[TenTec] TT Century 21 CW monitor osc problem

[Mike Bryce]

Okay….

first off is the audio filter working correctly? Do you have all three filter stages?

if you do, then the IC LM3900 is working. There are four sections to that amp, three are used for the audio filter. Yes, it’s possible that the fourth amp is toast, and I’ve seen that before, but let’s not suspect the IC right now.

Keying the sidetone is done by the “T” and “R” voltages from the control board.

They flip flop with “R” being high, and “T” low during receive, and on TX, they flip states.

The schematic I have isn’t the best copy in the world, yet it looks to me like the “R” voltage goes to the sidetone oscillator

First thing I’d do is check the junction of R1, D1, and R2 to see if it changes state when you press the key.

If not, check D1.

Like I said, it’s not likely that LM3900 has  a popped gate, but it certainly is possible. I’ve seen other ICs behave like that.

And that being said, Ten Tec was famous for having less than stellar soldering and it could very well be nothing more than a re-solder of all the pieces parts 
around the sidetone gate.

C1 sets the freq of the sidetone, and to me, it’s way too large. If you have to pull the pbc, consider changing that cap to something a bit smaller in value.

A scope would be very useful to tack the output from pin 10 though the sidetone volume. Again, suspect cold/cracked solder joints.
Resistor R5, capacitors C2 and C3 form a simple RC filter that knocks some of the corners off of the square waves from the tone generator. A cracked solder joint at C2 would shut the sidetone down as well.

Mike, WB8VGE

J e e p
o|||||||o 

A man with one clock knows what time it is. A man with two clocks is never sure.

> On Dec 16, 2019, at 10:22 PM, Mike Bryce <prosolar at sssnet.com> wrote:
> 
> It’s time to break out the scope!
> 
> Mike wb8vge 
> 
> 
> Sent from my iPad
> 
> Knowledge is knowing a tomato is a fruit; Wisdom is not putting it in a fruit salad.
> 
> 
>> On Dec 16, 2019, at 8:07 PM, admin <75a4 at cox.net> wrote:
>> 
>> Put this on the list only one reply .
>> 
>> Trying again
>> 
>> That person said most likely the LM3900 IC, yes I get it, but before anything happens, like removing front panel and all the other stuff , in order to remove board and desolder the 14 pin cap from board.
>> 
>> Any on had a bad cap or resistor, checked  the restores, in spec.
>> 
>> Thank very much
>> The Old Mossback K6RCL
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