Vega Selena 215 Poor FM

[OldTechFan96]

I have had this radio for a while and now feel confident enough to tackle its issues.

It works well on AM. FM is bad. It pulls in a few stations poorly.

The voltage on pin 1 of the VHF front end is 7.3V and it should be 4.3V. I've been looking at the service manual and I'm not too sure about where this voltage originates. The switching make it hard to work out as the circuit diagram is a bit different to what I'm used to.

I'm powering the radio via a benchtop PSU set to 9V.

Any help would be appreciated!

[paulsherwin]

These radios don't have outstanding FM at the best of times, especially considering the complexity of the design.

Presumably you've cleaned the turret tuner contacts, but I would suggest giving them another go. You may need to dismantle the mechanism to clean the contacts properly

[mickm3for]

Hi i had fault on two of these both low distorted fm both same fault 2x ge diode in fm det. i used oa91s Mick

[OldTechFan96]

Thanks for the pointers. I'll hook up my 'scope up to the input and output of the FM detector and see what's going on.

[orbanp1]

This is the circuit diagram I am referring to:
http://www.radiohistoria.sk/Oldradio.../Selena216.pdf
Stitch them together to a single drawing, that should help!

The voltage regulator for the FM tuner is located on the "A3 RF AMP - IF UNIT" module, it is based on the circuit of V11, V13, V14 transistors. It produces +4.4V on the collector of V13 transistor.
This voltage through R19 resistor (27 Ohm) goes to board connector "3", wiring loom id "3". At that point it is +4.2V.
Wiring loom "3" goes to the band change turret connector "3" on the "A2 SW-MW-LW UNIT". That unit also has the FM strip on the turret.
On the FM turret connector "3" goes to connector "18". It is also wiring loom id "18".
Wiring loom id "18" goes back to the "A3 RF AMP - IF UNIT" module, board connector "16".
On that board board connector "16" is tied together connector "22", wiring loom id "25".
Wiring loom id "25" goes to the FM tuner module connector "1".
Based on this you should be able to follow the supply voltage.

Hope I did not loose you in this explanation!

Regards, Peter

[OldTechFan96]

Thanks for that explanation. I've followed it through and I have measured 7.3V at each point. I don't seem to be able find 4.3V anywhere.

I've measured the voltages on the transistors you mentioned and they are either wrong or not quite right. They test good in my tester.

What does this part of the circuit actually do? I've attached a snippet.

[orbanp1]

As described earlier, V11 to V14 realizes a simple low power stabilizer circuit.
The voltages to be checked in the circuit are listed on the schematics.

First to check is the voltage on the stabilizer diode, V12, it is a 7GE2A-K selenium stabilizer diode. You should read around 1.46V to 1.6V on the diode.

If the voltage on the stabilizer diode is within range, it is possible that the trimmer pot, R30 in the schematic, developed some "unwanted change". Do exercise the pot (and/or clean), and then try to adjust the output voltage, 4.4V on the collector of V13.

If the stabilizer diode 7GE2A-K is faulty, it can be replaced with three small signal Si diodes in series, like 1N914 or equivalent, same polarity as shown in the schematics. This would give a bit higher reference voltage, but the output voltage adjustment is within the range of the trimmer pot.
If you are running out of adjustment on the trimmer pot, do change R29 from 12k to 10k. That should give you enough adjustment.

If any of the transistors are faulty, those are small signal Si NPN and PNP transistors, 2N3904 and 2N3906 or similar would work there without any problem.

Also enclose a simulation of the circuit with three Si diodes instead of that Russian stabilizer diode, it pretty much shows voltages in the same ballpark.
Also enclose a datasheet on that selenium stabilizer diode.

Hope this helps.
Regards, Peter

[OldTechFan96]

Thanks for the further explanations. I have been able to adjust the FM front end voltage to 4.3V via R30. The voltages for the three transistors are now correct. I will reassemble things and do more testing.

When I bought the radio in 2017 I twiddled some of the potentiometers on the right had side of the PCB. R30 was turned to an extreme. This must have turned V11 off and also explained the other wrong voltages.

