Selena Vega B215 Circuit Diagram / Schematic

[3Phase615V]

Hi,

I took the ground from the radio's metal chassis. Was that not correct? My measurement of tag 1 on the VHF UNIT was from chassis ground as well and that was about correct at 4.5V (4.2V on schematic).

I previously did the oscillator test with the radio tuned to a weak radio station at 107.8MHz and tuned the broken radio right across the band with no effect to the sound coming from either radio. I'll try what you suggest on this anyway to see if it reveals anything. Unfortunately my oscilloscope is broken at present,so I can't probe around for where the signal path is dying.

I was thinking of what you suggested on the IF UNIT, which I will try shortly.

Thank you so much for your help. Much appreciated.

[orbanp1]

Originally Posted by 3Phase615V

...
I took the ground from the radio's metal chassis. Was that not correct?
...

Sometimes they are not the same. Do measure the voltage between the radio's metal frame and the metal shield of an IF-can on the IF board. The shields of the IF-cans are tied to the ground.

Peter

[3Phase615V]

Okay, thanks, making progress. Completed the tests and it would appear the VHF UNIT is okay on the broken radio as I get FM coming through nicely on the working radio but if I feed the working radio's VHF UNIT to the broken radio, it's still dead. So now taking a look at the signal path through the RF AMP-IF UNIT board if I possible without a scope...

[orbanp1]

The IF section in that radio is mostly common for AM and FM. that means that most of the transistors there are probably fine.

The differences between the AM and FM path are as follows:
- For the FM-IF there is an extra pre-amp stage implemented by V6. Do check the voltages on that transistor.
- The IF amplifier is common for both IF frequencies, but the filters are different. The FM pass band filter is the top row of LC filters, the AM are the bottom ones.
- The decoders are different.
Do check the solderings in the FM filter and decoder stages!
- The audio signal from the FM decoder is amplified by V18, while the audio signal from the AM detector is not amplified by that stage. Do check that V18 stage out too.

The IF stage uses mostly Si-PNP transistors, while the tuner uses NPN transistor - here is your explanation why the metal frame of the radio is not at 0V DC.
Both the metal frame and the ground are the same potential from AC point of view, using capacitors, while they can have different DC potentials.

This would be a good time to have a scope, that would make finding the fault in that IF stage a lot easier!
You should fix your scope too! I am sure we could lead you through the repair of that too, should you require.

Peter

[3Phase615V]

Peter, again many thanks for you advice and guidance.

I've been probing around and getting voltages through the stages and actually another forum user (thank you, Paula ) suggested via PM that I check the voltage across C56, which I did. On the good set the voltage it's around 1.5V, but it's completely dead across the cap on the faulty set. Removing the cap from circuit shows it's working okay value and ESR-wise, I think, though it sends my component tester crazy after a couple of seconds under test (asterisks and + symbols appear on-screen).

The V6 voltages on both radios seem comparable (see below); however, there are one or two other transistors that are not. It also starts to get a little confusing around V13 on the schematic as one or two of the components and markings are hidden under components on the PCB. V12 on the schematic is indicated as a Zener diode but on my radios' PCBs V12 is marked beside a brown plastic boxed component (is it a transistor?) I've given the voltages for V13, assuming the number under R39 is a "13" making the marking V13. The R39 marking is half hidden behind the red ceramic you can see leaning slightly back over the marking (see photo).

Moving on, I removed V15 to test it and it appears okay, but on pulling V13 out (space circled in red in photo) the component tester shows an open circuit between the collector and base, if the PCB markings are accurate (I found V15 PCB markings were not). So, assuming V13 is the black-cased transistor, i.e. KT209Б, it looks blown. It has the number 310 on the front of it with possibly some Cyrillic below that number (see photo). What would be an equivalent for this component? Interestingly the voltages looked okay between the sets.


