Selena 215 - low VHF sensitivity (again)

[PaulM]

Hello, first time in 'domestic radios' (I think) - I normally inhabit the TV section!

For a very long time I have had a Selena 215 (the PNP transistor version) and as many people have noted, they are amazing on AM but rather insensitive on FM. I do like the audio quality too. Hats off to the USSR radio designers!

Having had difficulty recently receiving Lincoln City Radio (recommended!), I thought that I would have a 'quick look' to see what's what with the problem of low sensitivity.

Now, before I go down a rabbit hole, I thought that I would ask if anyone else has discovered what I've noticed.

What I found (entirely by fluke) was that putting a meter probe on VD5 cathode immediately produced a quantum leap in sensitivity and quality. I've attached the relevant bit of the schematic - the 1st IF.

Voltages are all spot on around VT6, VD5 checks, C5 is fine and tweaking L3.1 makes no difference (it peaks). I guess VD5 is there to stop very strong signals affecting the bias on VT6?

Short of permanently attaching a wire to VD5 cathode (!!!!), I'm not sure what to do without this turning into a time vampire at a time when things are very busy round here.

Anybody else been here?

Best regards,

Paul M
www.becg.org.uk
www.projectvivat.co.uk

[PaulM]

My first thought too was C5. Changed it, no difference so put the original back.

Paul M

[Andrew2]

An odd one! Could there be something amiss with the next stage (around VT7)? The meter lead may be radiating the 10.7 MHz IF signal and it's being picked up by the next IFT, by-passing the faulty IF stage? Clutching at straws, I know!

[Herald1360]

Try sniffing for unexpected RF frequency signals in the area. Something may be hooting and tamed by the extra damping introduced by the probe.

[Jon_G4MDC]

What if VT7 is the mixer and VT6 is the RF stage?

Then the test probe at the top end of C5 acted as an antenna supplying the signals which were not previously coming via VT6 as they should.

It would be good to see more circuit please.

[PaulM]

Hello All,

Thanks for the responses and full circuit attached.

The time vampire had his fill and I think that I've solved it. The FM is so much better now and I wonder how many more of these sets are afflicted?

The answer (I think) is that I was being led up the garden path by L3.1 seeming to reach a peak. It was, but a false peak (internal construction of IFT?). Adding a 22pF capacitor across C5 resulted in the peak occurring around the mid point of the IFT adjust range - a much more comfortable 'feel' and a so much stronger peak.

Now, I may be treating symptom here. The IF on FM is, I believe, 8MHz on these sets. What if the LO is wrong in the tuner head? That would lead to a false IF, but then everything downstream would be wrong too, so I somehow doubt that.

Short of putting it on the bench (no room - full of TV camera bits), I don't know if that's right or not. The time vampire is looking fully replete, I can receive Lincoln City Radio and all is well, so I'm not going any further just now. Manufacturing error? C5 was correct as per the schematic.

Thanks for the responses and I hope that this thread might be useful to others with 'deaf' 215s.

BTW. are these really 80s sets? It looks so 1970s. Made in Minsk, Belarus, I believe.

Best regards,

Paul M

[paulsherwin]

They are really a 70s design which stayed in production in various forms for a decade. They are odd things, a strange mixture of overdesign and shoddy materials and construction. The MW/LW performance is indeed very good though, thanks to the tuned RF stage. I've seen it argued that the FM performance is a bit feeble because they were originally designed for the lower frequency OIRT band used in the USSR.

[Jon_G4MDC]

I can see a diode ring mixer in there - no wonder it works well!
Glad you are happy with it now and yes the VHF RF Amp/Mixer are indeed elsewhere.

Is there any chance of a higher resolution version please - maybe make a pdf of it?

[linescan87]

I have three of these and two B212. They are all mediocre on VHF except one of the B215's, which is really sensitive! I've not done anything to it, it may be worth opening it up sometime to see if it's been modified.

[PaulM]

Sorry, that's the highest resolution that there is of the schematic (from a Russian source). It turned up as a TIF. It may translate better into pdf rather than JPG. I can try.

