Federal Limiter blown choke?

[substitute]

Hey all, I'm doing some work on a really old Federal limiter, it's not the normal AM 864. Initially the PT was dead, I replaced that, it worked fine, I returned it to the owner and of course he called me back saying it was making crazy noises and burning smells. It seems to me that the filter choke has now gone bad. First off, it's vibrating and making crackling noises, secondly I'm measuring 145v going into it but 5v coming out. The schematic specs it at 22hy 25ma 350r, I don't have an inductance meter but the resistance measures normal. There's two hammonds that are close, one's a little over one's a little under...

154E 20H 20 ma. 1666 300 0.25 C2H
157G 30H 40 ma. 595 400 1 C9H

So my questions are, what else can I test to be sure it's the choke, and which choke should I go with.

The schematic doesn't seem to be available online but here's a pic of what's inside the chassis

 

[trobbins]

Unless you have more details on actual normal operation (which now doesn't seem likely) then the 40mA rated choke would seem a safer option.

The 157G DCR is higher so there will be some additional voltage drop, but appears to be benign.  There is also a ripple trap capacitor across the choke - that will need to be modified to nominally resonate with 30H rather than 22Hy.

You may need to review how well the equipment is operating, and if any risks of over-current remain, as the choke is most likely stressed from over-current causing over-heating and an internal breakdown of insulation and arcing (not an uncommon end for vintage chokes).

You can measure the inductance with DC using a relatively DIY test setup - see link below.  Another worthwhile test is to measure the insulation resistance from the choke coil to choke core at at least 500VDC, but preferably at 1kVdc - but that needs a special meter which you are unlikely to have.
https://www.dalmura.com.au/static/Choke%20measurement.pdf

 

[abbey road d enfer]

Hey all, I'm doing some work on a really old Federal limiter, it's not the normal AM 864. Initially the PT was dead, I replaced that, it worked fine, I returned it to the owner and of course he called me back saying it was making crazy noises and burning smells. It seems to me that the filter choke has now gone bad.

It may, or not...
Your measurements (145V in, 5V out) indicate there is a short; you must sort this out before replacing the choke, that may not be fried. This short may be due to a number of things, in particular a smoothing cap gone bad, but it also can be a shorted tube.

 

[substitute]

You can measure the inductance with DC using a relatively DIY test setup - see link below

Thanks for that, and the note about the ripple cap, I'll have to read up on that.

It may, or not...
Your measurements (145V in, 5V out) indicate there is a short; you must sort this out before replacing the choke, that may not be fried. This short may be due to a number of things, in particular a smoothing cap gone bad, but it also can be a shorted tube.

Good point, all the filter caps have been replaced at least relatively recently (they are modern components) which of course doesn't mean they can't go bad, the tubes in this model are of the black metal old timey radio variety. I ordered a full set of replacement tubes when I replaced the PT, they are of course not new though.

 

[emrr]

You haven't told us the model # or the tube line-up. 

 

[substitute]

You haven't told us the model # or the tube line-up.

Model is CA-314

Tube line up
v1 6sk7
v2 6j7
v3 6f6
v4 6h6
v5 38184 replaced with ss rectifier plug (5y3 style)

I disconnected the leads coming off the downstream side of the choke and took some resistance measurements.

Point A = 3k
Point B = 40r (this is the lug on the silver sardine can looking cap not the choke L1)
Point C = 17m

It seems that there's an issue with the ripple cap that straps across the choke, which is  0.1mf 600v, I'm sure I have one of those lying around.

 

[emrr]

Single ended voice compression. 

Resistor style indicates pre-war. 

 

[PRR]

Disconnect the 0.1u and see if the smoke stops.

Personally I'd leave it out.

 

[radardoug]

I disconnected the leads coming off the downstream side of the choke and took some resistance measurements.

Point A = 3k
Point B = 40r (this is the lug on the silver sardine can looking cap not the choke L1)
Point C = 17m

Your measurements make no sense at all. A resistance measurement must be taken between two points.
When measuring the choke, there are two measurements to take, across the choke, and choke to ground. Across the choke should be ohms, maybe up to 1 K. To ground should be as high as possible, many megohms.

Your point C measurement, is that milliohms or megohms, and what is the other end of it?

You need to give us accurate and clear measurements if you want us to help you, and of course clear measurements will help you to figure out what is going on.

 

[substitute]

Your measurements make no sense at all. A resistance measurement must be taken between two points.
When measuring the choke, there are two measurements to take, across the choke, and choke to ground. Across the choke should be ohms, maybe up to 1 K. To ground should be as high as possible, many megohms.

Your point C measurement, is that milliohms or megohms, and what is the other end of it?

You need to give us accurate and clear measurements if you want us to help you, and of course clear measurements will help you to figure out what is going on.

All those measurements are from the noted point to ground, since we were discussing looking for a short I thought that was implied, apologies for being unclear.

Disconnect the 0.1u and see if the smoke stops.

Personally I'd leave it out.

Will do, thanks for stoping by prr.

 

[substitute]

Yup, disconnecting that cap did the trick. Working fine now. Thanks everyone.