Stage Line 500W amp repair project

[hevans1944]

... my interest lies with audio equipment, will those 2 pieces of kit be usefull?, if so i will buy them, thanks

IMHO, both instruments are not only useful, but also essential for any serious electronics hobby work involving audio.

Almost everyone gets their "feet wet" with audio circuits when beginning to learn electronics. Even if it's just a "crystal radio set" with a ceramic ear-piece, pulling in a nearby AM broadcast radio station and hearing it is a thrill that you will never forget if the electronics bug bites you.

Salvaging parts to build a crystal radio can be a challenge today, but it can still be done... even today, where everything is pre-packaged and instantly disposable. I built my first crystal set using an empty toilet-paper roll for a coil form. Grandfather supplied a jar of premixed shellac and a paint brush to spread the shellac on the cardboard tube. Today you might consider using a can of clear acrylic spray pain. It took several coats (and excruciating waiting for each coat to dry!) before the cardboard tube became stiff enough to support several hundred turns of fine enameled copper wire. The wire was hard to come by for a young boy in the early 1950s, but Grandmother somehow found a small spool and purchased it for me. Later, I learned to raid trash bins behind radio and TV repair shops for discarded radio and television sets that could be salvaged for parts... but I digress.

I notice from this picture (https://www.electronicspoint.com/forums/attachments/20201229_133846-jpg.50275/) that Santa was good to you last year. Can't complain about owning Fluke equipment, except for the price. What is the little thingy (also a Fluke) to the right of the Fluke DVM in the image you uploaded? Just curious.

 

[bikemech]

IMHO, both instruments are not only useful, but also essential for any serious electronics hobby work involving audio.

Almost everyone gets their "feet wet" with audio circuits when beginning to learn electronics. Even if it's just a "crystal radio set" with a ceramic ear-piece, pulling in a nearby AM broadcast radio station and hearing it is a thrill that you will never forget if the electronics bug bites you.

Salvaging parts to build a crystal radio can be a challenge today, but it can still be done... even today, where everything is pre-packaged and instantly disposable. I built my first crystal set using an empty toilet-paper roll for a coil form. Grandfather supplied a jar of premixed shellac and a paint brush to spread the shellac on the cardboard tube. Today you might consider using a can of clear acrylic spray pain. It took several coats (and excruciating waiting for each coat to dry!) before the cardboard tube became stiff enough to support several hundred turns of fine enameled copper wire. The wire was hard to come by for a young boy in the early 1950s, but Grandmother somehow found a small spool and purchased it for me. Later, I learned to raid trash bins behind radio and TV repair shops for discarded radio and television sets that could be salvaged for parts... but I digress.

I notice from this picture (https://www.electronicspoint.com/forums/attachments/20201229_133846-jpg.50275/) that Santa was good to you last year. Can't complain about owning Fluke equipment, except for the price. What is the little thingy (also a Fluke) to the right of the Fluke DVM in the image you uploaded? Just curious.

Hahahaha, yes when i was about 8 or 9 i started pulling electronic equipment apart to see how it worked, my favorite shops where second hand markets and charity shops where i could buy BROKEN stuff to dismantle without having to worry i would brake it, i did repair quite a lot of thing from record players to VCR when betamax was about hahaha, the new kit i have now is supplied by the company i work for, the smaller fluke on the right is a temp gun, i now work for a UPS and generator company installing backup battery banks and doing discharge testing on 48 and 110vDC systems, i also have another fluke meter and a clamp meter,
Pete

 

[hevans1944]

Back in the day (mid 20th century) there were two pieces of audio test equipment that were very useful: an audio signal injector and an audio signal tracer. You can build either one of these for your own personal use using inexpensive parts. A 555 timer chip makes a nice audio signal injector and a low-voltage op-amp can be cobbled up to make a signal tracer. Both instruments are connected to the circuit under test using low-valued, non-polarized, capacitors. You can use the instruments individually or together to trace the passage of an audio signal through the circuitry. The test frequency for the signal injector is not critical, a few hundred hertz or a kilohertz or so will do nicely. Waveform doesn't matter either... square, triangle, sawtooth, sinusoidal, pulses or whatever all work.

 

[bikemech]

Back in the day (mid 20th century) there were two pieces of audio test equipment that were very useful: an audio signal injector and an audio signal tracer. You can build either one of these for your own personal use using inexpensive parts. A 555 timer chip makes a nice audio signal injector and a low-voltage op-amp can be cobbled up to make a signal tracer. Both instruments are connected to the circuit under test using low-valued, non-polarized, capacitors. You can use the instruments individually or together to trace the passage of an audio signal through the circuitry. The test frequency for the signal injector is not critical, a few hundred hertz or a kilohertz or so will do nicely. Waveform doesn't matter either... square, triangle, sawtooth, sinusoidal, pulses or whatever all work.

I was with you all the way untill the wave fourms

 

[Martaine2005]

Here, you’ll find out about the waveforms @hevans1944 mentioned.

https://en.wikipedia.org/wiki/Waveform

Martin

 

[bikemech]

To check the output voltage, short the junction of R69 and C42 for the R.H. channel or R12 and C34 for the left hand channel to the nearest 0V connection on the PCB.

You will sometimes see 0V referred to as Ground. These are not to confused with Mains Earth.

Next make sure there is no load, test resistor or speaker, on the output of the channel you are testing.

Connect + lead of your DMM to the O/P of the Channel under test and the - lead to 0V at the power supply.

Set your DMM to 2V d.c. or whatever range you have in that region.

Turn on your amplifier and check the voltage reading on your DMM.

It should be less than ± 100mV or preferably less than ± 50mV. If you get substantially more then you have a problem.

