TRC-458, good receive, no transmit

[brandon7861]

They look the same, just isolated and checked. 5.7 Vf on both

That raises more questions than it answers. These should be around 0.6v, not 5.7v. This also contradicts the voltage measurements you made in post 47, so rather than assuming these are zeners and installed backwards, I am going to assume this is a measurement error.

 

[rixdafix]

I'm sorry my friend. That should read. 57v. Edit in progress...

 

[brandon7861]

You could certainly test C169 and C170 if you want to. Doesn't hurt to check.

I think its VR16. Consider the voltages you measured in post 47. With the pot wiper maxed, the lowest you could get the voltage is a little higher than the diode voltage. That's not right. VR16 and R219 are in parallel forming the lower half of a voltage divider with R180. As VR16 is turned (I assume clockwise from your post), the resistance of VR16 should drop so low that the voltage divider doesn't allow the diode to even turn on. With the wiper to ground like that (instead of the wiper being at the divider junction), it can burn the carbon only where the wiper makes contact without burning it out completely giving the impression that it is still good. Since the wiper is at ground, it is hard to notice this by measuring it in circuit. It should not be possible to get .6v with the VR turned all the way, just not possible. You need to find a new VR to put there and try again.

 

[rixdafix]

You could certainly test C169 and C170 if you want to. Doesn't hurt to check.

I think its VR16. Consider the voltages you measured in post 47. With the pot wiper maxed, the lowest you could get the voltage is a little higher than the diode voltage. That's not right. VR16 and R219 are in parallel forming the lower half of a voltage divider with R180. As VR16 is turned (I assume clockwise from your post), the resistance of VR16 should drop so low that the voltage divider doesn't allow the diode to even turn on. With the wiper to ground like that (instead of the wiper being at the divider junction), it can burn the carbon only where the wiper makes contact without burning it out completely giving the impression that it is still good. Since the wiper is at ground, it is hard to notice this by measuring it in circuit. It should not be possible to get .6v with the VR turned all the way, just not possible. You need to find a new VR to put there and try

Kind of wanted to put multiturns in here anyway. Having a hard time finding 300 ohm. Would 200 ohm work? As a test and assuming the rest of the circuit is ok, I should be able to short the two utilized terminals of VR16 and see very little if any voltage at TR43 base?

 

[rixdafix]

Well well. Shorted VR16 and measured 46.8mV at TR43 base, keyed up, AM. Gonna buy some pot s.

 

[brandon7861]

200ohm should work. If not, you can always increase R219 that is in parallel with it to compensate.

 

[unit_399]

Rixdafix - If you can't find 300 ohm, use a 500 ohm instead. Easier to find than 200. Also don't need to change R219 with a 500.

J.J. 399

 

[rixdafix]

We're on the air fellas! Multiturns installed, adjusted L30 per manual, haven't touched any other adjustments. AM deadkey 4 watts swings around 12. SSB peaks around 20. That final bias is still real sensitive. I bet VR16 is only dialed in a few ohms to set bias at 15mA. I still think I have a ceramic or a resistor that shorts under load. I also found NOS relays, my "spare" is scratchy. They'll be here in a few days.

I thought this had good receive audio before the recap but holy cow it sounds fantastic now. I've made a couple of local and dx contacts with good reports too so yeah. I'll get back into it in a few days and start testing ceramics and I'm sure it would benefit from a full alignment but for this weekend I think I'll enjoy some air time and just limit my key ups. What a cool old radio.

I really appreciate you guys helping me out with this. Once it's buttoned up, I'm gonna revisit an LSB issue with a beautiful TRS Challenger 850 I've got on the shelf...

Happy Independence Day and 73!
WDX561 standing by.

 

[unit_399]

I still think I have a ceramic or a resistor that shorts under load.
Happy Independence Day and 73!
WDX561 standing by.

No such thing as a shorted carbon resistor. Wirewound yes.

J.J.399