Crystal failure?

[brandon7861]

I am having an issue with that Lafayette I was hoping to fix up and sell. I did get it working, but on 7 of the 23 channels, it acts like a crystal is bad, but only occasionally. If I touch the crystal solder contacts a few times, I can get those channels to come alive. It is not a bad solder joint (because an equivalent beating with a non-conductive stick did not do the same), but what I do not know is whether the crystal is failing or if the oscillator transistor just lost a little gain over the years and needs to have the surrounding components adjusted to compensate.

The problem I face now is that if it is a crystal or two going bad, the ones I order need to be very close to the right frequency as there are no trim caps to adjust it in this radio. If I can tweak the negative resistance (gain) at the base of the transistor for that frequency (which I recently learned is possible), would I be buying myself any real time or is that crystal going to fail anyhow? How do crystals behave when they die? Do they slowly increase in Rs or do they just go dead?

Edit: Im hoping they just go dead so I know if a transistor swap or capacitor change will solve it. Come to think of it, the Royce I just fixed for Coffeeman had to have its base resistance changed because the transistor had a low Hfe. Maybe it didn't start out that low. What do the experienced techs say??? Transistor or crystal???

Thanks!!!

 

[midnight special]

years ago,,, we would come across that problem,, sometimes just heating the crystal up but not the solder joints would bring it back,,, man many years ago,,,,,,

 

[nomadradio]

Swapping another crystal will reveal if the crystal itself is cutting out. Doesn't have to be the right one, just the right frequency range for the oscillator circuit. Even if it doesn't deliver the correct channel frequency, you'll know what to blame if swapping it fixes the intermittent fault. And if the crystals are in sockets, this is a lot simpler to do.

73

 

[brandon7861]

Swapping another crystal will reveal if the crystal itself is cutting out. Doesn't have to be the right one, just the right frequency range for the oscillator circuit. Even if it doesn't deliver the correct channel frequency, you'll know what to blame if swapping it fixes the intermittent fault. And if the crystals are in sockets, this is a lot simpler to do.

73

No sockets, but I could try that. My only concern there is that every crystal has a different Rs. If it is a gain issue where the Rs is still acceptable but the -Ω at the transistor's base at that freq is not negative enough, swapping in a crystal with a slightly lower Rs may lead to relaible starting and me to believe it was the crystal when it really was a lack of gain.

I can and will rule out the transistor tomorrow, I am just getting my ducks in a row for tomorrows assult. A transistor going bad could have put the gain right on the edge of where oscillation can be sustained with that particular crystal, but what I want to know is if a crystal's Rs can change, and if it does, does that represent imminent failure?

I will pull the crystal and see what it does in a test oscillator. If it reliably starts in that, I will measure the Rs of the that crystal and compare it to the ones next to it. If they have similar Rs values, I can pull the BJT and look at its Hfe out of circuit. If that is within spec, I guess then the only other step would be to use the VNA and see what the base lead is doing at that frequency and try to shift the -Ω curve a little.

Just trying to learn a little about crystal failure...

 

[frankie-b]

hi brandon7861 what lafayette radio is it? i too have a lafayette ssb 75 with crystals in it and it too is having the same problem. i thought it was a bad solder joint also. i would move the crystals around and it would come on. i taped them into their sockets so i can't touch the crystal or the solder on it's plug. if i move the crystal bank it comes in. i've owned this since it came out. i was a hell of a radio back in its day. i did all it's mods. that radio has a gain on it's receiver now and has minimal splash now.

 

[brandon7861]

hi brandon7861 what lafayette radio is it? i too have a lafayette ssb 75 with crystals in it and it too is having the same problem. i thought it was a bad solder joint also. i would move the crystals around and it would come on. i taped them into their sockets so i can't touch the crystal or the solder on it's plug. if i move the crystal bank it comes in. i've owned this since it came out. i was a hell of a radio back in its day. i did all it's mods. that radio has a gain on it's receiver now and has minimal splash now.

This one is the HB-700. When I find out more about what is wrong with mine, I will report it here.

 

[Captain Kilowatt]

Sounds to me like the oscillator is running on the ragged edge of stability. Touching the crystal changes the capacitance and pulls it back into proper oscillation. That would explain why tapping it with a stick would not make it work but touching it does. I would suggest it needs either a new crystal or a tweak of the trimmer capacitor. The crystal may have drifted too far off frequency it is having trouble performing properly and the added capacitance from touching it is pulling it back into operations.

 

[brandon7861]

I got to it with the oscope today. One of the crystals, when switched in, would still cause oscillation, but the amplitude of the signal was roughly half for the channels that used that crystal. After cleaning the band switch contacts and cleaning the factory flux off the crystal board, I had equal amplitude oscillations with all crystals. I got lucky this time.

 

[frankie-b]

that's what i was going to do. i guess i'll try it. got nothing to lose at this point. thanks brandon7861