The VHF FM sensitivity is now as good as my Hacker RP37 (sort of contemporary), but it sounds better.

I wonder if this was a 'gotcha' on the production line - the response seemed to peak but not properly. A quality control issue?

Perhaps mine is just a 'one off', but I somehow doubt it.

I've been to Belarus in the early 2000s, and worked with some engineers in Minsk. I was actually pretty impressed with the people and the country wasn't as bad as many paint it. A curious place indeed, but I did enjoy my week there and I was made very welcome.

Best regards,

Paul M

[Pfraser]

Re. C5: is the removed part actually of the value marked on it? - possibly a component manufacturing or labelling error?

I had a B212 and found the FM sensitivity quite impressive; selectivity less so, in the busier reaches of the FM band in the 1980s. Then again, I was not far from the Sutton Coldfield mast at the time!

Moderate tropospheric lift conditions would bring in Viking Radio, fully quieting. I also seem to recall Solar Radio, from London. Signal Radio was always a usable signal, somewhere around 104MHz. Sunshine Radio from Shropshire came in just about fully quieting, on FM.

MW performance was certainly good, for a (luggable) portable. I didn't know about the tuned RF stage. The decent-sized ferrite rod probably helped too.

[PaulM]

I didn't actually put the removed C5 on the LCR bridge, but substituted with a known good 50pF and it was just the same. The time vampire had already had quite enough of a feed on this one . . . !

Best regards,

Paul M

[dan.mih]

Sorry for interfering. Not exactly a response to the questions.
I have found this modernization of the FM block.
https://www.kn34pc.com/construct/v_terziev_ukw.html

I did not have the time to build it yet, but it is intriguing. With the help of Google Translate it is easy to understand. A more detailed archive can be donwloaded.

Other useful details here:
https://www.kn34pc.com/construct/v_terziev_ukw_rx.html

[orbanp1]

Originally Posted by PaulM

...
Now, I may be treating symptom here. The IF on FM is, I believe, 8MHz on these sets. What if the LO is wrong in the tuner head? That would lead to a false IF, but then everything downstream would be wrong too, so I somehow doubt that.
...

Hi Paul,

Congrats on the successful repair!

The PNP transistor version radios are the B-210-11-13 series.
According to the service sheet I have on these radios the FM-IF is at 10.7MHz.

Regards, Peter

[PaulM]

Hello Peter,

Thanks for the information and the congrats. 10.7 MHz sounds more likely, but I'm prepared to accept anything reasonable with these USSR products!

The schematic that I have is annotated 0.5 Mc/s and 8 Mc/s (in Cyrillic) near L3.1 (the first IFT - the one in question). See my post #1. Could it be that they moved to 10.7 MHz but mine is actually 8 MHz - hence the need for more capacitance to bring the resonant frequency down? Back to production/documention errors? Guess we will never know.

Short of measuring it, I'm not going to know for sure and the time vampire isn't having any more out of me on this one!

It does sound good, so well pleased to have 'improved' it.

Back to the TV kit now . . .

Best regards,

Paul M

[orbanp1]

Hi Paul,

You are correct, the B-215 also uses PNP transistors in the IF-stages, except those are all Si-transistors!

The FM-IF on those radios are 10.7MHz too!

What you interpreted as as 8MHz on the drawing, that is IF sensitivity, in "mkb", that is in micro-volts, with Cyrillic letters.

Regards, Peter

[crackle]

The 215 that I had was fairly death on FM but brilliant on AM.

Mike

[PaulM]

Hello Peter,

OK, I could buy that. 'B' in Cyrillic is indeed a 'V', so yes. Hard to read the subscript and I just latched on to the '8' and '0.5' as likely to be IF frequencies (FM and AM) - seems logical(ish).

It also seemed that 8 MHz versus standard 10.7 MHz could account (somewhere) along the line for the effect seen as a production/documentation error.

The main thing is that it now works an awful lot better and may help other so-afflicted 215s.

Best regards,

Paul M