Collins 26W Interstage trafo?

Just got an Collins 26w that has a very thin sound
and it seems to loose it bass at the interstage trafo.

When feeding the input  a testsignal I measure fullrange at the primary sides at the interstage trafo
but at the sec sides the bassrange is gone.
And if I feed the testsignal at the sec coils it measures fullrange at the output.

Desolder and measured the trafo with a ohmmeter
and both coils on the primary are opened
and the coils at the sec side measures 1.1k each.

Should it not be totally quiet if both coils are opened on the pri side?

At the moment  it has  a UTC(no info on it) as interstage, no idea if its original or not.
The Limiter has definitely been worked on but looks like proper work.
A lot of resistors etc changed around before the interstage
so it´s looks like its been problems there.

Does anyone have any info on the Interstage transformer?
Or recommandation  for a replacement?

Hammond 835 or 850Q are the best choices I know of.  Original is a 1:1 in the 40K-50K range.  The Hammond perform well with 40K source Z, which won't be true of almost anything else modern, maybe none.  Original measures about 550H per side with a handheld meter at 120Hz, the Hammond are much higher.

> Should it not be totally quiet if both coils are opened on the pri side?

You think it is coils in there, but a transformer is a lot of capacitance. Hundreds of pFd.

100pFd leaking into 100K grid resistor is unity above 16KHz, falling at lower frequencies. So the top of the audio band is "fine", but middle is down and bass is way-way down.

before you buy a new one try and fix the old one, what do you have to lose?

take a 3/8" drill bit and remove metal from the pem nuts that hold on the terminal plate

lift the plate up and look for loose wires going from the xfmr to the plate

Love this forum!
Thanks for the info.

Yes I believe the interstage been replaced sometime. Also all wiring to it are new and the resistors before it
so it surely been some problems there....
So the question is if this is the second Interstage its ruined ?

Doug, will Lundahl 1635 or 1660 also work? I´m in Sweden so it´s simpler for me.
I also noticed that this is a early 26W with a 6N7GT as V102 instead of 5814A
and V101 V103 are labeled 6N7
can´t find any schematics for this early one  but should the resistors etc have the same value anyway besides the different pinout of the tube.

CJ, Yes thanks I have a go at the transformer.


Hard to tell if the input trafo is a LS series as its in a boxed
but it looks like the original transformer when looking at the solderjoints etc

Lundahl 1635 may work.  Inductance is stated as lower that the original somewhat, and response is referenced to a 4K source, which is much lower than what's present in the 26W, though that quoted response is much higher.  1660 has the wrong ratio for starters, and even lower inductance. 

R119/120 are the only change, 6K8 versus 18K.  The release times were shortened in the later version, but the labeling was not changed. 

That does look like the correct input on that version. 

The open UTC LS has two more pins that specified, but they may have left the unused off the schematic.  The lack of a UTC badge suggests a custom part, and the Collins ID was usually a decal that may be long gone.  If there's any trace of a decal on the side, you can consider it original and custom.  Am I seeing 4 pins strapped together?  You need to disconnect all those straps and ohm out every winding.  It may be you can change windings to other equivalent taps, that's a very long shot but should be investigated first before opening the case. 

Manage to open the interstage,and it looked like this.


So it´s probably something homemade,right?
and the core has not label either.
The wires are attached to the core so it´s probably inside the coils they open




I get the the Hammond 850QA don´t know what the A means but the 850Q are out of stock at Digikey.

850QA is the current part with wire leads.  The 850Q with hermetic terminals isn't made anymore. 

Someone stuffed something else in that UTC case, or tried a repair already.  Duct tape and foam aren't UTC materials!  No telling what it is, or if it even worked properly.  You could have popped that end plate off rather than hacksawing the entire case!

Someone stuffed something else in that UTC case, or tried a repair already.  Duct tape and foam aren't UTC materials!  No telling what it is, or if it even worked properly.  You could have popped that end plate off rather than hacksawing the entire case!
[/quote]

Yes suspected that, well the good thing is that I didn´t have to see a original transformer broken..

It was like that, just removed the screws and of the top came.

Received the Hammond today, and now its passing audio nicely.Thanks Doug.

Doesn´t compress properly yet though.
Sounds great with the V102 removed so either a bad tube in the SC or something else in there...

Does anyone have the Schematic for the early 26W with 6N7 as V102 instead of 5814A?