Working Set........Faulty Set

.....V6.....................V6......
E - 3.8V................3.73V
B - 3.05V..............2.98V
C - 0.227V.............0.291V

.....V18...................V18.....
E - 1.48V...............2.05V
B - 2V...................2.66V
C - 8V...................8.18V

.....V16...................V16.....
E - 4.1V.................4.72V
B - 2.08V...............4.1V
C - 4.7V.................1.7V

.....V15...................V15.....
E - 3.8V..................3.8V
B - 4.5V..................4.56V
C - 3.7V..................2.6mV (0.0026V)

.....V13...................V13.....
E - 8.1V..................8.1V
B - 7.27V................7.46V
C - 4.71..................4.73V

[orbanp1]

Do substitute C56 for another cap!
ESR measures the AC resistance of a cap, if the cap is shorted it will show up as "good ESR" on an ESR tester.
Depending on the tester, those funny characters on the cap tester tells me somehow that cap is not really up to snuff!

V11, V13, and V14 makes up a low voltage stabilizer circuit, it provides the +4.4V stabilized supply for the FM tuner, the AM oscillator (V9), and the AM-RF pre-amp stage (V8).
If the +4.4V is correct then the circuit works correctly.
You adjust the voltage with the R30 trimmer pot.
V12 is a selenium stabilizer, it gives about 1.3V to 1.6V.

If the +4.4V is OK, then V13 must be fine too. Probably something not right with the tester.
Again, those transistors in that circuit are generic low power Si-NPN-PNP transistors, V13 (PNP) can be substituted by a 2N3906 or similar, the NPNs with what you have already used.

BTW, the voltage marking on the schematics at V13 base and V14 collector is not correct, your voltage readings are the correct ones!

EDIT:
Looking at the schematics again, that 1.9V should read 7.9V, probably just bad graphics there, and the 1.5V should point to the base of V14!

Do check/replace that C56!

Peter

[ronbryan]

Assuming that the circuit Peter supplied in post#10 is the one you are working from, C56 is the FM discriminator stabilising capacitor, so unless the FM receiver section is working, it will not have any voltage across it, because there is no FM carrier to rectify. There is no harm in changing it, but if it tests ok then it probably is ok.

I have found that those red rectangular coupling capacitors in the FM band pass filter are unreliable and would be worth checking.

Ron

[3Phase615V]

Just waiting for some 2N3906 transistors to arrive. Will update when I've been able to replace the faulty transistor. It fails a diode test as well, so I believe it really is faulty.

[paulsherwin]

I'm sure you're right about the transistor, but have you cleaned the wavechange turret contacts thoroughly? These are a notorious source of trouble. It's possible to remove the turret completely for cleaning though it's a fair bit of work.

[3Phase615V]

Hi Paul,

Thanks for your suggestion. Yes, that was one of the first jobs a took care of and the contacts are clean and make good contact. Fortunately I've been able to get to all the contacts to clean them without needing to remove the turret.

[3Phase615V]

Oops! I appear to have made a mistake with the V13 - it is in fact working. I realised the component tester I may not have had the legs in consecutive holes before hitting the test button, so it was detecting only two of the legs on the transistor. Not sure how I managed to miss it again in the diode test, but there it is. So looks like V13 is okay and it is definitely marked as V13 on the PCB as I was able to get under R39 to check it.

[3Phase615V]

Just thought I'd update this thread with some news that FM has been restored to full working order. I generous friend spent some time with me and we identified the following components as having failed, C5 (51pF) and C27 (24pF), which are both of the red bead variety in this radio. C27 was found to be open circuit and following the replacement of it, it restored FM audio, but only at about half the volume output expected. C5 was then found to have increased to near 100pF. Replacing it restored the audio output to normal. Decided to swap out all the other red bead caps as C26 was also looking like it was drifting out of spec. C18 was also out of spec and replaced. Adjustment of the caps' corresponding inductors will of course be necessary bring the FM alignment back to optimal. Hope this helps.