Surplus store score! Collins 26c and 2x Gilfillan Bros/Bendix filters pics
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plumsolly
Well-known member
I decided to have a go at fixing my original Collins input tx. I figured it was already busted, so I didn't have much to lose. I froze it and dug out the potting. The problem was just one intermittent secondary, which could be affected by tugging on the wire, so I though it was probably an issue right where the lead met the coil. The following pictures are not for the faint of heart - Doug are you sitting down?
I did find an open connection at the point in question. I dug into it a little to see if I could find the coil to reconnect it. I didn't have any luck and may have damaged it further messing around in there, but I am not any worse off than when I started - I could still send it to Doc Hoyer to rewind or to CJ to dissect, if I ever get around to it.
In the mean time, I have really been enjoying my fuller-bandwidth 26c and I think I am going to give the Lundahl a try.
Best,
Ben
I did find an open connection at the point in question. I dug into it a little to see if I could find the coil to reconnect it. I didn't have any luck and may have damaged it further messing around in there, but I am not any worse off than when I started - I could still send it to Doc Hoyer to rewind or to CJ to dissect, if I ever get around to it.
In the mean time, I have really been enjoying my fuller-bandwidth 26c and I think I am going to give the Lundahl a try.
Best,
Ben
plumsolly
Well-known member
Case pics. Not too exciting, but it seems to be made of cast iron, which is neat.
Update on the plots of different input transformers: The Edcor was indeed broken - open secondary winding.
I went ahead and ordered a Lundahl LL7905. I will be back soon with a report on that.
Best,
Ben
Update on the plots of different input transformers: The Edcor was indeed broken - open secondary winding.
I went ahead and ordered a Lundahl LL7905. I will be back soon with a report on that.
Best,
Ben
plumsolly
Well-known member
Startling discoveries! A shocking admission! And one dramatic conclusion (sort of)!
I received my Lundahl LL7905 and started to do some testing with it. The first test I ran was with it wired as 1:11 with a 10K/600 pad. 1:11 is pretty close to the original, which is around 1:11.5/500:80K. The test showed some high frequency roll-off that I wasn't expecting (see below)
I decided to run the same test without the pad to see how it affected the response:
Notice the vastly improved response. At this point I went and investigated the pad, which is located on a mic/line switch in a breakout panel I have attached to my 26C. The panel also houses a bypass as well as attack and release controls. Upon inspection, I realized I had wired the pad incorrectly! I had managed to put the 600k resistor on the wrong side of the 5.1k resistors, making an upside down U-pad. With this arrangement, the input of the limiter was seeing a 10.6k output impedance, and my sound card was seeing a 600 ohm input Z - pretty far from ideal. I fixed it and resumed testing. Here is the same test with Lundahl and the correct pad:
This last one is very similar to the no-pad response - much better. But it is still not as nice as what I was seeing with my 600:600 UTC HA-108X with no pad and a little frequency compensation, seen here:
So what does my 600:80K UTC HA-101X look like with the correct pad? Here it is with the secondaries in series and no frequency compensation:
Much better than with the incorrect pad, but still shows the dip caused by having the secondaries in series. So how about the HA-101X with the secondaries in parallel? Here it is with some frequency compensation placed after V1:
Much better! But remember all these test are without the limiting engaged, so lets see what happens when the limiting circuit is in:
I guess I won't be needing that high-end rolloff I had introduced. I am at a pretty good place now - the frequency response exceeds the specs in the manual, probably due to the high quality input transformer, but I have lost some level on the front end. I went from a 1:11.5 transformer with around 21dB gain to a 1:5.8 transformer with 15dB gain. Will I still have enough level with my 10k/600 (25db) pad? I decided, at least for now, that I would - I had lowered the threshold and raised the side chain signal by fooling around with V3, and I can still get it into limiting just fine.
It is back in the rack now and sounding really good. Thanks to everyone who stopped by and helped out.
Best,
Ben
I received my Lundahl LL7905 and started to do some testing with it. The first test I ran was with it wired as 1:11 with a 10K/600 pad. 1:11 is pretty close to the original, which is around 1:11.5/500:80K. The test showed some high frequency roll-off that I wasn't expecting (see below)
I decided to run the same test without the pad to see how it affected the response:
Notice the vastly improved response. At this point I went and investigated the pad, which is located on a mic/line switch in a breakout panel I have attached to my 26C. The panel also houses a bypass as well as attack and release controls. Upon inspection, I realized I had wired the pad incorrectly! I had managed to put the 600k resistor on the wrong side of the 5.1k resistors, making an upside down U-pad. With this arrangement, the input of the limiter was seeing a 10.6k output impedance, and my sound card was seeing a 600 ohm input Z - pretty far from ideal. I fixed it and resumed testing. Here is the same test with Lundahl and the correct pad:
This last one is very similar to the no-pad response - much better. But it is still not as nice as what I was seeing with my 600:600 UTC HA-108X with no pad and a little frequency compensation, seen here:
So what does my 600:80K UTC HA-101X look like with the correct pad? Here it is with the secondaries in series and no frequency compensation:
Much better than with the incorrect pad, but still shows the dip caused by having the secondaries in series. So how about the HA-101X with the secondaries in parallel? Here it is with some frequency compensation placed after V1:
Much better! But remember all these test are without the limiting engaged, so lets see what happens when the limiting circuit is in:
I guess I won't be needing that high-end rolloff I had introduced. I am at a pretty good place now - the frequency response exceeds the specs in the manual, probably due to the high quality input transformer, but I have lost some level on the front end. I went from a 1:11.5 transformer with around 21dB gain to a 1:5.8 transformer with 15dB gain. Will I still have enough level with my 10k/600 (25db) pad? I decided, at least for now, that I would - I had lowered the threshold and raised the side chain signal by fooling around with V3, and I can still get it into limiting just fine.
It is back in the rack now and sounding really good. Thanks to everyone who stopped by and helped out.
Best,
Ben
plumsolly
Well-known member
lassoharp said:Full story at 11 ?
Sorry Lasso, I accidentally hit submit before I was done and had to go back and edit it.
-Ben
