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Old 22nd Aug 2020, 5:40 pm   #61
OldTechFan96
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Default Re: Vega Selena 215 Poor FM

Am I right in thinking that the voltage across C56 will increase with the cumulative amount of amplification? Therefore the voltage across C56 will be higher at the B of VT6 than the B of VT15.

EDIT: I understand now. When the signal is injected into the B of VT10 there is 1.5V across C56. This voltage drops to 180mV when the signal is injected into the base of VT7.

I'll look into the components between VT10 and VT7.

Last edited by OldTechFan96; 22nd Aug 2020 at 6:00 pm. Reason: More info.
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Old 22nd Aug 2020, 7:03 pm   #62
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Default Re: Vega Selena 215 Poor FM

You've pretty much got the idea now. Each additional stage of amplification should give a higher voltage on C56 for a given input voltage, or will require a smaller input voltage for the same output voltage.

The block of tuned circuits will have some loss, I wouldn't expect it to be more than about 10dB loss. The bulk of the IF gain comes from VT10 and VT15.

The signal level at VT10's base seems to be on the high side to obtain 1V on C56. I would look at C47 and C53 too. A quick way to check that the IF gain is about right is to connect your oscilloscope via a X10 probe to the collector of VT15. Connect the signal generator to the base of VT10. Adjust the signal generator's frequency for the peak output, which will be lower than 10.7MHz due to the additional capacitance from the oscilloscope/probe.

I would expect that you should see a voltage of 2v on the oscilloscope for an input voltage of less than 20mV p-p. If there isn't a peak frequency as you tune the signal generator and/or you need much more than 20mV for 2V on the oscilloscope, C47 is suspect.
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Old 22nd Aug 2020, 7:37 pm   #63
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Default Re: Vega Selena 215 Poor FM

Quote:
Originally Posted by frsimen View Post
Try next the injecting the signal into the base of VT7, which should give a good increase in the output voltage, followed by the base of VT3 which should also increase the output voltage. If you don't get an increase in voltage level at VT3 or VT7, the tuned circuit in the transistor's collector will be the source of trouble.
I have followed this through and here are the voltages measured across C56:

B of VT15: 1V
B of VT10: 1.5V
B of VT7: 100mV
B of VT6: 5mV
B of VT3: 65mV

I now understand that there will be a reduction in voltage between VT7 & VT10 due to the tuned circuits. Is this reduction about what you'd expect?

The lack of voltage at the B of VT6 suggests that there is an issue between VT3 and VT6. Am I on the right track? Where is this voltage being lost?

The 100mV at VT7 suggests that the 5mV is being amplified fine.
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Old 22nd Aug 2020, 9:19 pm   #64
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Default Re: Vega Selena 215 Poor FM

Are all these measurements with the signal generator output set to the same level?

If they are, the tuned circuit associated with VT6 is a likely candidate for the main source of trouble. The 51pF capacitor or possibly the diode could be faulty, as there seems to be a big loss of signal between VT6 and VT7. That is the sort of loss I was hoping to see.
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Old 22nd Aug 2020, 9:21 pm   #65
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Default Re: Vega Selena 215 Poor FM

Yes, all the same level.

Would C5 measuring way too high be enough to stop things working?
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Old 22nd Aug 2020, 9:28 pm   #66
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Default Re: Vega Selena 215 Poor FM

Yes, it certainly would. Try measuring its resistance with your DMM, you will probably find that it has quite a low resistance. It does sound like you've found the main culprit.
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Old 22nd Aug 2020, 9:43 pm   #67
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Default Re: Vega Selena 215 Poor FM

I'll pull it after the snooker finishes.

