Riga 103-2 - Crackly Bass Control, A Small Modification.

[Kat Manton]

Hi,

While working my way through this set and restoring the circuitry to how it's meant to be ("The Phantom" has attacked this set a little) I've worked out something which I thought I'd share:

The yellow lead to the bass control wiper had been connected to one end of the track by "The Phantom", returning this wire to the wiper resulted in a very crackly bass control; not the "dirty pot" noise, but the "DC passing through the track" noise, if you see what I mean.

Referring to the attached schematic for the Riga 103 AF amplifier, the first item under suspicion is the coupling capacitor C7 (1). This was found to be 5uF, not the 1.0uF as per the schematic. I replaced this with a 4.7uF 35V electrolytic.

This didn't cure the problem entirely, though. Closer examination of the schematic showed a DC path from the emitter of T1, through R12, R13, R8, R6 (the bass pot), then R14 to chassis. (Well, the DC path is technically the other way round, +ve supply to chassis, -ve supply rail, PNP transistors... I just think upside down when working on this sort of set.)

Knowing that the Riga 103 portable consists basically of the RF panel and one AF panel from the Riga 101 stereo mains receiver shoehorned into a large battery portable case; I took a look at the schematic for the Riga 101.

On the Riga 101, the DC path through the bass control is not there; so I've modified my 103 to match this, as follows:

• Remove 10k resistor R12 (2) - I just desoldered and lifted one end.
• Fit a 10uF electrolytic, as per the Riga 101 diagram (5) - there are holes present in the PCB for this; the +ve end goes to the track leading to external connection 3.
• Fit a 47k resistor between external connections 3 and 4 (6) - again, there are holes present as the tags for these connections are absent.

Result - no DC through the bass pot and no crackles when it is adjusted.

Another optional modification, with 3.3k resistor R8 (3) present, the bass seemed a little thin for a set of this size. This resistor wasn't present on my set anyway and isn't there on the Riga 101 circuit, so I've left it out; the bass response is now "adequate".

C20, the 500uF loudspeaker coupling capacitor (4) on my set had been badly replaced with a 1000uF axial type flopping about on long leads; I tried a 470uF in this position but felt that 1000uF produces another slight improvement in bass response, so I've fitted a radial lead 1000uF capacitor properly.

I hope this information may be of use to any fellow Riga 103 owners out there

Regards, Kat

[John Jones]

Hi Kat,

I've just been given an Astrad Solar, and having checked out the schematic, I'm amazed at the features this receiver has in the AM part; double-tuned RF tuned circuits, an RF amplifier, double tuned IF transformers, two bandwidths....

These are features I hadn't come across before in a portable transistor radio, and the switchable bandwidth is something I'd only read about until now.

Mine worked fine after the application of a little switch cleaner, and my only real complaint was that the bass control seemed a little weak as you described, so I'll probably apply the mods you've mentioned.

My only real worry is that if any of the transistors packs up, I might have trouble finding replacements, but I'll cross that bridge when I come to it, I guess.

Cheers,

John Jones

[howard]

Hello John,

I keep the Russian germanium transistors for this set, see the Radio Spares offered column. My experience with loads of radios from the former Soviet Union is that it's very unlikely you'll ever need one

Howard

[John Jones]

Originally Posted by howard

Hello John,

I keep the Russian germanium transistors for this set, see the Radio Spares offered column. My experience with loads of radios from the former Soviet Union is that it's very unlikely you'll ever need one

Howard

Hi Howard,

Thanks for the reassurances; I'll keep them in mind.

John