Technics SE-A1010 Amplifier Repair

[quantumtangles]

The VU meter bulbs (part reference XAMR131) in my Technics SE-A1010 audio amplifier stopped working ages ago. I finally decided enough was enough and resolved to replace them with LED bulbs.

I hoped (should say expected) to do a little maths, add resistors to the positive pins of the LEDs...and problem solved.

However when I tested the supply leading to the lamp array, I could not detect any DC coming into the VU meter lamp array at all. Nothing.

Alarm bells...so I checked for AC, and that was when I got a reading of just over 40 volts AC with the multimeter. Hello.

Hoping I was wrong (wrong about a nasty AC supply to the lamp array), I checked the schematic online. The voltage to the lamp array was on Page 17 [online at http://akdatabase.org/AKview/albums/userpics/10004/Technics SE-A1010 Service.pdf ] and makes it clear the array really is supplied with 40.2 volts AC.

So I have low voltage DC replacement LED bulbs...but the power supply to the defunct lamps is a relatively high voltage AC supply.

Stupid ideas followed. Maybe make a little bridge rectifier...rectify the AC... and add resistors to get it down to 2.8v for the LEDs. Crazy?

What about ignoring the AC supply rail completely...connecting the LEDs to a node somewhere in the DC part of the circuit, but I hesitated about doing that as well.

The problem is that the electrolytic capacitors in this machine are way way better quality than the crap I buy on ebay...because they are low noise components. Harmonic distortion might very well be the end result of a redneck fix.

Please let me know what you suggest. What is the best way of safely hooking up LEDs to light up the VU meters...without wrecking this beautiful piece of craftsmanship. Many thanks in advance for your suggestions.

 

[davenn]

Hi there quantumtangles
welcome to the forums

nothing nasty about using AC for a lamp supply tis quite common

or instead of rectifying it ( the 40VAC) you could just use the LED's on AC with the appropriate components.
It has been discussed more than once on these forums or do a google search on LED's running off AC

Dave

 

[quantumtangles]

Many thanks Davenn. I am checking out LEDs running on AC as you suggest.

 

[quantumtangles]

If it looks like a rectifier...

AC is dangerous...I don't like it. But I want to repair the amp so I am going to tame the 40v AC and make it perform useful work for my DC LEDs

There are bound to be other people with the same amplifier and the same problem, so this could be a nice thread to help them out as well.

First, do not even attempt this repair unless you are an electronic engineer. These voltages are very dangerous.

Secondly, here is what I have in mind (rather vague at the moment) after grabbing some more information.

Phase 1 (there is no phase 2)
Experiments
(for all your wtf moments)

1. With a breadboard and bench power supply, determine optimal voltage and current for the replacement DC LED bulbs.
2. Make a string of LED bulbs and check out the voltage drop with a multimeter.
3. Do some maths
4. Bling my string with suitable diodes.
5. Solder a resistor to each LED depending on the resistor values the math indicates.
6. Invite everyone I know to my house...to marvel at the redneck rectifier hot-glued into my amp.

OK. Its good to have a plan. I will let you know later on just how reality divergent it turned out to be



I can only explain it to you
I cant understand it for you

 

[(*steve*)]

AC is dangerous...I don't like it.

The fact that it's AC doesn't make it dangerous.

 

[quantumtangles]

AC

It depends on the power in Watts. If a kitchen oven is under load (turned up high for example), touching the wrong cables would probably be fatal because of the higher current draw x standard voltage. Slightly less dangerous if not under load...but still eye widening.

In any event, I built a bridge rectifier from 4 x N5406 Diodes. It rectified splendidly, pumping out no AC at all. For reasons buried somewhere in the datasheet...15 volts DC popped out the other side. (Note to self...get a new tuxedo for the ceremony in Stockholm)

But the amp hummed ominously and then (horror) began to crackle.

I might have to think before taking the next step (check the datasheets).

I toyed with the idea of posting a photo of the bridge rectifier online, but decided not to. If machines take over the planet, it could warrant summary execution (crimes against components)

 

[quantumtangles]

My first effort at a bridge rectifier (to replace the VU lamps in a Technics SE A1010 amplifier) was a flop. Harmonic distortion from the massive diodes I used made the exercise absurd..

I tried and failed to use a DC node on the circuit board to power an LED. It glimmered but was too dim to light up the panel (voltage drop under load).

However, persistence is the key in electrical engineering so I made another bridge rectifier. It works well without affecting sound output and the result is that the 40.2v AC supply from the bulb rail, rectified and with only one resistor and no further diodes, now provides 12.5v DC.

A 12v 50Hz LED bulb from my parts bin works perfectly with this set up (connected to the new DC supply from the amplifier (bulb is still on a breadboard but is connected to the new power supply from the amplifier).

Technical specs of the rectifier and LED bulb to follow with photos in a couple of days, hopefully with a photo of the new LED working in the renovated amp.

 

[quantumtangles]

With the 40.2v AC going through my second attempt at a bridge rectifier, I soldered a 12v 50Hz LED bulb to a 4.1k resistor. The LED worked, but when pointed it at the VU meters and turned the lights off, it was not lighting them up. The light was not bright enough.

It helps you realize just how powerful the original array of 5 x 8v AC bulbs really were.

Anyway, with progressively diminishing intelligence I increased the luminosity of the LED (by using progressively lower value resistors).

The LED got quite bright at one point (with a vanishingly low value resistor) but then there was a sort of pop and smoke appeared. Took a deep lungful of it out of curiosity. Accordingly, that ship has sailed. The 12v 50Hz LED is toast and the amp repair is on hold until after Christmas.

Here are some photos of what not to do

 

[tweakjunkie]

Sorry Quantum, the lethality of a circuit would only be dependent on load if there was a sudden arc near the point of voltage in question. Current isn't pushed, it's drawn. Accidentally touching the voltage source would simply add a very slight load to it, independent of what it is delivering to the rest of the oven. What Steve said is correct, AC isn't more lethal than AC, however when it's high frequency AC, that brings in a whole nasty other problem, RF burns. And they hurt, I'm speaking from experience.... Wasn't trying to show anyone up here. There seems to be a misconception that current is pushed.

Two main things determine lethality: The voltage of the supply (which cause current in a load) and the supplies' ability to deliver that current.

Hope this helps.

P.S. AC is fine for lighting supplies in audio, makes no difference.

 

[quantumtangles]

Sorry Quantum, the lethality of a circuit would only be dependent on load if there was a sudden arc near the point of voltage in question. Current isn't pushed, it's drawn. Accidentally touching the voltage source would simply add a very slight load to it, independent of what it is delivering to the rest of the oven. What Steve said is correct, AC isn't more lethal than AC, however when it's high frequency AC, that brings in a whole nasty other problem, RF burns. And they hurt, I'm speaking from experience.... Wasn't trying to show anyone up here. There seems to be a misconception that current is pushed.

Two main things determine lethality: The voltage of the supply (which cause current in a load) and the supplies' ability to deliver that current.

Hope this helps.

P.S. AC is fine for lighting supplies in audio, makes no difference.

'Wrong thinking' in my original post has already been corrected by experienced engineers here. So I have been reprogrammed...but even so, your comments are welcome.

Wrong thinking is difficult to shake off once it has taken root, but I do now understand my misconception. You also iterate current is drawn rather than pushed. Very counterintuitive to my mind but I get it even though it does not exactly leap off the pages of Maxwell and Kirchoff. Many thanks for an interesting post