Selga 404

[Neil Purling]

Sorry people. I made a cock-up there.
No signal
C: 7.97V
B. 8.9V
On a signal the reading is:
C. 6.5-6
B. 8.91V

[orbanp1]

Hi Niel,

There is something wrong with those voltages!

If you recall I mentioned that the referencing of the voltages in the schematics seems to be mixed up!
If these voltages are indeed referenced to the +9V line, then the voltages on the base of T7 and T8 should be around -0.12V.
Do check the voltage at the TR1 secondary center point, at TR1 primary winding, both ends.
From the +9V the transformer center point should be about the same as the bases, -0.12V or so, on the primary winding where the collector of T6 is connected, there should be -0.7V or so. The resistance of the primary winding of TR1 is only 260 Ohm.
That is the point where they switched to ground reference in the schematics, and in that case that voltage should read +8.3V (9V - 0.7V).

See if these voltages are correct, there could be a break in one of the transformer windings or a cold solder joint.
Also check out R27 100 Ohm resistor, and R23 330 Ohm thermistor, and their solderings!

Good luck, Peter

[Neil Purling]

Ought I to have been measuring with the DMM on AC, rather than DC?
AC range
T7/8 collectors 0V
T7/8 bases 0.4-2.1V varies with the signal
T7/8 emitters 0.3V-1.2V

DC range
bases negative 8.85V, does not change with signal
emitters minus 7.7 - -6.6

Can you make any sense of this??
I think I need a tea intake, urgently.

[Neil Purling]

Is R27 something that actually resembles a blue disc ceramic capacitor?
There's also some flat wire around a paxolin former between T7 and T8. Here my print-side diagram is unclear. It could well be R26, an unusually low value.
If you can make any sense of my measurements in post #23 your brain is less befuddled than mine, even after intake of tea.
I need some potential culprits to check out and change as necessary. If T5 or 6 themselves are at fault I do have substitutes.

[paulsherwin]

You should be using the DC range.

[orbanp1]

Hi Neil,

Hang in there, you will fix it!

The voltages are DC voltages as Paul mentioned, do measure them without any audio signal.
Also do measure the voltages with reference to the ground, you did not really tell, we can just guess, and some of the values are questionable.

On the T7, T8 emitters you should read close to +9V, the voltage drop on that 6 Ohm resistor at 3mA or 4mA current is about 18mV to 24mV. So you would see 18mV to 24mV less voltage on the emitters than 9V.
On the bases of T7 and T8 you should read about 0.1V to 0.3V less voltage than on the emitters (that is the forward bias voltage of a Ge B-E diode of the transistor), so that would be 8.9V to 8.7V (the 18mV to 24mV is neglected).
The actual value is not that important if you are in the ballpark, what is important that the base voltage is less than the emitter voltage (when you measure it to the ground).

The blue disk is the thermistor, it is a 330 Ohm resistor with negative temperature coefficient. 330 Ohm is the value at 25C degrees (if you warm it up the resistance drops).
The 100 Ohm resistor is that green cylindrical item like the other resistors.
That funny thing between the emitters and the 9V line must be the 6 Ohm resistor.

Peter

[Neil Purling]

I was using the wrong wire for the reference point!
Where is that emoticon with the dunce's cap for that 'brain-fart' moment?

T7 & 8 are really.
C -9.00
B -0.15
E -0.26

Does anyone know the wattage for R26, that strange wire-wound thing on the sliver of paxolin?

[Neil Purling]

!Hallelujah Moment!. By that I mean a huge choir in the Albert Hall + the mighty Father Willis organ giving their all.
The only 6R8 resistor I have is a big ceramic one, about 3-5W dissipation. That strange 6R resistor, R26 was reading 2.8K.
Bridging the pins with a 6R8 resistor restored normal operation instantly.

This little Selga has has nearly driven me to bang my head against the door frame at times.

I wonder if a 0.6W metal film resistor is going to be sufficient?

[Herald1360]

Unless the set takes more than 300mA from the supply and it all goes through the 6R8 resistor, yes.

[Neil Purling]

One of the MP41 transistors was defective to the point of getting hot & bothered.
Remember that I mentioned before that I had found the wires of them to be loose?
One of the MP41 spots (T8) has a AC128K there until some MP41 come from the USSR.
It now plays brilliantly and the RF alignment seems spot on.

[Neil Purling]

Is the earphone socket a standard jack, or specific to RRR sets? It looked like a 3.5mm jack, but the plug from my Sony headphones won't go in & I certainly am not going to use any force. The jack looks rather over-engineered.
The Selga did not come with earphones. I have never had much use for those earphones in 1970's radios as they were rather un-hygenic. I am wondering what the Selga came with. The battery area has a space to keep some.

[Neil Purling]

Does anyone know of a former Soviet Union source of a replacement volume potentiometer/switch unit & control wheel. Possibly a new unit?
The track of the volume control on the Selga seems to have damage in that the audio is distorted at one spot. Whichever Forum member originally brought this back to life for me mentioned damage. The plastic control had to be glued onto the 'pot, so possibly physical damage rather than wear & tear.
I am hoping that the volume control and the thumb wheel was shared by several models.

[Neil Purling]

Where were the Selga 404 & 405 in the hierarchy of Soviet radios in terms of quality of manufacture and of the sound reproduction?
I was wondering if it is worth sourcing a new old-stock control and thumb wheel.