Vintage amplifier resists several attempts to be stabilized. Support this channel on Patreon www.patreon.co... Email: johnaudiotech@gmail.com Thank You!
I built a lot of the SwTPC amplifiers back in the late 70s. They all require a few things to be stable. A secure, bundled, and symmetrical wiring harness to the output transistors is imperative. And then add flyback diodes between speaker output and the power rails. Clean the anodization from the RCA input connection. And we would wind large output inductors because we used the amps hard at 4 ohms for low and mid frequency drivers. Good luck.
I think a look at all those over long cables all linked together, that cannot be good. Might be worth untying them and see what happens. Afterall, you have tried everything else!
Looks like Q10, Q11, Q12 and Q13 were replaced with modern transistors. OPS transistors also seem to be replaced but not sure if they were replaced with old new stock or with modern transistors as well. Modern Transistors may have much higher power bandwidth than old legacy ones. This might lead to oscillation. Also, the wireing job in there looks terrible. Nothing is twisted to get field cancelation. This also helps oscillation to come on. Just my 2 cent worth of remote observation.
You hit the nail on the head with the modern transistor problem. These amps can be redesigned with more degeneration but I kind of think this guy doesn’t know how to do this.
@SkYLR It's a possibility. At least, that's where I would start to drill in. Most Audio guys seem to be DC guys and don't have any experience with RF stuff, and that might explain why most seem clueless when they have to tackle oscillation. Most also don't have much experience with PCB layout. Again, they just look at it from a DC aspect and make terrible routing mistakes while ignoring the signal return path. Stray capacitance and inductance are also commonly ignored. Most don't realize that there is much much more to this audio stuff than what's meeting the eye.
@@billspencer8540 I'm no RF nerd but I've cured a 40Mhz oscillation with stopper resistors in another design. The distributed layout of this amp is not kind to fast transistors. I'd need to convert the outputs over to sockets in leu of the push-on connectors as they aren't fit for adding local stability components.
Found this in the Audiokarma forums: "1. Add a 400pF cap from Q10 collector to ground, and Q11 collector to ground. 2. R14 must be upgraded to 1W. (nothing to do with stability, it was underrated.) That did the trick." - Dr*Audio Might be the same problem with yours
I did see that same post and tried that, but with no success. I read something on DIYaudio about an engineer who designed fixes for the amp, but I could not find anything on it.
Great video and I'm sure you will get there ! I think I might have a can of freeze spray out and go over the semiconductors, who knows ?.....cheers..Koo Kooky Chew.
I built Universal Tigers when I was in HS. It's possible that part of the problem may be the long wires. I got busy with college and eventually bought stable amps.
Modern transistors have too high GBWP. Need to reduce open loop bandwidth - can start with local compensation, base stopper resistors in some cases might solve it (outputs and drivers). Lots of options could turn into a good new series.
The wiring is giving me the Heebie Jeebies. Really amateur looking. It might not be a bad idea to put some distance between the wires to the base and the output. It's been awhile since I took a class, but I'm thinking if the amp has a phase shift of over 90 between output and feedback point its going to turn negative feedback into positive feedback, and it appears you have 90 just in the final stage. Just as a test, I'd suggest modifying the feedback so it's DC only but AC open loop. See if it still oscillates. If it doesn't, then you know the problem is a issue with the loop (gain and phase). If it does oscillate, then it should be easier to track down without the feedback confusing things.
Theres a dry joint at the bottom.right of the screen @14:58 (though i dont think its connected to anything meaningful). But if theres one...there may be more you havent spotted yet
I've a feeling it's going to be something weird. Like the substrate of the circuit board has somehow broken down and now it's in circuit. Just some deep seated problem that's pervasive. If every component checks out then it must be something else. Everything happens for a reason.
I still wonder about grounding, especially the RCA jack. A ground tied to the Al chassis is always suspect. I saw the oscillation much larger on the negtive rail, not so much on the positive. Huge RF signal on the Q8 collector, much more than above, this issue is mostly on the negative half. Make sure the wires on those big supply caps are tight! Would like to find some heavier lugs.
Yep, I'm seeing what you saw. The chassis ground seems good. Those big filter caps has low ESR, but I found them to have high ESL plus the long power leads that run to the back of the chassis for the fuses and then over to the board, so the film bypass caps I added to the board should help.
