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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 22nd Feb 2018, 10:53 pm   #1
Urumiko
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Location: Northumberland, UK.
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Question 1962 Dallas Scala Instrument amplifier.

Hi there,

This forum was recommended to me by one of the chaps over at sound on sound. I hope posts about guitar amplifiers are welcome here?

I am a complete amateur when it comes to working on amps and such.
I should probably say up front that I have learned how to ensure my caps are discharged before we go any further.

In a nut shell I bought blind at auction the above amplifier.

I got it home to try out and when turned on it crackled rather a lot, like i had very dirty pots and i was constantly turning them.

I plugged a guitar in and had a twiddle and it seemed to settle down a bit, either due to the twiddling of the volume knob to a clean spot or the valves warming up. I played it for all of 30s and it sounded surprisingly great.. like really great. then the volume seemed to steadily tail off to complete silence and now i cant get a sound out of it.

I was looking for some troubleshooting help.

I thought it would be the rectifier valve. But it turns out there is no rectifier valve.

Here are some photos:
https://drive.google.com/open?id=1CV...5tOxIJEzeOWvyy

All i've established so far is the big capacitor does get up to full voltage and then will slowly discharge over a mater of minutes when turned off.

Both valves seem to light up as pictured.

The speaker wire is poor but is not broken or shorting.

The input transformer looks errr old.

It will have had a tremolo unit at some point that has been removed.

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Old 22nd Feb 2018, 11:39 pm   #2
PJL
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Default Re: 1962 Dallas Scala Instrument amplifier.

Don't power it up again until it has been checked over. It looks in reasonable shape but will need a full service if you plan to use it regularly. This would involve checking all parts and replacing any that are faulty from age. We can provide advice if you want to DIY but you will need a soldering iron and digital multi-meter and plenty of patience. Alternatively, you could post in the 'repair and restoration services wanted' section.
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Old 23rd Feb 2018, 6:35 am   #3
Radio Wrangler
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Default Re: 1962 Dallas Scala Instrument amplifier.

It's all too easy to sound like a health and safety freak, but when buying an amplifier of unknown provenance, I wouldn't just plug it in and grab hold of an electric guitar connected to it. I'd do some basic safety checks and a visual inspection before plugging it in.

To do a proper earth-bonding and insulation test requires some expensive equipment, but you can do a lot to improve your chances of survival with just a multimeter, the next time you get an amp.

David
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Old 23rd Feb 2018, 8:53 am   #4
Nickthedentist
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Default Re: 1962 Dallas Scala Instrument amplifier.

Love that fag packet
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Old 23rd Feb 2018, 9:55 am   #5
Urumiko
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Default Re: 1962 Dallas Scala Instrument amplifier.

Hi all,

Yes I look upon this as a learning experience so would rather do this myself. I have a multi meter and soldering iron, prototyping board, and basic low voltage components at my disposal.

High voltage caps, valves, transformers, rectifier etc. I'd have to buy in unless I started stealing from one of my working amps.

I do have a beaten up old Marshall valve state combo I'd planned on converting to house a kit amp at some point so there are potentially some components in that I could steal.

I could certainly try a shotgun approach and replace all of the caps, valves and rectifier, but if I could learn anything along the way by testing the circuit for clues with some basic guidance that would be great.

I am up this book but its a bit expensive for this side of payday, and some interactive help is always best.

So far all i have established is that:
  • both valves light up,
  • The voltage between the +ve and -ve on the regulator remains high when turned off and slowly falls over a few mins till it levels out at about 9-10v.
  • I tested the voltage on the blue wire going to the input transformer, against the ground when in use and this is hot (couple of hundred volts) testing between that and ground causes a small audible pop from the speaker.
  • I can hear very quiet white noise coming from the speaker when turned on.


Yes I am aware of the high voltages within the amp and have learned how to discharge the caps as the first thing when opening her up.. also note the rubber gloves lol.

Yup, an old pack of fags and marigolds, sounds like a fun Friday night.

Last edited by AC/HL; 23rd Feb 2018 at 4:00 pm. Reason: Anglicised
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