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 Post subject: Re: Graymark 508 - "Boys Radio" ┐(´д`)┌
PostPosted: Feb Fri 16, 2018 5:31 pm 
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Here's our update (see below).

Just a little tidier than previously ... unf. we're only getting a very faint hiss out of the speaker.

We tried reversing one side of T3 to address potential oscillating, but didn't seem to affect anything.

Both transistor's hfe values steadied at Q1 - 135 and Q2 - 250.

Checked our diode is the right direction ...

Should we starting pulling components and checking individual values?

Thanks in advance for your help.

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 Post subject: Re: Graymark 508 - "Boys Radio" ┐(´д`)┌
PostPosted: Feb Fri 16, 2018 6:27 pm 
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Location: S. Dartmouth MA 02748-1225 USA
loucommando Can't' seem to understand why the loop-stick has gone from 6 wires to 4, though the four is correct. Also, one of the wires appears to be a twisted pair. Be sure the primary, the tuned section of the loop-stick winding is the greater number of turns or the tuning range will be incorrect, hence the hiss may well be shortwave. A hiss can also indicate the audio is working. Can you inject an audio signal into the wiper connection of the volume control and reference the battery positive for a signal return?

If you do not have a signal injector one can be made from common parts. Info is online. Such a test will confirm audio and not an oscillating condition in the audio.

Yes, check the components, resistors in tolerance, but, should work if only 5% out of the spec. %. Disc ceramic caps will leak especially the old low voltage types these will kill a signal or a battery. Note this guy has 11K across the battery as a voltage divider.

Do check the electrolytics, actually, replace them. The bypass electrolytic across the battery if leaking can also flatten a battery in a few days time.

I did not look-up the transistors, if you are confident they are O.K. replace them, if in doubt. Probably have to settle for modern plastic cased substitutes. Do not sub a silicon for a germanium, the circuit parameters will all have to changed and maybe won't go into existing PC board...

Last words.

The sensitivity of this radio is very poor. It must be less than 5 miles from a 5kw station. Much closer if a 1Kw station AND in the open, in a home with metal or stucco wire, brick, stone siding or foil barrier almost no hope of reception.

You can couple an indoor antenna using 5 to 10 turns of wire around the loop ferrite. one end of the wire goes to earth, the other end goes to 25 feet of wire around the room near ceiling. Do not connect external antenna directly to tuned circuit will shift the tuning range away from broadcast.

Anchor the loop-stick and any outboard wires, if an accident, parts could be mechanically ruined, like a broken ferrite...

Yep, pin-board looking much neater:D

I have much to do working with household files today. I'll catch-up this evening or tomorrow to see how it goes...

GL

Chas

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 Post subject: Re: Graymark 508 - "Boys Radio" ┐(´д`)┌
PostPosted: Feb Fri 16, 2018 8:32 pm 
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Hi Luke

I think I know where the problem is

In the actual physical construction of T3 the white dots are on opposite corners.

The white dots side of the coil has fewer windings.

Try this:

Rewire the coil as shown in the diagram below.

Agree with Chas, the sensitivity of these radios is quite poor, the nearest radio station is about 12 miles from my location and reception is weak from inside the house. Much better when I bring the radio outside.


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 Post subject: Re: Graymark 508 - "Boys Radio" ┐(´д`)┌
PostPosted: Feb Fri 16, 2018 9:43 pm 
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This is great. Thanks so much guys. We're learning a lot here.

@Chas - Will definitely check out a signal injector. This could reveal all. At Rocco's recommendation at the outset, we replaced all the electrolytic caps. Will run through the other components and test. Not sure how to test for transistor leakage. We plugged them into our meter, and they gave us hfe values. But we don't really know if they're ok. The values held steady once their temperature leveled off.

@Rocco53 - Thank you for the extra diagram. With the T3 reversing, we used jumpers to actually change its direction ... thinking that rotating it would wouldn't change anything. Luckily, it is currently hooked up as you have it drawn there. Additionally, we looked at the "printed board", and the "printed" connections match how we have them on the breadboard right now.

The hiss is very, very faint. You have to press the speaker to your ear to hear it.

I'm thinking that our troubles are on the amplification end of things.

Will let you know what we discover!

Luke


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 Post subject: Re: Graymark 508 - "Boys Radio" ┐(´д`)┌
PostPosted: Feb Sat 17, 2018 2:07 am 
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Do a discovery on the correct impedance for the speaker. I recall some of these boys sets used a high impedance speaker 30 ohms or so. Others didn't even use a transformer, direct to a pin drive speaker.

Chas

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