Binatone 5 Star Issues

[ChubbyChops]

Background:
At some point it has had some mods done which have been removed before I got it. I do still have the board that was removed, but what it is, is a mystery to me.
Because of this, there were a number of components missing and switches disabled.
I have gone through the radio to the best of my ability and now all switches and knobs function correctly.
The issue:
No RX or TX.....
The VCO won't go into lock. The voltage on TP1 is 6.38 volts and will not adjust, so this is where I'm stuck.
Anyone have any ideas please?

[Otter]

There may still be a component missing. Probably a resistor near the PLL.

[ChubbyChops]

OK thanks, I'll take another look.

[1968neil]

Check D3 varactor diode, VCO wont lock without it
Also check that there are no dry joints in the vco itself (the waxed area). These are 40+ year old radios now and dry joints are very common

Also the RED trimmer may be dirty, a liberal amount of switch cleaner and a few turns normally revives them, from memory its CT1.
Also check pin 18 of the PLL it should be 6.2v (supply pin) if this is high or missing then Zener D2 may have failed (possibly due to Reverse polarity)
Good luck


73's
Neil

[ChubbyChops]

Thanks Neil, yes 6.2v on pin 18 and I've gone over all the joints in the VCO block.
So with the help of you guys we now have receive.
I was missing C8 Otter, so you were spot on with that.
Once replaced I was able to set the VCO and tune the receiver, so that's done.
Now, there's still no TX, so any clues where to look next and what to check please?

[1968neil]

Have you got the service manual ? The Rotel RVC 240 manual is the same set up, go thru the alignment and If you have one use an oscilloscope to monitor the rf or a diode probe if you have one ?
Is the TX light coming on when you press the mic ?
My money is on CT1 having wax in it or just dirty.....
Try cleaning it then do a full alignment.
73’s
Neil

[1968neil]

Just a thought......
If the front end is ok then it’s possible that one of the chokes in the output stage is open circuit.
Or the 2078 may be duff ?
If the alignment path is good you’ll probably be looking at finals. Get the alignment sorted first as a good starting point.
Neil

[ChubbyChops]

Just a thought......
If the front end is ok then it’s possible that one of the chokes in the output stage is open circuit.
Or the 2078 may be duff ?
If the alignment path is good you’ll probably be looking at finals. Get the alignment sorted first as a good starting point.
Neil

2078 is Q6, right?
Yes I've done an alignment of the receiver.
CT1 swapped for new one - no difference.
No TX light on 5 Star, but when I key the mic it stops receiving and the meter needle drops backwards, fully left.
There's absolutely nothing received on a rig sitting right next to it.
Thanks for the help so far

[1968neil]

Assuming you have 4.0v on TP1 when in transmit (CH40) (adjusted with CT1) Then the next stage would be to scope the path from there.

Q5 Is the 2SC2078 Final
Q6 is the Pre-driver 2SC1846
If the TX light isn't working check Q17 2SA719 PNP this is the TX Switch. (assuming the bulb is ok)
There should be 8.6v on the Collector in TX and if you put your meter on the junction of R89 (3.3K) and Q17 (Base) you should see it switch.
If you can see it switch at this point there is a good chance that Q17 is duff.
I have seen that transistor fail a few times over the years.

I know it may sound daft but have you checked the Low power switch ? If it's set to low and RV5 is duff there will be very little output. This is very common in that chassis.

Keep me posted, well get there
Regards
Neil

[ChubbyChops]

Hi Neil
I'll check for Q17 switching later when I get a few mins, but the point about the TX light is that there isn't one.
The voltage on TP1 is 4.0v on receive (Ch40) but when I key the mic it goes to 6.2v and cannot be adjusted with CT1.
This is the same regardless of channel when transmitting.
Receive on Ch1 is 2.25volts and it will receive on all channels, so channel selector is OK.
Thanks for sticking with me.
I'm learning much and is appreciated.

[Otter]

Check that the base of Q2 is (almost) 0V on Rx, and ~0.7 to 0.8V on Tx. If that checks out, and since you have replaced CT1, replace Q2.

[1968neil]

Otter beat me to it, Try replacing Q2 that would stop the vco locking on tx

[ChubbyChops]

Base on Q2 on receive is 8.37volts, drops to 0.1 when mic keyed.
Collector and emitter are always 0v in any state.

[Otter]

Base on Q2 on receive is 8.37volts, drops to 0.1 when mic keyed.
Collector and emitter are always 0v in any state.

That's the collector at 8.37V. That transistor is working, so It's something else in that part of the circuit. It pretty much has to be C21, CT1 or adjustment of CT1.
It's very unlikely to be C21 unless someone has replaced it with the wrong value.
Did you replace CT1 with another red trimmer capacitor or was it a different colour?

[ChubbyChops]

I'll check C21. Looks like it should be 15pf?
CT1 was replaced with another red trimmer.