Welcome! Need support, you got it. Or share your ideas and experiences.
152.131.9.132
Just picked up a Dynaco ST-70 which was in good working condition. It had the the new power capacitor mod but other than that was stock. The unit seemed to run fine. I decided to replace the board capacitors with Orange drops and install diode mod to replace selenium rectifier. I also removed the power capacitor mod and reinstalled a new multi can capacitor with new bias caps and new resistors in the bias and power section.
The unit ran great for a few weeks. I could never get the bias up to original 1.56 as it would max out before but I read I should have the bias set to a lower value so I kept it around 1.4 or so (can't remember of hand what other threads advised).
I also noticed later that it appeared there was some sot of diode mod on the rectifier tube which I thought at first was a mistake but realized upon further reading some people recommend this mod. Unfortunately, it appears that one of the diodes was soldered across so effectively removed it from circuit.
A few days ago I brought the unit inside to see how it sounded with the living room set up. I ran the unit for a few hours without issue and it sounded great. I had the unit at almost half volume with the cover on and it didn't seem to run hotter than it usually does. In the garage i never had the cover on but inside there is more potential for burns.
The next day I went to turn on the unit and there was nothing. No sound or even a hum from the transformer. I checked the fuse and didn't seen any evidence of it being blown but it is white so I can't see the filament. I put a new one in and still nothing. I found a few places on the internet that talk about testing the transformer. I'm not getting any AC reading across to black filaments.
Thinking I may have fried to transformer but I am unsure. I was wondering if anyone had some insight into where the source of the problem may be and how I can go about testing the transformer.
Thanks
Follow Ups:
The testing of the power tranny can get quite extensive. Another source of the power issue could just be the on/off switch had its day, unless it was already replaced. Also, if it had the power supply board mod, why did you change the quad cap? That is usually disconnected.
You have to see if the primary wires to the power tranny is getting AC voltage. I would also change the fuse even though you might think was good. It uses a slo-blo. make sure you put in the proper amperage value. Don't go higher. It is sized for the circuit.
As for the bias supply. You can put in a larger value resistor(3-5K larger) in there to let less negative voltage to pin 5. Could be just those tubes need a larger range across the pot. 1.4VAC is good for both the tubes. With the higher VAC voltages coming out of the wall outlet the tubes will draw more than they should if kept at the stock 1.56VDC. Also, the bias resistor from the tube pin 8 to ground might be out of tolerance. That should be changed to a 10 ohm and then bias both tubes at .80VDC(40 MA per tube if they are matched).
You should also measure pin 5 of both the output tubes to see if you have the proper negative voltage. Should be about -36VDC or so(I forget totally.
The big thing right now seems to be the power tranny not turning on. I really don't thing you did anything to blow it up. That's what the fuse protects, among other things.
Hope this helps!
Thanks for the reply. Ended up bypassing the power switch and the unit worked fine. I did check the various points to assess the transformer status and they are all within spec. Now it's a matter of getting a new switch but happy it wasn't anything major.
With regards to the quad cap. I replaced it because I removed the capacitor mod. I wanted to return the unit close to stock to see how it sounded. If I do end up doing modifications I want to be able to enjoy the changes as I go along.
Plus the person who had the unit before wasn't the best with the soldering iron so I didn't know how much I could trust and figured I would start from square one.
Thanks again for the help and you were spot on with the on/off switch
Good to hear it all worked out. Very rarely is the power tranny ever an issue. Good idea starting from scratch and trying all the different mods out there. The best two mods I ever did to that amp was to move the choke from where it is to just have it do the front end. Makes the amp much faster. Another beneficial mod is either to get a front end board that has individual bias pots for each output tube or install two more pots of the same type(or replace the two original with the same as the ones you install)for individual biasing. Allows for unmatched tubes.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: