Bush SRP41 Mains Transformer

[Techman]

I'm a little confused as to why you were advised to get a 24 volt transformer when the amplifier needs around 26 volts - perhaps folks weren't looking at the circuit diagram or followed the link to that other thread that I posted. It's a pity someone didn't spot this, as I'm not on here on a regular basis and often don't monitor ongoing threads. On the plus side, you've obviously exchanged that transformer for one of more appropriate voltage.

The reality is that in your case it would have been easier to have got the original transformer rewound to original specification and kept with the original rectifier diode system. I think you've definitely made a mistake with the wiring and am guessing that part of the bridge rectifier is being shorted out and you may even have incorrect polarity on the output, if so, let's hope no extra damage has been done, as I don't like the sound of that "no sound output but transistors getting very hot".

Why not forget the bridge and use the original rectification system. A clear close up of the connections to the bridge rectifier and the bottom of the smoothing capacitor would help.

[ms660]

Re, my previous post, I've expanded the image somewhat, the connections now look ok if +ve rect goes to the red tag on the capacitor and -ve rect goes to the plain tag.

Lawrence.

[brodier54]

It looks a bit higgledy-piggledy but I'm pretty certain I have it wired correctly. I have the + and - terminals connected to the red and plain terminals on the smoothing cap and the two wires which I have marked blue go to the transformer. I've attached some close ups here as well as a picture of a spare rectifier I got.

If I remember rightly it's quite difficult to get a 27.5v transformer now so I was told 24v would be enough. Then when it was realised I would have to use a bridge rectifier which would increase the voltage I got an 18v. I probably would have been better getting the original rewound. I thought replacement might have been quicker but here we are.

As for the rectification I don't think you can use the original two diodes on a transformer that isn't centre tapped. Correct me if I'm wrong though.

[Techman]

Originally Posted by brodier54

As for the rectification I don't think you can use the original two diodes on a transformer that isn't centre tapped. Correct me if I'm wrong though.

I thought you had where you've got that link wire between the two secondary windings?

[ms660]

Re: Post#43: Compared to a two diode full wave rectifier the bridge rectifier will have less output for the same voltage input as there are two diode forward voltage drops to consider for each half cycle, with the full wave two diode (centre tapped transformer) there is only one diode forward voltage drop to consider for each half cycle.

Typically 1.4 volts and 0.7 volts respectively.

You're correct, two diode full wave rectification is not possible without a centre tapped transformer.

Lawrence.

[brodier54]

I've just done a quick bit of reading, am I right in understanding that any transformer with two secondaries is 'centre-tapped'? If that is the case how should I replicate the wiring with 4 output pins where the original only had 3 ( shown in the attached picture ).

[Nuvistor]

Using that transformer as 9-0-9 in a full wave will only give 13 volts or so off load.

[ms660]

With those two separate secondary windings in your transformer as supplied there is no centre tap unless the two windings are linked correctly, the link between the two secondary's as you've configured in yours is the Centre tap, the secondary thus giving 9v-0-9v (18v across the whole):

https://www.electronics-tutorials.ws...nsformers.html

EDIT: Post crossed.

Lawrence.

[brodier54]

Thanks for linking the tutorial Lawrence I think that makes sense. Given what you've said could I strip a bit on insulation out of the middle of my centre tap and use that with the two diode full wave rectifier. If I did would I run into issues with low voltage as nuvistor mentioned?

[Techman]

It's already been confirmed in post #30 that the amplifier will work down to 10 volts so go for original two diode rectification with what you've got - the lower voltage will be kinder at this stage if there's any other faults in the amplifier. No need to strip the wire link, just solder to the nearest terminal, if you see what I mean.

[ms660]

Originally Posted by brodier54

Thanks for linking the tutorial Lawrence I think that makes sense. Given what you've said could I strip a bit on insulation out of the middle of my centre tap and use that with the two diode full wave rectifier. If I did would I run into issues with low voltage as nuvistor mentioned?

Yes as per what Frank said.

If you go two diode full wave centre tapped secondary with that transformer you will only have 9 volts going into the rectifier, with full wave bridge you can have 18 volts going in.

Have a read through this:

https://www.electronics-tutorials.ws/diode/diode_5.html

Then this:

https://www.electronics-tutorials.ws/diode/diode_6.html

Lawrence.

[brodier54]

That makes sense Techman, I suppose it's connected to that centre tap regardless of which terminal you use. If I find that does work well should I swap back from the 18v to the 24v? since I would get a bit more juice out of that.

[Techman]

Make sure you connect everything exactly as in your picture in post #46. Getting the diodes the wrong way round would be a bit of a disaster. Once you've got it working like this, you can decide on whether you're happy with the results or whether to try it with the bridge rectifier configuration - I note that you have a spare to play with.

Edit: Post crossed with yours - yes, you were correctly thinking the same!

[brodier54]

Right so I've set it up with the two diodes and it seems to power it okay, it provides 13v dc so I might swap in the 24v at some point. However I'm still not getting any throughput to the speakers. I would guess this would be something to do with the amp. While I was testing the transistors I might have reinstalled the transistors wrong. Would there be any easy way to diagnose this?

[Techman]

If they're AD161 & 162 they'll only fit one way round, and as for being in the wrong places, you can work it out from the circuit diagram

[Herald1360]

How many angels can dance on this pinhead?

The circuit info says 27.5Vdc. A bridge rectified and large capacitor smoothed 24Vac will give rather more than this, a 27V transformer even more. The amp is reputed to be running right on its upper voltage limits at 27.5V.

The amp works fine on somewhat lower voltages and will be less stressed, at the expense of lower output power capability- at half the volts you get a quarter the power.

The 18V transformer should give about 24Vdc (maybe a little more off load because of regulation (it's designed for 18V on full load)). This will give the amp less stress and won't noticeably affect output power. What's not to like about it?

As a matter of interest, has anyone got an original player and would they be prepared to measure some actual voltages- dc and ac (inc. mains input at the same time) for our edification?

[Techman]

I agree with what you say, Chris. Knowing what we know now, the best option would have been to have just sent the transformer to Ed and got it rewound. I did a quick search on-line last night for a good replacement transformer and couldn't find anything suitable at what would be a sensible price and available in the UK. I personally wouldn't be happy with it running at reduced power, even for the sake of reliability, but most folk probably wouldn't notice the difference in the lower output wattage. I now suspect from what the OP has said, that he may have got the output transistors in their wrong places, hence the 'no sound and a very hot transistor', but he will be able to work out which should be where from the circuit and hopefully no extra damage has been done, thanks perhaps partly to the reduced voltage.

[brodier54]

So I had a good look at the transistors and they actually seem okay. I took some pictures before I removed them and they match up okay. I'm feeling a bit defeated at this point. It turns out there is a place in Edinburgh that does vintage kit repair so I might get a quote from them.

[julie_m]

Your wiring looks all wrong.

Get a new bridge rectifier, in case you've knackered that one. Then connect the two AC terminals to the transformer, and the + and - terminals to the capacitor.

There isn't a common connection with this transformer, because you only have a total of 18V available as opposed to 18V + 18V.

EDIT: Oops, missed a bunch of posts. Ignore me then.

[John10b]

Some actual readings from the mk 1, the one with fuse fitted.
Mains Transformer P = 240ac: Sec. 20vac - 0 - 20vac, I = 18mA.
Output at FS1, cathode of two rectifiers = 27vdc.
The two output transistors, AD162 run very cool.
Hope this is of some help. I’ve taken picture of transformer, if I can send it to some one they can upload onto forum ( I’m using iPad and can’t load pics ok)
Cheers
John