Eddie hy fi
Active Member
theres no one but you guys , thanks
Dynaco 70 restore
- Thread starter Eddie hy fi
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- dynaco 70
Transformers are simple to test. Better to test what you have than buy another expensive part on a suspicion.
Eddie hy fi
Active Member
Where does the audio signal come out of the PCB ? and over to the out put tubes ? , is it via 22 and 23 ? ,, can i swap those to the left to see if its a PCB issue ?
Eddie hy fi
Active Member
i can do that without a generator , with audio playing threw my test preamp on my bench
Could be the output transformer but not likely I think. I worked on an ST70 recently. Had a completely dead channel. Was just a failed solder connection of the bias resistor to ground. These aren't very complicated circuits so it usually doesn't take that long to track down a solid fail. Do you have a volt/ohmmeter?
You could. It's just easier to look at and sync to a clean signal set to a single frequency. You can also download a tone generator for your phone or use this one if you have a way to connector your computer of phone to your input.
https://www.szynalski.com/tone-generator/
If you can already do that, then you should.... That's how you will find out where your channel is failing.
Eddie hy fi
Active Member
yes i have several volt / ohm meters
Eddie hy fi
Active Member
and im able to set the bias on both channels , would i be able to do this if it was a bias resistor to ground issue ? - i can use the meter good , if you talk me threw where to test
If the bias is adjustable then it isn't the bias resistor. That was just my example of the types of fairly simple things that will kill the output.
You can do a resistance test of the output transformer. With the power supply drained down you can measure the primary. Check resistance from the CT (red wire which is connected to B+) to each of the other four leads at the EL34 sockets. You can measure the secondary at the speaker terminals without any load connected, each output tap relative to ground. The secondary will be low resistance windings below 1 ohm. Compare each measurement to the other channel. This should tell you at a basic level if the output is ok.
John
You can do a resistance test of the output transformer. With the power supply drained down you can measure the primary. Check resistance from the CT (red wire which is connected to B+) to each of the other four leads at the EL34 sockets. You can measure the secondary at the speaker terminals without any load connected, each output tap relative to ground. The secondary will be low resistance windings below 1 ohm. Compare each measurement to the other channel. This should tell you at a basic level if the output is ok.
John
Eddie hy fi
Active Member
just did half of this , at the speaker jacks it is point 8 on both sides , at the 8 ohm out , got to get now , but will check the el sockets next and later , ive all so re flowed some points on the PCB near and around number seven and that tube socket -- thank you guys , your really cool for even giving me your time , i need to pay it back and do in my dayly life here
Output from the board goes to pin 6 on the EL34 sockets. Thats eyelet 22 and 23. There should be DC voltage and AC signal at that point. If you get normal voltage at the bias test points, I'll assume that means the negative DC is approximately where it belongs.
if the output transformer was open, you'd have no plate and screen voltage, so no voltage across the 15.6 ohm resistor. If it biases, the transformer isn't open.
If the transformer was shorted, you'd likely get some small amount of output and see very high current draw in the output stage. If its not doing that, fair bet its not shorted either.
This is the signal path on a stock 70
Check at the various points along the path and see if what you get matches this. I would not suggest poking at pin 3 of the EL34 tubes though, pretty solid bet the voltage is greater than what your scope is rated to deal with. You can use a 630v cap at whatever value is on hand as a DC blocker though. I'd start at the input and trace it through to eyes 22 and 23. Let us know where the signal stops.
has an explanation of the signal path too, maybe you'll find that helpful.
if the output transformer was open, you'd have no plate and screen voltage, so no voltage across the 15.6 ohm resistor. If it biases, the transformer isn't open.
If the transformer was shorted, you'd likely get some small amount of output and see very high current draw in the output stage. If its not doing that, fair bet its not shorted either.
This is the signal path on a stock 70
Check at the various points along the path and see if what you get matches this. I would not suggest poking at pin 3 of the EL34 tubes though, pretty solid bet the voltage is greater than what your scope is rated to deal with. You can use a 630v cap at whatever value is on hand as a DC blocker though. I'd start at the input and trace it through to eyes 22 and 23. Let us know where the signal stops.
has an explanation of the signal path too, maybe you'll find that helpful.
When you test the EL34 sockets just remember to remove the tubes. ;-) With the amp off and the EL34s removed, if you measure from pin 3 of one socket to pin 3 of the other, you should read about 200 ohms, As gadget says, I seriously doubt the output transformer is bad. If you can hear hum from that channel, that alone indicates it's working.
Hang in there, it's a real pain to diagnose these things but you will figure it out.
avguytx
Lunatic Member
All these guys helped me a lot with mine with troubleshooting and it was finally figured out. Ended up being a small flake of solder across the terminal post where the selenium rectifier once was and replaced with a diode. Original owner had left it and I never saw it till I started removing sections of wiring to find out if main power transformer was good. Sometimes it's the tiniest piece that gets you.
Eddie hy fi
Active Member
Output from the board goes to pin 6 on the EL34 sockets. Thats eyelet 22 and 23. There should be DC voltage and AC signal at that point. If you get normal voltage at the bias test points, I'll assume that means the negative DC is approximately where it belongs.
if the output transformer was open, you'd have no plate and screen voltage, so no voltage across the 15.6 ohm resistor. If it biases, the transformer isn't open.
If the transformer was shorted, you'd likely get some small amount of output and see very high current draw in the output stage. If its not doing that, fair bet its not shorted either.
This is the signal path on a stock 70
Check at the various points along the path and see if what you get matches this. I would not suggest poking at pin 3 of the EL34 tubes though, pretty solid bet the voltage is greater than what your scope is rated to deal with. You can use a 630v cap at whatever value is on hand as a DC blocker though. I'd start at the input and trace it through to eyes 22 and 23. Let us know where the signal stops.
has an explanation of the signal path too, maybe you'll find that helpful.
yes i read 194 ohm from pin 3 to 3 --on both sides , so if signal feeds threw 22 and 23 , then on the dead left side its number one and 2 that im looking at with evil eyes !
Eddie hy fi
Active Member
True that transformer video was way cool , and the sound quality on the right channel was way cool too , now if we can just fix the leftPlus it will learn ya sump'n along the way.
Eddie hy fi
Active Member
im going to test again and if the left does not work , i will unplug the left RCA input patch cord and touch it to number one and maybe then number two and see if any sound comes out of the left --------crude i know , but it may point me to a PCB issue--------------im surprised that the pin 6,1k resistors , on the EL34a , are not some point of many mods , that people have come up with , no one even says to check or replace them ?
They carry no current so the value really isn't very critical. If they went open you'd have no DC bias at the grid and the current in the output stage would be extreme. Since its not doing that, we can fairly safely assume that they aren't bad enough to cause zero audio signal to get through.
Eddie hy fi
Active Member
True , and ive tested all four and the read on spec ,
Eddie hy fi
Active Member
So my test produced faint audio out the left , that was clear , just not amplified , so it seems to be some kinda issue with the left hand side of the PCB yet all the resistors and caps check out , thats the update for now , ill dig at it moor later , maybe have to put a new PCB in , not sure what moor i can do to this old one , but will go over it again for the sixth time