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Old 23rd Sep 2020, 12:36 pm   #1
Mathers
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Default Bush SRP31 - No Sound Since Rectifier Replacement

Hello all.
I have an early Bush SRP31 record player that I am resurrecting. My mother purchased this machine new in 1958. Everything was working okay, except that the rectified DC output voltage was too low, at 39v. I therefore dismantled the unit and replaced the selenium rectifier with a 1N4007 diode with an inline 330ohm 10w resistor. This has now raised the output voltage to around 215v, however there is now no sound output at all. Upon powering up there was some intermittent cracking from the speaker, but since then, no sound whatsover. Is it possible that raising the DC output voltage to the correct level, has caused another already weak component to now fail and if so, where should I start looking? The unit is still dismantled, but this should presumably not have any bearing on the sound output at all? Any advice would be very gratefully received.
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Old 23rd Sep 2020, 12:51 pm   #2
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

Probably something silly like a wire off or bad connection - check carefully!

Otherwise, check valve voltages against data sheet etc. 'Should' be an easy fix.
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Old 23rd Sep 2020, 1:14 pm   #3
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

Just a second thought - Your HT voltage tells me that the valves are passing current as they should - the voltage would be a lot higher if they weren't loading the supply, so it's likely to be something silly like a wire off somewhere. Check the pickup cartridge wiring and do a 'buzz' test (look it up if you don't know) with the volume full up.
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Old 23rd Sep 2020, 2:22 pm   #4
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

Does this model have a make/break contact on the Ext. stereo amp/speaker socket? That may have gone bad.
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Old 23rd Sep 2020, 8:35 pm   #5
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

Hello,
Thanks for the replies.
I carried out a buzz test this evening and there is indeed a low level buzz on full volume, from the red wire in the headshell that connects to the cartridge. If I want to feed in another audio source in, to test the sound output, such as an iPod, where would be the most suitable place to connect it? Regarding the coaxial sockets at the back, this model is unusual in that, there is no fly lead visible and hence neither socket has anything actually plugged in to it?
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Old 24th Sep 2020, 12:36 pm   #6
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

I would expect a loud buzz from the very decent amplifier fitted to this RP.
Further investigation is required.
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Old 24th Sep 2020, 4:53 pm   #7
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

I agree with John on this one, there should be a loud buzz.

You'll now need to do the same test at the top end of the volume control - look at the circuit diagram.
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Old 25th Sep 2020, 3:46 pm   #8
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

Thanks.
I'll continue to investigate over the weekend.
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Old 28th Sep 2020, 10:32 am   #9
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

Hello, so i've now carried out a buzz test both at the centre connection of the volume control and at the other side of R2, where the grey cable from the cartridge is soldered. The result in both cases was again, just a quiet buzz on full volume.
I then noticed that the tops of the two valves were completely different in appearance. One is black and the other a light grey. I then removed and examined them, to discover that the grey topped in was cracked and in fact part of the glass casing came adrift when I did so.
Being a vacuum tube, does this mean that it will now, no longer be functioning correctly?
If so, could this be the cause of the symptom of very low output?
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Old 28th Sep 2020, 1:02 pm   #10
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

Yes and yes.

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Old 28th Sep 2020, 1:28 pm   #11
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

I thought it was going to be something simple being caused by you having the guts removed while you were working on it, but I thought it would be a simple wire off type fault or bad connection and not that you'd managed to smash the glass of a valve. This shows how careful you need to be when working on delicate vintage electronics. However, most of us have probably done something similar at some time or other - I myself managed to bust a valve while working on an old telly last year!

There's two valves in your player and each valve actually has two valves inside each glass bottle, so there's actually the equivalent to four valves altogether - which you should notice when you look at the circuit diagram. So in effect you've actually lost two valves with that one broken one. The ideal situation would be to replace them both with a brand new/old stock matched pair, but in reality you'll probably get away with just replacing the one broken one, but in this case it would be better to locate a good used/second hand one to best match up with the existing good one that's left.
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Old 28th Sep 2020, 3:13 pm   #12
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

Thanks Chaps.

