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

[James Green]

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