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Old 29th May 2021, 3:02 pm   #1
Gabe001
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Default Pye black box 1004 transistor background noise problem

Good afternoon

Just after a bit of help

I've ended up with a black box 1004 and a whole lot of vinyl from a house clearance. A new challenge for me as I am brand new to transistors and their woes. I am also brand new to record players and vinyl, but that's a different story. In any case, one need to start somewhere.

In a nutshell, I've replaced MR1 with a suitable bridge, c11, c12 and c13 and turned on. F1 wasn't blown to start with.

I've got nice loud sound when I hook up my phone to the pick-up. None of the transistors get particularly hot.

The problem is that I have a volume dependant background noise, which is quite loud. It's best described as "white noise" similar to that from a television that's not tuned into a station. It's present even when no audio source is plugged into the pick-up.

I got my Velleman out and did some signal tracing. All's clear until the junction of C9 to GD1/VT4 base, where the white noise appears for the first time. See diagram highlighting test points (X denoting the presence of the white noise)

These tests suggest to me that the noise is coming from the emitter of VT3, which will probably need to be replaced. Am I missing something obvious? Apologies for the stupid question but as I said I'm new to transistors.

Thanks for your advice

Gabriel
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Old 29th May 2021, 3:06 pm   #2
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Default Re: Pye black box 1004 transistor background noise problem

Yes, probably a noisy transistor. If it's varying with the volume control, then that suggests the culprit is before that point.
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Old 29th May 2021, 5:46 pm   #3
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Default Re: Pye black box 1004 transistor background noise problem

Yes, there was always a bit of "Germanium Rustle" on these. Check also the value of the components around VT3 and VT4. If the transistors are the orginals, they will be Newmarket. If you need to replace any of them, you may need to use the nearest possible equivalents if you can't source these hard-to-find Newmarkets. If you really are "new to Vinyl", do bear in mind that the cartridge fitted into the BSR UA15 atochanger may not be stereo-compatible.
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Old 29th May 2021, 6:02 pm   #4
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Default Re: Pye black box 1004 transistor background noise problem

The volume control is split, between right after the pick up and between transistor one and two. At least that's my interpretation.
The output from it appears noise free
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Old 29th May 2021, 7:01 pm   #5
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Default Re: Pye black box 1004 transistor background noise problem

Quote:
Originally Posted by Edward Huggins View Post
Yes, there was always a bit of "Germanium Rustle" on these. Check also the value of the components around VT3 and VT4. If the transistors are the orginals, they will be Newmarket. If you need to replace any of them, you may need to use the nearest possible equivalents if you can't source these hard-to-find Newmarkets. If you really are "new to Vinyl", do bear in mind that the cartridge fitted into the BSR UA15 atochanger may not be stereo-compatible.
Thanks Edward

The transistors are all original. I found 2 NKT224s on eBay for £6 which I've bought. I'll check the values of the components as you've advised.

I know about the cartridge, it's a mono ronette - I've got the BSR X5M bookmarked as a stereo compatible mono cartridge which should be a good replacement but I haven't starting doing anything on the deck yet.

Gabriel
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Old 30th May 2021, 6:11 pm   #6
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Default Re: Pye black box 1004 transistor background noise problem

This is the reply from Jerry sent by email (cathoderay57 - posted with his permission)

I think that since both Paul and Jerry are saying that the problem lies before VC I think I will look at VT1 before VT3. The reason why I can only hear the noise using the Velleman after VT3 is very plausible.

