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 Post subject: PRC-47 Transmit issues.
PostPosted: Feb Sun 04, 2018 5:30 pm 
Member

Joined: Mar Thu 15, 2012 10:28 pm
Posts: 99
Location: Madison, WI 53704 USA
I posted about this transceiver when I first got it. I was looking for pinouts for the U-77 connector. I found the answers I needed and posted the pinouts.

I picked up a whip antenna with legs and counterpoise. I also home brewed a SO-239 adapter so I can connect a standard antenna or dummy load. I also have a handset, morse code key, aviation headset with a mic & a high amperage 28VDC power supply.

I have gone in and switched out the mechanical filter so it now does LSB instead of USB.

I connected everything up as per the operators manual and using my SO-239 adapter I connected a Bird 43 watt meter and a Bird termaline dummy load. I set up the tuning as per the operators manual and put it into tune mode. I heard the T/R really switch and the switching transistors start their 400 Hz hum. However neither the power meter on the PRC-47 or the Bird 43 register any output. The needle doesn't even wiggle. Since it was advertised as functioning properly I contacted the guy I got it from. He came over and observed the same behavior. He was surprised it didn't transmit and said he would get it working for me. He took it home with him.

We have been in fairly regular communication and he has discovered the same problem with the other PRC-47s he owns. He has been working on the problem off and on for the last few months but has been unable to resolve the problem. I just exchanged emails with him and he says he is trowing in the towel. He plans to return it to me in a couple weeks.

I have a good original copy of the service manual which has a pretty good trouble shooting section. I plan to use this and see if I can resolve the problem myself. I expect I will have lots of questions and hope I can find answers here. I have looked for a more relavent forum, but have been unable to locate one.

I'm looking forward to this project I will let you all know when I get the radio back and set up on my bench.


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 Post subject: Re: PRC-47 Transmit issues.
PostPosted: Feb Sun 04, 2018 5:44 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5625
Good luck with it and I hope you get it operational. Mine was in good shape when I bought it but I also did the LSB modification. The transistors in the inverter are a known high failure item so checking that they are actually producing HV is a good place to start. The final tube is also known to be fairly short lived when operated in the high power position. Check to see if excitation is making it to the final.

One of the better sites covering issues is this one in case you haven't already come across it: http://k6jca.blogspot.com/2009/09/prc-4 ... tions.html

I haven't had mine on the air in a couple of years so I will plan to get it out again this Spring. Below are photos from its first testing after I went through it and installed the filter for LSB.

It is sometimes hard to believe that I will soon be teaching that little girl in the photo how to drive. The years are flying by.

Rodger WQ9E


Attachments:
PRC-47 panel.JPG
PRC-47 panel.JPG [ 142.97 KiB | Viewed 828 times ]
Anna transit case.JPG
Anna transit case.JPG [ 245.97 KiB | Viewed 828 times ]
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 Post subject: Re: PRC-47 Transmit issues.
PostPosted: Feb Mon 05, 2018 12:02 am 
Member

Joined: Mar Thu 15, 2012 10:28 pm
Posts: 99
Location: Madison, WI 53704 USA
Roger,

Thanks for the link. It looks like there are a couple things to look at. The AGC mod transmit problem may be a clue to follow. I emailed the blog site to the seller just to see is this is any help to him.

It looks like there are a lot of good mods there too.

One other mod I have been kicking around is making USB/LSB switchable without losing CW capability. Here is my concept:

Find a way to sandwich in both the USB and LSB filters.
Add a 28VDC relay to switch out the 2 filters.
Control the 28VDC relay using a magnetic reed switch glued to the inside of the aluminum cabinet. A small plate of steel should be glued to the outside & a pull apart connector used to tie the reed switch into the circuit so the case can be removed without ripping it out of the circuit.

Then to switch from USB to LSB simply place a magnet on the small steel plate. This closes the reed switch and energises the relay.

I have experimented with this concept using a similar aluminum case, a reed switch, a small plate of steel and a magnet. It seems to work!

What are your thoughts?

Jim
KG9MM


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 Post subject: Re: PRC-47 Transmit issues.
PostPosted: Feb Mon 05, 2018 2:37 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5625
Jim,

That sounds like a workable approach as long as you can find space for both filters. Magnetic reed switches ARE great for a lot of vintage gear projects. I used one several years ago along with a relay to repair an old timer's Hallicrafters receiver that was missing the AC switch on the response switch.

Rodger WQ9E


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 Post subject: Re: PRC-47 Transmit issues.
PostPosted: Mar Mon 05, 2018 9:32 pm 
Member

Joined: Mar Thu 15, 2012 10:28 pm
Posts: 99
Location: Madison, WI 53704 USA
OK, here is an update:

The guy I got my PRC-47 from was unable to get it transmitting so he returned it along with 2 other PRC-47s & said use anything from these to get a working unit. For reference I will refer to them as Units #1 (mine), #2 and #3.

Equipment used:
Bird 43 wattmeter
Bird termaline Dummy Load
30-watt regulated power supply adjustable from 12VDC to 33VDC

Note:
I read somewhere that a high AGC line can cause the PRC-47 to not transmit & that running it at lower voltage may get it to transmit.

