Trio KA 3500 DC Off set - UK Vintage Radio Repair and Restoration Discussion Forum

novicerepairer · 6th Oct 2022, 5:33 pm

Hi, I have a Trio KA3500 that I have not checked the DC offset on, as I have only recently learned the importance of checking before use with speakers.
The measurements I got with my multimeter set on max 200m amps was: left channel -57.0 and -38.0 for the right.
Would someone be able to tell me if this is Ok to use, as I wouldn’t know how I would be able to do the repairs to change these figures, but wasn’t expecting them to be that high.
Also, I used the red lead on multimeter to the red terminal on the speaker output but got a minus symbol on the multimeter?? Did I do this right??
Thanks in advance, Colin.

Radio Wrangler · 6th Oct 2022, 5:51 pm

The DC offset is really a voltage. The current would depend on what load you would later connect. Had there been a bad offset you would have destroyed a cheap multimeter, or blown a fuse in a posh one.

Set your multimeter on a voltage range for this measurement. Pick a high voltage range to start with as you don't yet know what you will find, then if the reading is low, switch to lower and lower voltage ranges.

For this measurement, the polarity doesn't matter as you are looking to check that the DC offset is within a hundred or so mV either polarity, and you can ignore the sign, just look at the magnitude.

General practice is to put the -ve mead of a voltmeter on a connection nearest to earth, like the black output post on an amplifier.

There are some amplifiers operated as bridges where both speaker connectors are live, but swing in opposite directions. Not very common apart from high power stage amps, and car audio

David

novicerepairer · 6th Oct 2022, 5:56 pm

Thanks for your advice on saving damage to the multimeter. I will definitely use you way of working in the future. Thanks, Colin.

novicerepairer · 6th Oct 2022, 7:08 pm

I wonder if anyone could confirm, if this figure is acceptable for use, or if it is in need of a repair before using?
Thanks, Colin.

Radio Wrangler · 6th Oct 2022, 7:35 pm

It doesn't look bad, but as we don't know the overall loop resistance of the circuit through the amplifier, through the multimeter and along the test leads, no one can know for sure.

Common multimeters (but not all!) are a basic 200millivolt measuring instrument shorted with a 1 Ohm resistor to make a 200milliamp current range.

IF this is so, then you have 57mV and 38mV of offset but with a 1 Ohm load, so this doesn't sound bad. A voltage measurement would have avoided the uncertainty.

David

novicerepairer · 6th Oct 2022, 9:25 pm

The loop resistance bit has lost me I’m afraid, It is a digital multimeter from Maplins that was about £30, but I have used this amp quite a lot in the past with no problems. There is a couple of capacitors that look a bit leaky though.
I have not replaced them as I have not done anything that involved before!

novicerepairer · 6th Oct 2022, 9:42 pm

Some pics hopefully!

Radio Wrangler · 6th Oct 2022, 10:30 pm

You were measuring current on your multimeter when you should have been measuring voltage. Then you asked if the current reading indicated whether the amplifier was OK or not.

The relationship between current and voltage in a circuit is resistance... and Ohm's law.

It gets a bit more complicated as there may be non-linearities. So it's best to measure the wanted thing directly.

David

joebog1 · 6th Oct 2022, 10:38 pm

The "gunk " around the caps is contact glue. Its put there so the caps are not pushed upwards when the board is in the solder bath. There is also some glue that was used in computer PSU's that burned up and became really conductive. So much so it was known to catch fire.

The glue looks tacky, and the practice is also tacky in my opinion, BUT they care about profit not ease of repair or asthetics.

The caps can still be removed but require quite a pull AFTER the leads are completely desoldered from the PCB.
"Sometimes " the glue can be prised off with a sharpish screwdriver, but if its clean underneath the cap its probably best left alone.

Joe

novicerepairer · 6th Oct 2022, 10:54 pm

Oops, I see what I have said now. So sorry, I am sure it was the 200millivolts setting and Dc setting I used for the readings.

Radio Wrangler · 7th Oct 2022, 7:47 am

Ah, then the amplifier is fine.

David

novicerepairer · 7th Oct 2022, 8:18 am

Thanks so much, I didn’t know if the 58mv was a bit high and could cause speaker problems.
Thanks for your knowledge, I will fire it up again at the weekend. Sorry for my school boy error in my first message. Regards, Colin.

6th Oct 2022, 5:33 pm	#1
novicerepairer Tetrode Join Date: Jun 2020 Location: Basildon, Essex, UK. Posts: 94	Trio KA 3500 DC Off set Hi, I have a Trio KA3500 that I have not checked the DC offset on, as I have only recently learned the importance of checking before use with speakers. The measurements I got with my multimeter set on max 200m amps was: left channel -57.0 and -38.0 for the right. Would someone be able to tell me if this is Ok to use, as I wouldn’t know how I would be able to do the repairs to change these figures, but wasn’t expecting them to be that high. Also, I used the red lead on multimeter to the red terminal on the speaker output but got a minus symbol on the multimeter?? Did I do this right?? Thanks in advance, Colin.

