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TPRD-1554 Troubleshooting

Eric Grabowski
 

I'm working on this VHF duplexer our club acquired from another Ham. He doesn't know any of the history before he received it, so I don't know if this was factory ordered for the 2 meter band or if it was originally built for a high band commercial application and retuned somewhere along the line. Since I've never worked on any Telewave cavities, I'm hoping for a little help from those of you who have.

As received, each cavity was tested separately using a DSA815 spectrum analyzer with tracking generator and a high quality 50 ohm load. The insertion loss of each was 0.7 dB and the notch depth was at least -35 dB.

The only problem encountered was while tweaking the notch capacitor on the cavity closest to the antenna TEE in the receive branch. Touching the trimmer with the insulated tuning tool caused the notch to flatten out at about the 15 dB level. Turning the capacitor cause the trace to jump around wildly. Attempting to tweak it a little then removing the tool trying to luck out and hit the notch frequency was a waste of time. The tuning definitely appeared to be nonlinear.

I pulled the loop and it all seems to be intact. The solder joints look shiny and solid. So the question is: Does this indicate a bad capacitor?

The only suspicious area is the top of the plate which has a light layer of green residue, which is probably from the humid environment it lived in. By the way, the plates on all the other cavities have the same amount of green residue and they tune up fine. So might it be possible that this particular capacitor is not making a good physical ground connection to the plate? I should add that all of the capacitors had their protective caps installed so I don't believe any dust particles are to blame.?

I'm also curious about the cable lengths. The inter cavity cables are the same length for both the transmit and receive branches. They are RG-214 precisely 12.125 inches long measured tip to tip. I would have expected the lengths to have been slightly different because of the different frequencies, but maybe at a 600 kHz split the difference is negligible since it's only about one tenth of an inch or less. Comments?

The other length I'm curious about is what Telewave refers to as the phasing cables. These are the cables between the cavities and the antenna TEE. Their documentation states these cables should be 1/2 wavelength at the reject frequency. The lengths of the RG-214 cables on this duplexer are both the same and only 13.5 inches long tip to tip.These seem way short since 1/2 wave cables at these frequencies is 26 inches and change. Comments?

73 and aloha, Eric KH6CQ


? ?

 

Any time a cap jumps around it is bad and the green junk is a sure sign of water



On Saturday, January 16, 2021, 8:20 PM, Eric Grabowski via groups.io <ejgrabowski@...> wrote:

I'm working on this VHF duplexer our club acquired from another Ham. He doesn't know any of the history before he received it, so I don't know if this was factory ordered for the 2 meter band or if it was originally built for a high band commercial application and retuned somewhere along the line. Since I've never worked on any Telewave cavities, I'm hoping for a little help from those of you who have.

As received, each cavity was tested separately using a DSA815 spectrum analyzer with tracking generator and a high quality 50 ohm load. The insertion loss of each was 0.7 dB and the notch depth was at least -35 dB.

The only problem encountered was while tweaking the notch capacitor on the cavity closest to the antenna TEE in the receive branch. Touching the trimmer with the insulated tuning tool caused the notch to flatten out at about the 15 dB level. Turning the capacitor cause the trace to jump around wildly. Attempting to tweak it a little then removing the tool trying to luck out and hit the notch frequency was a waste of time. The tuning definitely appeared to be nonlinear.

I pulled the loop and it all seems to be intact. The solder joints look shiny and solid. So the question is: Does this indicate a bad capacitor?

The only suspicious area is the top of the plate which has a light layer of green residue, which is probably from the humid environment it lived in. By the way, the plates on all the other cavities have the same amount of green residue and they tune up fine. So might it be possible that this particular capacitor is not making a good physical ground connection to the plate? I should add that all of the capacitors had their protective caps installed so I don't believe any dust particles are to blame.?

I'm also curious about the cable lengths. The inter cavity cables are the same length for both the transmit and receive branches. They are RG-214 precisely 12.125 inches long measured tip to tip. I would have expected the lengths to have been slightly different because of the different frequencies, but maybe at a 600 kHz split the difference is negligible since it's only about one tenth of an inch or less. Comments?

The other length I'm curious about is what Telewave refers to as the phasing cables. These are the cables between the cavities and the antenna TEE. Their documentation states these cables should be 1/2 wavelength at the reject frequency. The lengths of the RG-214 cables on this duplexer are both the same and only 13.5 inches long tip to tip.These seem way short since 1/2 wave cables at these frequencies is 26 inches and change. Comments?

73 and aloha, Eric KH6CQ


? ?

 

If it's the cavity on the antenna side, more than likely it took a lightening hit.

Replace the capacitor and you should be fine. Just make sure you get the highest voltage one you can find.

I would pull the other loops apart and clean up the green stuff.

