TheRadioBoard

I picked up a Science Fair Globe Patrol radio 28-206
on Ebay.

http://home.mchsi.com/~litterbox99/temp/globe1.jpg

http://home.mchsi.com/~litterbox99/temp/globe.jpg

I hooked up a long wire ant (20ft) in the basement.
I know this isn't the best, but I should pick up a local
AM station.

Well, nothing... not even a static rush. I repaired
several cold solder joints, but no effect. I can get
some oscillations when I turn main tuning and
bandspread.

I'm new to RF... I can repair & diagnose pins, vids,
slots and computers from the late 70's on up.

The circuit is simple enough to shot gun, but I don't
want to do that. I know early transistors have
unique characteristics that today's do not.

Perhaps someone could point to a regen faq or
basic theory so I know where to start.

A regen should oscillate at the frequency it's tuned to. Tune another AM radio between stations. Turn the GP's regen control fully CW, and tune it across the band. You should hear the oscillating detector on the second radio. If not, check wiring to the bandswitch. We can go on from there.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Measure the DC voltages in the circuit. The base emitter voltages of the Germanium transistors should be around 0.2V.

OK,

I measured the transistors with reference
to the ground terminal. This ground, according to
the schematic, is actually Vcc +8vdc. Weird,unless
there is an error in the diagram.


Here's my results;

Collectors on all three transistors is 8vdc.

Q1

E=0vdc
B=179mv

Q2

E=.600vdc
B=.745vdc

Q3

E=2.6vdc
B=2.6vdc

Many old germanium radio and amplifier circuits were designed with negative power supplies, so that the ground rail was always the positive side of the supply. Don't know why ...



This is reasonable and expected.



Probably OK ...



This does not sound right ... possibly a shorted transistor here

One thing you can do to work out which parts of the radio may be working or broken is to signal trace the radio. For example, use a crystal earphone, or a good pair of high impedance headsets, and connect the wires to the volume control at the top and bottom ( ground ) of the control. If you hear static - or something that tunes or acts like a radio trying to work, then the RF section is probably good, and the audio amp is bad ( that would be Q2 and, most likely, Q3 ).

Signal tracing of this type is usually done with an audio amplifier that has a test probe as an input. You can move the probe through the signal path of the radio to see where the signal disappears.

Tony

I placed a crystal earphone across the volume pot.
I can hear very faint reception on AM band.
Very poor reception and not tunable across the
band.

I removed the earphone and used a set of amplified
speakers in the phone jack. Volume maxed on the
speakers, still week sound.

If I crank the RF gain (regen) sound improved
but then breaks into oscillation, back it off a bit
and its ok. Doesn't seem like it's tunable on AM
band. We have several AM stations I should pick
up, but it's best at the end of the band 1490khz.

I need to get a long wire in the attic but it's 90 out
right now, so the attic's cooking. I'll pull the transistors
and test them for now.

If the detector is oscillating but not tunable, I'd try to find out why not. It would be helpful if you could find out what frequency it's oscillating on, by tuning another AM radio across the band while it's oscillating.

It's safe to assume this radio never worked properly because it was wired incorrectly. Again, try to trace out the wiring around the bandswitch for starters. Also check the tuning capacitors for shorted (bent) plates, a common problem with these radios.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Hello,

I think there is an error on D1. The anode has no DC-connection.

Gerd

_________________
Hüllkurven wollen demoduliert werden

D1 is the detector/demodulator. Rectifies the envelope of the incoming signal after it's amplified by Q1, and feeds the detected AF to Q2.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

The manual for the battery-powered version of this radio is online. http://www.ohio.edu/people/postr/bapix/RSGlobeManual/ I see the bandswitch is soldered to the circuit board. Eons ago I worked as a tech at the Heathkit store in NYC, working on ham gear. Egad, the errors I uncovered! It's entirely possible that resistors, capacitors, or even band-select inductors, were transposed.

Given that this was an entry-level kit, and you found cold solder joints, you can assume the builder was inexperienced. You know Q1 is oscillating, which is a good sign. Before yanking hard-to-replace germanium transistors out for testing, I'd spend time tracing the circuit to make sure the builder didn't make a mistake. This radio may have never worked properly, and just been set aside.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

How can D1 do that, without a closed path for DC? There is no way for the rectified AF-signal!

This will work:



and this also



but the shown does´nt!
Becaus there is no way for the DC-part of the signal.

Gerd

_________________
Hüllkurven wollen demoduliert werden

The bottom of the Volume control connects to the [edit] B+ (ground) bus (not used to seeing PNP transistors! )[/edit]. You can trace it back to the center tap of the power transformer.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

C12 and D1 present what is referred to as a Floating Detector ... which is actually a very viable circuit.

Tony

Yes, I can trace , but one leg of D1 is "hanging on the air"

On every AM-demodulation process you get 2 components, the AF and a DC-Component. But in this schematic is no way for DC, and you can not get the AF-component.

Gerd

_________________
Hüllkurven wollen demoduliert werden