Although this problem is way beyond my basic troubleshooting skills, I"ll give the old college try, If you bypassed the output transistors with small value disc capacitors, wouldn't this shunt the oscillations to ground and keep from causing trouble in the speakers, or bypass all transistors in the signal path.
UK Stay with it John. It's a sign of a good professional when he admits his mistakes, so few do. I'm not in your league, you lost me when I clicked on your channel. Nevertheless over the years you have helped. Regards.
Many years ago, I ran films for a small (110 seat) theater. Had a Pioneer Spec 2 driving a pair of JBLs for the sound. I found about 1W/channel was the right volume. If it put it up to 10W, I got complaints.
I don't have a clue why is someone wasting money trying to fix this piece of junk. At least chassis and transformer could be reused for some decent amp.
SWTPC branded hardware is collectible due to their place in electronics history. They were microcomputer pioneers. In an alternate Universe they'd be the dominant tech company today. But that's not what happened in our timeline. They lost and Apple won.
@@HillsWorkbench yes they didn't have the two Steves that Apple did. But really at one time they were no more crackpot than Apple was. Heck I'd say SWTPC was more above the boards. Jobs and Woz got their start selling phone phreaking boxes. Was blue boxes if memory serves me.
![ARGENTINA vs. CHILE [3-0] | RESUMEN | ELIMINATORIAS SUDAMERICANAS | FECHA 7](http://i.ytimg.com/vi/De3AWmXzhuA/mqdefault.jpg)
I built a lot of the SwTPC amplifiers back in the late 70s. They all require a few things to be stable.
A secure, bundled, and symmetrical wiring harness to the output transistors is imperative.
And then add flyback diodes between speaker output and the power rails.
Clean the anodization from the RCA input connection.
And we would wind large output inductors because we used the amps hard at 4 ohms for low and mid frequency drivers. Good luck.
I think a look at all those over long cables all linked together, that cannot be good. Might be worth untying them and see what happens. Afterall, you have tried everything else!
Yeah, I agree, cut the tie wraps! The quickest way to make tracing the wiring easy. Still think there could be an error.
Nice video John!
Looks like Q10, Q11, Q12 and Q13 were replaced with modern transistors.
OPS transistors also seem to be replaced but not sure if they were replaced with old new stock or with modern transistors as well.
Modern Transistors may have much higher power bandwidth than old legacy ones. This might lead to oscillation.
Also, the wireing job in there looks terrible. Nothing is twisted to get field cancelation. This also helps oscillation to come on.
Just my 2 cent worth of remote observation.
You hit the nail on the head with the modern transistor problem. These amps can be redesigned with more degeneration but I kind of think this guy doesn’t know how to do this.
@SkYLR It's a possibility. At least, that's where I would start to drill in. Most Audio guys seem to be DC guys and don't have any experience with RF stuff, and that might explain why most seem clueless when they have to tackle oscillation. Most also don't have much experience with PCB layout. Again, they just look at it from a DC aspect and make terrible routing mistakes while ignoring the signal return path. Stray capacitance and inductance are also commonly ignored. Most don't realize that there is much much more to this audio stuff than what's meeting the eye.
@@billspencer8540 I'm no RF nerd but I've cured a 40Mhz oscillation with stopper resistors in another design. The distributed layout of this amp is not kind to fast transistors. I'd need to convert the outputs over to sockets in leu of the push-on connectors as they aren't fit for adding local stability components.
@JohnAudioTech I'm glad you figured the oscillation problem. Adding components may spoil frequency and/or transient response.
Found this in the Audiokarma forums:
"1. Add a 400pF cap from Q10 collector to ground, and Q11 collector to ground.
2. R14 must be upgraded to 1W. (nothing to do with stability, it was underrated.)
That did the trick." - Dr*Audio
Might be the same problem with yours
I did see that same post and tried that, but with no success. I read something on DIYaudio about an engineer who designed fixes for the amp, but I could not find anything on it.
@@JohnAudioTech Try looking for books by Douglas Self;iirc he used these as examples,maybe...
Great video and I'm sure you will get there ! I think I might have a can of freeze spray out and go over the semiconductors, who knows ?.....cheers..Koo Kooky Chew.
I love to build my own amp too, using simple circuit with STK series, lm12, tda and some push pull out put.
I built Universal Tigers when I was in HS. It's possible that part of the problem may be the long wires. I got busy with college and eventually bought stable amps.
