rockinrob86

EP-2 tube Echoplex erase circuit choke issue
« on: July 21, 2020, 12:14:42 PM »
I have an EP2 that isn't erasing.  I've gone through the circuit and replaced all electrolytics, etc. 

In the erase bias circuit, the choke is fried.  These have a simple circuit with a 6c4 tube, going through a dual choke with a 25 ohm side and a 42 ohm side (Ignore the drawings in the below schematic, I just found a random one on google.  The oscillator circuit is on the middle right hand side). 

What can I use for a replacement part for this?



squarewave

Re: EP-2 tube Echoplex erase circuit choke issue
« Reply #1 on: July 21, 2020, 02:46:26 PM »
That could be tough. You should try to obtain an original part.

The problem is that that "choke" is really a transformer with a ~1.3 turns ratio and must be a specific impedance at ~70kHz. There is a chance that there is a part on Mouser that would actually work. Inductance of the side with the 2n cap is probably ~2.7mH to make 70kHz. But the bias can vary a quite a bit if you don't have to match an existing trap circuit (if that is installed then you need to match it pretty closely to hit the notch).

If I were trying to fix this and I could not find a suitable replacement part from EchoFix or whatever site like that, I would just tweak the oscillator a little to accommodate whatever part I can find on Mouser and also create a matching trap while I'm at it.

The biggest problem is probably that the trap inductor and bias transformer have to be for ~70kHz, shielded and high Q which is a little unusual. And the turns ratio might be a factor but a 1:1 might work. Many chokes and inductors that you'll find on Mouser or whatever mega parts sites are designed for much higher frequencies. Something like Bourns RL181S-272J-RC is good for a trap but you would need to find the transformer equivalent. Probably a transformer like you might find in an SMPS power supply would be right.

 

rockinrob86

Re: EP-2 tube Echoplex erase circuit choke issue
« Reply #2 on: July 21, 2020, 08:18:25 PM »
Thanks, although that’s some bad news for sure!


 

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