The appeal of a high bandwidth vertical amplifier relates to both amplitude accuracy over a range of frequencies as well as slew rate and overshoot resistance.

Slew what?

For an HF station monitor you'll be observing variations on an AC sine wave with moderate rise and fall times. On a test bench, square waves can a be a challenge to display accurately as a true square wave can go from 0 to 60 volts instantaneously on the rise with the inverse rate on the fall. The bandwidth at the transition can be factors of 10 greater than the nominal frequency and that's why Stereo Review liked to measure the square wave response of an audio amplifier... Slew rate of a near instantaneous rise, overshoot, DC response on the flats (top & bottom), slew rate of a near instantaneous fall. In terms of bandwidth that's one hell of a ride to display perfectly and the truth is no amplifier can reproduce a square wave perfectly. Not the amplifier under test, the oscilloscope, or the function generator.

So why do you need 100+ MHz of bandwidth to observe an SSB, CW or digital signal at 28.400 MHz?

You don't.

No HF amplifier or PA section compliant with Part 97 radiates a true square wave. If it did the spurs and harmonics would be fugly, and a reasonably modern KenYaeCom HF transceiver has switched lo-pass filters to make sure out of band components are suppressed. I agree with W2AEW that a 20 MHz 'scope is adequate for HF signal monitoring. It's also true that clean used 60 MHz 'scopes have been historically both plentiful and affordable. The Tex 22xx series was very popular and for good reason........... They trigger reliably far above their bandwidth spec, are easy to work with, and if you want an X-Y display "can do" at the flip of a switch.   

Good Catch!

One other rare but completely frustrating issue that masquerades as a bad AC ripple condenser is the ol' heater to cathode short in the 1st audio tube or phase inverter. B+ looks clean, yet there's a hummmmmmmmmmm that just won't quit. Sometimes a tap of the tube will blip the hummmmmmmmmm and that's a clue, other times the set plays fine until it warms up for five minutes or so. Then the hummmmmmmmmmmm starts to sneak into the audio.

Freeze spray can be a useful diagnostic tool but never, ever let it splash on the glass of a hot tube....... Instant stress fracture followed by extreme gassiness.

Yuck.   

Survey Says:   YES!

But there are two things to consider when assessing risk:  Probability and Severity.

While the severity of a mini-nuclear blow out is potentially high, IMHO the probability is low.  Remember that it takes a fair amount of moxie to steam a twist-lok can to the point of rupture and the chances of the secondary winding / rectifier combo in a typical receiver having the Joules to make that happen are low. Diathermy machine, different ballpark. What is more likely is the little black rubber plug in the phenolic cap on the base blowing out / drying out far enough to release nasty goo from 1962..........

On a related note, many moon ago I tore down a hi-amp computer power supply. Beer can sized electrolytic felt like it had a golf ball inside when I pulled it. Mr. Fluke sez it's wide open, best guess is the innards globulated in the heat of catastrophic failure.

To look at it you'd never know there had been a problem...   

...you're about to get an education in German radios.

Sometimes difficult to work on because the point to point wiring is free-form compared to a Filco, more tuned circuits with some double-tuned, and the piano key function switches are a marvel of potential confusion. They're also real big on double and triple section tubes. I have an old Grundig that's a mono radio with a stereo amplifier so an external turntable can do the left-right thing through the DIN jack.

Plays fine, but the audio output tube is a 9 pin jobbie that's like two 6AQ5's in the same envelope. Rare in the day and probably unobtanium today. So if that tube ever goes toes up............

BTW:  The T-Funk you have was not top of the line but certainly worth restoring. Don't expect super reception or big audio as it's an AM/FM five tube superhet, and that's a lot to ask from five tubes. But, on the flip side, five tubes are your idea of a good time... Even if it does have a transformer power supply.

You'll also learn that Rohren = Vacuum Tube, Verstarker = amplifier, UKW = VHF (oder "FM")

If you can't find a free schaltplan, poke around here for similar models: http://www.4tubes.com/SCHEMATICS/Radios/Telefunken/index_img/

Not sure I'd say "very", but the shielded balanced line will have approximately the same characteristics as the coax it's made from in terms of loss and voltage ratings. Should be somewhat more forgiving of a mismatch as well. And don't forget that you can do this with RG-59 or RG-6 for a 150 Ohm balanced line. Given the cost and availability of RG-6 this might be worth considering. Especially if the line needs to run underground or in a setting that would be difficult for ladder line. The shields should be tied together at both ends and some references will tell you they should be grounded at both ends. I prefer grounding only one end as that's an old audio trick for avoiding a ground loop, which could be completely irrelevant to this concept....

BTW:  Now that I think about it, "very" high loss if we're talking about RG-174 in parallel.   

If you want to test the transformer first, remove the 5Y3, 5U4 or whatever the rectifier tube might be, then give it 120 vac straight up the power cord. This will juice the transformer and filaments with an opportunity to check the secondary windings. Do not worry about the actual voltages as long as they're in the ballpark... With no load they won't match the squidmatic values... But any center tapped windings should be checked for balance.

Both sides equally hot? That's good. Filaments lit? Even better.

You will see opinions regarding a low-voltage conditioning cycle through a Variac to re-form the electrolytics, but I wouldn't drop the coin for a Variac if I didn't have one. You can haywire an Edison lamp socket in series with one side of the AC line then use a 40 Watt lamp to give it a lo-volt experience. A 60 watt could be better, or maybe a 75 watt jobbie. What's the rated draw for the radio? Remember that with filaments at half power or less the B+ through the 5Y3 is going to be considerably below nominal and a weak electrolytic might be un-stressed at that level. So you really won't know if it's good until it sees full tilt DC.