You will next need to check the quiescent current but we will deal with that on another post.

I can not find C42 or C34, are then on the left and right amp schematic ?
And are tou asking me to connect R69 and C42 togeter or test each one to 0V ?

Edit, found C42 and C34, there on the main PCB

 

[bikemech]

To check the output voltage, short the junction of R69 and C42 for the R.H. channel or R12 and C34 for the left hand channel to the nearest 0V connection on the PCB.

You will sometimes see 0V referred to as Ground. These are not to confused with Mains Earth.

Next make sure there is no load, test resistor or speaker, on the output of the channel you are testing.

Connect + lead of your DMM to the O/P of the Channel under test and the - lead to 0V at the power supply.

Set your DMM to 2V d.c. or whatever range you have in that region.

Turn on your amplifier and check the voltage reading on your DMM.

It should be less than ± 100mV or preferably less than ± 50mV. If you get substantially more then you have a problem.

You will next need to check the quiescent current but we will deal with that on another post.

is this what you meant, now take readings ?, used fuseable resistors just incase i was doing it wrong

 

[bikemech]

View attachment 50544 View attachment 50545 is this what you meant, now take readings ?, used fuseable resistors just incase i was doing it wrong

If this is correct the readings are,
RIGHT CHANEL:
--probe on OVP T7
+probe to T9 = --0.008mvdv,
+probe to T10 = 124.65mvdc

LEFT CHANEL:
--probe to T2
+probe to T4 = --0.006mvdc
+probe to T5 = 42.091mvdc,
hope this is right, although i did not short R12 and C34 when i tested the left chanel, if this is essential i can redo the left

 

[WHONOES]

Hi again. No need to repeat tests. Results seem reasonable though the R.H Channel is a little on the high side.

The idea was to short the junction(s) of the component mentioned to 0V so that there is absolutely no input to the amps whatsoever and the input is effectively grounded.

You now need to check the quiescent current as described in post #422. This is quite important.

Going back to the items of test equipment. You should aim to buy the very best items that you can afford if you are really interested in delving into audio.

A good Sig' Gen' that gives sine and square wave, a small DSO (Digital Storage Oscilloscope) and DMM is the minimum you should aim for.

 

[bikemech]

IT's good that you have the right hand side working now. Have you tested with any audio or a signal generator?
It would be worth removing the fuse from the + supply and inserting your DMM across the fuse receptacle and switched to a current range, probably 200mA, and then switch on and see what current is flowing in the O/P stage. I would expect it to be in the range of 10 to 50mA. Do this with no load on the output and with ideally the the power Amp input shorted. If not turn the volume control down to minimum and the measure. Make a note of the value.

would this be correct, dont normally use this function on this meter, it has lots of setting but there is only a few i need to use,
Pete

 

[bikemech]

View attachment 50548 would this be correct, dont normally use this function on this meter, it has lots of setting but there is only a few i need to use,
Pete

This was taken across fuse F2 + terminals

 

[bikemech]

Hi again. No need to repeat tests. Results seem reasonable though the R.H Channel is a little on the high side.

The idea was to short the junction(s) of the component mentioned to 0V so that there is absolutely no input to the amps whatsoever and the input is effectively grounded.

You now need to check the quiescent current as described in post #422. This is quite important.

Going back to the items of test equipment. You should aim to buy the very best items that you can afford if you are really interested in delving into audio.

A good Sig' Gen' that gives sine and square wave, a small DSO (Digital Storage Oscilloscope) and DMM is the minimum you should aim for.

I will probably purchase the 2 items mentioned by @bertus in posts 432 and 434 when i get paid this month, i think these will help once i learn how to use them correctly, anymore suggestions welcome

 

[bertus]

Hello,

When you buy the nano scope, have a look at these videos:

Bertus

 

[bikemech]

Hello,

When you buy the nano scope, have a look at these videos:

https://www.ebay.co.uk/itm/HAMEG-OS...-/114631236179?_trksid=p2349624.m46890.l49292
Would this be any good ?

Bertus

 

[bertus]

Hello,

That is a 20 MHz analog scope.
That would also do fine for audio work.
It has 2 channels, so you can also compare signals.
I have an old Tektronix 454 scope.

Bertus

 

[bikemech]

Hello,

That is a 20 MHz analog scope.
That would also do fine for audio work.
It has 2 channels, so you can also compare signals.
I have an old Tektronix 454 scope.

Bertus

Is it for sale lol

 

[bikemech]

Is it for sale lol

This one looks quite old but easy to use

 

[bertus]

Hello,

Mine is even older (1971):
https://w140.com/tekwiki/wiki/454

This is the one from your video (1989):
https://w140.com/tekwiki/wiki/2205

Bertus

 

[bikemech]

Hello,

Mine is even older (1971):
https://w140.com/tekwiki/wiki/454

This is the one from your video (1989):
https://w140.com/tekwiki/wiki/2205

Bertus

The nano one looks good, but will it stand the test of time like the one you have, but then again given the price of the nano, would that matter

 

[WHONOES]

IMHO I think you would be better off with this Mds-3200a dds nc dual channel function signal generator frequency meter ttl wave Sale - Banggood.com Sig' Gen'. As it is a bit more future proof.
And the following Scope Hanmatek DOS1102 Digital oscilloscope with 2 Channels and Screen 7 inch / 18 cm, TFT-LCD Display, Portable Professional Oscilloscope Kit with 110mhz Bandwidth 1GS/s Sampling Rate: Amazon.co.uk: Business, Industry & Science.
More expensive than the one previously suggested. The last thing you want to be doing is to buy a cheap device only to discover it's not up to scratch and you find yourself buying the one you should have bought at first. It's irksome and wastes money.