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Old 13th Mar 2023, 3:50 pm   #1
112stereo
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Default Bush SRP51 Record Player

Can anyone help please, I have just acquired a Bush srp51 record player which was seized solid and very dirty for my daughter who wants a record player to play her stack of old singles.
I have throughly cleaned it throughout, stripped the deck cleaned and lubricated where needed and now runs as good as ever, however, the amp does play and sounds quite good but when in idle it crackles like mad but silent as soon as the stylus touches the record.
I seem to remember reading somewhere that the output transistors cause the problems and a substitute direct equivalent can be used , but I can't find the thread and don't have any equivalent books any more.
Can someone please suggest what I can do !?
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Old 13th Mar 2023, 10:34 pm   #2
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Default Re: Bush SRP51 Record Player

Some of these had a muting switch on the deck, check that it's clean! Intermittent contact here would definitely give the problem you describe.
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Old 13th Mar 2023, 11:24 pm   #3
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Default Re: Bush SRP51 Record Player

Thank you, I forgot about that.
Will check that tomorrow.
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Old 14th Mar 2023, 12:49 pm   #4
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Default Re: Bush SRP51 Record Player

The mute switch is ok, the three pots are ok, the crackling is definitely somewhere in the amp .
Have checked and changed a few suspect caps and soldered a few suspect joints which made absolutely no difference.
If left on for a while the crackling starts intermittently then builds up to quite loud then there is a bit of a thump all goes quiet for about 3 seconds before it starts again.
I am sure I have seen this fault on this site before but can't find it.
Failing that I will just need to start changing the transistors till hopefully it stops.
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Old 14th Mar 2023, 12:56 pm   #5
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Default Re: Bush SRP51 Record Player

Tin whiskers in transistors? I have had this in a number of amplifiers of this era.

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Old 14th Mar 2023, 1:04 pm   #6
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Default Re: Bush SRP51 Record Player

That's what I was thinking, Chris. I think I had this problem in an Ultra player and it was one of the OC -type transistors IIRC.
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Old 14th Mar 2023, 2:02 pm   #7
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Default Re: Bush SRP51 Record Player

Do you mean the BC type transistors ?
BC114 or BC 132 might be the problem!
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Old 14th Mar 2023, 3:08 pm   #8
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Default Re: Bush SRP51 Record Player

Does this help .?
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Old 14th Mar 2023, 3:14 pm   #9
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Default Re: Bush SRP51 Record Player

Sorry, I was thinking of the earlier version of the SRP41, which used germanium transistors. I suppose the BC xx ones can go noisy, but not so likely. The two presets go intermittent and out of spec.
If you disconnect C7, is the fault still present? That would give some indication of which section of the amp. is causing the trouble.

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Old 14th Mar 2023, 4:44 pm   #10
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Default Re: Bush SRP51 Record Player

Thank you.
I will try that tomorrow.
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Old 15th Mar 2023, 1:51 pm   #11
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Default Re: Bush SRP51 Record Player

Disconnected c7 fault still there exactly the same.
So fault is in the output section, I suppose its trial and error now as I don't have an oscilloscope.
I had better order some transistors, perhaps I should get all three and be done with it..?
What do you think?
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Old 15th Mar 2023, 2:29 pm   #12
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Default Re: Bush SRP51 Record Player

Mark the position of the preset pots. RV4 and RV5, then work them to and fro, returning them to their original positions. best done with a little Servisol. It would also be a good idea to check that the mid point voltage (junction of R14 and R15) is half the rail voltage and the quiescent current of TR4 and TR5 is 6 - 7 mA. Those AD161 and AD162 transistors are usually quite robust. Are R14 and R15 OK?

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Old 15th Mar 2023, 4:49 pm   #13
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Default Re: Bush SRP51 Record Player

Thank you.
It might not be until the weekend as we are busy.
Will post a reply as soon as I have done the checks.
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Old 16th Mar 2023, 3:44 pm   #14
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Default Re: Bush SRP51 Record Player

Both R14 and R15 need replacing, but I applied some heat to the output driver and the crackling stopped !
Left it to cool completely and the crackle came back, looking for a replacement.
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Old 16th Mar 2023, 5:53 pm   #15
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Default Re: Bush SRP51 Record Player

Excellent!
I'm not sure what the difference between a BC119ua and a BC119 is. I suspect they are different spec.
The BFY52 is shown as an equivalent for the BC119ua.