I understand the trouble that high value resistors cause in circuits, but what about high value capacitors? How does too high capacitance influence the working of a circuit?
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Old 22nd Aug 2020, 9:54 pm   #68
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Default Re: Vega Selena 215 Poor FM

If it were genuinely high in value it would lower the resonant frequency of the tuned circuit. However, a leaky capacitor when read on many capacitance meters will read high in value as the leakage causes the meter to give a false high reading. The actual value may be correct still, but the leakage resistance will lower the Q factor of the tuned circuit (possibly by a lot). Reduced Q will lead to reduced stage gain or in a bad case even attenuation.

Either way, a replacement is required.

Enjoy the snooker!
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Old 22nd Aug 2020, 11:13 pm   #69
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Default Re: Vega Selena 215 Poor FM

I have pulled C5 (51pf) and it tests open with an ohm meter. With my component tester I get erratic readings that range from 21pf to 49pf. I will check it again in the morning.

Funnily enough, I suspected C5 a few hours ago and removed it. Its capacitance then measured 140pf. I did not know how bad this was and put it back. It seems I can't replicate that reading at the moment.

These component testers do have limitations when it comes to measuring small pf capacitors. The lowest mine will register is 25pf. The manual does recommend measuring small capacitors in parallel with large capacitors. I'll do this tomorrow.

Thanks for the explanation of the way capacitance affects tuned circuits.

Last edited by OldTechFan96; 22nd Aug 2020 at 11:21 pm.
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Old 23rd Aug 2020, 12:09 pm   #70
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Default Re: Vega Selena 215 Poor FM

I think it is best to replace that capacitor, I don't like components that measure differently each time you try. You need to use a NP0 or C0G dielectric ceramic capacitor to replace any of the IF tuned circuit capacitors as these have the best temperature stability. Choose 5% or better tolerance types for this position. 51pF is a value where a less temperature stable type may also be found, so you will need to be careful with what you select.

The diode is also worth checking, in case that has gone short circuit and look carefully around the IF transformer for breaks in the track on the secondary side too.
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Old 23rd Aug 2020, 2:40 pm   #71
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Default Re: Vega Selena 215 Poor FM

I've been able to find a 51pf capacitor from a spare radio chassis. I will install it and do more IF tests. I lifted one side of diode VD5 and it tests good. What is VD5 for? Overload protection or an FM limiter?
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Old 23rd Aug 2020, 3:42 pm   #72
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Default Re: Vega Selena 215 Poor FM

Oh that's good. Hopefully the radio will perform rather better with that fitted. The diode provides both of those functions, by damping the tuned circuit on strong signals.
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Old 23rd Aug 2020, 4:01 pm   #73
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Default Re: Vega Selena 215 Poor FM

C5 has been replaced. The PCB looks fine and L3 is not open. There has been a change in the voltages measured across C56:

B of VT15: 1V
B of VT10: 1.6V
B of VT7: 10mV
B of VT6: 270mV

The voltage at the B of VT7 is lower than it was before and the voltage at the B of VT6 is larger. I can't quite tell if this is progress. Other components are probably bad too.

It looks like VT7 is amplifying a lot more this time, which is good.

I wonder if we are looking at a fault with in the tuned circuits, since so much signal is being lost here?

I'll also go through the procedure outlined in post #62.

EDIT: I suppose that I could inject the signal into the tuned circuits and see where the signal drops the most?
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Last edited by OldTechFan96; 23rd Aug 2020 at 4:07 pm. Reason: More info.
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Old 23rd Aug 2020, 4:15 pm   #74
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Default Re: Vega Selena 215 Poor FM

I think you are right, you have certainly got much more gain now from VT6. There seems to be something wrong in that main block of tuned circuits between VT7 and VT10.

The other approach you could try is to apply the signal to the base of VT7 and using the oscilloscope check what you measure on the collector or VT7, then move on to each of the tuned circuits to the right on the diagram. I would expect a big drop in signal level ( much more than 1/2 the level drop ) when you find the faulty tuned circuit. Either approach should reveal where the fault is.