Another thing I tried, was to swap the valves around in the holders. Once swapped there was then a very low level output from the turntable, whereas in their original positions, there was no discernible output at all.

I will procure at least one replacement valve and report back, with the result.
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Old 28th Sep 2020, 4:31 pm   #13
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

That's correct, that's how it should be, as each valve has an output section, but only one of the second sections is used as a preamplifier, the other being the splitter. Don't run it with only one valve, as you risk damaging the other one!
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Old 19th Oct 2020, 2:13 pm   #14
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

Hello,

Well, I finally managed to obtain a replacement ECL83 valve and it has resolved the issue.

A couple of questions,

Is it normal for the valves to run quite hot, too hot to touch in fact?

Would it be possible to test the capacitor values in circuit, using a multimeter, with the valves removed for instance?

Thanks
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Old 19th Oct 2020, 3:01 pm   #15
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

ECL83s are tiny output valves. They will get hot, I expect you could touch them briefly though. They dissipate about 9 to 10 watts if working in their correct 'envelope'. (Assume Heaters @ 6.3v x 0.6A = 3.78 watts plus HT @ 200v x 28mA = 5.6 watts so 9.38 watts total approximately.) Imagine touching a hot 12 watt bulb for comparison.

You need to measure the voltage on their cathodes (pin 7) it should be about 5 to 6 volts. If so all is well.
If more there is a problem. Next measure the voltage on their pins 8. Any voltage here indicates the 0.1uF coupling caps C7 and C8 are leaky and likely C5 too.

Not easy to test caps in circuit. You can only get an idea at best and that depends on their location.
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Old 19th Oct 2020, 4:16 pm   #16
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

Okay thanks, I will check the pin voltages and come back with the result.
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Old 23rd Oct 2020, 2:51 pm   #17
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

Quote:
Originally Posted by Mathers View Post
Is it normal for the valves to run quite hot, too hot to touch in fact?

Would it be possible to test the capacitor values in circuit, using a multimeter, with the valves removed for instance?
These are both 'output' valves and will run hot enough to cause you end up in casualty with a nasty burn if you happen to grab hold of one!

There's also another valve inside the same glass on each one, so this may add to the heat. This is also a reason for being particularly careful when putting one of these record players back together that you make sure that there's no wiring that's going to get its insulation melted by being against the valve glasses. Also, the hot wire wound resistor that you fitted in series with the new diode rectifier will run very hot in the same way, so the same precaution regarding the wiring applies.

These particular record players have high quality coupling capacitors fitted (probably better than any replacements you may fit), so generally never need replacing, but a check for any significant positive voltage on the grid of the output section of each valve should be done as a matter of course just in case. There's a couple of cathode decoupling electrolytics that you could think about replacing, but often they're ok. There's a small brown or black Hunts paper capacitor in there, but its position in the circuit means that it's harmless where it is and replacing it will be pointless, even though it's probably slightly leaky by now.

Hope that helps.
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Old 26th Oct 2020, 11:40 am   #18
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

So i've now checked the voltages on all of the valve pins and all appear to be consistent with those indicated on the circuit diagram. They're all slightly lower, but that is presumably because the HT voltage is approximately 5% lower at around 205v.

I've also attempted to fettle the dead tweeter, however the plastic membrane inside has lost around 50% of its conductive coating and it now just crackles very quietly! I'm therefore going to leave it out of circuit.

Thanks for your assistance everyone.
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Old 26th Oct 2020, 5:06 pm   #19
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

If you take that Electrostatic out of circuit, do make sure you well insulate the 2 wires feeding it. Of course you could fit a 3" cone tweeter instead.
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Old 26th Oct 2020, 8:15 pm   #20
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Default Re: Bush SRP31 - No Sound Since Rectifier Replacement

Have you checked the cathode voltage on the 2 op valves pin 7, it should be about 12v.
Its important and will indicate if the audio coupling capacitors are leaking or not.
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