Introduction

A vintage Romanian top of the line audio amplifier. These are scarce already. I wrote a long story about it on the Personal Weblog, article number #83. If you like stories, make sure you read it. Otherwise, I hope my restoration work is inspirational for you.

Technical Data

This amplifier has the following technical characteristics.

Parts List

I have assembled a parts list for this amplifier. In my restoration I attempted not to touch original output transistors. They are of a very good quality. However, I have updated the majority of them small signal transistors with vintage counterparts in metallic cans. The restoration targets mainly exchanging the electrolytic capacitors and the wiring. I have also decided to swap all the resistors in this amplifier. But this is not normally necessary. So for the purpose of this overhaul, the list of capacitors is assembled below.

The schematic value corresponds to what normally can be found in the electrical schematic diagram. The recommended value is what I replaced the former part with. Where I found appropriate, I have chosen a film capacitor replacement instead of an electrolytic. My reasons are reliability in time. Less electrolytic capacitors, less time-ticking bombs.

Descriptions and Parts Listings

Please note that on this board, every capacitor from the table below needs to be double ordered. In the table only left channel electrolytic capacitors are listed and we need to replace both channels.

The turntable and reel-to-reel tape recorder preamplifier (Preamplificator P.U. MAG.) board is coded 1 and is located in a Faraday cage made of thick steel in the top-right part of the steel chassis. You cannot miss the thick and heavy metallic box.

The Volume board is coded 2 and implements the volume regulator with the loudness correction. You can find it in the front-center part of the steel chassis. It is held in place by the nut of the volume potentiometer axle.

The Tone Control board is coded 3 and is located in the front-center region of the steel chassis, next to the Volume board. For an easy maintenance access, you need to extract this board. Or you could work your way through the bottom of the amplifier chassis. It is held in place by the nuts of the tone shaping potentiometer axles.

There are twp Power Amplifier boards, one for each channel. Both are coded 4 and they occupy the top-center of the steel chassis, between the output transistors heat sink and the array of filtering capacitors. You recognize them easily as they really do look like power amplifier modules. For the trained eye, at least.

The Power Supply board contains only one filtering capacitor. This board is coded 5 and can be located in the lower-left part of the steel chassis, next to the power transformer shielding cage.

The Power Supply filtering capacitors are mounted on the Chassis. There is an array of six large filtering capacitors in the middle of the steel chassis.

The construction of this amplifier is straightforward and poses no problems for the restoration.

Restoration

This amplifier has a straightforward 1970-style construction.

General Considerations

Working on this unit exposes you to electrical hazards. There are lethal voltages inside.
Severe accidents and possibly death by electrocution might occur. I am qualified and skilled at electronics and I have been doing audio gear repairs for over 20 years. If you lack experience, please take these articles as just a knowledge base. Do not attempt to repair something that you cannot handle as there is a high chance of doing further damage while also possibly suffering accidents.

Good tools are a must for a quality restoration. I use eutectic soldering alloy and a temperature-controlled soldering station equipped with various tip shapes. I have both standard and precision desoldering pumps and desoldering wicks of various widths. To clean the flux, I use isopropyl alcohol and high-purity acetone.

Empirically, I found that soldering at a temperature of exactly 300 °C is safe for these vintage printed circuit boards. I have never lifted any pads and I never wait more than a couple of seconds with the hot tip on any pad. While working on the chassis, I use between 360 and 440 °C. Flux fumes are extremely toxic and should be avoided at all costs.

Every replacement part is brand new, from a reputable manufacturer, ordered from the U.S.A., Japan, or Germany. In addition, I only use parts that are suitable for specific circuit sections, after inspecting and comprehending the original schematic diagrams. Last but not least, I have years of experience backing up my choices and actions.

Sometimes, I like to take unnecessary actions, such as replacing perfectly working components for futureproofing, or completely stripping down PCBs of parts for a better cleaning. While tedious, I have fun while doing so, and I also learn a thing or two in the process.

