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Old 18th Apr 2019, 11:20 am   #1
kitchen10
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Default Bush SRP31D Restoration Questions

Hello!

I have recently acquired a Bush SRP31D portable, fitted with a BSR Monarch UA16 autochanger. I'm totally new to this kind of set, typically I lean more towards the solid-state hifi end of things, and this isn't particularly either of these with the expection of the rectifier! I'm looking to get this portable ship-shape. Hopefully my background in electronic engineering will help me along the way, although I'm a newcomer to valve amplifiers so I'm somewhat out of my transistorised comfort zone!

All in all, it's in pretty good shape - the autochanger works fine (although the automatic record size selector is in need of a good degreasing), the amplifier works and the tone controls both noticeably affect the tone, however there are a few points I'm not so sure on:
  • Very occasionally, the amplifier starts making a hissing sound that increases in volume and eventually disappears after a pop is heard from the speaker. This seems to happen only when the amplifier has been running for a while.
  • I've noticed that the Grundig electrostatic tweeter doesn't seem to work, which seems like a common issue with these models. I've researched some threads that cover the refurbishment of these tweeters, but I fear that I don't have the delicate touch needed for the procedure.
  • If I turn the volume knob past the '5' marker, the output starts to distort in a rather unappealing manner; is this par for the course for this model? I did read another post stating that the original speaker isn't really up to the job, but I'm not sure how to get around this.
  • Currently an Acos GP96-1 cartridge is fitted rather than the original Sonotone 8T4A. The Acos seems to do the job, but I would like to replace it with the stock cartridge - these seem to be somewhat difficult to find, however! Does anyone have one sitting unused in a cupboard?
  • The BSR is obviously due an overhaul. I can't seem to find the service manual for this changer anywhere online.
  • The long silver trim along the bottom of the unit with the control legends is missing on my unit. I appreciate that it's a long shot, but would anyone happen to have one floating around?

I suspect I'll be ok with overhauling the autochanger as I know my way around the later BSRs (thanks to a misspent and not particularly long-ago youth spent playing about with late-70s disco consoles), but I would really appreciate any input on the above points.

Thanks in advance,
Henry
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Old 18th Apr 2019, 12:23 pm   #2
Herald1360
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Default Re: Bush SRP31D Restoration Questions

You might find this useful- at any rate there's a circuit there that isn't available "up top".


https://www.youtube.com/watch?v=3GZE_wnM3mI


If you think of a pentode as a JFET with an extra gate that simply has to be connected to HT and decoupled you can relate to them in a semiconductor fashion. A triode doesn't have the extra gate and has (sort of) a resistor between source and drain, though the analogy falls down if things are biassed to cutoff.


Another way is to think of the pentode as a voltage controlled current source and the triode as ditto with a parallel resistor.



The heater is simply there to make the valve actually work by heating the cathode until it can emit electrons.



The ECL86s are getting somewhat expensive now, so hopefully yours are OK.


If they're paper dielectric types, replacing both output valve grid coupling capacitors is a must otherwise just check that voltages round the circuit make sense. The common cathode bias resistor and bypass capacitor need to be in good condition too.
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Last edited by Herald1360; 18th Apr 2019 at 12:31 pm.
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Old 19th Apr 2019, 3:35 pm   #3
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Default Re: Bush SRP31D Restoration Questions

I would not rush to revert to the original Sonotone 8TA. Whilst these were good cartridges in their day, the (newer and lower compliance) Acos GP96/1 should serve you well.
You are unlikely to hear any difference. You might also find the that the record player sounds sweeter with that old hissy electrostatic speaker disconnected. The limiting factor in these quite high powered players, is that very small 6" x 4" Celestion main speaaker.
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Old 20th Apr 2019, 6:44 pm   #4
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Default Re: Bush SRP31D Restoration Questions

I have a spare Grundig electrostatic speaker from a Bush record player I stripped, it is untested but you are welcome to it FOC. Drop me a PM with your address if you want it and I will post it.

Peter
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Old 20th Apr 2019, 10:11 pm   #5
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Default Re: Bush SRP31D Restoration Questions

I totally disagree with Edward. I repaired the tweeters in both my SRP31's both C and D versions and also in the separate amplifiers used for stereo. They sound much better with the tweeter than without it. I used pieces of a mylar happy birthday banner I had lying around to repair them
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Old 23rd Apr 2019, 1:21 pm   #6
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Default Re: Bush SRP31D Restoration Questions

Herald1360, thank you for the analogy! Is there a way of testing valves without a dedicated valve tester, or should I work off the assumption that they're ok if they work? I'll have a poke around the chassis and check the caps and resistors when I'm home.

Quote:
Originally Posted by Edward Huggins View Post
I would not rush to revert to the original Sonotone 8TA. Whilst these were good cartridges in their day, the (newer and lower compliance) Acos GP96/1 should serve you well. The limiting factor in these quite high powered players, is that very small 6" x 4" Celestion main speaaker.
Thank you, Edward. I'm glad I won't be needing to track down a Sonotone! I do think I'll try and repair the tweeter rather than disconnecting it, as I'd like to get it sounding more or less as it would have originally. I noticed on another SRP31D thread that there was some discussion on replacing the speaker with a more appropriate unit - I've tried to find a replacement but the 3 ohm coil makes things somewhat difficult. Do you know of anywhere I'd have a chance of finding one?

Quote:
Originally Posted by Electronpusher0 View Post
I have a spare Grundig electrostatic speaker from a Bush record player I stripped, it is untested but you are welcome to it FOC. Drop me a PM with your address if you want it and I will post it.
PM sent, thank you Peter!

Quote:
Originally Posted by regentone001 View Post
I repaired the tweeters in both my SRP31's both C and D versions and also in the separate amplifiers used for stereo. They sound much better with the tweeter than without it. I used pieces of a mylar happy birthday banner I had lying around to repair them
Interesting repair material, Steve! I'd read elsewhere on VR that the issue with these tweeters is the foam sandwiched between the two mylar sheets, what did you replace the foam with?

I am on the lookout for the matching AU31D amplifier unit, I have already posted a wanted ad in the classifieds.
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Old 23rd Apr 2019, 8:31 pm   #7
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Default Re: Bush SRP31D Restoration Questions

Some would say that the best test for a valve is whether it works OK in the circuit designed for it.....


A simple check of electrode voltages against any service data will show up any serious problems and unless the cathode current sits excessively high or low there's unlikely to be much to worry about.
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Old 23rd Apr 2019, 9:17 pm   #8
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Default Re: Bush SRP31D Restoration Questions

Although it slightly annoys me when I read the comment that a particular component is a "change on sight", there's one exception in this model of record player, and that is the rectifier. A lot of them that you come across will have already had this component replaced with a single diode and series 'watty wirewound' power resistor. Some will have just had a diode only fitted, which is bad news as regards the valves, as they'll be thrashed to death, so always fit a resistor before or after the diode.

There's no point in taking voltage readings from around the valve bases until you've checked the HT voltage at the rectifier on these players, as if it's the original rectifier, it'll be virtually non-existent at below 40 volts in most cases. The record player will still work in this state but will sound terrible with no bass.

There's virtually no capacitors to change in this model, as the grid coupling capacitors are either Mullard mustard or disc ceramic types in this player and unlikely to be faulty. Cathode decoupling capacitors could be replaced, but I've yet to find a faulty one in this type of player.

Good clear photos of both over and under the chassis from end to end will tell forum members what the state of play is with your player. A forum search will tell you all you need to know regarding replacing that half wave rectifier if it still has the original one fitted.
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Old 25th Apr 2019, 4:31 pm   #9
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Default Re: Bush SRP31D Restoration Questions

Following advice on here, and picking up on Techman's point, here's a simple way to address the rectifier: https://www.vintage-radio.net/forum/...d.php?t=153857

I've now done this procedure on three separate SRP31Ds and it evened up the sound between all three of them without any need to change other components.

Regarding the cartridge, I had the Acos - which I believe is highly rated around here - in a Hacker Gondolier I acquired, but it wasn't performing up to scratch in dynamic range (I tested it by A/B testing running a line from one of the SRPs fitted with a Sonotone). In the end I replaced the Acos in the Hacker with a BSR12M. Separately, the Sonotone on one of the SRPs had badly corroded/broken contacts, and that was replaced with a BSR SC11M. Simply a case of what cartridge I could find at the right price!
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Old 8th May 2019, 12:14 pm   #10
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Default Re: Bush SRP31D Restoration Questions

Apologies for taking so long to reply! I've been drowning in work unfortunately. Anyhow, I've now whipped out the chassis and taken a few photos of the chassis, underside and topside. I've noticed that the driver has a small hole in the paper near the edge - could this be a potential cause for the poor performance at medium to high volume?

Link to chassis photos

Quote:
Originally Posted by Herald1360 View Post
Some would say that the best test for a valve is whether it works OK in the circuit designed for it.....
That makes a lot of sense, the valves seem good in that case. The intermittent 'whoosh' noise has now disappeared too, I guess the old girl just needed easing back into life.

Quote:
Originally Posted by Techman View Post
Although it slightly annoys me when I read the comment that a particular component is a "change on sight", there's one exception in this model of record player, and that is the rectifier. A lot of them that you come across will have already had this component replaced with a single diode and series 'watty wirewound' power resistor. Some will have just had a diode only fitted, which is bad news as regards the valves, as they'll be thrashed to death, so always fit a resistor before or after the diode.
It seems that the rectifier has already been replaced with a diode which is nice, assuming it's been done right! I've taken some close-ups of the job the previous owner has done, accessible via the above link.

Henry

Last edited by Station X; 9th May 2019 at 9:40 am. Reason: Link fixed.
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Old 8th May 2019, 5:26 pm   #11
Edward Huggins
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Default Re: Bush SRP31D Restoration Questions

I would not have thought that the small hole in the Celestion 6" x 4" unit would cause a problem. Easily patched with tissue paper/Copydex. However, this is a very small unit to handle the potential power of the amplifier and you may just be overloading it.
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Old 9th May 2019, 6:43 am   #12
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Default Re: Bush SRP31D Restoration Questions

That link only shows a loudspeaker, is it me?
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Old 9th May 2019, 8:58 am   #13
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Default Re: Bush SRP31D Restoration Questions

I can only see a picture of a speaker via that link, too. Do you have to sign up to that site and be logged in to see other photos? If that's the case, why not post them on the forum via the 'Go Advanced' button below the reply box?
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Old 9th May 2019, 9:41 am   #14
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Default Re: Bush SRP31D Restoration Questions

I've fixed the link in post #10.
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