#1:Uses 2N2287 switching transistors. Both test good. I tried to tune it at voltages from 22 to 30VDC and got no output.
#2: Uses 2N1166 switching transistors. I did not test them. I tried to tune it at voltages from 22 to 30VDC and got no output.
#3: I did not open the case yet so don't know what switching transistors are used in it. I started out at 28VDC and was able to tune it up to 20-Watts on low power and 60-watts on Hi power.

This is my base line. I will post periodic updates and I make new discoveries.


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 Post subject: Re: PRC-47 Transmit issues.
PostPosted: Mar Fri 09, 2018 8:56 pm 
Member

Joined: Mar Thu 15, 2012 10:28 pm
Posts: 99
Location: Madison, WI 53704 USA
The next update:

Today I removed the PL-177WA power output tubes from PRC-47 #1 & PRC-47 #2 and tested them in PRC-47 #3. All three PL-177WA tubes produced full power when properly tuned up in PRC-47 #3.

So far I know my switching transistors are good and my PL-177WA tube is good.

I guess that's progress.


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 Post subject: Re: PRC-47 Transmit issues.
PostPosted: Mar Sun 11, 2018 12:03 am 
Member

Joined: Mar Thu 15, 2012 10:28 pm
Posts: 99
Location: Madison, WI 53704 USA
Here is another update:

I have PRC-47 #1 (mine) and PRC-47 #3 The working one up on my bench. I started going through the short circuit tests. I found and fixed 2 problems with PRC-47 #1

First test point A5J1 to ground should measure 40K Ohms. It measured 30K Ohms in #1 and 44K Ohms in #3. I adjusted A5R4 so it now matches spec.
Second, A5J2 is supposed to be 50K Ohms #1 measured 18K Ohms #3 measured 50K Ohms. I traced the problem to a dirty PA power switch. After cleaning it I now get 50K Ohms. This may be the solution.

Still of concern are the following:
A5J7 should measure 15 Ohms. #1 measures 24.5 Ohms and #3 measures 13.8 Ohms. The manual says a high reeding may indicate the Inductor L1 may be open. It’s not open. I switched the power supplies between #1 and #3 and the problem remained with #1. Since L1 is in the power supply chassis L1 must be OK.

A5J8 should measure 30 Ohms. Both #1 & #3 measure close to 300 Ohms. This according to the manual it is likely a defective voltage regulator assembly TB1. I tried the power supply from PRC-47 #2 and it measures high too. Is the failure of TB1 a common problem?

A5J9 should be 8 Ohms. #1 is 22 Ohms & #3 is 11 Ohms.

A6J2 should be 200 Ohms. #1 measures 307 Ohms, #3 Measures 339 Ohms. The manual says Probably open A6L9. I have not yet checked A6L9.


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 Post subject: Re: PRC-47 Transmit issues. Regulator
PostPosted: Mar Sun 11, 2018 9:22 pm 
Member

Joined: Mar Thu 15, 2012 10:28 pm
Posts: 99
Location: Madison, WI 53704 USA
I pulled the regulator out of one of the 3 power supplies. I checked the parts and this is what I found:

The 9V zener diode was open.
Q1 a 2N697 transistor was open.
Q2 a 2N697 tested OK
Q3 a 2N1485 tested OK

I didn't have any 2N697 transistors on hand, but found a cross reference site that recommended a 2N3053. I have several of these.

I also found a working 9V Zener diode in my junk box. I am now soldering it back together.

The Original Zener diode was a 1N935A. It looks germanium. I don't think the one I am replacing it with is germanium. I hope this will not cause a problem.

It's kind of a pain digging out the regulator. It's a good thing I have another power supply to look at when putting it back together.


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 Post subject: Re: PRC-47 Transmit issues. Regulator installed
PostPosted: Mar Sun 11, 2018 10:35 pm 
Member

Joined: Mar Thu 15, 2012 10:28 pm
Posts: 99
Location: Madison, WI 53704 USA
I just finished putting the regulator back together and have it installed in the power supply. Now A5J8 to ground measures 24.5 Ohms. The manual states 30 Ohms and prior to the repair it measured close to 300 Ohms. I plan to look for some inexpensive original transistors and zener diodes to repair the remaining 2 power supplies on ebay or elsewhere.


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 Post subject: Re: PRC-47 Transmit issues.
PostPosted: Mar Mon 19, 2018 8:15 pm 
Member

Joined: Mar Thu 15, 2012 10:28 pm
Posts: 99
Location: Madison, WI 53704 USA
I have to rethink the voltage regulator problem. I must have been measuring something incorrectly. I have been out of town for a few days and I just started working on these radios again. Just to get back into it I re-checked all the resistances & found a couple things I didn't see before.

First, The voltage regulator repair did not lower the A5J8 to ground measurement. All three radios still measure about 260 Ohms.

Either my repair using equivalent parts was not successful, Something else is causing the resistance to go high or there is an error in the service manual.

I did find 2 bad transistors and one bad Zener diode. I ordered replacements original part number parts & will try repairing it again.

Second, A5J2 is suppose to measure 50K to ground. I discovered something else. PRC-47 #3 (working) measures 50K when the output power switch is in the low position. In the off and high position it measures about 35K. PRC-47 #1 (Mine still not transmitting) measures 50K in the low power position and around 18K in the off and high power position.

The manual does not provide initial settings for these measurements.

Does anyone know what the initial settings should be?


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