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Author Topic: Update on Londoner L100A amp  (Read 1112 times)

psafloyd

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Update on Londoner L100A amp
« on: January 04, 2019, 12:56:51 PM »
I posted on this amp last year but for a number of reasons, it got shelved.

I have launched a new topic as it is so Ling since I posted I feared people might not see it.

Anyway, had it apart today and think I understand why the speakers were being bypassed via the speaker out.

There is a second board behind the back panel with a large burn mark under one of these components marked 155-318.

Can you advise what these are? I'm struggling to identify them on the schematic (attached).

I am also struggling to find an on/on latching DPDT toggle that is rated at 6A 125V. I can get one, but seemingly only from China, so that can't be right. Can you recommend a trusted supplier you use for components that might carry these? The ones I have used for guitars and effects pedals don't seem to carry them.   

There are a number of potentiometers with broken shafts I was looking to replace. They are 47K board mounted pots with three pins but I can't work out – and I've been looking for an answer – whether these should be log or linear pots.

Any advice on these greatly appreciated as ever.

Many thanks.

         
« Last Edit: January 06, 2019, 12:15:00 PM by psafloyd »

phatt

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Re: Update on Londoner L100A amp
« Reply #1 on: January 04, 2019, 09:35:47 PM »
Need more info,,
Is it a resistor?  a Capacitor? what does it look like?
Try and take a pic of the part.
Phil.

psafloyd

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Re: Update on Londoner L100A amp
« Reply #2 on: January 05, 2019, 05:04:55 PM »
Need more info,,
Is it a resistor?  a Capacitor? what does it look like?
Try and take a pic of the part.
Phil.

I did upload some pics as well, but it would appear that didn't work. Will do it again.


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psafloyd

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Re: Update on Londoner L100A amp
« Reply #3 on: January 06, 2019, 12:15:41 PM »
Need more info,,
Is it a resistor?  a Capacitor? what does it look like?
Try and take a pic of the part.
Phil.

Have managed to attach the pics this time.


g1

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Re: Update on Londoner L100A amp
« Reply #4 on: January 06, 2019, 04:05:36 PM »
Those are power resistors (1ohm, 5watt).  They are shown at the emitters of the power transistors in the 100S schematic.
If they measure 1 ohm or less with your meter, they are probably fine.
The pot looks like it says 'lin' on it, which would be linear.

psafloyd

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Re: Update on Londoner L100A amp
« Reply #5 on: January 07, 2019, 05:39:56 AM »
Those are power resistors (1ohm, 5watt).  They are shown at the emitters of the power transistors in the 100S schematic.
If they measure 1 ohm or less with your meter, they are probably fine.
The pot looks like it says 'lin' on it, which would be linear.


OK, so if they register <1ohm, they should be OK. Can I measure them in situ, or do I need to detach one of the legs? (Sorry for the noob questions.)

The other components look OK, ie not obviously melted or charred, so perhaps it is historical or it just overheated some time.

I imagine I should probably check the big blue capacitors while I am in there , though I don't see any bulging, etc.

Thanks also for your eyesight. My near sight has become very middle aged over the past year and I missed that when I could normally read 7pt text in the dark!

But I haven't found any 47k pots and wondered if that means they are not readily available. Of course, it could be my eyes can't register the decimal as I know 4.7k are easily had, however, if 47k, am I right to think I cold replace those with a similar 50k pot. After all, that is within 10% of the tolerance set for many components?

Thanks again.   

   

phatt

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Re: Update on Londoner L100A amp
« Reply #6 on: January 07, 2019, 07:26:52 AM »
Yes 50k pot is the same thing.
i.e. when a schematic calls a resistor 5k you use a 4k7 resistor.
Except for maybe the bias it's rare in these circuits to need exact values.

To find out if the pots are linear or log just use your meter to measure end to end,, jot that down ,, now turn the knob to halfway and measure from wiper tab (Centre) to one end then the other.
If it's a linear pot it should read close to half the total value from both ends.
If Log,, it will read about 10~20% from bottom to centre and 80~90% from centre to top tab.

The burn marks are a common result as they are power resistors and do get very hot. Would be better to mount them on insulated stand offs but amps are made to a cost. If the heat is extreme the board can burn up and it can de laminate the copper tracks underneath but yours looks ok.

