Advice request on setting a variable resistor in an Alba 532 record player. - UK Vintage Radio Repair and Restoration Discussion Forum

alpharadj67 · 29th Oct 2019, 2:35 pm

Good afternoon.
I am a hobbyist and amateur and have been working on an Alba 532 record player. It was in poor shape when I got it, needing several new components, including a bridge rectifier and a smoothing capacitor. Unfortunately I physically damaged one of its variable resistors (represented on the schematic as ‘VR4 100K’. I replaced it with a new 250K preset variable resistor (‘TRIMMER, POT, 250KOHM, 10%, 1TURN’). I chose 250K because this was the upper limit stamped on the original. I have no expertise and am at a loss to know how to set this thing or what to set it to. The only advice given on the schematic is ‘Adjust VR4 for Equal Clipping’. I only have a hand held multimeter of the type bought fairly cheaply in most DIY shops. I would be extremely grateful of any advice as to what to do. Many thanks in anticipation.

'LIVEWIRE?' · 29th Oct 2019, 6:08 pm

My (educated)guess would be that VR4 is used toset the bias on an early stage of the amplifier, and the output should be monitored with a 'scope, whilst injecting (say) a 1kHz sine wave at the input, so that a perfect sine wave is present at the output, so the VR should be adjusted, IMHO, util the o/p begins to clip-i.e. that the top & bottom of the sine wave start to flatten, then back off the adjustment so that the clipping is eaual on bith the top and bottom of the wave. The input signal should be at a low level. If you only have a multimeter you will not be able to carry out the above procedure, and the only suggestion I can make is to adjust VR4 for minimum distortion.

Cobaltblue · 29th Oct 2019, 6:24 pm

I would guess that this is going to set the mid point voltage on the output pair.

There's probably another low value pot to set the quiescent current through the output pair.

If that's the case set it so that the voltage on the speaker with reference to the ground is a little less than half the supply .

So with your Meter on a suitable range the supply across the smoothing capacitor should be around 24V DC.

Now set that pot so that between the speaker and Common is around 11.5V DC

That should be close enough.

Cheers

Mike T

alpharadj67 · 30th Oct 2019, 5:42 pm

Thank you both very much for this help! As I'm very much learning at this stage, this advice is going to take me a bit of time and thought to absorb and understand - although the lack of ownership of the equipment Livewire mentions is an obvious disadvantage. Will try to put into effect Cobaltblue's suggestions and report back in a day or. Many thanks again for this help.

crackle · 30th Oct 2019, 8:09 pm

The Service details for this player is in the Radio Museum, https://www.radiomuseum.org/r/balcombe_532.html there is a brief description on adjusting VR4 & VR3.

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I think I still have one of these kicking around in bits somewhere.

Mike

Herald1360 · 31st Oct 2019, 11:47 am

The setup info isn't a great help if you don't have a 'scope, but you should be fine if you adjust VR4 for 11.4V at the speaker and VR3 for a voltage across one of the 2.2ohm emitter resistors of about 15mV (0.015V). No more than 22mV max.

Circuit can be found here:

https://www.service-data.com/section.php/569/1/a10242

or in the R&TV servicing books if you don't already have it.

alpharadj67 · 31st Oct 2019, 2:10 pm

Many thanks for this. I have found the voltage across the smoothing capacitor to be 23.7V which I am very happy with. The query that I have is – when you say “between the speaker and common”. Do you literally mean to connect the multimeter probes to the 2 wires that feed the speaker and take the reading from that point? and should I disconnect the 2 wires from the speaker?

Look forward to hearing from you.

Thanks again

'LIVEWIRE?' · 31st Oct 2019, 8:28 pm

In this case, yes, since Alba fitted the coupling electrolytic in the ground side of the speaker rather than, as was usually the case, between the output transistor emitters and the speaker. Under no circumstances should the speaker itself be disconnected. You should get a reading of between 11 & 12 volts (DC) at the junction of the two emitter resistors and the speaker,'live' side, and it's this voltage which is adjusted to give 11.4v as Herald 1360 points out

Herald1360 · 1st Nov 2019, 1:29 am

Quote:

Originally Posted by 'LIVEWIRE?'

You should get a reading of between 11 & 12 volts (DC) at the junction of the two emitter resistors and the speaker,'live' side, and it's this voltage which is adjusted to give 11.4v as Herald 1360 points out.

Not quite.... voltage of 11.4V can be measured between either side of the loudspeaker and the negative (ground) supply. There should be no measurable voltage between the emitter resistor junction and the speaker itself since they're connected directly to each other.

The resistance of the speaker won't affect the reading since it's very low compared with the meter loading.

'LIVEWIRE?' · 1st Nov 2019, 9:44 am

Why the 'not quite,', Chris? 11.4v is between 11 & 12v after all, and the exact voltage measured will depend, upon other things, on how close to the ideal setting VR4 is when the reading is taken, and what impedance testmeter is used. Because the Electrolytic in this design is on the 'earthy' side of the speaker rarher than the more common 'live side, yes the same voltage will be measured at both speaker connections as the DC resistance of the speaker is too low to make any readable difference, but I still feel my original comment is correct. Admittedly I should have said 'between the junction of the two emitter resistors and common', but I hope that would be understood by the OP.

Herald1360 · 1st Nov 2019, 12:00 pm

Quote:

Originally Posted by 'LIVEWIRE?'

.....

Admittedly I should have said 'between the junction of the two emitter resistors and common', but I hope that would be understood by the OP.

That was my point, not the precise voltage itself..... from the tone of the rest of the thread it seemed to me that measuring between the wrong points would cause yet more confusion.

