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Advice on setting bias...

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monkeypuzzle View Drop Down
Young Croc
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    Posted: 01 August 2020 at 2:59pm
Hi folks, I'm after a bit of advice with setting the bias on my Turners. I've been slowly going through them and replacing the electro caps and trimmer pots as they're getting to 40+.

I've measured across the emitter resistor (0.15r) on most of them and I'm getting between 0 and 12mv giving anywhere between 0 and 80 ma quiescent. They start to get warm at this higher level. I've set them all to 5mv (or as close as a single turn pot will get!) giving 33.3ma as a safe starting point and they sound okay on a the limited testing I've been able to do.

Theres no information out there about these amps out there although i do now have a schematic and the output and driver transistors are Turner branded so I can't get any info on them. Is there a way with a scope to see when they're into the linear operating range?

Cheers.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 01 August 2020 at 7:15pm
What do they measure across the little bias resistors on each rail? I think the Turner circuit is probably similar to the other quasi-comp. designs of the same period,RSD's,HH 5500d,DC300a etc?

On the HH,bias voltage is 0.6v ,the DC300a is between 0.30 and 0.35v.

Edit, im guessing the Turner-branded outputs are just rebadged 2n3773 ,or MJ15003's?


Edited by jbl_man - 01 August 2020 at 7:22pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote monkeypuzzle Quote  Post ReplyReply Direct Link To This Post Posted: 01 August 2020 at 7:37pm
Cheers, I'm not quite sure which ones you mean, here's what I've drawn up for the 300 and 500 series taken from a diagram I found online and then edited to the later boards that all mine seem to have. 300 series have 50v rails and 1 pair of output devices per ch, the 500 series has 62v rails and four devices per ch
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 01 August 2020 at 9:29pm
Ok,had a problem reading that,had to blow it big (eyes not so good these days  Geek )

Measure the voltage across R30 and R32.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 01 August 2020 at 9:36pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote monkeypuzzle Quote  Post ReplyReply Direct Link To This Post Posted: 01 August 2020 at 11:16pm
Originally posted by jbl_man jbl_man wrote:

http://www.russelltechnologies.co.uk/images/turner.b302.sch.pdf

Yep, that’s where I got the original from but the board he’s drawn from is an earlier version and all the component numbers are wrong and some of the values are different on the second revision of the board which most (post ‘77) boards seem to be. I’ll check what I get on those resistors and load up a higher res version if I can. Thanks mate. I’ll post up some pics of the racks soon Smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 02 August 2020 at 7:27am
From the diagram they should be 68ohm,will be quite small,quarter or half watt ones..if you still cant find them,follow the wire from the base connection of the TO3 outputs back to the pcb.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 02 August 2020 at 9:00am
On his original board,it should be these two resistors, see what they read across them.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote monkeypuzzle Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 5:29pm

Second try at uploading the image and I've edited it a little. For some reason converting from pdf and uploading as jpg gives rubbish results... I'm going to be in the workshop tomorrow afternoon I hope so will get measurements then. In the meantime I have one on the bench at home thats giving weird readings...

So its got dc across the output, I've swapped out every diode as I had them spare on the shelf, I've popped out every transistor and tested off the board and they all seem correct... I then thought I'd go through the resistors on the board and all seemed fine until i get to R18. It should be 10k, its partner R22 reads 10k on the board, and so do R18 and R22 on the other boards I have here with other faults. If i take it off the board it reads 10k but when I test it on the board I get 4.9k with my meter leads one way around and 2.2m with them the other way... really weird... 

I'm sure its something really obvious but I just keep missing it.... any ideas?


Edited by monkeypuzzle - Yesterday at 6:08pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 5:47pm
How much DC is at the output? How many volts? Very carefully check the PCB tracks,there might be a hairline short somewhere, or a hairline fracture, particually check around the solder connections of all PCB transistors......also check continuity on the TO3 fly-leads. If all the transisitors/diodes are testing ok,it has to be a PCB track fault somewhere.

By the way,double check all the output T03's again....including testing them backwards....i had this issue with the DC300a,they all tested fine in the normal manner, but when i tested them the reverse way round, two had slight leakage.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote M4trix Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 6:05pm
I'm attempted ....I'm attempted to simulate this amp in Multisim ! Heck ! Tongue Angry 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote monkeypuzzle Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 6:13pm
What happened?

I tried to simulate using systemvision.com and didn't get anywhere. I don't have any other simulation software. As I pretty well drew up the plan for myself, be aware that not all the points where lines cross are joins....
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