Dynaco 70 restore

[Eddie hy fi]

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.

Think i just found the issue , a trace on left side of PCB is not getting threw to the small couple cap , this must be it , going to run a thin small hard jump wire to it now

 

[Eddie hy fi]

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.

Ok so my fix is kinda crude but so is my old PCB , lets hope it works now

 

[Eddie hy fi]

Its fixed now , that was it , PLays ! both sides ! level equal on both ! --- but i now have two issues to deal with , one issue is i have a miss match of output tubes going , with three svetlana tubes and one mullard , ---- the other issue is strange , im not able to set bias now because it keeps jumping around , im letting it warm up now and will try again , but there was some talk about adding a 4.7 k resistor to the modern diode , no one seems to do this , what is the deal with that , i have the resistor too , it came with rebuild kit , but i did not add it in line with diode , should i have done this ??

 

[Eddie hy fi]

Now able to set the bias , it has settled down amp playing on its second cd now , it was a long hall with this one but we got it now ! thank you guys and karma too

 

[gadget73]

Good deal, I've had cracked traces cause me issues before. I overhauled my preamp and it went dead on one side, took me a few hours but finally chased it to the same sort of thing. Fixed it more or less the same way too, used a piece of cut-off resistor leg to bridge the broken spot.


Can't have anything playing when you try to bias it, otherwise it will jump. Also some small fluctuation is normal, the last digit will often dance up and down a bit as the line voltage shifts slightly. Not that big a deal.

 

[zackthedog]

Hurrah!

My SP14 Aikido-style preamp recently lost sound in one channel--or so I thought. I looked and looked and looked, tested the output with a DMM and was getting signal on both channels, couldn't figure it out. Finally went through the whole circuit checking all voltages and discovered that one of my fancy V-Caps had totally shorted, sending 139VDC to the input of my Williamson amp, driving the input tube waaaaay beyond cutoff! Didn't really think to check for DC at the output! Fortunately only had it on for moments before shutting it down again to troubleshoot. It's a good thing tubes are tough. ;-) Also glad I didn't hook my USB scope to it! Pffffft! The input 6SN7 on the monoblock for that channel tested fine afterwards, so I got away with it this time.

Makes a good argument for input caps on the amps, I'd probably better install some!

 

[Pio1980]

Hurrah!

My SP14 Aikido-style preamp recently lost sound in one channel--or so I thought. I looked and looked and looked, tested the output with a DMM and was getting signal on both channels, couldn't figure it out. Finally went through the whole circuit checking all voltages and discovered that one of my fancy V-Caps had totally shorted, sending 139VDC to the input of my Williamson amp, driving the input tube waaaaay beyond cutoff! Didn't really think to check for DC at the output! Fortunately only had it on for moments before shutting it down again to troubleshoot. It's a good thing tubes are tough. ;-) Also glad I didn't hook my USB scope to it! Pffffft! The input 6SN7 on the monoblock for that channel tested fine afterwards, so I got away with it this time.

Makes a good argument for input caps on the amps, I'd probably better install some!

Covered in the base line thread for the Dyna 70 and applicable to other DC coupled tube amps.