6th Oct 2022, 5:51 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Trio KA 3500 DC Off set The DC offset is really a voltage. The current would depend on what load you would later connect. Had there been a bad offset you would have destroyed a cheap multimeter, or blown a fuse in a posh one. Set your multimeter on a voltage range for this measurement. Pick a high voltage range to start with as you don't yet know what you will find, then if the reading is low, switch to lower and lower voltage ranges. For this measurement, the polarity doesn't matter as you are looking to check that the DC offset is within a hundred or so mV either polarity, and you can ignore the sign, just look at the magnitude. General practice is to put the -ve mead of a voltmeter on a connection nearest to earth, like the black output post on an amplifier. There are some amplifiers operated as bridges where both speaker connectors are live, but swing in opposite directions. Not very common apart from high power stage amps, and car audio David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

6th Oct 2022, 5:56 pm	#3
novicerepairer Tetrode Join Date: Jun 2020 Location: Basildon, Essex, UK. Posts: 94	Re: Trio KA 3500 DC Off set Thanks for your advice on saving damage to the multimeter. I will definitely use you way of working in the future. Thanks, Colin.

6th Oct 2022, 7:08 pm	#4
novicerepairer Tetrode Join Date: Jun 2020 Location: Basildon, Essex, UK. Posts: 94	Re: Trio KA 3500 DC Off set I wonder if anyone could confirm, if this figure is acceptable for use, or if it is in need of a repair before using? Thanks, Colin.

6th Oct 2022, 7:35 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Trio KA 3500 DC Off set It doesn't look bad, but as we don't know the overall loop resistance of the circuit through the amplifier, through the multimeter and along the test leads, no one can know for sure. Common multimeters (but not all!) are a basic 200millivolt measuring instrument shorted with a 1 Ohm resistor to make a 200milliamp current range. IF this is so, then you have 57mV and 38mV of offset but with a 1 Ohm load, so this doesn't sound bad. A voltage measurement would have avoided the uncertainty. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

6th Oct 2022, 9:25 pm	#6
novicerepairer Tetrode Join Date: Jun 2020 Location: Basildon, Essex, UK. Posts: 94	Re: Trio KA 3500 DC Off set The loop resistance bit has lost me I’m afraid, It is a digital multimeter from Maplins that was about £30, but I have used this amp quite a lot in the past with no problems. There is a couple of capacitors that look a bit leaky though. I have not replaced them as I have not done anything that involved before!

6th Oct 2022, 9:42 pm	#7
novicerepairer Tetrode Join Date: Jun 2020 Location: Basildon, Essex, UK. Posts: 94	Re: Trio KA 3500 DC Off set Some pics hopefully! Attached Thumbnails

6th Oct 2022, 10:30 pm	#8
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Trio KA 3500 DC Off set You were measuring current on your multimeter when you should have been measuring voltage. Then you asked if the current reading indicated whether the amplifier was OK or not. The relationship between current and voltage in a circuit is resistance... and Ohm's law. It gets a bit more complicated as there may be non-linearities. So it's best to measure the wanted thing directly. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

6th Oct 2022, 10:38 pm	#9
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Trio KA 3500 DC Off set The "gunk " around the caps is contact glue. Its put there so the caps are not pushed upwards when the board is in the solder bath. There is also some glue that was used in computer PSU's that burned up and became really conductive. So much so it was known to catch fire. The glue looks tacky, and the practice is also tacky in my opinion, BUT they care about profit not ease of repair or asthetics. The caps can still be removed but require quite a pull AFTER the leads are completely desoldered from the PCB. "Sometimes " the glue can be prised off with a sharpish screwdriver, but if its clean underneath the cap its probably best left alone. Joe

6th Oct 2022, 10:54 pm	#10
novicerepairer Tetrode Join Date: Jun 2020 Location: Basildon, Essex, UK. Posts: 94	Re: Trio KA 3500 DC Off set Oops, I see what I have said now. So sorry, I am sure it was the 200millivolts setting and Dc setting I used for the readings.

7th Oct 2022, 7:47 am	#11
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Trio KA 3500 DC Off set Ah, then the amplifier is fine. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

7th Oct 2022, 8:18 am	#12
novicerepairer Tetrode Join Date: Jun 2020 Location: Basildon, Essex, UK. Posts: 94	Re: Trio KA 3500 DC Off set Thanks so much, I didn’t know if the 58mv was a bit high and could cause speaker problems. Thanks for your knowledge, I will fire it up again at the weekend. Sorry for my school boy error in my first message. Regards, Colin.