Joe - WA7JAW
 

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Yep bad cap. I have had that happen a couple times. Also the BPBR coupling loops for the VHF telewave cavities for 2M are a tad longer than those use in the commercial region. There was a write up awhile back out of VK land that covered this.?

I have found the 4 cavity filter lacks enough isolation. I suggest adding two 5¡± bandpass cavities on the RX leg. I found this to work very well.?

Last year I built a 5 channel TX combiner 1-10¡± band pass and 2 or 3 BPBR cavities per channel. RX side was 2 5¡± BPBR and 2 5¡± Bandpass. Everything had >90 dB of isolation in just the filters. The. Another 40 dB of isolation. Between the 2 two bay stacked VHF Dipoles. All the systems really work. No desense and the receivers have excellent Bandpass isolation.?

My .02.?

Jim.?


On Jan 16, 2021, at 5:45 PM, Jack Mcelyea/n4jem via groups.io <n4jem@...> wrote:

? Any time a cap jumps around it is bad and the green junk is a sure sign of water



On Saturday, January 16, 2021, 8:20 PM, Eric Grabowski via groups.io <ejgrabowski@...> wrote:

I'm working on this VHF duplexer our club acquired from another Ham. He doesn't know any of the history before he received it, so I don't know if this was factory ordered for the 2 meter band or if it was originally built for a high band commercial application and retuned somewhere along the line. Since I've never worked on any Telewave cavities, I'm hoping for a little help from those of you who have.

As received, each cavity was tested separately using a DSA815 spectrum analyzer with tracking generator and a high quality 50 ohm load. The insertion loss of each was 0.7 dB and the notch depth was at least -35 dB.

The only problem encountered was while tweaking the notch capacitor on the cavity closest to the antenna TEE in the receive branch. Touching the trimmer with the insulated tuning tool caused the notch to flatten out at about the 15 dB level. Turning the capacitor cause the trace to jump around wildly. Attempting to tweak it a little then removing the tool trying to luck out and hit the notch frequency was a waste of time. The tuning definitely appeared to be nonlinear.

I pulled the loop and it all seems to be intact. The solder joints look shiny and solid. So the question is: Does this indicate a bad capacitor?

The only suspicious area is the top of the plate which has a light layer of green residue, which is probably from the humid environment it lived in. By the way, the plates on all the other cavities have the same amount of green residue and they tune up fine. So might it be possible that this particular capacitor is not making a good physical ground connection to the plate? I should add that all of the capacitors had their protective caps installed so I don't believe any dust particles are to blame.?

I'm also curious about the cable lengths. The inter cavity cables are the same length for both the transmit and receive branches. They are RG-214 precisely 12.125 inches long measured tip to tip. I would have expected the lengths to have been slightly different because of the different frequencies, but maybe at a 600 kHz split the difference is negligible since it's only about one tenth of an inch or less. Comments?

The other length I'm curious about is what Telewave refers to as the phasing cables. These are the cables between the cavities and the antenna TEE. Their documentation states these cables should be 1/2 wavelength at the reject frequency. The lengths of the RG-214 cables on this duplexer are both the same and only 13.5 inches long tip to tip.These seem way short since 1/2 wave cables at these frequencies is 26 inches and change. Comments?

73 and aloha, Eric KH6CQ


? ?

 

As received, each cavity was tested separately using a DSA815 spectrum analyzer with tracking generator and a high quality 50 ohm load.
The insertion loss of each was 0.7 dB and the notch depth was at least -35 dB.
35 dB notch depth is a bit low with loops set for 0.7 dB IL.

Touching the trimmer with the insulated tuning tool caused the notch to flatten out at about the 15 dB level.
That indicates the capacitor is shorting out. Piston trimmer caps are rather fragile beasts. There are several concentric plates in the piston and stator. The slightest eccentricity will cause them to short. It is almost guaranteed to need to be replaced. You can unscrew the rotor all the way, and, using a magnifying glass, look closely at the plates on both sides. I bet you'll see either a slight distortion, pit, or something that will be the reason for it shorting out.

I'm also curious about the cable lengths. The inter cavity cables are the same length for both the transmit
and receive branches. They are RG-214 precisely 12.125 inches long measured tip to tip.
I would have expected the lengths to have been slightly different because of the different frequencies,
but maybe at a 600 kHz split the difference is negligible since it's only about one tenth of an inch or less. Comments?
Between adjacent cavities - cable lengths would not vary because the frequencies are so close. Between cavities and the antenna tee, the correct length is a function of Z of the opposing cavity at its notch frequency (i.e. at the opposing side's pass frequency), with the goal of rotating that Z to be back at the T to make it appear as an open circuit. Many times, but not always, the lengths to the tee end up being equal.

