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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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3rd Oct 2018, 4:46 pm | #61 |
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Re: Bush SRP41 Mains Transformer
Thanks John, Going by the transistors and the lack of fuse I think I have a mk 2. The transistors do run nice and cool when I use the diodes instead of the bridge rectifier as well. At some point I will need to sort the power input so that it's closer to the spec but at the moment the lack of amplification is the problem. That being said, your transformer seems to give a combined 40vac which is way more than the 18vac I'm feeding it could the power be so low that the amp isn't even starting?
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3rd Oct 2018, 5:37 pm | #62 | |
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Re: Bush SRP41 Mains Transformer
Quote:
I stand by my original method that I described in that other thread linked to, that I would have first tested the amplifier on an external power source. I myself would have used my bench power supply with built in current limiting, but an ordinary person can't be expected to obtain a specialised power supply just for one record player repair, so that's why I advised using three 9 volt batteries from the pound shop strung together to give 27 volts. Only after this initial testing would I have even thought about looking for a replacement transformer, but that's just the way I do things. I think that something went wrong with that bridge rectifier, or the connections to it, and that's what has caused the problem you're having now. I think you'll have to test those transistors and the driver, or perhaps first take some accurate voltage readings from them all, which will all be lower than normal, but which may give an indication due to the relationship between them all. |
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3rd Oct 2018, 7:27 pm | #63 |
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Re: Bush SRP41 Mains Transformer
I'll get hold of some 9v batteries and see how it behaves. As for testing the transistors, I already removed them and they gave satisfactory measurements from the diode setting on my meter. If I'm testing them in circuit should I just be measuring each pin for a voltage proportional to what is shown on the circuit diagram? Also when you say driver do you mean the wire inputs to the speaker? I tested those for DC voltage and it was dead.
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3rd Oct 2018, 7:51 pm | #64 |
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Re: Bush SRP41 Mains Transformer
I think Techman means the Driver Transistor which is the one before the output pair.
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3rd Oct 2018, 8:22 pm | #65 |
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Re: Bush SRP41 Mains Transformer
Thanks for that. All quite clear, now. 18V and a bridge should be just fine. Max output power will be down by about 20% or 1dB. Barely audible!
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3rd Oct 2018, 8:34 pm | #66 |
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Re: Bush SRP41 Mains Transformer
Thanks Edward, I tested all 5 transistors out of circuit. I'll see if I can find out what the transformer is feeding it while it's running
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3rd Oct 2018, 9:00 pm | #67 |
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Re: Bush SRP41 Mains Transformer
Right so I don't honestly know what these measurements mean but the transistor gave the following measurements.
BC119ua driver transistor base to collector 4.6v base to emitter 0.66v collector to emitter 5.3v, a lot of crackling AD162 power transistor emitter to base 0.27v collector to base 5.36v collector to emitter 5.66v AD161 power transistor emitter to base 0.3v collector to base 6.26v collector to emitter 6.57v |
3rd Oct 2018, 9:34 pm | #68 |
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Re: Bush SRP41 Mains Transformer
There seems to be a black blob connected across the transformer seconary and the reservoir capacitor. I do not see a bridge rectifier.
A bridge rectifier should be connected from the wiggly connections to the transformer and the + should be connected to chassis and the - connected to the fuse. Work your way slowly along the circuit. Take out fuse. What is the fuse rating? Test the transformer on its own. Add the rectifier, check output. Add reservoir capacitor. If everything runs cool with the correct voltages, connect a 100ohm resistor across the fuse holder to limit current and check voltages. I suspect that you have connected the rectifier DC output terminals to the AC transformer. Things should not get hot unless you are outputting a lot of sound. Germanium transistors do not like high temperatures |
3rd Oct 2018, 10:19 pm | #69 |
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Re: Bush SRP41 Mains Transformer
Hi Trevor I'm not quite sure what you mean by black blob unless you are referring to the picture I uploaded with the bridge rectifier connected. I've tried a few configurations so far including with a bridge rectifier and I definitely had the dc outputs going to the smoothing cap and the 'wiggly' ac terminals going to the transformer. After encountering issues using the bridge rectifier I have swapped back to the original two diode rectification.
There is no fuse in the mark 2 player to test. The current system may be under powered but it does provide consistent voltage. I think something further along the circuit has broken but I think finding what that is might be beyond my expertise. |
3rd Oct 2018, 10:59 pm | #70 |
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Re: Bush SRP41 Mains Transformer
I don't think there's a lot of point in powering up with batteries at this late stage, but if you get the amplifier fault sorted, then you could always use three batteries to see for yourself how it performs soundwise on full voltage, so I would work with the reduced power supply you've got for the time being.
You said you had some crackling when testing voltages round the driver transistor. If this was from the loudspeaker, then I'm very much liking the sound of that, as it 'may' be an indication that the amplifiers output stage is working and you've got some other fault, such as a disconnection somewhere on the audio input. You don't say whether you can hear any slight hum from the speaker when the amplifier is powered up. It looks like you've taken those voltage readings the wrong way. each one should be taken with respect to chassis, or the negative line. As for testing those transistors out of circuit, you may have already done this correctly, but you need to think of each transistor as two diodes back to back. The AD161 is an NPN transistor, so it will be like two diodes with their anodes connected together at the base, and their cathodes respectively at the collector and the emitter. The AD162 is a PNP transistor, so the two cathodes are connected together at the base, with the two anodes connected respectively at the collector and emitter. The driver is another NPN type so test as such. As an example, to test the first transistor which is a AD161 NPN type, you would expect your ohm meter to show conduction from both base to emitter and base to collector, obviously with your positive probe on the base. You would obviously test in the reverse for the PNP type. There should be no reading in either direction between emitter and collector on the particular transistors used in this amplifier. Any reading will be down to leakage within the device which, depending how bad, may mean the transistor has had it, just as any backward reading on either junction would mean the same - if that all makes sense. John has given you a typical current draw reading (18 mA) which you could check out with your meter in series with the supply on a 'high' enough current range - ie, start high - 10 Amp range (just in case) and work down if necessary. I may not be able to get on-line much over the next few days depending on certain things, but I'm sure other members will be able to advise you - you should be able to sort it in the end. Last edited by Techman; 3rd Oct 2018 at 11:18 pm. |
3rd Oct 2018, 11:33 pm | #71 |
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Re: Bush SRP41 Mains Transformer
Thanks again for the suggestions techman. Your instructions for testing the transistors are the same as I found elsewhere so I think they are alive and well. I'm on holiday with the other half for the next few days so I'll follow up on your other instructions when I get back.
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3rd Oct 2018, 11:39 pm | #72 |
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Re: Bush SRP41 Mains Transformer
No problem, have a good time. Things are sounding a bit more promising, so fingers crossed for a good result in the end.
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7th Oct 2018, 7:32 pm | #73 |
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Re: Bush SRP41 Mains Transformer
Hi Again,
I found out that the stereo output on the player is working. This means that I can cross the cartridge input off of the list of culprits. Also I gave Techman's reply another read and I was hoping to clarify a couple of things. What is a voltage reading taken with respect to the chassis? I was just measuring voltage between transistor pins. Also How can I take a current reading in series with the transformer without disconnecting one of the wires? Also in response to the question there is no hum from the speaker when I start the player up. Like I said though when I am poking around the circuit with the meter probes it crackles a bit. |
8th Oct 2018, 7:36 pm | #74 |
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Re: Bush SRP41 Mains Transformer
Hi Brod, crackles when probing round an audio amp are normal.
To measure current with a normal DVM/multimeter you must break into the circuit and insert the meter on the correct range "across the break", making sure it is set for AC or DC current as appropriate. Current measurement damages more meters than many other measurements; if the meter is set to current and connected across voltage measurement points it is often goodbye meter. It is often possible to deduce current by measuring the voltage dropped across a resistor in the part of the circuit of interest. These tricks will come with experience. Ed |
9th Oct 2018, 10:08 pm | #75 |
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Re: Bush SRP41 Mains Transformer
Hi again,
So hopefully I haven't misinterpreted Ed's instructions. I disconnected the pink wire from the smoothing cap and reconnected it with the ammeter as a bridge. That measured 600ma which is an awful lot more than what john quoted. Also I might just be grasping at straws but I saw other threads mentioning 'bias pots' potentially causing issues. I assume these are the little red things in the attached image. I measured the resistances as 39kohms on RV4 and 1.2k on RV5. the datasheet has these listed at 150k linear and 50 linear respectively. Is this potentially an issue? |
10th Oct 2018, 8:27 pm | #76 |
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Re: Bush SRP41 Mains Transformer
Hi Brod, yes it looks as if you got it right. I believe 600mA is in the right area, but others (with the schematic) will know better.
The presets look like bias setting pots, again the instructions for setting them should be on the service data. Their settings will influence the amount of current drawn from the power supply. Ed |