HRO-60 Circuit Mystery

[ashart]

I've been repairing a lovely-looking but not-yet-fully-working HRO-60 that I just bought, and have been making good progress.  However, I've encountered a circuit peculiarity that I'm hoping some experienced HRO-60 maven can explain.

According to my schematic, the plate of V5, the 2nd converter, has a 3pf cap, C135, the other side of which is connected to the junction of C32, an 0.01 cap, and R14, a 2200 ohm resistor that goes to B+.

Why there??

The 0.01 C32 effectively grounds all AC at the junction, and no DC can make it through the 3pf cap, so why is the 3pf cap connected there and not just connected to ground???

Was it merely convenient to use that particular point in the receiver for an AC ground point?

Is it just a schematic blunder?

Do I need to go back to engineering school?

Relevant answers hereby solicited.

Tnx.

-al hart, w8vr
  

[w3jn]

If you trace the switch out, it appears the 5 pF capacitor is actually switched in across the primary of L12, probably to compensate for detuning that transformer when the second converter is switched in.

[ashart]

3JN:

Yes.  At first blush, the cap was seen as permanently connected as I had stated, but upon considering your response, the plate is functionally out of the circuit when the 2nd converter is switched out.

You said 5pf and I read my admittedly-fuzzy internet-downloaded schematic as 3pf - I'll check that out too.

I suppose there is some plate-to-other-electrodes stray capacitance even when the plate is switched out, but your observation that the 5pf cap is switched across the L12 primary indeed makes some sense.  

Yet, where the lower end of L12 is AC-grounded by C32, then the 5pf cap would be switched across the primary of L12 anyway if its lower end were merely grounded.  Perhaps it's physically wired there to minimize lead length?  At higher freqs, It wouldn't take much wire length for its XL to compromise the effect of 5pf.

Now I gotta get that silly 2nd converter working.  The HRO seems to be working just fine on the lower bands (subject to final re-alignment), but the higher-freq coils using the 2nd converter are as dead as the proverbial doornail.  I haven't looked at it yet, and hope it's just a wiring or component problem and not the switch.  I'm off at this very moment to warm up my voltmeters and scope!

Thanks again for your wise observation!

-al hart, w8vr.

[ke7trp]

Ahhh.. A reciever mystery.  According to Pork fat, I am the only one with Reciever mysterys. 

I cant add much to the discussion about the circuit. I am just a kid.  However, My HRO 50T had 2nd conv trouble. I found the risistor at the screen was toast.  When I went to check it with the meter, it crumbled.  After replacement it fired up.  When those bypass caps popped and cracked years ago, It likely took the resistor out.   I was told to check all the resistors and after doing that, I found a handfull of bad ones.

After saving a few recievers this year, I learned to just check/replace every component ONE tube at a time.  Its funny how the thing starts to work better and better.

Good luck and please take video or at least some pictures!  I would love to see the sixty on the bench.

C

[W1DAN]

Gee...

I thought the HRO-50 was a single conversion! :-)

Dan

[ke7trp]

Ha.. Yes It was the 2nd IF amp! 

C

[ashart]

Well, when last walking out of my workshop, the HRO-60's 2nd converter was quite dead.  When I returned to the project the next morning, the stupid thing was working just fine.

No amount of poking, wiggling, pounding, line-voltage fiddling, turning it on and off, heat-gun application, nor cold-can spray, would make it stop. 

I think I'm going to give up radio and run away and join the circus.

-al hart, w8vr

[The Slab Bacon]

Ahhh.. A reciever mystery.  According to Pork fat, I am the only one with Reciever mysterys. 

Hmmmmmmm.....................   

[KM1H]

Replace R-31 just on spec; Ive had 3 or so that were intermittent, drove me bonkers the first time.

Carl