Today we will analyse the intrinsics of a DIY construction we succeeded to purchase on the Central Market for 5€. Culturally, the build is a very characteristic Russian one (or should I say – Soviet). The power plug notes its era, most likely mid-80’s.
This item amazed me for its contradictory essence. On the one hand, some best engineering practices were used (or unwittingly mimicked), and at the same time, many qualifying practices of DIY work were blatantly ignored – such a contrast is rarely seen. Is is noticable that a design going far from normal Soviet aesthetics, was a special goal for the author. E.g. the knobs are heavy because of fully made of Aluminium.
The rubber foots had been cut and glued very “roughly”. Some struggle in the authors soul is visible regarding the power transformer – because toroid transformers were luxury at Soviet time, the author has tried to squeeze some extra 4mm out of the construction by a rectangular opening for the transformer. Last but not least, cooling openings probably are a fast ad-hoc solution to the pecularities or the thermal regime of the device.
To give the construction some stability, the front and back are designed based to the casted alloy frames which very likely are a cut pieces from some military produced measuring device. The bolts used, are made of some copper alloy (like brass), the unlimited use of such is a sure giveaway of the Soviet period.
DIN-connectors have been used. For some unclear reason, most home electronics of the Soviet era abandoned 1/4″ tip-ring-sleeve phone jacks in late 1960-s and used DIN-connectors instead. DIN-connectors were used even for speakers (up to 15W) and headphones.
Here we see the style how front panel details were fastened by means of epoxy glue. Soviet power switches of “tumbler” type were of extremely rough design thus people made some efforts to conceal those.
It is hardly recognizable that the lower and back panels are made of two-sided FR-4 grade (almost!) printed circuit material. Here one can see how the “proper” grounding was achieved between these panels. That’s nice to see, while the author probably knew nothing about the Star grounding topology.
This is how the insides of the amplifier look like.
On the left side, there is the power supply, on the right side, two amplifier blocks (left, right) and a timbre block (bass, high). The construction is extremely sturdy due to the extra bridge in the middle. It is clearly seen the author has met some military design examples in his life. There are 3 replaceable blocks using military grade connectors. A big design fault – the blocks are not fastened and they could slip out from the connectors (they actually were) if shaken. The paper had been put between the left and right channel to avoid the short circuit between the heatsinks (this is a more generic problem with all non-isolated designs).
Both channels of the power amplifier use a Russian chip К174УН7 which was a clone of a not so popular ТBА810AS chip (other chips of the same class like TDA2003, A210K, LA4420, are much more known). К174УН7 was of enormous popularity in Russian home electronics while the special pinout and need in a non-standard heatsink actually made them rather hard to use in DIY. In our case, the autor probably has made a mistake while tracing the pin-out. The PCB is mirrored and thus the IC together with the heatsink has placed on the opposite side of the PCB.
The PCB is a little bit too dense … to have the heatsink of that size, the PCB should have been twice so large. The blue wire is shunting the a capacitor 😉 I imagine the pain in the ass while soldering that IC to the place 😉
This is the helicopter view of the PCB for the power amp. As one might see, holes were drilled manually while centre marks were not used. As one might see, there is no chance for the heatsink to be placed on this side.
The preamp block is based on Russian К174УН10 chip which is a precise clone of TCA740 chip. There were at least two military grade capacitors used in audio chain. The bloody truth is that the quality of Soviet capacitors varied wildly depending on the manufacturing date (around payday everyone at the factory was drunk). [Anecdotes about the Stepanakert Capacitor Factory in Nagorno-Karabakh Republic omitted this time.]
Let’s look at the same PCB from the bottom side. As one might notice, the PCB had been traced and etched by the manual method. To achieve the depth “clearance”, some grinding has been applied.
The mechanical construction is very interesting and mimics some military grade equipment. However, on could notice that the left and right channel modules are mounted in a complementary way, under a 180 degree angle … which could be an indication of some thermal problems.
Disregarding some artisanal features, the overall repairability of the device is relatively high.
The quality of the soldering work is relatively low.
Might be, the most interesting puzzle is related to the PSU. As one might see, there are two diode bridges fed by separate transformer windings. However, the output of the two КЦ405 diode bridges are connected in parallel, together. Considering the Vcc only is 15V for the particular IC, it effectively doubles the losses on the diode bridge. However, the problem lies in maximum current for the bridge which only is 1A. Most likely the initial thought had been to power the channels separately (an idea relatively common in Hi-Fi equipment, which this device certainly isn’t), but then the natural question will arise about powering the preamp 😉
This is the side view of the amplifier. The transformer is based on ШЛ core.
As the reader might notice, I never switched on the amplifier. Why? Because I really hate the sound of К174УН7 chip in a standard configuration (without an extra transistor on the input).
karu sisemus 