So Q?
Is the amp working?
if so don't replace parts willy nilly as you may cause a new problem.
Fix the pots first then see how it sounds.
Phil

psafloyd

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Re: Update on Londoner L100A amp
« Reply #7 on: January 07, 2019, 09:45:13 AM »
Yes 50k pot is the same thing.
i.e. when a schematic calls a resistor 5k you use a 4k7 resistor.
Except for maybe the bias it's rare in these circuits to need exact values.

To find out if the pots are linear or log just use your meter to measure end to end,, jot that down ,, now turn the knob to halfway and measure from wiper tab (Centre) to one end then the other.
If it's a linear pot it should read close to half the total value from both ends.
If Log,, it will read about 10~20% from bottom to centre and 80~90% from centre to top tab.

The burn marks are a common result as they are power resistors and do get very hot. Would be better to mount them on insulated stand offs but amps are made to a cost. If the heat is extreme the board can burn up and it can de laminate the copper tracks underneath but yours looks ok.

So Q?
Is the amp working?
if so don't replace parts willy nilly as you may cause a new problem.
Fix the pots first then see how it sounds.
Phil

Yes, the amp is working, or was. I am only going to change the pots and put on a new power switch which has failed – if I can find one at 6A 125V as all these seem to be mini DPDTs. Any ideas gratefully received.

When I last looked at the amp, it was powering on but making no sound. So I switched the jumper lead run from the speakers to the external speaker out on the back (in previous post).

I hadn't worked out why this had been jerryrigged before as it was sticking into the slave socket when I got the amp. Then it worked and pretty well, but I assumed the reason for it being done like this might be something to do with a problem elsewhere which this might bypass. That doesn't really make sense to me, as it seems to just loop the signal from the speakers back into the amp (and no doubt an external one). But if it works, why would that be bypassed? Anyway... 

There is some horrible sound when turning the depth pot on the double potted parametric EQ (which changes in tone with the EQ setting), though this may have improved with a thorough cleaning of the pots.

I can't test the reverb as the controls are broken off, but the tank looks in very good shape.

phatt

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Re: Update on Londoner L100A amp
« Reply #8 on: January 07, 2019, 12:28:36 PM »
If the original switch can't be found you may need to look at mounting a different type of switch.
I assume the original is a round hole mount with metal toggle.
There are plenty of 10Amp plastic package rocker switches around now but they are mostly square cutout type.


Re the speaker socket.
Who knows why,, without seeing the unit I have no idea but a lot of amps do have a switching setup so that when the external speaker jack is used it turns off the internal speakers.
This avoids the low ohms issue that can often burn out the power amp.
If the parametric EQ pot is stuffed that will cause wacky sounds, the amp may even oscillate and squeal like a pig. :o
In my experience, the can of cleaner will only be a temporary fix and likely cost more than a new pot. 8|
Phil.

psafloyd

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Re: Update on Londoner L100A amp
« Reply #9 on: January 07, 2019, 05:00:37 PM »
If the original switch can't be found you may need to look at mounting a different type of switch.
I assume the original is a round hole mount with metal toggle.
There are plenty of 10Amp plastic package rocker switches around now but they are mostly square cutout type.


Re the speaker socket.
Who knows why,, without seeing the unit I have no idea but a lot of amps do have a switching setup so that when the external speaker jack is used it turns off the internal speakers.
This avoids the low ohms issue that can often burn out the power amp.
If the parametric EQ pot is stuffed that will cause wacky sounds, the amp may even oscillate and squeal like a pig. :o
In my experience, the can of cleaner will only be a temporary fix and likely cost more than a new pot. 8|
Phil.

Yes, round hole metal toggle. Seen on/off/on ones, but most on/on are 15A. But most switches are the plastic rocker style as you said. Would mean cutting a larger hole in the panel.

The EQ pot works, but the depth pot next to it makes awful noises when moved.

Thanks again.


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g1

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Re: Update on Londoner L100A amp
« Reply #10 on: January 08, 2019, 02:32:16 PM »
You don't need a 6A switch, you need a switch rated 6A or more.
If the 15A will fit, use it.
If not, post some pics and measurements (bushing diameter), I'm sure there are switches available that will work.

psafloyd

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Re: Update on Londoner L100A amp
« Reply #11 on: January 09, 2019, 06:13:38 PM »
You don't need a 6A switch, you need a switch rated 6A or more.
If the 15A will fit, use it.
If not, post some pics and measurements (bushing diameter), I'm sure there are switches available that will work.

Thanks for the advice.


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