Radios Serviced By Observation

[Steve - WB3HUZ]

Although geared toward BC sets, some good tips here.

http://www.antiqueradios.com/features/service.shtml

[KD6VXI]

"Touch the grid cap of the first audio tube, or the grid terminal. This usually can be easily located as the grid lead comes from beneath the chassis."

Showing a bare finger touching the anode of an external anode glass tube.

NICE!

Other than the OBVIOUS safety omissions, good stuff, so nuff.


--Shane

[Steve - WB3HUZ]

I think that's a grid cap - no HV.   

"Touch the grid cap of the first audio tube, or the grid terminal. This usually can be easily located as the grid lead comes from beneath the chassis."

Showing a bare finger touching the anode of an external anode glass tube.

NICE!

Other than the OBVIOUS safety omissions, good stuff, so nuff.


--Shane

[k4kyv]

I have touched the grid cap with my finger many times in receivers to check to see if the audio stage is working. That's about the simplest signal tracer in existence.  If the stage is OK, touching the grid cap usually produces a hum in the audio output.  Sometimes you can hear the background noise decrease when you touch the grid cap of an rf or if stage. Another trick is to firmly hold a metal object like a screwdriver or nail, and touch the metal to the cap.  If the stage (af or rf) is working, it will usually make a click or pop.  Touching the grid cap of an rf stage or mixer will sometimes inject enough signal to bring a dead receiver back to life.  That tells you that something is wrong at the antenna input circuitry - usually a burnt out coil from a lightning pulse.

I can't think of any radio receiving tube that has a plate cap coming out the top.  Some early TV sweep tubes did, though.  I wouldn't worry about tube caps in old broadcast receivers, but I wouldn't recommend poking at top caps in tube type TV sets unless you know exactly what you are doing.  Some of the plate voltages in early TV's were close to what you would expect to find in 100w class transmitters.

[N4LTA]

Touching the grid wire in an audio amp is a great first check to find a problem in a multistage amp.

Knowledge is a very important part of safety.

[k4kyv]

Touching the grid wire in an audio amp is a great first check to find a problem in a multistage amp.

Knowledge is a very important part of safety.

Yeh, there isn't a lot of physical distance between pin 5 and pin 3 on a 6L6.   

[The Slab Bacon]

Touching the grid wire in an audio amp is a great first check to find a problem in a multistage amp.

Knowledge is a very important part of safety.

Yeh, there isn't a lot of physical distance between pin 5 and pin 3 on a 6L6.   

OOOOOOOuuuuuuuuwwwwwwwwww..................I hate when that happens! ! !  

[KC4VWU]

'Tis true, but what about a leaky blocking cap from the preceding stage? I like to double check for HV just to be safe. Usually, just touching a probe tip or screwdriver with an insulated handle to the grid cap will give an indication.

Phil

[W1RKW]

or a grid to plate short.

[N4LTA]

A grid to plate short would manifest itself pretty quickly without and handling. A leaky bypass would tingle a bit.

If you don't want to do it don't do it - but please  - I don't intend to put on a flame proof suit, use lead-free solder, put in a fan to suck up solder fumes, use only green cleaners etc.... ad nauseaum.

The guy was using technique that has been used for years - it works - he isn't stupid -

If you are scared of DC voltage above 50 volts - then go play solid state.

If you don't understand how a circuit works - don't work on it - it is be futile and maybe dangerous (to you and the circuit).

And yes - pin 3 is close to pin 5 - but pin 4 usually stings nearly as bad!

I have plenty  of first hand experience.

[N0WVA]

Boat anchors and many older solid state rigs can be successfully troubleshooted with nothing more than observation and a VOM. And if you know your circuitry and have good eyes, most of the time one doesnt even need a schematic.  The VOM is the most versatile tool on the bench. From alignment to checking transistors, if you dont have a VOM your dead in the water.

[k4kyv]

My problem with VOMs is that inevitably I will eventually forget to switch the range and try to measure HV with the ohms or milliamps function.  I have managed to avoid blowing up my Fluke DVM thanks to its built-in fuse and other safety features.  I destroyed a previous Fluke when I accidentally put it across the full transmitter plate voltage trying to measure about 400 volts. I couldn't count how many analogue VOMs I have ruined over the years.
 

[N4LTA]

God bless Fluke!

I have heard that fast beep beep beep way too many times with about 400 volts on the Ohms scale.

They survive - the Chinese $9.95 jobs don't.

I did manage to blow up a Model 27 after 20 years or so.

[KC4VWU]

Geez man, bag all safety issues and don't bother learning or experimenting. That's some pretty good advice.

There are a lot of guys that aren't still around because they got a little complacent about their safety practices. On the other hand, I've clip leaded power supply circuits together on the bench before, but I wouldn't necessarily advise someone else to do it. Constantly reminding each other to be safe has probably kept a lot of us alive this long.

I know the practice is an age old technique; just trying to hash out the pros and cons. No problems.

73, Phil   

[KM1H]

I couldnt survive without clip leads and exposed resistors hanging out of a chassis as Im fine tuning the voltage. Its all part of the aura and patina surrounding hollow state.

They taught us in Navy ET School to be creative in an emergency which means when the exact part needed wasnt stocked on the ship. Ive rewired sockets, jury rigged IF or power transformers and whatever it took. Once I had the radar IF strip hanging out with a bunch of clip leads and parts attached to them. We were entering the English Channel and no radar was not an option. It helped that Id been a home brewing ham for 4 years before joining. The skipper and other officers came by and just shook their heads and it was decreed that I no longer had to stand any watch or Shore Patrol details.

Ive rebuilt many radios, especially consumer grade, with nothing more than a 260/630 and a tube manual or ARRL HB for pinouts. Once youve done a few it becomes as natural as breathing

Im damn careful around transmitter HV but I do get careless with 250VDC. With a wood top bench, wood floor plus rubber mat, I dont sweat the small stuff    A little tingle keeps me awake.

Carl
KM1H

[N4LTA]

Wait a minute - Who EVER said bag all safety and don't learn about anything?

Having knowledge means  "don't learn or experiment"

As Carl says  - I don't get real worried at 250 volts or less with a hand in the pocket not standing barefoot on a concrete floor - we were talking about a low current receiver grid circuit - not a 100 watt transmitter.