The other length I'm curious about is what Telewave refers to as the phasing cables. These are the cables
between the cavities and the antenna TEE. Their documentation states these cables should be
1/2 wavelength at the reject frequency.
Telewave is decidely wrong.

The lengths of the RG-214 cables on this duplexer are both the
same and only 13.5 inches long tip to tip.
These seem way short since 1/2 wave cables at these frequencies is 26 inches and change. Comments?
Optimized cable lengths for the ham band:

Between cavities: 12.125" RG214 tip to tip

Tee to first cavity on each side: 12.625" RG214 tip to tip

The latter works out to about 84 degrees...nowhere near 180 degrees.

--- Jeff WN3A


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Eric Grabowski
 

Jeff,

Thanks for you comments and especially for the cable lengths.

There was no evidence of arcing on the capacitor's body so I don't think it was damaged by a lightening strike.

Do you have any idea what the value and voltage rating of the capacitor should be?

And could you recommend a preferred vendor?

73 and aloha, Eric KH6CQ

On Sunday, January 17, 2021, 5:52:39 AM HST, Jeff DePolo WN3A <jd0@...> wrote:


> As received, each cavity was tested separately using a DSA815 spectrum analyzer with tracking generator and a high quality 50 ohm load.
> The insertion loss of each was 0.7 dB and the notch depth was at least -35 dB.

35 dB notch depth is a bit low with loops set for 0.7 dB IL.

> Touching the trimmer with the insulated tuning tool caused the notch to flatten out at about the 15 dB level.

That indicates the capacitor is shorting out.? Piston trimmer caps are rather fragile beasts.? There are several concentric plates in the piston and stator.? The slightest eccentricity will cause them to short.? It is almost guaranteed to need to be replaced.? You can unscrew the rotor all the way, and, using a magnifying glass, look closely at the plates on both sides.? I bet you'll see either a slight distortion, pit, or something that will be the reason for it shorting out.

> I'm also curious about the cable lengths. The inter cavity cables are the same length for both the transmit
> and receive branches. They are RG-214 precisely 12.125 inches long measured tip to tip.
> I would have expected the lengths to have been slightly different because of the different frequencies,
> but maybe at a 600 kHz split the difference is negligible since it's only about one tenth of an inch or less. Comments?

Between adjacent cavities - cable lengths would not vary because the frequencies are so close.? Between cavities and the antenna tee, the correct length is a function of Z of the opposing cavity at its notch frequency (i.e. at the opposing side's pass frequency), with the goal of rotating that Z to be back at the T to make it appear as an open circuit.? Many times, but not always, the lengths to the tee end up being equal.

>? The other length I'm curious about is what Telewave refers to as the phasing cables. These are the cables
> between the cavities and the antenna TEE. Their documentation states these cables should be
> 1/2 wavelength at the reject frequency.

Telewave is decidely wrong.


> The lengths of the RG-214 cables on this duplexer are both the
> same and only 13.5 inches long tip to tip.
> These seem way short since 1/2 wave cables at these frequencies is 26 inches and change. Comments?


Optimized cable lengths for the ham band:

Between cavities:? 12.125" RG214 tip to tip

Tee to first cavity on each side: 12.625" RG214 tip to tip

The latter works out to about 84 degrees...nowhere near 180 degrees.

??? ??? ??? ??? --- Jeff WN3A


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I worked on a TPRD-4544 many years ago that was not tuning properly. I had a feeling the coax used to form the coupling loop was somehow defective. I ended up buying two new coupling loop assemblies brand new from Telewave. They had them in stock, complete, ready to screw into the cavities. I probably had to tell them whether they were for the high-side or the low-side, and the approximate frequency range, all obtained from the label. Something tells me they were around $50 each. I installed them, the duplexer tuned up perfectly, and exceeded specifications.
Eric Grabowski
 

Bob,

Thanks for that info. I'll keep it in my back pocket in case my repair doesn't work out.

73 and aloha, Eric KH6CQ

On Tuesday, January 19, 2021, 2:26:19 AM HST, Bob M. <wa1mik@...> wrote:


I worked on a TPRD-4544 many years ago that was not tuning properly. I had a feeling the coax used to form the coupling loop was somehow defective. I ended up buying two new coupling loop assemblies brand new from Telewave. They had them in stock, complete, ready to screw into the cavities. I probably had to tell them whether they were for the high-side or the low-side, and the approximate frequency range, all obtained from the label. Something tells me they were around $50 each. I installed them, the duplexer tuned up perfectly, and exceeded specifications.

 

Do you have any idea what the value and voltage rating of the capacitor should be?
I'm out of town on a job so I can't check my notes, but from memory it should be 1-14 pF, Johanson 5402 or similar.

And could you recommend a preferred vendor?
Jeff's parts stock. Email me direct, I'll be back in town this weekend.

--- Jeff WN3A




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