[crackle]

My 215 had poor FM sensitivity, but excellent AM reception.

Mike

[OldTechFan96]

I've checked the voltages on the transistors in the FM front end. The voltages on VT2 match what is given in the service manual.

VT1 (SM spec.):

E: 3.3V (1.05V)
C: 85mV (3.45V)
B: 2.5V (1.7V)

VT3 (SM spec.):

E: 3V (1.3V)
C: 0V (3.8V)
B: 2.3V (1.9V)

The above voltages are wrong and I'm not sure what conclusions to make from this. The radio servicing books I have don't seem to go into how to interpret wrong transistor voltages.

[paulsherwin]

You could have a transistor fault of course. The 215 uses the KT315 family of silicon planar transistors, one of the first Soviet consumer products to do so. The USSR was very slow to develop cheap mass market silicon transistor technology - they were a decade behind Philips/Mullard and over 15 years behind the American big boys. They aren't the best performing transistors of their type in the world. That said, I'm not aware they have a reputation for failure.

[OldTechFan96]

I've been able to pull and test VT1 and VT3 and they test good. They show up as PNP and are unmarked.

I'll do some checks around the collector circuits. Looks like a good place to start as they both measure 0V.

[Sideband]

Originally Posted by OldTechFan96

I've been able to pull and test VT1 and VT3 and they test good. They show up as PNP and are unmarked.

When you say they 'test good', in what way do they 'test good'? As an example, I have had a humble BC108 fail in a circuit. Taking it out and doing the usual resistance tests on a meter showed that it did indeed measure correctly as a good transistor should. However a replacement (which measured exactly the same resistance-wise), got the set working perfectly. Making up a test circuit later proved that the faulty transistor had virtually no gain at all.

If you are just relying on resistance measurements for checking the transistors, that may not be sufficient.

If you are using a more sophisticated tester, it is presumably giving you a gain result. How do you know it is correct?

[buggies]

Originally Posted by OldTechFan96

I've checked the voltages on the transistors in the FM front end. The voltages on VT2 match what is given in the service manual.

VT1 (SM spec.):

E: 3.3V (1.05V)
C: 85mV (3.45V)
B: 2.5V (1.7V)

VT3 (SM spec.):

E: 3V (1.3V)
C: 0V (3.8V)
B: 2.3V (1.9V)

I think it is suspicious that the voltages measured + specified total about 4.3 volts e.g VT2 E 3V (1.3V) and the emitter voltages are above the bases.
Is it possible you are measuring from the wrong point? (4.3 v rail tag 1 instead of tag 2 "ground")

[OldTechFan96]

I used a component tester to check the transistors. The gain figures for VT1 and VT3 are 102 and 68 respectively.

I understand where you are coming from. I once had an issue with an audio output stage where the faulty transistor tested fine with a DMM but was revealed to have a gain of only one. Replacing the transistor solved the issue.

EDIT: I'll do some more checks buggies. My negative lead was clipped to the chassis, which connects to the VHF tuner head via pin 2.

[ronbryan]

Post #12 says the component tester identifies VHF tuner transistors VT1 and VT3 as good and type pnp. Was that a typo? The circuit diagram shows them as npn.

Ron

[OldTechFan96]

Both transistors tested as PNP. I'll upload a photo later.

The results are unusual as I was expecting NPN transistors.

[OldTechFan96]

Here is the result from the component tester. They are both unmarked apart from some blobs of paint.

I can't find any service data for this radio that references PNP Si transistors in the VHF front end.

I've also included two photos that demonstrate an easy servicing position for both PCBs.

[paulsherwin]

That doesn't look like a KT315 series transistor, so I've no idea what's going on.

https://en.wikipedia.org/wiki/KT315

The analyser numbers do look right for a PNP silicon transistor though.

[OldTechFan96]

The Trader Sheet for the Selena 210/2 does show PNP transistors in the VHF front end but I suspect they are Ge. Not really much help.

My Selena is an 1987 model and must be newer than the service date that I can find.