KB KR021 "Cobra"
 

I have been repairing one of these and the fault turned out to be the first IF transistor, a AF127. It was short (4 ohms) between base and emitter. I cant see any thing in the way of faulty components which could have contributed to this.
I have replaced the transistor and the voltages all seem OK, but when my hand goes near the aerial the voltage on the collector goes up to almost 8 volts.

Do AF127 transistors just go faulty like this?

I believe this is an early version of the radio, the serial number, which looks like 1277/-04951 and the numbers in italic are hand written.

This particular example, being an early one has Caroline, R. 390, R. City, and R. London marked on the dial. Later examples (eg. serial number 1277/-17506) did not have the Pirate stations marked.

MW is spread over two bands, this probably made selecting these stations easier.
I will post some photos later when I have put it back together.

Its interesting to note there appears to be no detector diode in this circuit.
Mike

Re: KB KR021 "Cobra"
 

AF127's in my experience, seldom fail at all, but maybe after 50 years or more, that one had just 'had enough!' What is puzzling me is how is the A.F. recovered? Like you, I see no sign of a detector diode. Maybe the third ?AF127? acts as a detector. The components between it an the volume control seem to be a conventional filter circuit, with Rx13 as the load? Is this by any chance the transistor equivalent of an 'anode bend' detector - or am I barking up the wrong tree?

Re: KB KR021 "Cobra"
 

Detection is due to the base/emitter bias voltage, so yes, same idea as anode bend.

Lawrence.

Re: KB KR021 "Cobra"
 

Yes Lawrence is correct, I have been reading the Trader sheet 1841 for this radio. It explains that TXk2 acts as a "collector bend detector".
Attachment 167177
I have come across this circuit a couple of times and blow me I cant remember what the alternative resistor Rk4 is supposed to do. I seem to remember that it had something to do with local and distant reception, and it was mentioned in notes somewhere, that it would be fitted or not by the dealer depending on where the radio was to be used, but I cant find any reference to it now.

Could a large interference signal cause TXk1 to fail base-emitter short, if the radio was set for distant reception?

Mike

Re: KB KR021 "Cobra"
 

I have the RGD-badged version of this set called the RR221 Rover. It's serial number 1359/01117 and has 'R.London', 'R.City' and 'Caroline' on the dial. The service manual simply says that "TXk2 functions as a detector amplifier' and goes on to point out that the DC level at the collector varies with signal strength which is fed back to TXk1 via Rk3 to provide AGC

I am amazed by how modular it is. Audio amplifier and IF strip on plug-in PCBs. Without desoldering anything you can remove those, the ferrite rod assembly, the frequency changer PCB, etc. It can't have been cheap to make.

Re: KB KR021 "Cobra"
 

On this latest example of this radio the RF module which contains the wave change switch has broken away from its plastic mountings which hold the spring clips the screws fix into. Its not a big problem I just need to glue the case mouldings back which hold the captive pressed spring screw sockets, I am reinforcing it with epoxy.

Mike

Re: KB KR021 "Cobra"
 

Hi Tony, It should also have R. 390 on the dial, in the bottom half of the dial under the red Scotland. It appears to be the same dial as this KB.
https://www.radiomuseum.org/r/rgd_rover_rr221.html

Re: KB KR021 "Cobra"
 

My normal way to repair such plastics, assuming the plastic dissolves in dichloromethane ('plastic weld') is :

Put the bits together

Run a brush dipped in dichloromethane along the cracks. Hold the parts together but don't squeeze them hard, you do not want to force all the softened plastic out of the join

Cut a piece of thin cotton fabric (e.g. an old dress shirt) to fit over the repair. Put it in place, then brush it with dichorlomethane and force the fabric into the softened plastic.

The result is normally strong enough for any reasonable use. I can't remember where I got that trick from but it's not original to me.

Re: KB KR021 "Cobra"
 

Yes, R.390 is there too.

I got it as 'just another transistor radio' but I found the modular construction to be interesting so I am pleased I have it.

Re: KB KR021 "Cobra"
 

As promised here are some photos of the KB KR021 "Cobra";
This is the early version with 4 pirate stations on the dial. Attachment 167190 Attachment 167191 Attachment 167192
The red lettering for the BBC stations Light, Third, North, Home, Scotland, Wales etc. has faded with age.

This is the later model with BBC1 BBC2 & BBC3 marked on the dial. Attachment 167194 Attachment 167193

Mike

Re: KB KR021 "Cobra"
 

Here are some photos of my RGD RR221 "Rover", here you can clearly see the original red lettering for the old BBC station names.
Attachment 167195 Attachment 167196

Re: KB KR021 "Cobra"
 

The KB service manual (CRTS) for the KB KR021 and RGD RR221 is dated January 1967, so if the original production run started around the same time and was the one with the Pirate stations, it is likely it was soon superseded with the dial without the pirate stations because the Marine offences act became law at midnight on Monday 14 August 1967.
I believe the BBC launched its new national stations named Radio 1, 2, 3, & 4 also in or around 1967.

Mike

Re: KB KR021 "Cobra"
 

I notice in that service manual that the 'Scale (Self Adhesive Paper)' is listed as a spare part. I wonder if, when the pirate stations were removed if that kept the same part number (so if you ordered a replacement you got the revised one, even for an older set) or if the new scale had a different part number (in which case what happened if you ordered the original one)?

Why were the pirate stations removed from the tuning dial. Was it that by having them there it could be claimed the radio manufacturer was assisting said pirate station which was illegal?

Re: KB KR021 "Cobra"
 

I would think it could have easily been argued it was advertising the Pirate stations, which would have landed the manufacturer in trouble. It was quite likely that the radio licence of the day did not authorise the reception of unauthorised transmissions, rendering anyone who listened to the unauthorised pirate stations liable to prosecution as well.

Mike

Re: KB KR021 "Cobra"
 

Advertising the frequencies of unlicensed broadcasters could have potentially made them complicit in any offence The Authorities chose to prosecute.

The fact that some of the former pirate DJs were eventually employed by the BBC says a lot, really. I guess with the Cold War raging, it was more important for the airwaves to be under some sort of control, no matter how superficially distasteful some of the content might be to the stuffed shirts in charge, than a free-for-all with optional political propaganda.

Re: KB KR021 "Cobra"
 

The comments in the last few posts regarding the Pirate Stations are no doubt correct, since the Radio licence only authorised listening to 'licencd broadcasting and Amateur stations', but, AFAIK, no-one was ever prosecuted for listening to Caroline, London, et al!

Re: KB KR021 "Cobra"
 

I don't think they would have dared!

The government of the day knew full well that The Kids who were listening to pirate radio were the exact same The Kids that The Scary Reds wanted to recruit to their own ends. And at least to date, the transmitting ships had not been of the heavily-armed variety ..... Perhaps it's not so ironic who was responsible for the Marine Offences Bill, but a carefully-chosen figure to send a crafted message. But that's about as political as it's safe to take things out here, I think; PM me if you're interested in continuing that line of discussion, and we can adjourn to the safety of a blog.