Does anyone have one of these?

I need a schematic for the little fused attenuation pad that attaches to the RF input/output.

I lost this accessory.

Thanks,

Eddie

20 db attenuator as I recall. Same on the CE-5.

Download the manual for the CE-5 here ( warning, it's a 22 meg download):

http://www.rfstuff.com/manuals/cush_ce5.pdf

The attenuator is on page 6-115. But it's easier to buy another on eBay. Somewhere between $ 10-15 usually. The resistors are usually toast due to some bonehead transmitting into the Cushman but they are available from Mouser or DigiKey.

Here's a thread from another forum with good comments about the pad, specifically "don't try to build it":
http://www.eham.net/ehamforum/smf/index ... ic=36571.0

It's a 20dB pad, so the output voltage with the pad in place is 1/10th the value without the pad, provided your load is actually 50 ohms. If your load is not 50 ohms, the generator output with the pad will be lower than the calibrated value, but the actual level is not known. It can be calculated if you know the actual load impedance.

The 1/10th amp fuse was there because the service monitor was used with two-way radios. If you accidentally keyed the transmitter while connected to the generator output, you would blow the fuse (if it was there) or the attenuator.

For our use with vintage receivers the fuse is not needed (but you do need a DC blocking capacitor).

- Leigh

I wouldn't let that comment in the previous forum post regarding "not trying to build it" dissuade someone from trying to clone a CE-3 pad. The CE-3 attenuator is fairly crude and would be very easy to duplicate. I've rebuilt them numerous times for both CE-3 and CE-5 analyzers when our shops were using them. They weren't precision devices by any stretch of the imagination and were intended to be field repairable. The only trick is to make sure the resistors are purely carbon comps and not spiral trimmed to avoid unwanted inductances.

Pete

Thanks for the link to the CE-5 manual. The 20 db attenuator appears a simple pie resistive circuit. I modified the circuit to place the 1/32 A PICO fuse as the first element.(As opposed to after the first 62 ohm resistor. This way the first 62 ohm resistor is protected as well. I don't think its placement is that critical to the impedance.

I built the circuit tonight, and measured the input and output resistance to be actual 50.77 ohms.

The specified values were 220 ohms in series with the fuse input to output, with 62 ohms input to ground, and 62 ohms output to ground.
1/4 watt, 5% tolerance.

I wasn't too hard to build.

I read your posts on generator test leads and using a blocking capacitor to protect test equipment.
I wanted to keep my Cushman original so I followed their diagram as far as using a fuse.

What do you think about the use of a fuse as protection? A capacitor gives more protection and doesn't need replacing, but it does alter impedance.

Eddie


.

After some thought...I see the necessity of a blocking series capacitor when working with vintage/ tube high voltage circuits. Otherwise one would be constantly replacing fuses.

The capacitor only blocks a DC voltage. The fuse was intended to protect the attenuator resistors from a transmitter being accidentally keyed into the signal generator port! It is possible for a transmitter to damage the internal attenuator circuits, hence Cushman used a sacrificial, and repairable, external attenuator.

Pete

You forgot one very important consideration: the resistance of the fuse. That was taken into account when Cushman designed the pi attenuator values. Moving the fuse probably affects the calibration.

I'd bet that fuse measures between 20 and 30 ohms resistive. A 20 dB pi attenuator would use 61 ohm resistors and a 248 ohm resistor. The extra resistance comes from the fuse being in series with the 220 ohm resistor in the Cushman attenuator.

Pete

Good point Peter. I didn't measure the fuse resistance, and moving it creates a voltage divider. I'll try changing to the original design and make some measurements. I couldn't source the original fuse, so I substituted a different manufactuer.

Thanks again,

I don't really see how the CE-3 is going to be of much use on old radios since it's a VHF-UHF instrument and the coverage ranges are segmented rather than continuous, as I recall. There was an "IF / HF" plug-in which provided for use on IF and something like 1-30 MHz frequencies but I can't recall whether that worked with the CE-3 as I only used them on the CE-4 Series, and I suspect you don't have one anyway.

The main problem with the fuses in my experience is that some person has already put a 5 Amp in there after blowing the stock one, and then burned up the resistors. As Peter points out, the fuse is not where you would expect it to be. There are two or three different part numbers and layouts for this attenuator, they are somewhat different. One has the fuse showing through a hole in the housing, another has the fuseholder mounted on the housing. Cushman shows "phantom" capacitors in some of the attenuator schematics, which the parts list calls something like "overall circuit capacitance" and represents spacing to ground plane of the various traces. That may be where the "don't try this at home" warnings come from. But does it really matter all that much? I doubt it.

One annoying issue with instruments like this and the Systron Donner competitors is that they do not have pure outputs. In other words, the output (and input) has spurious signals and so on, away from the frequency of interest. This doesn't present a problem with something like a single frequency police radio, but using the instrument to tune around a broad spectrum and analyze unknowns can get frustrating.

Vishay claims their current product carbon film 1/8 Watt through-hole resistors are non-inductive. So far I believe them. If they aren't, it's outside the range of anything I have to measure them with.

I wasn't planning on using my CE-4 to service antique radios. Although I do restore a few antique radios and other electronics, I also dabble in Vhf communication receivers, etc. My actual trade is an Automation technician for the Natural Gas Industry. I find this forum to be a great resource for all types of electronic equipment.

As I stated I lost my attenuator at some point. I used to carry the Cushman CE-3 in my service van when servicing commercial land mobile radio many years ago. I remember the layout was pretty straight forward. I just couldn't remember the part values. Thanks to you guys, I have the manual and did find the page 6-115 with the attenuator..

Fortunately I had a Die cast Palmora box with connectors and resistors in my junk box. I 'm having a problem finding the fuse: 1/32 A GFA Buss fuse. I remember the original was a little round clear cylinder, but the available crosses to one quite different with axial leads. There are several to chose from. I guess I will have to measure and experiment, I calculate the fuse resistance needs to be 39 ohms which when combined with the 220 ohm resistor, shunted with 62 ohms on each end yields 50.02 ohms.

It probably isn't all that critical, but it is a fun challenge to make it the best I can.

Eddie

Hi Eddie

I was a communications department leader for 31 years, and I have repaired CE-3 and CE-5 service monitors since the 1960s. In fact, I own a CE-5 and a CSM-50A/TG; although I rarely use them, or have a need to use them in my home shop. The CE-3 was an awesome instrument decades ago, but the lack of
coverage for the 440 - 450 MHz ham bands limits their popularity.

Pete

If you are homebrewing the attenuator then you would also need the bi-pin fuse socket which is a bit hard to find.

You could use a picofuse and solder it to the circuit, somewhat less convenient but it works. I don't know how close the resistance is on those.

There is a Cushman Yahoo Group with some files and some members who have parts rigs and scrap stuff. For some reason it seems to be on Yahoo Canada but the basic Yahoo Groups search will find it.

Once upon a time a few decades ago it was a dream to someday have a personal Cushman CE-3. Time marches on; last year I bought a fully working and clean CE-6A for $ 25 at the electronics flea market. An original manual cost me $ 40 on eBay.