Regentone BT23 "Regenteener"

[Crackle]

This is a strange little radio, (ebay photo)    

the circuit board bears some resemblance to the KB WP11 "Carioca", but there are some strange changes to the AF feedback circuit (and RF circuit). It appears to have positive feedback resulting in very high AF gain, which frankly does not seem to be required, it sounds better with the feedback disconnected.
Here are the AF circuits for the Regentone BT23    

And the KB WP11    

Apart from the AF117 transistors needing the usual treatment of being "zapped" there was another weird fault.

The reception was very weak, just audible on full volume. But if the car aerial wire was touched it burst into life at a very loud and distorted level, so turn the volume back down and it sounds OK, until you rock the tuning off slightly then it suddenly goes quiet again, turn up the volume re-tune and touch the car aerial wire and it bursts into life again, completely repeatable every time.
After changing every electrolytic, which were not very good anyway, it was still behaving the same. So I tried various other transistors in TX1 and TX2 positions, still the same.

I eventually found the problem in a little variable resistor in the secondary circuit of IFT1. The service manual gives a clue, but I don’t profess to understand exactly what is happening and why have the diode there at all.    

Regentone BT23 RF circuit    

KB WP11 RF circuit    

[Terry]

Is the AF feedback supposed to be positive, I wonder?

Perhaps a production error reversed the connections to the primary of the driver transformer, switching the desired negative feedback to positive!

As for the pot in the IF circuit,  the diode is intended to switch the 680Ω effectively in parallel with the primary of IFT1 to cause damping in the event of particularly high signal levels being received. The pot adjustment is, to me, the unusual bit.

Interesting to note that, in two otherwise identical IF designs, the position of the pot moves from secondary to primary! In both cases, however, it determines the point at which the diode conducts. From a DC point of view, the diode is between the two collectors but the KB circuit adjusts the collector voltage of TX1 whilst the Regentone alters the collector voltage of TX2 by adjusting its bias!

If you can reliably date both designs it should be a pointer as to which version works best!

Update: I didn't notice the two AF circuits but have now looked at them. I suspect that the KB design came first but when the design team decided to bring the first AF stage into the loop they forgot to tell the PCB designer (assuming it is on a PCB!) to reverse the phase of the transformer ...

[pwdrive]

The action of the AGC damping diode can be found here:

http://www.vintage-radio.com/repair-rest...tages.html

Also in this Roberts manual:

http://www.doctsf.com/documents/schemath...S/R700.PDF

Lawrence.

[Terry]

Yes, Lawrence, both circuits being essentially the same but without the adjustment - both are designed to operate automatically when required although I would have liked to see an RF bypass capacitor across the 2μF electrolytic in the first circuit you pointed to!

[Refugee]

Have those three electrolytic capacitors been changed around the mixer stage?
I would check D1 next if that does not work.
It is a bit like a local/distant control on a 405 TV by the looks of it. It is biasing the IF amp transistor to get something like the effect of a variable MU pentode.

[Crackle]

Hi Ref
Yes all the electrolytic capacitors have been changed one at a time, although some measured as high as 10 ohms on my Peak ESR tester, changing them made hardly any difference to the operation of the radio. Each one I tested and changed I thought, ah this will be the cause, but no. I was thinking of leaving the fault and offer it as a challenge for anyone to fix at this Sundays get together here in Basildon.

I originally found by trial and error that adjusting the preset R7 in the IF section fixed the issues. I then read what it did in the service instructions ( hadnt printed that page out). I have been back and adjusted it to give 4 volts across R9 the 2.2k resistor, but I found it caused the radio to give a sort of popping effect in the audio, it sounded best with the resistor set to give a voltage of 2.9v on R9.

Next up will be a Regentone BT21, just arrived today, slightly earlier then this Regenteener, and boasting 7 transistors, one of which seems to be doing not much more than acting as a detector, a good marketing ploy.

Thanks guys for the information on the function of the diode, and the links to the web pages, some reading for tonight.


Mike

[Terry]

 It is biasing the IF amp transistor to get something like the effect of a variable MU pentode.

No, not at all!

By altering the bias and, therefore, the collector current, it is altering the voltage drop across R9 for no other reason than to set the voltage at which the diode conducts.

In the KB circuit it is R5, in the mixer supply,  which is variable - again for the same reason.

The references that Lawrence quoted both explain what the diode does: it is just that, unusually, these sets have an adjustment for what, in other designs, is a fixed parameter.

[Crackle]

I have had another look at the feedback issue, this time with more sensible eyes.
In my previous tests I made a mistake, I was not just disconnecting the feedback, I was disconnecting C19 the 100uf bypass capacitor in the emitter circuit of TX3, that was the reason it went from loud to quiet.
If I just disconnect R25, 22k, from the speaker then there is a slight increase in volume and it sounds a little fuller tone.  So I guess I was wrong and it is negative feedback after all.

Mike

[Crackle]

I have finally put this little radio back together after a long distraction working on my KB Discomatic Jukebox.

There were 2 corners missing from the top of the case which I had decided to try and repair.    

I used epoxy filler    

then cut and filed to shape.    

I had problems matching the original colour by mixing Humbrol paints and had 3 attempts at painting it, I finally settled on as it is now as being the closest I could manage.

Here it is, close up to the repair it is still obvious, but from a distance it looks better than it did.    

[Mike Watterson]

I'd not call the fabulous Discomatic a distraction.
As usual, a marvellous repair.