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

[Tim]

Hi Jim - Regarding filament power:  Note that the PA filament is powered by
either 6.3 VAC RMS when the radio is powered by 115 VAC 400 cps - OR - a
6.3 VRMS square wave when the radio is powered by 28 VDC.  But the simple
test is pull the lower PA compartment cover off the front panel and see the
white hot PA filament glow while in transmit.  You should hear the 400 cps
power oscillator whine while in transmit, verifying that the HVPS power
oscillator is working...The PL-177 is directly heated "instant on" to save
battery power as others have noted.

Another hint to save your HVPS oscillator transistors while troubleshooting
the transmitter is to temporarily replace the 20 amp front panel fuse with
a 5 amp fuse - and only test in low power mode until you get it all sorted
out.  That 5 amp fuse will save your transistors during a fault..

Looks like you are into yours "up to your elbows" so you probably already
know all of this..

Good luck - they can be a pain to troubleshoot without the module cable
extender kit...

Tim
N6CC

On Wed, Apr 4, 2018 at 12:49 PM, James Green <jagreen3 at sbcglobal.net> wrote:

> 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
>
>
>
>
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