Modern transistors have too high GBWP. Need to reduce open loop bandwidth - can start with local compensation, base stopper resistors in some cases might solve it (outputs and drivers). Lots of options could turn into a good new series.
The wiring is giving me the Heebie Jeebies. Really amateur looking. It might not be a bad idea to put some distance between the wires to the base and the output. It's been awhile since I took a class, but I'm thinking if the amp has a phase shift of over 90 between output and feedback point its going to turn negative feedback into positive feedback, and it appears you have 90 just in the final stage. Just as a test, I'd suggest modifying the feedback so it's DC only but AC open loop. See if it still oscillates. If it doesn't, then you know the problem is a issue with the loop (gain and phase). If it does oscillate, then it should be easier to track down without the feedback confusing things.
Tigers repair is not for the faint of heart! they can be quite troublesome!
Ground Loops can be very problematic to solve;they can destable a high frequency amplifier.
Theres a dry joint at the bottom.right of the screen @14:58 (though i dont think its connected to anything meaningful). But if theres one...there may be more you havent spotted yet
Mounting tabs for the little heatsinks.
A lot of techs will reflow every solder joint as a matter of course before they even begin to troubleshoot. Just to rule out bad solder joints.
Be interesting to find out what's causing the oscillation.
I've a feeling it's going to be something weird. Like the substrate of the circuit board has somehow broken down and now it's in circuit. Just some deep seated problem that's pervasive. If every component checks out then it must be something else. Everything happens for a reason.
That amp looks like a messy rats nest. If it worked it could be forgiven. But it doesn't so I'm holding it against the unit.
I still wonder about grounding, especially the RCA jack. A ground tied to the Al chassis is always suspect. I saw the oscillation much larger on the negtive rail, not so much on the positive. Huge RF signal on the Q8 collector, much more than above, this issue is mostly on the negative half. Make sure the wires on those big supply caps are tight! Would like to find some heavier lugs.
Yep, I'm seeing what you saw. The chassis ground seems good. Those big filter caps has low ESR, but I found them to have high ESL plus the long power leads that run to the back of the chassis for the fuses and then over to the board, so the film bypass caps I added to the board should help.
Very nice video
Would you consider adjusting the feedback sacrifice a little bit of gain?
I'm Glad that worked out.
If you look at video time 13:48 there are two leads that are not soldered completely. Look at the bottom of the video.
Almost lost me till I heard that Beatles reference...:) Did you have the other amp for some comparison?
What about a 470uf between the inverting input and ground and see if that stops the problem.
More Ocelot than Tiger.
Although this problem is way beyond my basic troubleshooting skills, I"ll give the old college try, If you bypassed the output transistors with small value disc capacitors, wouldn't this shunt the oscillations to ground and keep from causing trouble in the speakers, or bypass all transistors in the signal path.
UK Stay with it John. It's a sign of a good professional when he admits his mistakes, so few do. I'm not in your league, you lost me when I clicked on your channel. Nevertheless over the years you have helped. Regards.
I may have schematics for them...
burned resistor between Q11 and Q13 also
how many watts are suitable for 8 inch 200watt speakers?
1 watt.
@@peterlarkin762 🤣
Many years ago, I ran films for a small (110 seat) theater. Had a Pioneer Spec 2 driving a pair of JBLs for the sound. I found about 1W/channel was the right volume. If it put it up to 10W, I got complaints.
@@russellhltn1396 I mean, how many watts is the ideal power amplifier for 200w l and r speakers 8 ohm
Goo goo g'joob
I don't have a clue why is someone wasting money trying to fix this piece of junk. At least chassis and transformer could be reused for some decent amp.
SWTPC branded hardware is collectible due to their place in electronics history. They were microcomputer pioneers. In an alternate Universe they'd be the dominant tech company today. But that's not what happened in our timeline. They lost and Apple won.
@@1pcfred Looking at that PC board, I'm not seeing a quality operation.
@@russellhltn1396 you've seen the avionics boards I've made? Where was that?
@@1pcfred Unfortunately, they folded up pretty quickly after Daniel died.
@@HillsWorkbench yes they didn't have the two Steves that Apple did. But really at one time they were no more crackpot than Apple was. Heck I'd say SWTPC was more above the boards. Jobs and Woz got their start selling phone phreaking boxes. Was blue boxes if memory serves me.