KB KR019

[Crackle]

I bought one of these radios at a boot sale on Saturday, you can see one like it in the radio museum. KB KR019
It looks just like the one in the museum. The case is some kind of thick card covered covered in plastic film. Where the back hinges open the plastic has cracked as in all these types of case.

There has been some obvious tampering with the trimmers and wiring. One of the IF slugs was broken, which I repaired by dismantling the can and replacing it with a slug from an IF transformer I got from Pamphonica.
2 resistors had been cut. One leg of R14 in the discrimninator circuit and another mystery 150k resistor fitted across L8.
There are holes and pads in the printed circuit board for the 150k resistor, but it is not shown in the schematic and not present in an identical Regentone TR419 that I happen to have.

There is quite a lot of hiss as soon as the set is switched on, coming from the audio section after the volume control.
MW was receiving something, but not very well, LW was just picking up R4, and FM was dead.

I have started to check the voltages and got as far as TX8 where the collector voltage was almost the same as the emitter voltage. On checking the resistor R36 I found it to be marked as a 150k one. Then I found that R37 was a 10k one. so I have swapped these round using, what seemed to me as micro surgery.
What do you think, do you feel the radio would have worked on FM at all with R36 & R37 fitted incorrectly in the factory.

I have no idea what TX7 & TX8 are there for, what function do they serve in the circuit.

Mike

[Mark Hennessy]

I suspect TX7 is the FM oscillator. It looks like the oscillator is coupled via C40 into the emitter of mixer TX5. If I'm right, TX6 is the first FM IF. TX4 is the RF input amp.

The connection via R38 is interesting. It looks like it might be AFC, given that it's fed from the output of the discriminator. Varying the bias on TX8 changes the capacitance of its collector-base junction (a bit like a varicap diode).

If I'm right about this, then yes, it should have worked from new, but with no or poor AFC. Whether this would have been noticed is obviously up for debate.

Interesting circuit - quite different to the norm. Thanks for posting

[Crackle]

Hi Mark
It would have helped it I had read the manufacturers notes, which I was doing whilst you posted.
They dont say specifically that TX7 is the FM oscillator, but they do describe TX8 as being controlled by the voltage from the FM ratio detector, and that TX8 is part of the AFC circuit, its degree of conductance alters the effect of C49 across L29, and hence gives an AFC control of the oscillator..
So you are correct.

Hmm, time for some more voltage checks and then try with a 10.7MHz FM signal to see how far forward through the circiut I can go with that.

For some strange reason my repaired/modified Leader signal gen has stopped giving a modulated signal, so its get the massive old Advance out and warm it up.

I wonder is this the first ALL silicon FM portable with AFC.
I wish I could find a circuit diagram for the predecessor of this radio, the KB KR017. It would be interesting to compare with this radio. The KR017 only has 9 transistors so it must be missing the AFC function.

Mike

[Mark Hennessy]

I wonder is this the first ALL silicon FM portable with AFC.

Germanium, naturally

RM.org suggests 1965 for the KR019. The Hacker Sovereign RP18 was 1964, and that has AFC...

Anything earlier? Of the top of my head, the earliest FM set I have is the Dynatron Commodore, but no AFC with that one.

[Geordie McBoyne]

With regard to TX8, would there be sufficient capacitance between the collector and base so that the transistor functions as a Miller reactance device. Perhaps the 2SA100 was chosen because the base to collector capacitance was the right value for a reactance transistor operating at VHF.
http://www.radiomuseum.org/tubes/tube_2sa100.html

Geordie McBoyne.

[Crackle]

Germanium, naturally

Oh silly me, I looked at the polarity with the negative ground and for some reason assumed silicon NPN, with out even noticing the transistors were PNP.

Mike

[Crackle]

It looks like it is not the first to employ AFC then.
I have not got anywhere with making the FM work, I injected a 10.7MHz signal modulated with a 1kHz tone into the RED test point. (eventually when I found it, because on this KB radio the test point was GREEN, it is red in the Regentone version)
There was very little signal received on the output, but the FM transformers did peak up, sort of just about.
When I switched to AM there was a very strong signal on the OP which I was able to find a small null for on L18, but the AM signal was still very strong on the OP. Maybe I need to test my old Advance SG63F on a known working radio.

I have been checking the voltages around the transistors, here are the results.
   

Do any of the voltages indicate they are sufficiently out to cause FM to not work. The voltages in red are my readings.

thanks
Mike

[pwdrive]

Which is the green test point in that schematic?

TX2 base/emitter looks a bit low to me.

Lawrence.

[Refugee]

I have come across a 2SA69 and it has a very familiar look about it.
It absolutely sucks of a badge engineered AF117.

[Crackle]

Hi Lawrence
TP1 is on the emitter of TX5.
And yes there is a distinct similarity to the AF11x range of transistors.
I may have reduced thye image a little too much, if any of the numbers are difficult to read there is the full service information here.
http://kbmuseum.org.uk/kb_service_manual...019_kb.pdf

Mike