Soviet made 70s or 80s Amplifier Repair - Page 1
Forum section: electronics / repair

author: dominicM
author_rank: Regular Contributor
timestamp: August 22, 2019, 12:26:45 pm
content: So I have this old speaker set and amp that worked well until it didn't after 5 years of attic storage. Speakers still work well and are from 1986, the amp may be from the same time or maybe older. I recall the speakers sounding really great but with a shitty amp I have now they sound like dog shit so it would be really nice if I could repair and restore this amp like I did with the speakersI checked the main fuse and 2 on the board inside, one was blown but that was a long time ago and it was "fixed" with a wire. It draws ~6.4W with or without speaker attached, power indicator bulb is on. No visible damage or smell. There is a relatively quiet buzz from a speaker and it does get louder with volume increase. I checked the cables and I can hear a faint audio signal from the speaker when amp is powered off but none when powered on. This tells me signal is reaching the amp. There also used to be a kind of sweeping sound when amp was powered on but there is none now. Don't really know where to start here, any suggestions?

author: MasterT
author_rank: Frequent Contributor
timestamp: August 22, 2019, 01:02:32 pm
content: I see, IC's are stamped '78, amplifier likely the same - more than 40 years old. You will have to replace all electrolytic, its often failed in first 3-5 years of functioning, definitely dead now.Common practice in to find an electrical drawings, and check DC test points with multi-meter .

author: PKTKS
author_rank: Super Contributor
timestamp: August 22, 2019, 01:14:07 pm
content: Actually I would replace **ALL** caps since these caps arefrom early 70 or may be even earlier..Piece of cake in this type of board - almost everythingcan be replaced or closely upgraded.Nice piece for collectors.Paul

author: tunk
author_rank: Super Contributor
timestamp: August 22, 2019, 01:32:43 pm
content: Corrosion in switches and/or potentiometers?

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 22, 2019, 01:52:50 pm
content: Look at those bad-ass stud rectifiers! Eye candy, indeed.But I digress..............I would also start, like the previous poster has indicated, by attempting to clean both the switches and potentiometers.Also, with a multimeter, you will have to check that the supply voltages in different parts of the circuit are correct. Difficult without a schematic but not impossible.Many Soviet era ICs have similar Western counterparts. I bet those ICs are quad opamps. If they conform with the more or less standardized quad opamp package, V+ should be on pin 4 and V- on pin 11.The exception to the rule at least on the west) was the popular audio grade RC4136, with V+ on pin 11 and V- on pin 7.

author: free_electron
author_rank: Super Contributor
timestamp: August 22, 2019, 02:44:43 pm
content: slightly off-topic ... but : does anyone have pictures of a 'modern' russian made piece of kit. something with surface mounted parts. like a board made in the year 2000 or 2010 ? made with russian parts. i'm curious to see what weirdness they come up with these days.

author: dominicM
author_rank: Regular Contributor
timestamp: August 22, 2019, 02:54:54 pm
content: I fear I may be in over my head hereWill try though, hopefully you guys can guide me. Based on the symptoms in OP, which board is the likely at issue here and what should I check first in that board? Remember that is failed in storage. I would think caps are unlikely to fail when not in use. Corrosion on connectors/pots seems unlikely too since signal is reaching the speakers and volume port does affect idle buzz that is present.With the help of google translate here is the translation from image 2:LAYOUT OF THE MAIN UNITS 21. PU board2. UM board3 Filter condensation unit4. Filter board UM 325 0025. Block of power transformer and rectifier6. Block of terminal transistor.Will try and get more accurate translation from someone later.

author: Bud
author_rank: Super Contributor
timestamp: August 22, 2019, 03:10:16 pm
content: Electrolytic caps can and do fail in storage.

author: james_s
author_rank: Super Contributor
timestamp: August 22, 2019, 03:24:03 pm
content: Quote from: free_electron on August 22, 2019, 02:44:43 pmAs I recall from talking to a former coworker who was from one of the former Soviet countries, after the fall of the iron curtain they started getting goods from China and elsewhere. I doubt you'll still find the sort of unique stuff that developed in isolation when the Soviet Union was closed off from the outside world.

author: dominicM
author_rank: Regular Contributor
timestamp: August 22, 2019, 03:36:54 pm
content: I guess all components can fail in storage, but I wonder how much less likely they are to fail in storage..? It would be good to at least for now eliminate caps as the ultimate cause it would be difficult to troubleshoot. There are so many of them and they seem to have thick casing so maybe they cannot bulge meaning they could all be failed but look normal. Just looking for a good place to start, maybe get lucky and find the issue quickly.

author: dominicM
author_rank: Regular Contributor
timestamp: August 22, 2019, 04:44:47 pm
content: I took a closer look at the 2 vertical boards, closest to heatsink in the first image. I would think these boards aren't the issue since they both would have to be bad for both channels to have the same defect but then there is Murphy... Boards seem identical. Both have a 27ohm resistors with browning in the middle but both measure 27ohm so seems ok. Both also have a can component with long legs. On one board 1 one of the 3 legs is cut/unconnected not sure why seeing as the 2 boards are otherwise identical. It doesn't look like it snapped. Some of the zener diodes also look bit different with some micro cracks in glass and minor discoloration. Measured values are different on Board 1, Z1. Could this be the issue, and is it likely only issue or just symptom? Don't want to make thinks worse by replacing something that shouldn't be replaced.Zener measurements:Board 1:Z1: OL-0.227Z2: 0.215Board 2:Z1:0.661-0.205Z2: 0.222-0.623

author: 2N3055
author_rank: Super Contributor
timestamp: August 22, 2019, 10:15:04 pm
content: Quote from: free_electron on August 22, 2019, 02:44:43 pmDon't have photos, but ones I've seen look pretty much mainstream. They adopted all EN norms as GOST, they use same components. ST MCU's are popular.I have no clue how high end mil grade looks, I had a look at some industrial electronics. It was very nice engineering..

