AM   #2
d2134 is offline d2134  Romania
diyAudio Member
 
Join Date: Mar 2007
Send a message via Yahoo to 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.
  Reply With Quote
Old 1st July 2007, 12:48 AM   #3
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
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.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 1st July 2007, 01:00 AM   #4
diyAudio Member
 
Join Date: Sep 2006
Default 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.
  Reply With Quote
Old 1st July 2007, 01:43 AM   #5
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
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?
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 1st July 2007, 02:08 AM   #6
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
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.
  Reply With Quote
Old 1st July 2007, 10:09 AM   #7
diyAudio Member
 
Join Date: Sep 2006
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 didnít 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.
  Reply With Quote
Old 1st July 2007, 11:52 AM   #8
diyAudio Member
 
Join Date: Apr 2003
Location: Portland,Oregon
Blog Entries: 4
Send a message via AIM to DigitalJunkie
Quote:
Originally posted by hailteflon
If you didnít 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
  Reply With Quote
Old 1st July 2007, 08:24 PM   #9
diyAudio Member
 
Join Date: Sep 2006
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.
  Reply With Quote
Old 1st July 2007, 11:42 PM   #10
diyAudio Member
 
Join Date: Sep 2006
Default 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?
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
motorboating jayme Tubes / Valves 16 2nd May 2007 12:56 PM
Is my reg motorboating (as the bishop said to the queen)? Triophile Solid State 8 2nd April 2006 10:57 PM
What causes motorboating with 4 ohms? tmblack Solid State 3 14th February 2005 03:03 AM
Motorboating in leak 20 bronto64 Tubes / Valves 5 13th August 2004 10:49 AM
LM4766 motorboating / static soundNERD Chip Amps 3 26th September 2003 09:01 AM


New To Site? Need Help?

All times are GMT. The time now is 04:12 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2