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Old 16th Mar 2023, 7:24 pm   #16
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Default Re: Bush SRP51 Record Player

R14 and R15 both needing replacing suggests there has at some time been too much current down the output transistors.

RV5 sets the voltage between the bases of the AD161/AD162 pair. This sets the voltage between their emitters, which is the voltage across R14 and R15. Ohms law then gives the bias vurrent down the pair.

Look at RV5. It's connected so that if the slider makes a bad contact or hops, skips or jumps due to vibration or failing springiness, the resistance across the whole thing goes to max. Max voltage between bases, max voltage between emitters and the transistors try to send huge current through themselves R14, R15. What burns out first!

This is a common and very bad bit of design. Worth learning and looking out for.

So I'd be rather suspicious of RV5 and get a new one.

R14 and R15 play an important role in this amplifier. They are the chief agents acting against 'thermal runaway' long the bane of early transistor amplifiers. If TR4 and TR5 get hot, their threshold voltages for starting to turn on, decrease. They become easier to turn on, and the bias current increases without the setting pot being changed. The increased current makes them hotter....and that puts the current up more... and so on, and so on.

Having big enough R14 and R15 doesn't stop this happening, but it does stop it running completely out of control.

Having biggish R14 and R15 wastes some of the supply voltage, wastes some power but on a mains powered set it's affordable. Designers of battery powered sets aren't so lucky. They have to save every bit of voltage swing, every bit of power so they have to go close to runaway conditions. You'll find thermistors in those sets and various attempts to compensate the effects of temperature on the bias current in their power transistors.

While we're at it, RV4 is used to set the DC voltage at the junction of R14 and R15 You'll see it marked as 13 ish volts on the diagram. This positions things so that the power transistors can swing that point about as far positive as they can swing it negative. This is an important adjustment. Get it much wrong and the amplifier will hit the end stops sooner, but on one direction only. If RV4 makes the voltage jump around rather than smoothly vary, then replace RV4.

Those three transistors are being made to do a lot. More recent designs use more devices, but they exploit this to make bias more stable and things easier in general. It's a bit of a hang-on from the valve days when the number of valves was miserly and the attitude lived on because early transistors weren't cheap. Once they became cheap, many people didn't notice and still played by the old rules.

So with some experience, you can read a circuit diagram and muse about what was going through the mind of the guy designing it. You can see successive eras as people learned more and more about how to exploit transistors, how to make sure they stayed stable.

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Old 18th Mar 2023, 1:04 pm   #17
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Default Re: Bush SRP51 Record Player

I totally agree with what you've said as I want this to be a long term repair.
Most parts I want to replace I can find except RV5 which appears to be a 50ohm minature!
I've found the RV4 at 200k but the 50ohm is very elusive so far and it seems the most important.
I have cleaned both pots changed TR5 and set it up according to the service sheet and it plays once again.
I ran it for about an hour at a medium volume the output transistors get warm equally and I can still touch them both and happy with the temperature.
I am going to replace the pilot bulb with a small 240 volt neon which should take some loading off the transformer as I noticed this player doesn't have a separate tapping for the bulb, I think it's a late model as the deck plinth is not wood, but plastic !?
Thank you everyone for your help as always its good to know someone out there is always happy to help.
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Old 18th Mar 2023, 1:38 pm   #18
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Default Re: Bush SRP51 Record Player

Just as a recap I can find several 100ohm pre-set pots, do you think it would work if I put a 100 ohm fixed resistor across it ?
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Old 18th Mar 2023, 2:59 pm   #19
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Default Re: Bush SRP51 Record Player

I think the 100ohm pre set should be OK as it is. Both presets are wired as variable resistors. The 50 ohm will be 0 - 50 ohms and the 100 will give 0 - 100 ohms. At some point it will give the right resistance.
Glad to see progress was made and that the nasty noises have gone away.
I have come across wood and plastic plinths with these - not sure which is newer.

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Old 18th Mar 2023, 5:45 pm   #20
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Default Re: Bush SRP51 Record Player

Wonderful, thank you.
That's saved me a lot of searching.
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