Restoration

First, I disassembled all of the mechanical parts such as front face, buttons and knobs. I proceeded to cleaning all these parts with care in order not to destroy any of them.

I have stored these parts away from dust for some time, until I finished with the electronics.

Then the chaos installs. I did a list with all the resistors and capacitors in this amplifier. Close to 250 distinct parts in total. I spent more than an hour at the electronics shop to order them all. Lucky I had in my spare box a lot of quality capacitors, both film and electrolytic so I saved quite some money as these get expensive. I bought all the required resistors, with tolerances between 1 and 5 %. For the tone corrector and both preamplifier boards, the resistors are all rated 0.25 W. For the dual mono power amplifier boards, I have chosen resistors rated at 0.5 W. These are more appropriate for this purpose and fit better.

The following picture presents the original state of the amplifier. Record this image in your mind so you can compare it to the final result.

One of the two power amplifier boards. Nasty resistors, bad capacitors, and crappy small radiators. Crazy wiring present.

The power supply board -- before.

The power supply board -- after.

Crazy wiring on the sources and record sources selectors. For instance, red goes to ground. Occasionally, red also goes to left channel. Or right. Or both. Same for all other wires.

High capacity filtering capacitors. All built by IPRS Baneasa. Even they have been in service for close to 30 years, the ESR is still within specifications. But crazy wiring strikes again. Small gauge wires for power lines. Even smaller for the output power transistors. Random wire gauge for the other connections. Nice routing, however. I like it and I am going to keep this layout, improving where I can.

This is the volume preamplifier board. It is populated with a variety of assorted resistors. Some low quality capacitors and weird wiring. There is almost no shielded cables for small signal and the hum level is still minimal. Amazing.

I started replacing the obsolete capacitors with some quality stuff. I used genuine Rubycon Japanese electrolytic capacitors and Philips MKT / MKP orange drops. I also used some Nichicon polyester film capacitors that I had lying around.

On the tone corrector I find the same assorted resistor forest. The axial capacitors are also present in place of radial counterparts. Some of the green rectangular film capacitors are very bad in terms of mechanical build. When I desoldered them, the plastic package cracked from the heat. And I used my solder pump and 40 W soldering iron as always. Fine tools, used with care; but bad parts that simply disintegrate!?

You can see a glimpse of my tiny workplace. I am using the tablet to lazy compute resistor values according to color code. I also have a tab with the schematic diagram opened for references. The fact is that I discovered wrong value parts in the amplifier instead of what it is written in the schematic diagram. Strange. For the reconstruction, I used the original parts from the schematic diagram.

Resistor chaos! This is just a small part of the total number of resistors in this amplifier.

On the tone corrector I started replacing the old parts with new ones.

The big capacitor tray from underneath. I like the inscription reading M.I.M.U.E.E. - I.E.I. I have absolutely no idea what the first abbreviation stands for. The first abbreviation stands for Ministerul Industriei de Masini si Unelte Electrice si Electronica. This translates to Ministry of Electrical and Electronic Machines and Tools Industry. The second abbreviation is the name of the factory that produced the A-350 amplifier, Intreprinderea de Electronica Industriala. Again, in English this translates to Industrial Electronics Enterprise.

Late night work in progress. I like keeping my workplace clean so I could find the parts easily. Less clutter is always a good practice.

I eventually finished replacing most parts on the volume preamplifier and tone corrector boards. The phono preamplifier board is still in its original shape. On the finished boards you can see the quality parts. I also used tantalum in the signal path. The majority will say this is stupid. I believe so, myself. But since I saw it in some 1980s HI-FI equipment with a nice sound timbre, I decided to give it a try. And just because I had some computer-grade tantalum capacitors, I decided to temporary implement this solution. Once I will receive some good signal path capacitors, I am going to upgrade them right away.

Volume preamplifier -- detail.