"Hi Gabriel,

Hmm, not sure your diagnosis is necessarily right. The problem I have is that the noise is dependent on the volume control setting. If VT3 were producing the noise then I’d have expected it to be present all the time even with the volume control set to zero, because VT2 and VT2 are running at constant gain (the volume control is only varying the ac sound signal, not the bias of the transistors on account of the use of dc blocking capacitor C7). You are probably hearing noise through the Velleman at the junction of C9 and VT3 collector / VT4 base and not earlier in the circuit as the sensitivity of the Velleman might not be sufficient to detect the noise until after 3 stages of amplification in the BB circuit. Therefore, my money is on VT1, a NKT226. The NKT226 data is identical to that of the NKT224 but the 226 is a low-noise version, used because it is right at the front end and therefore any noise it generates will get max amplification. That could be the vital clue! Since you have bought replacement NKT224s if one doesn’t fix the noise in position of VT3 try one in position VT1 and see if it makes any difference. I think there was some variability in the noisiness of transistors so you might get lucky! The other possible sources of noise are base bias resistors so you could also try replacing R5 and R4 with low-noise metal film resistors. Any noise in the base circuit of the first stage gets max amplification. The base bias resistors in the later stages are unlikely to be the cause of the problem. Good luck, and let me know how it goes
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Old 30th May 2021, 6:21 pm   #7
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Default Re: Pye black box 1004 transistor background noise problem

You could try a low noise silicon type such as a BC559 in the TR1 position, since it isn't DC coupled so the different bias requirements may not matter.
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Old 30th May 2021, 6:33 pm   #8
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Default Re: Pye black box 1004 transistor background noise problem

Someone write in an earlier thread that it only works well if you replace like for like.

In any case considering they cost pennies, should I use the 559A, B or C?

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Old 31st May 2021, 8:33 am   #9
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Default Re: Pye black box 1004 transistor background noise problem

Hi Gabriel, the BC559 has a dc current gain hFE within the range 110-800 whereas the A, B and C variants have closer gain tolerances within the same range, A being lowest gain and C highest. The NKT226 has hFE of only 50 and so I'd go for a lower gain variant. I have plenty of BC557 transistors which is a higher voltage rating than the BC559 but might work. I can pop one in the post FOC if you want. Cheers, Jerry
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Old 31st May 2021, 9:09 am   #10
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Default Re: Pye black box 1004 transistor background noise problem

Quote:
Originally Posted by Gabe001 View Post
Someone write in an earlier thread that it only works well if you replace like for like.

In any case considering they cost pennies, should I use the 559A, B or C?
The output stages are DC coupled so the transistor types are indeed pretty critical. The first stage is effectively DC isolated (I think - working from memory here) so should be much more tolerant.

Try whatever small signal PNP types you have to hand. It should at least confirm that TR1 is the culprit.

Jerry's BC557s should work well enough, but aren't low noise transistors so may be a bit noisy.
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Old 31st May 2021, 12:44 pm   #11
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Default Re: Pye black box 1004 transistor background noise problem

Thanks Jerry for the BC 557 offer. If you would kindly pop one in the post I'll be happy to cover any costs. I'm enjoying my first transistor project despite my diagnostic faux Pas. I may try putting a scope on it to see it it picks up anything.

Spent the morning polishing the case. It came on little legs (? original I think) which I've removed, so if anyone would like them drop me a pm
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Old 31st May 2021, 2:43 pm   #12
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Default Re: Pye black box 1004 transistor background noise problem

I don't remember these ever being specified, or supplied, with legs. They may have been added by a previous owner. As to your reference in Post#8, it might have been me that recommended "like for like". However, if there are later, possibly non-Newmarket, lower-noise options for the AF stages then do try them, especially if they do not require much, or any, alterations to the operating conditions.
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Old 31st May 2021, 6:32 pm   #13
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Default Re: Pye black box 1004 transistor background noise problem

Thanks Edward, and Jerry for posting me one. Will do and report back.

Picture of legs attached for what it's worth
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Old 31st May 2021, 10:23 pm   #14
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Default Re: Pye black box 1004 transistor background noise problem

If you drop in a silicon sub for VT1, it would be worth increasing R4 to get the emitter voltage back up to the 1.1V shown in the table. Otherwise the signal headroom at VT1 collector will be compromised.
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Old 5th Jun 2021, 8:21 pm   #15
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Default Re: Pye black box 1004 transistor background noise problem

Thank you all for your help and advice so far. I've made good progress I think.

