[MRCA] More on the PRC-47 no transmit problem

[Ray Fantini]

Been years since doing anything with something as old as a PRC-47 being most stuff i work on thees day is all solid state but what the hell, couple questions.


One- Dose the radio work in receive? and is it in frequency correct?

Just like most transceivers this is real important. the same IF and RF sections (A3 and A4)with the exception of the PA are shared by the receiver and transmitter and if it works in receive and not transmit that excludes a lot of big problems. Also the frequency control is the same for both so if you can receive with no issues would assume that everything in frequency control is good, think that was A7 and A8 on those things. 

Two - did you try it in CW ?

Again most if not all SSB transmitters use a audio oscillator to generate the CW audio and feed that to the modulator. Rig up a CW key and use that for troubleshooting the transmit side and that's way better then messing around with a microphone and talking into it. Trying to do anythig with a SSB transmitter using a microphone is a waste of time. 
In the shop i use a two ton generator in place of a microphone and almost anyone who works on SSB transmitters is set up for that. Remember that the audio is what generates the power in the side band and using your voice will result in readings all over the place.

three- do you hear the HV supply coming up when you key the transmitter?

If the HV supply is never coming online there is an interlock or lockout occurring. will look around for the shop manual and maybe try to see what the keying sequence is for that radio and get back to you on this. But needless to say most of the transmitter control is good if the HV supply is activated being that's almost always the last thing to come on.

Hope this is all not just a waste of your time but find that if you cannot answer thees basic questions first your just shooting in the dark otherwise. Until you get the problem localized tube base voltages and resistance measurements wont be doing you much good, at least that's how I see it and be for warned that a lot of people will be glad to tell you that i am not exactly the sharpest tool in the shed.


Ray F/KA3EKH


________________________________________
From: mrca-bounces at mailman.qth.net <mrca-bounces at mailman.qth.net> on behalf of James Green <jagreen3 at sbcglobal.net>
Sent: Wednesday, April 4, 2018 3:49 PM
To: mrca at mailman.qth.net
Subject: [MRCA] More on the PRC-47 no transmit problem

I have pawed through all the components in the radio along the filament voltage circuit and have found nothing wrong with any of them. I must have wiggled something the right way by accident because I now get filament voltage.

So still no transmit.

I suspect either the A8A1 Audio Amp Module or the A8A2 Amplifier Modulator. I opened up A8A1 and tested all the transistors in circuit. They all look ok. Also the diodes OK too. There are a lot of lytic caps that may be bad, but I don’t want to start pulling them to test them except as a last resort.

I decided to measure the innies and outies of A8A1 to see if it is getting fed properly and what it is putting out. All the input voltages are ok.

The manual I have is kind of vague on what the outies should look like. The manual does say I should see 0.25VAC-RMS at A1J7. I get about 0.022VAC-RMS. The manuals says to adjust A1R27 the microphone amplifier gain to get the desired result. This is a multi turn pot. I ran it from one extreme to the other and saw no response at all at A1J7.

The manuals mentions low or no audio signal at the balanced modulator input (J3-3). J3-3 is kind of hard to measure without piercing the insulation on the back of the connector so I looked for a common point on the circuit. The measurements were inconclusive due to the fact that I don’t know what a low reeding would look like. I got about 2.7VAC-RMS. On the scope the frequency wandered around quite a bit.

After that I spotted the table in Figure 3-82 that showed expected resistances and voltages for the terminals on the filter box FL2. I went through the process of comparing voltages. I think there are some typos in this table. For example, Terminals 1 & 2 are carry the filament voltages. Measurements are to be made under 2 conditions. Powered by 26.5VDC or 115VAC. The table suggests that when using 26.5VDC you should measure 7VAC to ground. However when using 115VAC you should measure 2.9VAC to ground. This must be incorrect. That would mean the filament voltage would be twice as high using 26.5 VDC than using 115VAC.

The same is true for terminal 2. Since one end of the filament(s) are connected to term 1 and the other term 2 you would get double the voltage of either terminal measured to ground. I get about 3.7VAC to ground and about 7.4VAC measuring from term 1 to term 2.

Another typo is at term 10 you should get 5.2VDC RMS. Wait! What is the root mean square of a DC signal?  It makes no sense. Anyway, I got about 20VDC in receive mode and 0VDC in transmit. I only got about 33mVAC-RMS in transmit. I’m not sure what to do with this.

Of note, Term 4 should have -17VDC. I got 0VDC. Terms 5, 6 & 7 should all read -105VDC. They measure -38VDC for terms 5 & 6 & -108VDC for term 7.

I ran these experiments without the PL-177WA tube in the circuit. I’m worried about burning it out with all this monkeying around. This probably affected the results of terms 4,5,6 & 7. I’ll measure them again with the PL-177WA in circuit. Perhaps tomorrow.

The big question for today is Why do I keep blowing F-3 the 1/2 Amp fuse on the main chassis.

Jim




______________________________________________________________
MRCA mailing list
Home: http://mailman.qth.net/mailman/listinfo/mrca
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:MRCA at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html