Tone corrector. Solder side -- detail. I used extra care while soldering and the results can clearly be seen. At the end I have used some isopropyl alcohol to clean-up all traces of flux. Looks industrial build.

Power selector resistor divider network. Before and after.

Take a look at this array of filtering capacitors. These are nice new-old-stock (NOS) new-in-box (NIB) Philips blue hexagonal capacitors. All are rated 6800 uF / 63 V. These things must have cost a ton of money. Lucky I have this friend that is dealing such parts and I got a big discount.

Old versus new. I appreciate the new capacitors have the same diameter as the old ones. However the new parts are slightly shorter than the old ones. But they fit like a glove.

The phono preamplifier is partially restored. This is a killer board with a lot of resistors and capacitors. I had to hand-form all these resistors to the squared shape. Notice that I have not started replacing any transistor on this board, yet.

The right amplifier -- detail. This board is partly finished. Some parts are still pending replacement.

The program sources and record the sources selectors selector received new wiring. I had to custom fabricate every piece of wire then dress it in varnish. Looks very good, built using bulletproof RF shielded cable, immune to noise.

Here is a view from the underside of the amplifier. The boards and the wiring look a lot better these days.

This is how the inside of the amplifier looks after the work I did so far. Comparing to the first picture of the internals, I would say that I am making some progress.

These are the parts that I removed from the amplifier.

And this is the amplifier carefully stored in its place. The restoration work is not finished. I still have to work on the phono preamplifier board.

Revisiting Transistors

Just because I got a big bag of golden-legs, unmarked transistors that look like BC107, BC108, and BC109, I decided to revisit some of the boards of this amplifier and upgrade the existing plastic package BC171 transistors with these things. I even tried replacing BC413 transistors by the unmarked ones but it is a trial-and-error procedure. Most of them have internal pop-crack noise. They are not good for audio amplification. Thus, I ended up replacing only the transistors that don't affect the sound. For the BC413 however, I used BC109C. For some reason I like the looks of small-signal metal can transistors. Maybe because the first transistor that I was given when I was a child was a BC107?

Anyway, at this point I worked through the bottom of the amplifier, using a pair of medical scissors to hold transistors while soldering on the other side. I did not want to remove this board again, concerned about stressing the tracks by repeated connection wires desoldering.

Here is a detail picture where you can see the golden-leg transistors.

At the moment I am using the A-350 as my main audio amplifier. But as I was saying at the end of the previous article, I still have to work on it. As I am expecting a lot of new Nichicon FG and KZ series capacitors, I am planning some replacements here and there. There are also some other updates scheduled so please revisit this page sometime soon.

Revisiting Capacitors

Finally, my order has arrived and I found some spare time to revisit various capacitors in this amplifier. So I am starting with the amplifier boards where I have replaced the 10 uF / 50 V and 2.2 uF / 50 V radial capacitor with axial counterparts as it was originally designed. I have used Nichicon axial capacitors for this purpose. Here they are.

Next up, is the preamplifier board. Again, I have used Nichicon axial capacitors for supply rail filtering purposes and Nichicon KZ MUSE series in all other cases.

Detail on Nichicon capacitors.

An interesting fact is that if we take a look at the construction of the printed circuit boards of this Romanian amplifier and those of an early 1970s vintage Japanese amplifier, we can surely tell that the quality is on par.

Even on the solder side.

Then I have proceeded to replace the capacitors on the volume preamplifier board and the tone control board.

I have used only high quality capacitors, mostly Nichicon KZ MUSE series. The sound is as good as it can be for this circuit topology. Reliability is increased as well.

Parting Ways

Since I haven't had time to finish this amplifier because of other stuff I've been dealing with, I decided to sell it. It was sitting on the TODO list for far too long. Thus, it went away to somebody who will complete it and put it to use it.

Please note that all the work presented on this site is non-commercial. This is my hobby, and I do this in my spare time. Since I freely share my knowledge, if you like my work, please consider helping me buy a transistor or a capacitor for my projects.

Thank you!