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phil99

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Joined: 11/02/2018
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Posted: 04:51am 18 Jan 2023

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Those readings suggest an open circuit somewhere in the path:-

+30V - R23 - Q8 - R19 - Z1 - R13 - R14 - -30V

Measure the resistance of each resistor, track, wire and solder joint.
.
Edited 2023-01-18 14:53 by phil99

Dinosaur

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Joined: 12/08/2011
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Posted: 05:40am 18 Jan 2023

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Hi All

phill99 I have been very thorough with that and all tracks and resistors measure perfectly. I did discover that R14 is 2.7k and not 2.2k as marked on the drawing.
However the good board is identical.

EDIT: Having difficulty replacing Q8 & Q9. Direct replacement is BC528 but not getting any hits. The critical spec is 500mA current. Most of the quoted "replacements" only are 100mA. Any suggestions on this ?
EDIT: BC640 , will replace Q8 & Q9 but phil99 you may still beright about an open circuit somewhere. Probably the Z1
Edited 2023-01-18 16:42 by Dinosaur

Regards
Hervey Bay Qld.

TassyJim

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Joined: 07/08/2011
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Posted: 06:57am 18 Jan 2023

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If Z1 was open circuit, the voltage at Q2 and Q8 would be closer to +30V, not negative. The open circuit is close to the +30V rail = Q8

On the good board, 0.491V drop across 47ohms is about 10mA.
If you add up all the resistors in the chain = 6k
6K across 60V also give 10mA so the static current is 10mA through Q8.
I would give a 100mA transistor a try if you can't locate a BC528.

Jim

VK7JH
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phil99

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Posted: 07:11am 18 Jan 2023

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A BD140 perhaps? Jaycar has them.

TassyJim

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Posted: 07:27am 18 Jan 2023

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phil99 said A BD140 perhaps? Jaycar has them.

That should do the job nicely.

VK7JH
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Dinosaur

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Posted: 03:12am 19 Jan 2023

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Hi All

Replacing Q8 & Q9 did the job of bringing the boards in line.
There is a 0.1V DC level difference (original Q8 and Q9), but when I put 1vpp into both channels they both amplify to a Q1 base voltage of about 60v.
Will probably replace the good board Q8 & Q9 as well so I can adjust the boards to the same voltages.

Now I have wait until the replacement Q1 & Q2 come in.
But in the meantime I will put the pre-amplifier board back in and see what sort of voltages I get at Terminal 3 input when I play a record.

I have learned a lot from our discussion and have really appreciated the input from you guys. Was certainly better than just swapping all transistors out.

Would like to replace the stylus head as this one is expensive and hard to get.
Any audiophiles out there that can suggest something ?

Will update when I finish.

Regards
Hervey Bay Qld.

pd--
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Joined: 11/12/2020
Location: Australia
Posts: 122

Posted: 09:48am 19 Jan 2023

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Cartridge
https://www.radioparts.com.au/category/av-accessories/turntable-access/cartridges
Styli
https://www.radioparts.com.au/category/av-accessories/turntable-access/styli

Dinosaur

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Posted: 10:39am 19 Jan 2023

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Hi All

Thank you for the links pd--
I thought paying $50 for the stylus at our local music store was a ripoff.
Obviously I have not kept up.

Regards
Hervey Bay Qld.

Dinosaur

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Joined: 12/08/2011
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Posts: 311

Posted: 01:59am 31 Jan 2023

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Hi All

Deleted post.
Have to go back to the beginning and verify things.
Edited 2023-01-31 16:28 by Dinosaur

Regards
Hervey Bay Qld.

Dinosaur

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Posted: 09:20am 11 Mar 2023

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Hi All

As promised below is what eventually happened.

After replacing all transistors on Both Driver boards, I kept blowing fuses and
almost immediately the Power driver transistors.

Spent days hooked up measuring voltages to confirm nothing was out of range.
Yet the Collector to Emitter always ended up at zero ohms.

When replacing the transistors and turning the Volume down to 0, it would sit there for
twenty seconds, then blow a fuse.
If I tried to increase the volume in that time, it would immediately blow the fuse.
The Power transistors would not overheat, yet all indications were that the transistor
went first and then the fuse.

As soon as one side blew (say + 32), the biases on the (-32) would also immediately blow.

On the output side of each of the boards is a impedance matching transformer with taps
to connect to the speakers. I don't have an impedance bridge and as the winding resistance is very low, it is hard to tell if anything is wrong.

Reading on the Jukebox website, discovered that shorts in these windings is typically
what causes these kind of failures.

In the end I ripped out the pre-amplifier and the 2 channels of power amplifiers and replaced them with 2022 versions from Jaycar and it all works perfectly.
Perhaps not the same output power, but for indoor use, it is adequate.

Regards

Regards
Hervey Bay Qld.

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