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 23, 2019, 08:09:42 am
content: My process would be to check voltages, then clean pots and switches, then put an audio frequency into the input jack and trace it through the circuit with a scope or signal tracer.Where it stops is where your fault is.Fix the issue then replace all electrolytic capacitors and finally connect the speakers and start blasting out some beats.

author: dominicM
author_rank: Regular Contributor
timestamp: August 23, 2019, 08:37:57 am
content: That makes sense, only problem is I don't have an oscilloscope. Could zener diodes as measures in previous post be the issue?

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 23, 2019, 02:10:47 pm
content: Many moons ago, when I was a college student who could not afford a scope and repaired audio amps as to get a little money, I acquired a small circuit which basically was a high impedance audio amp which drove a frugal crystal headphone.The circuit was nothing more than a Jfet and a bipolar transistor, nevertheless it was enough to trace the signal around. It was somewhat bulky because it required a 9V battery, and because thru hole themselves components were large.Nowadays, you could build something similar and much smaller from one of the many available amp ICs designed to run from 3V. That and the combination of SMT components, and you might be able to fit everything inside a discarded pen.

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 23, 2019, 05:42:53 pm
content: Mr Carlson on youtube has demonstrated this kind of device in a video or two.

author: dominicM
author_rank: Regular Contributor
timestamp: August 24, 2019, 11:26:11 am
content: Here is also an image of the model though I couldn't fin a trace of this model online. Kind of odd seeing as it's at least somewhat popular with serial number is over 10k.

author: Canis Dirus Leidy
author_rank: Regular Contributor
timestamp: August 24, 2019, 02:27:47 pm
content: Quote from: dominicM on August 24, 2019, 11:26:11 amUse the Russian, Luke!

author: chekhov
author_rank: Regular Contributor
timestamp: August 24, 2019, 02:29:15 pm
content: Hi, it doesn't seem to be really unique model, but it looks like there is even a schematic for that on this page -http://ldsound.ru/vef-101-stereo-uku-210-m-vef-101-stereo/I haven't verified everythong, but that looks like your model VEF-101-stereo

author: Kleinstein
author_rank: Super Contributor
timestamp: August 24, 2019, 04:11:27 pm
content: Quote from: dominicM on August 22, 2019, 04:44:47 pmThe numbers look like doing the typical diode test in circuit. The numbers are no really saying much for the zener. It is likely quite some effect from the rest of the circuit. A Zener diode would usually give some 0.4-0.8 V in one direction and over range in the other.That both boards read different kind of suggests some defect, but not necessary the zener - it could well be an electrolytic capacitor that connects there.I don't know the quality of the Russian capacitors. The older ones from the 1970s may not be as bad as from the 1990s.Usually no worries with film caps, they usually last quite long. It is more likely that carbon resistors could fail (e.g drift up) - not sure if there are some used.Checking the supplies would be one of the first test, maybe only after replacing the large caps.Another point that can be checked is the output transistors. If they fail they usually fail with a short and this can be tested in circuit.

author: Domestic
author_rank: Newbie
timestamp: August 24, 2019, 04:14:34 pm
content: Amplifier circuit

author: dominicM
author_rank: Regular Contributor
timestamp: August 24, 2019, 05:30:57 pm
content: That was quickVEF-101-stereo does look identical to my amp. I don't understand where the "VEF" came from, don't the Cyrillic characters translate as UZF?

author: Canis Dirus Leidy
author_rank: Regular Contributor
timestamp: August 24, 2019, 05:39:13 pm
content: Quote from: dominicM on August 24, 2019, 05:30:57 pmThis is the Cyrillic transliteration of the (abbreviated) name of the Latvian factory:Valsts Elektrotehniskā Fabrika

author: dominicM
author_rank: Regular Contributor
timestamp: August 24, 2019, 06:00:13 pm
content: Quote from: Canis Dirus Leidy on August 24, 2019, 05:39:13 pmNo clue how anyone could figure that outMy dad likely bought it in Latvia while visiting friends there.Edit: I see now that the letters are in the logo.

author: dominicM
author_rank: Regular Contributor
timestamp: August 24, 2019, 06:01:36 pm
content: Quote from: Kleinstein on August 24, 2019, 04:11:27 pmIdle power usage is 6w and indicator light is on, wouldn't that eliminate power supply short circuit?Looking closer at board 1, the leg of the 3 pin can may be snapped and not unconnected like I thought initially as I now see corroded leg soldered in. How it snapped in storage I do not know.

author: bsw_m
author_rank: Frequent Contributor
timestamp: September 04, 2019, 10:24:49 pm
content: First and may be last step to repair this AMP - replace all electrolytic capacitors on PCB'sThey are very old and very bad model (K50-16).After replace 98% chance, that amp going work/

author: picburner
author_rank: Frequent Contributor
timestamp: September 06, 2019, 11:38:08 am
content: This resistor (R48?) seems to have had some hot flash

author: Pinkus
author_rank: Frequent Contributor
timestamp: September 06, 2019, 12:19:44 pm
content: It's a pity that the internet can't transmit any smell yet. I would really like to smell the aroma of this old piece of electronic.