Looking at it that way, doing the lo-volt startup is needless.

Especially if you assume the electrolytics are on your to-do list. Which is not a bad assumption. I've been known to test the transformer first, replace the electrolytics next, then do my first power up at full voltage. Just like I'll be doing from that point forward. If something smokes, better it should go before I finish the rebuild........... 

So there's this Irish guy who moves to Cleveland.

Takes a walk one night and comes across a place advertising itself as an authentic Irish pub not more than a block away from his new apartment. Walks in, looks around, takes a seat at the bar and asks for three pints of stout. Bartender pours one pint and tells him he'll pour the second pint when he's ready for it. Guy says no, what I want you to do is pour three pints so I can line them up on the bar in front of me.

Bartender does that, then notices his new customer likes to take a sip from each pint in a rotating sequence.

Asks the guy why. Tells him he's one of three brothers and with him in Cleveland, another in Seattle, and the youngest in Dublin, having three pints lined up like that is the next best thing to them being together and sharing a drink. So the new guy becomes a regular and the bartender knows to line up three pints whenever he sees him coming through the door.

Until one evening when the customer stops him from drawing the third pint. No more, he says, from now on only two pints.

The bartender pauses for a moment, then asks if his brothers are OK.

Yeah, sure, they're fine, but it's me that has the problem. Went to the doctor this afternoon and he told me I need to stop drinking.................................

Which reminds me of a story about British money..........

As we all know, a Zombie's favorite meal is human brains. So this Zombie moves to a new town. On his first weekend he stops by the local Zombie food mart to see what they have to offer. In the deli section he sees that farmer brains are $12 a pound, accountant brains are $18 a pound, and lawyer brains are $42 a pound. He mentions to the clerk that he didn't realize lawyer brains had become that expensive............

Oh yeah, they've always been high. You wouldn't believe how many lawyers it takes to harvest a pound of brains.

Or, as I like to say: " A pound doesn't go as far as it used to when you're in need of brains...."

Allegedly, back in the 1930's when Crosley Broadcasting had a waiver from the FCC to operate WLW in Cincinnati, OH at half a megawatt, the brightness of streetlights in that part of town would pulsate slightly in step with the modulation.

You can find more here:  http://www.ominous-valve.com/wlw.html

Even at 50 gallons the station still puts out a manly signal, allegedly due in part to the Blaw-Knox transmitting tower which is extra wide in the center of its length... And that's supposed to be where the current node in the radiation pattern lives. Phattest part of the tower radiates the phattest part of the RF. Or, at least that's the plan...........

You have to remember that in New Jersey it's possible to buy certain goods at a significant discount........

They're usually described as having fallen off the back of a truck.

 

Without looking at the schematic, I have a hunch the key phrase in the original question was: "...to the source of Q2 in the VFO section"

If the condenser is there only as a DC block the value isn't critical. But, as you're aware, larger condensers will pass lower frequencies more readily and 5 mmf is a very, very small amount of capacitance.  Which leads me to suspect the 5 mmf value was chosen to act like a high pass filter in a circuit relying on the overtone of an oscillator.

I wouldn't stray much more than 50% and that takes you all the way up to 7.5 mmf.  Which isn't very far, and if Heathshkit could have subbed a more common value (like 68 mmf) I suspect they would have.......   

...that settles it!  I'm going to take up a collection at my local Hamfest next month to get a few more for you.

Apparently they're more appreciated in the UK than New Jersey.  And to think that Bendix built them in Baltimore....   

Yeah, but has it been rust and fungus proofed?

If you open up the battery compartment to find mushrooms growing there, I'd say that's your answer................

In other news, I'm one of the lucky bastards who can say they own a BC-221. I also regret not having used it in many moon.  My Bad.  No doubt on long winter nights the metal tubes inside long for the warmth of a filament glow only they can see... But, just for grins & giggles, I surfed over to BAMA to download this bad boy:  http://bama.edebris.com/download/military/bc221/bc221.djvu

OMG !!!  The BC-221 really is a fine quality analogue signal generator with potential accuracy far in excess of lesser designs like the IGGY-42 you spent so much time.... Uhhhhh... How should I say... " Polishing A Turd " ...?  Live & Learn.  All I know is that someone with exquisite taste in electro-militaria collectables is going to find a home for that BC-221 and never regret it........ Or need to clean out 69 years worth of fungus inside.

 

Years ago one of the Ham magazines did a write up on some of the more unusual items that Unca' Sam had on his big olive drab test bench. The BC-221 was one of them...  Not because of the design sophistication or exceptional parts quality, but because each meter had an extensive calibration procedure.

IIRC, each unit coming off the line was tuned to a long list of calibration frequencies.  The knob settings for each frequency was recorded on a typed reference card specific for that meter, possibly in duplicate.  The field operator would tune an unknown signal then compare the knob settings against the calibration card.  This was a very time consuming manufacturing process and given the large number of BC-221's on the surplus market the question became how-in-the-hell were they able to calibrate that many meters before the war ended?

It's one thing to crank out combat boots in quantity, but a boot doesn't need to be documented in detail........

I posted the URL as this particular example appears to have aged gracefully and thanks to the fungus proofing has historic provenance.  For all I know the cal card is also dated, but one glance tells you it's the real deal from 1944.  Which is why Vinnie wants it.