VT1 replaced with nkt226
VT2 replaced with nkt 224

All components to the left of VT2 on the circuit diagram replaced, except R13 and the tone and volume control, with gradual improvement. I didn't have a 33nf cap for C3 so I used 47nf. Also, I used ordinary resistors I had to hand, not the low noise variants.

MR1, c11, c12 and c13 are also new - I had replaced them prior to first switch on.

The background hiss is still there, but much attenuated compared to when I started out. I'm not sure if the set is meant to be completely silent, in my case the background noise is drowned out very quickly but you can hear it when I pause the music in the video. As explained before it is inaudible at zero volume and then appears quickly and stays constant. Disconnecting the base of vt2 seems to silence the set.

I'm posting a video link below, playing from an old phone directly into the pick-up. Could you let me know if there is still room for improvement? (ps ignore the squeaky volume control, it sounds worse than it actually is). I'm not sure whether some minimal background hiss is normal to a degree with transistors, and what to expect from the 1004. The sound quality is great, with very good volume and no distortion, although I'm no audiophile.

Video here

https://youtube.com/shorts/OBq_gsyelFk?feature=share

Thanks
Gabriel
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Old 5th Jun 2021, 9:10 pm   #16
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Default Re: Pye black box 1004 transistor background noise problem

Hi Gabriel, probably as good as it gets, although hard to hear the hiss through my crappy tablet speakers! When I use my Quad22 pre-amp with stereo Quad II PAs there is quite significant background hiss but using an op-amp pre-amp there is none. Part of the charm of EF86 valves and cracked carbon resistors I guess. In your case, further work is probably subject to the law of diminishing returns. Did you try the BC557? Cheers, Jerry
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Old 6th Jun 2021, 8:04 am   #17
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Default Re: Pye black box 1004 transistor background noise problem

Hi Jerry

Thanks for your continued interest

No I didn't try the bc557, mostly because I don't want to risk damaging the new 226 as I unsolder the legs from the board. I cannot take it out of circuit without removing it. In retrospect I should have tried the bc557 first

I think I'll probably replace the last micromite electrolytic and call it a day

An interesting observation is that whilst the hiss virtually disappears when the volume is a minimum, with the volume up at maximum it seems to improve as well. Same effect if I short R11. I suppose this is because proportionally more of the audio signal is bypassing VT1. I opened the R11 end of the volume pot yesterday and gave it a good clean but it didn't make a difference. I'm not quite sure what the point of the dual volume pot is.

Gabriel
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Old 6th Jun 2021, 10:55 pm   #18
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Default Re: Pye black box 1004 transistor background noise problem

The dual volume pot is a "loudness" control of sorts. The increase in value of R2 as volume is reduced by R11 will give more bass in the cartridge output as the load on it imposed by the relatively low input impedance of the transistor amplifier is reduced by the increasing series resistance of R2.

How effective this is at boosting bass as volume is reduced would be an interesting simulation exercise!

The "ganged pot" loudness control features (in a different implementation) in some other Pye record player amplifier circuits.
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Old 7th Jun 2021, 2:06 pm   #19
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Default Re: Pye black box 1004 transistor background noise problem

Thank you Chris
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Old 14th Jun 2021, 5:57 am   #20
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Smile Re: Pye black box 1004 transistor background noise problem

Just to report back that I've completed the Pye 1004 refurbishment, just awaiting a new stylus. Lubricating the deck was no problem, I followed the guide here. It was a great learning experience.

It was fitted with an Acos GP91-3SC cartridge which, despite its excessively high output, works well enough. I won't dare crank it up to max as it will upset the neighbours!

There is still some background hiss but it is drowned out by the music when the volume is up, and is not present when the volume is low, so it's not intrusive.

The front label was missing, so I've done a 3 d printed "replica", although the font isn't completely matched.

You can see and hear it here
https://youtube.com/shorts/W93G7oFl__M?feature=share

Thanks for the help. I'm pretty pleased, I can get rid of my ION record player now and keep this which fits nicely with the house decor.

Ps - I don't have any particularly valuable records

Gabriel.
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