TRC-458, good receive, no transmit

[rixdafix]

D51, R304, R205, R219 and the trimpot VR16 should all be checked.

All check fine. I also found my non conductive heat sink compound and got the final installed. This rig is back together and triple checked for solder blobs. I don't have a watt meter yet, would my rf volt probe tell what needs to be told at the antenna out? I'm assuming I'll need to check bias currents first? Haven't fired it up yet but got some good vibes going on here.

 

[brandon7861]

The square of voltage divided by resistance is power. Ohms law.

Since your RF probe is a peak voltage detector, you multiply that reading by .707 to get rms volts. Then, square that value and divide by the resistance of the dummy load to get watts.

The bias should be checked/set first.

 

[rixdafix]

I let the power supply run for about 20 minutes with the fuse removed and it held steady at 13.8v so that's cool. Set radio to USB, keyed mic, no audio, TP4 showed about 68mA, VR15 brought it down to 40mA no problem. TP2 however shows a little over 3mA and adjusting VR16 hardly has an effect. Hmm.

 

[brandon7861]

I let the power supply run for about 20 minutes with the fuse removed and it held steady at 13.8v so that's cool. Set radio to USB, keyed mic, no audio, TP4 showed about 68mA, VR15 brought it down to 40mA no problem. TP2 however shows a little over 3mA and adjusting VR16 hardly has an effect. Hmm.

Check D51 and R180.

Check R205 too.

 

[rixdafix]

All check good. I had my leads connected wrong at TP2 by the way, I'm only getting 0.28mA at key up. VR16 still makes no change and I did retest it. I'm done for the evening. I'll recheck my solder work and check some voltages tomorrow to make sure I didn't create a problem. Sure appreciate the input.

 

[rixdafix]

Ok I rechecked my work, all looks good. Voltages look good. Driver bias still good at 40mA. I'm using a different meter as the first one was showing an unadjustable .28mA at TP2 in the milliamp range but when I switched to 10 amp range and moved the lead to the correct jack on the meter, it read around 150mA at when briefly keying down. That's with VR16 centered, I can only get it down to around 100mA. I thought i was reading the meter wrong, nope.

 

[brandon7861]

When you checked D51, did you lift a leg? That cannot be tested in circuit because if it is an open circuit, your meter will see the b-e junction of the final and you'll think the diode tests good. It must have a leg lifted when you test it.

 

[Tech5]

Since you cannot bring down the idling current to zero and adjusting VR16 has no or small effect, I would suspect a burnt resistive track caused by a short in TR43 collector to base or a bad ground connection to its viper and probably is also D51 gone open as was pointed out earlier.

 

[rixdafix]

When you checked D51, did you lift a leg?

Yes sir. Every component I've checked has been isolated even if it meant removal. I'm quick with this iron and solder sucker though. I just finished checking c172, d66/d67, c169 just because they're in the vicinity. All good. Man we're on page 3 with this thing I'm probably wearing you guys out.

 

[rixdafix]

Since you cannot bring down the idling current to zero and adjusting VR16 has no or small effect, I would suspect a burnt resistive track caused by a short in TR43 collector to base or a bad ground connection to its viper and probably is also D51 gone open as was pointed out earlier.

Vr16 does change it, midway was about 150mA, all the way down was around 100mA. I'll reflow the wiper terminal. Vr16 checked good, going from wiper to unused terminal and taking it through full travel shows almost zero to 290 ohms and pretty smooth for watching on digital mm. I'll certainly recheck d51 again, easy enough. I wish I could say I was misreading the meter.

 

[Tech5]

When the viper is in its most upper position in the schematic the base is basically shorted to ground at DC so there should not be any current flowing through TR43 collector to emitter unless it is faulty.

 

[rixdafix]

When the viper is in its most upper position in the schematic the base is basically shorted to ground at DC so there should not be any current flowing through TR43 collector to emitter unless it is faulty.

Gotcha. I was wondering if the final I installed from the other radio is actually bad. A simple diode test across the junctions showed good but I imagine there could still be an issue with it. I don't think I transferred too much heat soldering it. I'll poke around a little and probably pull TR43 back out. I know the originals are unobtanium and the topic is covered in numerous other threads so I'll root through the forums for a suitable genuine non ebay substitute and give you guys a break. Thanks so much.

 

[Tech5]

Like a friend of mine said when he got into RF: "this is black magic"! In some cases it seems at first quite obvious what could be wrong, but when it comes to electronic and especially RF stuff you will have to be humble due to the fact that dealing with what seems easy is many times the opposite.

When you have TR43 out you may take the opportunity to measure the voltage, or ohms if you prefer that, on its base with VR16 set to minimum, IE viper to ground. L44 is in parallell with R179 so it will only add less than an ohm to the measurement so it should be very low ohm readings and basically no voltage at all. The measurements will show if the resistive track in VR16 is damaged or if there is something else that we will have to take another look at. Oh and it is pretty hard to see drop outs in a potentiometer when using a digital multimeter and with that being said an analog is at times my first go-to.

 

[rixdafix]

Like a friend of mine said when he got into RF: "this is black magic"! In some cases it seems at first quite obvious what could be wrong, but when it comes to electronic and especially RF stuff you will have to be humble due to the fact that dealing with what seems easy is many times the opposite.

When you have TR43 out you may take the opportunity to measure the voltage, or ohms if you prefer that, on its base with VR16 set to minimum, IE viper to ground. L44 is in parallell with R179 so it will only add less than an ohm to the measurement so it should be very low ohm readings and basically no voltage at all. The measurements will show if the resistive track in VR16 is damaged or if there is something else that we will have to take another look at. Oh and it is pretty hard to see drop outs in a potentiometer when using a digital multimeter and with that being said an analog is at times my first go-to.

I experimented a lot with electronics as a kid after getting my first Science Fair 60 in 1 kit. Got real interested in digital stuff and started breadboarding logic gates and sound synthesizers (man I miss my SN76477s) but RF circuits made my brain hurt at the time, kinda skipped over it. Then I discovered girls and mini bikes and electronics took a back seat for a few decades.

The amount of knowledge so many of the members have is impressive and collectively makes this site a really valuable resource. I am humbled and very appreciative of everyone's contributions to this thread, to the whole site for that matter.

I ordered three Hugo 2SC1969s from ICA, should be here in a few days. I'll do the testing you mentioned after I pull TR43. Agreed on the analog meter. I had a nice old Simpson I used in my transmission shop days for checking throttle position sensors. It grew legs one night apparently and I never saw it again. Man it could find the tiniest little glitch. I'll get another some day.

Thanks again, 73

 

[rixdafix]

I've got TR43 unsoldered, awaiting a replacement. It tested fine on diode check but I know that doesn't neccesarily mean it's good. VR16 tests fine also. With TR43 out of the circuit, is there anything I can check before installing new final? Also, I only eyeballed that L30 slug. If that was far enough out of adjustment could it cause the high bias current I was seeing at TP2? My gut tells me no because adjusting L30 is the step AFTER setting bias in the manual but....