Bush SRP41 Mains Transformer

[brodier54]

Hi again,

All of my bits and pieces arrived today. I managed to reattach all of the capacitors I tested without too much bother I hope ... given the advice I'll probably just order an LED for the indicator.
I had a question about the capacitor. The existing blown one had three pins for output to the circuit. The new one I have bought technically only has two once I put the two secondaries in series.
I've attached pictures of the old transformer connected and a picture of the new one. How should I be wiring this?

[ms660]

If you're looking for full wave rectification at 24 volts DC then that transformer won't do it with the existing rectifiers so far as I can make out.

Lawrence.

[Herald1360]

^^^WHS^^^

To use your new transformer:

You'll need to connect the two 115v primaries in series on the mains side (link the two central tags). Connect the incoming supply to the outer 115v and 0v tags.

Secondary side similarly series up the two secondary 12v windings. Then you'll need (ideally) a wire ended 2A or so rated bridge rectifier. Connect the ~bridge wires to the outer 12v and 0v tags, then extend the + bridge wire to reservoir cap +ve and the - bridge wire the reservoir cap -ve. Connect the wire left over from the centre tap on the old transformer to the reservoir cap -ve.

You could use the original rectifiers plus a couple more to make up a discrete component bridge but I reckon the encapsulated all in one route is less hassle!

[brodier54]

Hi Chris, I thought it might have been something to do with the diodes. Thanks for confirming that. I'll get a bridge rectifier ordered. Fingers crossed it should work after that.

[ms660]

Remember that the DC voltage at the reservoir capacitor (the output from the rectifier) will be noticeably higher than the AC voltage input from the transformers secondary.

Lawrence.

[brodier54]

Hi Lawrence, why would the voltage from the rectifier be higher? The transformer secondaries 12+12 give me a combined 24Vac. would the rectifier not just give me 24v dc?

['LIVEWIRE?']

Lawrence (or anyone else) feel free to correct me, but, unless I'm mistaken, the off-load rectified DC voltage would be about 34v (ACv x 1.414) for a F.W. rectifier with Capaitor filter. A transformer with an o/p voltage of 16-17vAC (DC x 0.707) is needed to provide 24vDC.

[brodier54]

Hi livewire, thanks for clarifying that. That's a bit of a nightmare. Will I be able to reduce that 34v with a resistor (or any other means) or will I have to buy a transformer with a lower voltage?

[Restoration73]

If a suitable transformer cannot be located, a 0-9,0-9 (or 9-0-9 centre tapped) @ 12VA
is more common, and I have seen 8v+8v transformers. If the secondaries are separate, just wire in series in phase, output can be confirmed with a meter.

[buggies]

At least two other forum threads (in 2009 and 2017) dealt with SRP41 wax-dripping mains transformers. In one case it seemed to be a leaky output transistor pulling excess current and in the other it was a dodgy bias pot causing a similar fault. There was also mention that a 12 volt transformer and bridge rectifier gave 13.5 volts DC. The amplifier worked ok down to 10 volts on a power supply.

[brodier54]

Hi George, I did skim over the latter thread. If I'd paid more attention I would have ordered the right one the first time. I've ordered a 9v+9v anyway but it's good to know I can try use a single secondary on the 12+12 if I get stuck. I'll see about testing the pot and transistors as well. Would the transistors be on the PCB? I couldn't spot anything that looked like one.

[rontech]

By the way re the awful smell, selenium compounds can be quite toxic.

[Herald1360]

The service info for the set suggests 27.5v DC is the nominal output from the power supply. So the max voltage from the transformer needs to be 20v AC or less for a bridge rectifier and about 19-0-19 for a centre tapped full wave like the original.

To keep supply down to 24v DC would need around 18v AC for a bridge and 17-0-17 for centre tapped.

[brodier54]

Hi again, I'm still waiting on my rectifier in the post so I haven't managed to get the player up and running yet. In the meanwhile though I tested all of the transistors. 3 of them seemed fine but 2 of them are a bit suspect. The two in question are the AD161 and AD162 germanium transistors which I think power the amp.
I measured the voltage drop using the diode setting on my multimeter and they both measured at about 0.13v on the appropriate terminals. from what I have read germanium transistors should read between 0.2v and 0.3v. Is this cause for concern? Also the AD162 had a fine coating of white powder on it's base. Could this be a leak?

[Ed_Dinning]

Hi Brod, germanium transistors test "leakier" than silicon for a variety of reasons and may be perfectly OK. The 0.2, 0.3v you refer to is the forward voltage of a Ge diode that forms the transistor. A Si transistor (diode) would read about 0.5v.
It is a common way of telling the 2 types apart.

The white powder may well be silicon grease, use as a thermal compound on the base of many power transistors, that has dried up.

Ed

[brodier54]

Hi Ed, thanks for that. I'm at a bit of a loss now though. I've tested just about everything and I'm not sure what could have caused the transformer to go. Unless of course it was the transformer itself. I'm a bit nervous about putting it back together again and the same thing happening.

[Herald1360]

Don't worry too much about killing a new transformer- unlike semiconductors, even a dead short on the secondary won't kill it instantly. You'll either hear its distress, or if it's a less severe overload problem, you can detect it by running for a while and checking the transformer temperature to see if it's overheating.

If you like, you could put something like a 22R wirewound resistor between the reservoir capacitor and the amplifier. With no signal it should stay fairly cool and you can check the current by monitoring the resistor voltage. If the amp is shorted, the resistor will protect the transformer ( possibly by self immolation) but again you should have plenty of time to turn off- green vitreous WW resistors can run red hot for quite a while!

If there is an amp problem, just come back again with any questions.

[brodier54]

Hi again folks, I've made some progress but not without any issues. I connected the new 18vac transformer and the bridge rectifier which seem to provide about 20v dc which was less than I expected.
I turned the player on and the turntable seems to work. I put on an LP and it was quietly playing the track but it didn't seem to get as far as the speaker. Also the rectifier must have been getting very hot as the plastic was starting to melt slightly. I'm not sure how to follow up on these symptoms.
I'm assuming one of two things is happening, either the fault which killed the first transformer is still there or potentially I have made a mistake when putting the circuit back together. Any suggestions? I have attached a few pictures of the work I have done so far. You might be able to tell I'm a bit of a novice.

[brodier54]

Oh, I forgot to mention my hand brushed against the AD 161 transistor while it was operating and it was very hot. From what I understand they often run hot as standard so that could be nothing.

[ms660]

It might be the photo but your wiring/connections don't look right to me? The rectifier doesn't sound very well, you might need another.

Post crossed.

Lawrence.