It may be quicker to change all those capacitors for long term reliability.
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Old 23rd Aug 2020, 6:42 pm   #75
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Default Re: Vega Selena 215 Poor FM

More progress:

I observed a large drop in voltage when probing C27 (24pf). I removed it and its capacitance was measured as 7000pf! I decided to replace C27, C32 and C19 as they all meaured very high. C45 was within tolerance so I put it back.

New measurements:

B of VT15: 1V
B of VT10: 1.75V
B of VT7: 900mV
B of VT6: 1.2V

Things are looking much better now. I like that there is not as much of a drop over the tuned circuits.

I'll reconnect the FM front end and see what I can pull in.
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Old 23rd Aug 2020, 8:48 pm   #76
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Default Re: Vega Selena 215 Poor FM

I reconnected the FM front end and things started to look promising. Plenty of stations could be pulled in but when a station was peaked it would distort heavily.

I noticed that the capacitor (C53) across L15 in the discriminator was of the same type that I replaced in the IF section. It measured 5000pf and was replaced.

Replacing C53 cured the distortion issue and the radio is working really well on FM. I will do more testing later on.

I still think the radio needs a bit of fettling as the signal strength meter just about makes it half way with a strong signal. Also, instead of the 'FM hiss' you hear when a station is off tune you hear a rasping like sound.

In total I could pick up 16 stations, including a low power hospital station.

This has been quite a good learning experience. Many thanks to those who have contributed to this thread so far. Especially to frsimen and orbanp1!
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Old 23rd Aug 2020, 9:25 pm   #77
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Default Re: Vega Selena 215 Poor FM

Well done on persevering with this and getting it working again.

The set will benefit from a realignment as you have changed quite a few of the capacitors on the IF transformers. That won't be too difficult with your Heathkit signal generator, there may even be a published procedure, although I've not seen one.
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Old 24th Aug 2020, 10:26 am   #78
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Default Re: Vega Selena 215 Poor FM

Yes, congratulations. These sets aren't easy to work on at a component level.
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Old 24th Aug 2020, 11:58 am   #79
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Default Re: Vega Selena 215 Poor FM

Congrats on the repair!

In the tuneup of FM radios the very first IF transformer, that is on the FM tuner board, usually has quite an impact on the quality of reception.
In can be tuned up by ear. Tune to a weak FM station, and adjust the slugs of L5 and L6 for max output (assuming the rest of the radio is tuned up reasonably well).

I did find the document that lists the usual problems with these radios, and it also mentions the bad filter capacitors too!
(Looks like the doc was translated from Russian by some software, so it is bit "cryptic", but can be made sense of.)

Congratulations again!

Regards, Peter
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Old 25th Aug 2020, 7:08 pm   #80
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Default Re: Vega Selena 215 Poor FM

Miscellaneous Vega Selena Service Data:

Here is all of the Vega Selena service information I could find on the internet.
  1. 1 This is a 61 page service manual (from orbanp1) that covers the 215 and 217. I've compressed it so it can be uploaded to the forum. It is still readable, although the English translation is a bit hard to follow at times. No circuit included.
  2. 2 Circuit diagram for the 215, 216 and 217. Si transistors. IC audio amplifier. Two PNP transistors in the VHF front end. Also includes PCB and component layouts. 8 pages.
  3. 3 Circuit diagram for the 216. Si transistors. IC audio amplifier. Three NPN transistors in the VHF front end.
  4. 4 Circuit diagram for the 214. Si transistors. IC audio amplifier. Three NPN transistors in the VHF front end.
  5. 5 Trader Sheet for 210/2
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File Type: pdf Selena_B215-217_SM-compressed.pdf (605.9 KB, 112 views)
File Type: pdf selena_b-215_b-216_b-217_sch.pdf (2.45 MB, 115 views)
File Type: pdf Selena216.pdf (2.79 MB, 112 views)
File Type: pdf OKEAN214schema.pdf (486.4 KB, 89 views)
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