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Sorting out an RCA BTF-20E1 modification

S

scappy

We have an RCA BTF-20E1 in backup service. It was the station's main transmitter until 1999. It runs good, making our 19kw TPO with plenty of headroom and good efficiency.

Recently, I had the opportunity to compare it to another BTF-20E1 that was removed from service and discovered that mine's had some mods made along the way.

What stands out is that somewhere along the way, after new, non-PCB caps were installed, two caps were disconnected from the circuit. The caps in question are marked IC3 (4uF) and IC4 (10uF). These are in the screen supply, if I'm not mistaken.

Of course, the engineers before me left no documentation about the modification.

What would be a valid reason to remove these caps from the circuit?
 
Wonder if they shorted and were disconnected as a 'temporary' measure to get the station back on?
 
BobOnTheJob said:
Wonder if they shorted and were disconnected as a 'temporary' measure to get the station back on?
Click to expand...
I suppose that is possible. They are still in the cabinet, just disconnected. I haven't tested them yet.
 
Jon Scaptura said:
BobOnTheJob said:
Wonder if they shorted and were disconnected as a 'temporary' measure to get the station back on?
Click to expand...
I suppose that is possible. They are still in the cabinet, just disconnected. I haven't tested them yet.
Click to expand...

Jon,
As I recall, if you kept all of the modifications sent out by RCA over the years the volume would exceed the size of the original manual by several times. Removal of the screen supply filter capacitors doesn't ring a bell as a mod and could send 120 hz ripple into the screen, yet the transmitter would still work. Based on this, I would have to agree with BOTJ post that it may have happened during troubleshooting of a screen grid circuit issue and never re connected. Screen issues were rare but not unheard of, not as much as the well heralded PA cavity meltdowns as a result of worn finger-stock on the PA tuning and loading or what was thought to be very high third harmonic issues with the cavity. I would check them out as plate to screen arc may have occurred and that would exceed their voltage rating. As I remember, RCA employed a dedicated screen supply. It was Gates that used voltage divider off of the HVPS for screen (gutless wonders). Please correct me if I am wrong about that. Its been long time since I worked on those things and I am glad about that!

Best,

w/
 
Yes, I think those capacitors had to be replaced to get it back on the air. Do remember that it's a RCA BelchFire series rig. Tuning one is like milking a cobra for venom. Tune for MINIMUM SMOKE!
 
Yup! The RCA FireBall Series!! Been there, had one... We had the damn thing tore down quite a bit to clean and replate several parts on that pos chasing a spurious problem one time. Turns out it was a bad cap. The exciter could be muted and it would still put out power on another freq!
 
BTF20E1: I've been at home on the phone with another stations engineer while he cranked on the IPA tuning/loading telling him when the big spur would go away or come back. A carrier around +1 MHz and only down 20 dB.

Never had trouble with the screen supply on the one RCA I maintained. Burned up 2 plate transformers before RCA sent a proper high altitude blower for it.

Replaced a couple of the mechanical time delay relays with solid state ones. Before that it was fun to have someone watching as I gently tapped in a certain place and it would finally turn on.

The doorknob caps on the PA grid would come apart. Spent a lot of time bending those straps making sure there was no stress on them. What a pain.
 
In general, taking capacitance off of a screen grid supply circuit will "stiffen" the RF impedance of the circuit,
and probably reduce output. It may have been done to "fix" a problem.
Perversely, this may have stopped a spurious oscillation, just not in the best way...
That said, it doesn't appear to be the right way to fix a problem, and the unbypassed
screen grids may even create distortion as the audio modulation "loads and unloads" audio on them.

DC supply circuits need filter capacitors to ensure that the RF signal is only coupled through the amplifier
circuits in the intended path, not by any leakage or pickup through the power supply DC circuitry.
 
boiseengineer said:
BTF20E1: I've been at home on the phone with another stations engineer while he cranked on the IPA tuning/loading telling him when the big spur would go away or come back. A carrier around +1 MHz and only down 20 dB.
Click to expand...
If this model existed in the early 1970's, it must be what 105.1 in Cincinnati was using at the time. Every year when the weather warmed up, the signal was almost +/-1 mhz wide (it was down, but still clearly audible at 6 miles). A call to the station & it soon disappeared. Never happened when the weather got cold, just when it warmed up. Never had the misfortune of messing with one of these...living proof that God watches out for me.
 
"RCA, where FM means Fire Maker."

Not to derail the thread, but just a sidebar...What was the rig that caught fire on the NAB convention floor years ago? Don't think it was an RCA...
 
"RCA, where FM means Fire Maker!" Good line and the definitive quotable of the week. Brain trust, would be nice to know which make of transmitter which caught fire on the NAB show floor.
 
Nothing like walking into the transmitter room and seeing the right side of an RCA 40kw rig looking like an arc welder is working inside the PA cavity. Then, to top it off, having the DJ at the studio/transmitter combo site say "Oh, the readings on the "B" transmitter don't look right". A perfect way to start your day! Ah, the good ole days of RCA.
 
So I guess my takeaway from all of this is that I should invest in a fire extinguisher (or two!) for the transmitter site before testing those caps in the transmitter...

As for making power and being clean - it makes full TPO (19kw) with room to spare, and efficiency is better than the original factory spec. We did find spurs, but those were cured with new caps in the exciter. I guess we're not in that bad of shape, and perhaps I should leave well enough alone as long as it is working and legal.
 
A seperate isolated screen supply, if nothing else, does insure that there is no likely path for RF through common DC circuits.
If the exciter as used today is "cleaner" and maybe has more drive (with a good match) than those of the day this
rig was built, if it is making the power and any spurs are controlled, do not worry.
Assuming other parameters are within reasonable range.
If you DO decide to put this capacitance back into the circuit, I wouldn't worry, because today's cleaner exciter
signal will still probably be keeping spurs down. On the other hand, I bet you could back off the drive
if you reconnect the caps. Probably would not make any difference, but I would expect you could get a longer life out of tubes if ...well, the tubes would last just as long but they'd have more "reserve power" in operating curve range
if the screens had adequate rf bypassing. "You could drive 'em harder, longer as they aged.
With screens bypassed , they will hit a more definite wall in life/output.
 
With one of those darn things, if it works, I wouldn't even open it up! LOL! Seriously, be careful of the interlocks on that old POS. Ours worked about half the time correctly. RCA, the most rusted name in electronics!
 
OKCRadioGuy said:
With one of those darn things, if it works, I wouldn't even open it up! LOL! Seriously, be careful of the interlocks on that old POS. Ours worked about half the time correctly. RCA, the most rusted name in electronics!
Click to expand...
A picture's worth a thousand words...leave it alone! http://www.pbase.com/image/62939921
 
stacker said:
A picture's worth a thousand words...leave it alone! http://www.pbase.com/image/62939921
Click to expand...

Wow, great pic! Suppose it ignited itself? Or did some frustrated engineer fire-bomb it because he was tired to hitting the plate on button with a 10 foot stick?
Click to expand...
I have to wonder if the insurance companies have a special premium rate just for RCA FM's? Makes me love those old CCA's all the more...What a difference 1 letter can make!
 
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