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OT: Bad Epoxy? in an older Rohde & Schwarz receiver


greenboxmaven
 

I believe there was some sort of relationship between Tektronix and Rohde & Schwarz, so maybe I'm not too far off topic.�� The discussion of the transformers in the 549 and my experiences with an epoxy coated "doorknob'' HV condenser in a 453 have caused me to think I might have a similar problem with a Rohde & Schwarz EK-57 receiver. The receiver and it's simpler EK-56 version have 12 VCO modules that are well known for being a serious annoying problem. They are very intermittent, refusing to start most of the time Other times they start immediately and produce a robust output. I replaced every component in one of them, and was rewarded with strong output, but it faded and stopped after a few days.� Has anyone seen circuit board material in Tektronix equipment become leaky at RF frequencies?� In this case the modules cover a total span of 40 to about 80 MC. in six bands. From all appearences, the oscillators are loaded down so much they can't start, sometimes they can be started with a blast from a signal generator. If I wanted to dismantle one of the modules to the bare circuit board (they are all the same) and get some new ones made, who would be a good place to go to?

����� Bruce Gentry, KA2IVY


Kurt Swanson
 

Bruce,

Years ago I was involved in an issue (not TEK or R&S) where we had a similar issue with an oscillator. Turned out a black rubber cushion that was put under the coil was becoming slightly conductive with time and temperature, and was causing the "Q" of the tuned circuit to drop. Since it was determined that it was only needed as an assembly aid, and served no other functional purpose once assembly was complete, the field repair solution was to just pull it out and discard it!

Bottom line - keep an open mind here.

Regards - Kurt


 

The VCO (coil; varicap diode; resistor, & ceramic caps) in a 27MHz 40 channel Pearce CB radio is also epoxied inside its metal case. In this case clear, so it makes a nice mantel piece not unlike an insect in amber.
Regards


greenboxmaven
 

Although definately a different material well known for serious leakage, there are several Kenwood rigs both ham and audio that had a rubbery encapsulant applied to oscillators that totally stopped them. Fortunately, patient scraping removes it and restores full operation.  There were many plastics and encapsulants developed in the 1970s that became utter disasters later on.

     Bruce Gentry, KA2IVY

On 2/16/21 17:53, David Collier wrote:
The VCO (coil; varicap diode; resistor, & ceramic caps) in a 27MHz 40 channel Pearce CB radio is also epoxied inside its metal case. In this case clear, so it makes a nice mantel piece not unlike an insect in amber.
Regards




Brad Thompson
 

greenboxmaven via groups.io wrote on 2/16/2021 8:08 PM:

Although definately a different material well known for serious leakage, there are several Kenwood rigs both ham and audio that had a rubbery encapsulant applied to oscillators that totally stopped them. Fortunately, patient scraping removes it and restores full operation. There were many plastics and encapsulants developed in the 1970s that became utter disasters later on.
Hello--

Add to the list "Sony Glue" (AKA  chloroprene glue):

https://www.edn.com/make-mine-melamine/

https://www.edn.com/make-mine-melamine-part-2/

I have great respect for Tektronix's engineering, and I doubt that it was used in the
scopes proper. However, modular power supplies purchased from outside suppliers
may have included the stuff.

73--

Brad  AA1IP


Bert Haskins
 

On 2/16/2021 9:21 PM, Brad Thompson wrote:
greenboxmaven via groups.io wrote on 2/16/2021 8:08 PM:

Although definately a different material well known for serious leakage, there are several Kenwood rigs both ham and audio that had a rubbery encapsulant applied to oscillators that totally stopped them. Fortunately, patient scraping removes it and restores full operation. There were many plastics and encapsulants developed in the 1970s that became utter disasters later on.
Hello--

Add to the list "Sony Glue" (AKA  chloroprene glue):

https://www.edn.com/make-mine-melamine/

https://www.edn.com/make-mine-melamine-part-2/

I have great respect for Tektronix's engineering, and I doubt that it was used in the
scopes proper. However, modular power supplies purchased from outside suppliers
may have included the stuff.

73--

Brad  AA1IP
Add to the list "Pyle Driver" a really nice sounding speaker that used a glue around the speaker cone that turned to dust after just a few years.

Cost me a bundle.

It seems like they got away with it, must have had friends on the hill.

Bert