Signet PDA 200/2 Loop amplifier repair

[gw0udm]

Hi all

This is pretty obscure but I thought it worth a try. I'm helping a local church with a big upgrade and like a lot of churches it has an induction loop for hearing aids which is driven by a (relatively) new amplifier which is a PDA 200/2 manufactured in the UK by Signet. The system wasn't working and whilst the amp powers up and the audio circuits are working there is no drive to the loop itself.

Opening it up it looks very simple - power supply, some audio circuits, an LM1875 and two BUZ11s. There are two internal fuses and one of them was blown, and it was connected to one of the BUZ11s which was open circuit on all the leads. I tried replacing it and whilst the amp seemed to work for a few minutes the fuse blew again and the new transistor is also now open circuit and presumably toast.

Unfortunately there is no service documentation at all - I've checked the output from the main regulator which seems OK and the rest of it is working fine (audio side etc, all the lights come as you'd expect etc).

I don't know if anyone has any experience of working on amplifiers like this - I'm really loath to chuck it out as I feel it's probably something simple. I'm going to try and speak to the supplier and see if they can help, the product is end of life now but they were still making them relatively recently.

This is the actual model:

https://soundinduction.co.uk/induction-loop-systems/signet-pda-200-2-200m-professional-induction-loop-amplifier.html

I know this is a bit of a long shot but if anyone had any advice or ideas i'd be glad to hear them!

James

[coppercone2]

check voltages.

desolder and check all the big transistors

[MathWizard]

Do you have an adjustable PSU with current limiting ? With it unplugged, sometimes it's ok to attach wires to the output of a voltage rail, like a 5/12V rail, and then with the current limit set to like 10-20mA, start slowly turning up the voltage. And then with a DMM try and figure out where the current goes, and what voltages don't make sense, like a cap that won't charge up if it's shorted.

I wouldn't start that way, I'd map out the general layout, and check the rectifier, diodes, caps, BJT/Mosfets or other chips for shorts with it unpowered.

[gw0udm]

Thanks. I don't have a current limiting PSU unfortunately but I'll take your advice and work round the board.

I've checked the rectifier and it seems OK, and from checking so far the voltages appeared about right but I need to measure them properly.

I've tried to get some info from the manufacturer but no success as yet. I'll keep trying though

Thanks!

James

[ElectronicsJoe]

I have just bought a PDA200/2 off Ebay. Advertised as 'switches on' but not further tested. It has not been delivered, but I am half expecting to need to do a functional repair. This being the case, I am just researching any available circuit information. (I will reverse engineer if necessary, but it is tedious and time consuming). I was just wondering how gw0udm, you progressed with your repair?