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In Reply to: Dynakit Stereo 70 Troubleshooting posted by david.garfinkel@gmail.com on September 22, 2020 at 14:46:17:
Again with the god damn caps. If it has gone out completely the only way a cap can cause it is by shorting out. A DMM will tell you that in a second.
It is more likely that a power supply dropping resistor has opened up, they do dissipate heat and it eventually gets to them. Like the output stage gets pretty much full voltage or the power supply, but earlier stages run on a lower B+ and that is dropped by a resistor.
And the 5AR4 will not affect the filaments.
Caps that are weak, which means high ESR will not fry the power transformer.
Loss of bias will destroy the output tubes and main rectifier before the power transformer is in any danger. The only way is if you change the fuse 80 times. Of course that means a set of tubes every time as well.
I got 45 years of SUCCESSFUL experience in the business. If anyone wants to argue with me state your case and this becomes electronics school.
Now, back to your particular amp. Get a DMM. Now first take a wire and short out every one of those can cap terminals to ground. After that, read each one with the DMM, it should start low, almost shorted but in time it will raise in resistance ad usually read open when it it done. Use the ->|- scale for this.
If nothing shows shorted, meaning it never rises in the reading on the DMM then it is time for a live test.
So you got it apart and it is time to plug it in. If you didn't get smoke before you are ot going to get it now, and not seeing smoke is good.
Now you are playing with fire, maybe a gun would be a more apt analogy. Do not touch anything with both hands. You can be killed. Many actualy put one hand in their pocket when doing live tests to make sure. If you gt a shock or a burn in one had it is not that big a thing, but both hands can make it pass through your body an stop your heart. I'm told this is detrimental to you heath. :-) Just mane SURE it cannot pass through your body. If you chose to use gloves those regular hospital gloves are not that reliable. Do not forget or disregard what I say here, I do not want people, well at least most audiophiles to die because of my advice.
You got it where you can get to the inside. Put the black lead fro the DMM on the chassis. Then you plug it in and let that 5R4 warm up a bit. stick one hand in your pocket. Put the DMM on the 1,000 volt scale till with the DMM black probe on the chassis. (that would also be fine for the ohms checks you already did) Except for the ones that got to ground you should see voltage on all of them, ranging from as high as 600 volts down to maybe around 100. If you have a dead one or more, pull all the tubes except for the 5AR4. Recheck. If the voltage is still gone then you probably have an open power dropping resistor. Look from the main filter that comes right off the 5AR4, there will probably be a coil there somewhere. But the resistors are what you need to scrutinize.
If you do get voltages on all those lugs though the next step is to start putting in the tubes back one by one. If one causes the volts to drop to zero that is probably the culprit.
The one right between where you have voltage and not is probably the culprit. It is probably marked in value, so go to Digikey or Mouser and order one. (If you are not in the US sorry, but you have the equivalent almost everywhere) Since it is not political a Google search will be sufficient.
If you do it post back all the results and we can figure out how to confirm this. I also do not want to cost audiophiles unnecessary money, you need that money for more gear. Get it now before the Wife leaves you because then you will have no money.
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- RE: Dynakit Stereo 70 Troubleshooting - JURB 14:51:58 09/23/20 (0)
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