The instructions on the schematic itself already take for granted that the person working on the set would know where to reference the voltage measurements and have already caused some confusion by assuming test equipment that was unavailable to Alpharadj67

Techman · 1st Nov 2019, 1:13 pm

Quote:

Originally Posted by alpharadj67

The query that I have is – when you say “between the speaker and common”. Do you literally mean to connect the multimeter probes to the 2 wires that feed the speaker and take the reading from that point?

I'd be far more worried about the OP doing this!

It sounds to me like the OP is thinking that the probe is connected to the two speaker wires at the same time, which of course will short out the output stage and possibly be game over for the amplifier.

This is why we have to be so careful as to how we word things when advising an unskilled person.

29th Oct 2019, 2:35 pm	#1
alpharadj67 Triode Join Date: Jan 2018 Location: Bristol, UK. Posts: 14	Advice request on setting a variable resistor in an Alba 532 record player. Good afternoon. I am a hobbyist and amateur and have been working on an Alba 532 record player. It was in poor shape when I got it, needing several new components, including a bridge rectifier and a smoothing capacitor. Unfortunately I physically damaged one of its variable resistors (represented on the schematic as ‘VR4 100K’. I replaced it with a new 250K preset variable resistor (‘TRIMMER, POT, 250KOHM, 10%, 1TURN’). I chose 250K because this was the upper limit stamped on the original. I have no expertise and am at a loss to know how to set this thing or what to set it to. The only advice given on the schematic is ‘Adjust VR4 for Equal Clipping’. I only have a hand held multimeter of the type bought fairly cheaply in most DIY shops. I would be extremely grateful of any advice as to what to do. Many thanks in anticipation.

29th Oct 2019, 6:08 pm	#2
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Advice request on setting a variable resistor in an Alba 532 record player. My (educated)guess would be that VR4 is used toset the bias on an early stage of the amplifier, and the output should be monitored with a 'scope, whilst injecting (say) a 1kHz sine wave at the input, so that a perfect sine wave is present at the output, so the VR should be adjusted, IMHO, util the o/p begins to clip-i.e. that the top & bottom of the sine wave start to flatten, then back off the adjustment so that the clipping is eaual on bith the top and bottom of the wave. The input signal should be at a low level. If you only have a multimeter you will not be able to carry out the above procedure, and the only suggestion I can make is to adjust VR4 for minimum distortion.

29th Oct 2019, 6:24 pm	#3
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,859	Re: Advice request on setting a variable resistor in an Alba 532 record player. I would guess that this is going to set the mid point voltage on the output pair. There's probably another low value pot to set the quiescent current through the output pair. If that's the case set it so that the voltage on the speaker with reference to the ground is a little less than half the supply . So with your Meter on a suitable range the supply across the smoothing capacitor should be around 24V DC. Now set that pot so that between the speaker and Common is around 11.5V DC That should be close enough. Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

30th Oct 2019, 5:42 pm	#4
alpharadj67 Triode Join Date: Jan 2018 Location: Bristol, UK. Posts: 14	Re: Advice request on setting a variable resistor in an Alba 532 record player. Thank you both very much for this help! As I'm very much learning at this stage, this advice is going to take me a bit of time and thought to absorb and understand - although the lack of ownership of the equipment Livewire mentions is an obvious disadvantage. Will try to put into effect Cobaltblue's suggestions and report back in a day or. Many thanks again for this help.

30th Oct 2019, 8:09 pm	#5
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: Advice request on setting a variable resistor in an Alba 532 record player. The Service details for this player is in the Radio Museum, https://www.radiomuseum.org/r/balcombe_532.html there is a brief description on adjusting VR4 & VR3. I think I still have one of these kicking around in bits somewhere. Mike

31st Oct 2019, 11:47 am	#6
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Advice request on setting a variable resistor in an Alba 532 record player. The setup info isn't a great help if you don't have a 'scope, but you should be fine if you adjust VR4 for 11.4V at the speaker and VR3 for a voltage across one of the 2.2ohm emitter resistors of about 15mV (0.015V). No more than 22mV max. Circuit can be found here: https://www.service-data.com/section.php/569/1/a10242 or in the R&TV servicing books if you don't already have it. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

31st Oct 2019, 2:10 pm	#7
alpharadj67 Triode Join Date: Jan 2018 Location: Bristol, UK. Posts: 14	Re: Advice request on setting a variable resistor in an Alba 532 record player. Many thanks for this. I have found the voltage across the smoothing capacitor to be 23.7V which I am very happy with. The query that I have is – when you say “between the speaker and common”. Do you literally mean to connect the multimeter probes to the 2 wires that feed the speaker and take the reading from that point? and should I disconnect the 2 wires from the speaker? Look forward to hearing from you. Thanks again

31st Oct 2019, 8:28 pm	#8
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Advice request on setting a variable resistor in an Alba 532 record player. In this case, yes, since Alba fitted the coupling electrolytic in the ground side of the speaker rather than, as was usually the case, between the output transistor emitters and the speaker. Under no circumstances should the speaker itself be disconnected. You should get a reading of between 11 & 12 volts (DC) at the junction of the two emitter resistors and the speaker,'live' side, and it's this voltage which is adjusted to give 11.4v as Herald 1360 points out

1st Nov 2019, 9:44 am	#10
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Advice request on setting a variable resistor in an Alba 532 record player. Why the 'not quite,', Chris? 11.4v is between 11 & 12v after all, and the exact voltage measured will depend, upon other things, on how close to the ideal setting VR4 is when the reading is taken, and what impedance testmeter is used. Because the Electrolytic in this design is on the 'earthy' side of the speaker rarher than the more common 'live side, yes the same voltage will be measured at both speaker connections as the DC resistance of the speaker is too low to make any readable difference, but I still feel my original comment is correct. Admittedly I should have said 'between the junction of the two emitter resistors and common', but I hope that would be understood by the OP.