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BG8000 fuse issues

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ALF
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ALF posted on Mon, May 1 2017 7:02 AM

greetings all,

a new arrival, a BG8000, which had a partial re-cap job done by the previous owner.

I noticed C24 was put in the wrong way as its positive should connect to the F1 ?!

anyhow, I corrected that, completed the re-cap job incl C27 - see pictures - and now F1 is constantly blowing

even with my DBT ?

I checked D16 to D19, all measured ok.

did I overlook something ? any ideas or suggestions are most appreciated :-))

cheers

ALF


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Lonnie
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Hi ALF,

I think you need to explore your measurements. Repeat the measurements you took yesterday which gave +ve voltages are they still +14.3 & +23.6V?

Use a different ground position?

BTW did the fuse blow or not?

If it didn't I'd add back in R75 but leave P6 disconnected and see if it blows the fuse.

 

Regards

Lonnie

Dillen
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Has the transformer been connected one pin off to the side? Difficult to see in the photo.

And get that dreadful foam away from electronics - and CMOS in particular.
You are doing more damage than repairs. Get an antistatic mat.

Martin

ALF
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ALF replied on Thu, May 4 2017 10:00 AM

Hi Lonnie,

yes, I used a different ground position which exactly bugs me, as it delivers different readings !

that is why I atteached a picture to show which ground position I chose today  - perhaps you would suggest a better choice ?

the fuse did not blow, so I re-installed R75, the fuse still ok but then p6/1 shows about 12V ! Which is not what we want to see.

with different ground patches on the main board I can find -14.7V at D26 but it confuses me - which ground I should measure against ??

there is something else I found odd because it does not look how the original transformer box was put back in its space ?

you can see the typical green/white earth-wire coming out of the box and gets connected to the chassis - odd because that is not the case

in my other BG8000  plus space for the box is very tight and this wire has almost nowhere to go - will upload a picture shortly.

ALF

ALF
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ALF replied on Thu, May 4 2017 10:04 AM

Quite right Martin,

something I should have done a long time ago - the mat is now on top of my to do list !

no, the transformer is correctly connected but as I pointed in my previous post , I find that green earth wire hanging out of

the transformer box rather odd - it surely can't be original ?? Space to re-insert that box is so tight !

ALF

Lonnie
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Lonnie replied on Thu, May 4 2017 10:31 AM

Hi ALF,

The green wire is grounded to the back right hand corner of the bottom chassis. There is a stud attached to the chassis and a nut screws the green earth wire on this stud.

Regards

Lonnie

ALF
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ALF replied on Thu, May 4 2017 10:43 AM

exactly, that is how i found it but I am pretty sure my other BG8000 does not have that set-up - perhaps something missing

or is it different from country to country ?

I pass on the pics as it seems correct but still feels odd as the space is so tight !

coming back to the original issue :

can you point out a more "suitable" ground patch so measurements can be compared ?

and with the double voltage at P6/1 (just over 12V) after re-installing R75 is this pointing to 0IC1 as a suspect ?

regards

ALF

Lonnie
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Lonnie replied on Thu, May 4 2017 10:59 AM

Hi ALF,

I'd recommend you cool your jets until you get your anti-static mat. A replacement CPU can be hard to track down.

Regards

Lonnie

ALF
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ALF replied on Fri, May 5 2017 3:24 AM

Yes, I know - guilty as charged ! 

The mat is now in place....finally.....and the jets getting ready to be warmed up again.

first step is putting the correct fuse in place and watch what happens....

ALF

Lonnie
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Hi ALF,

Excellent, you must have a good electronics store near by..

You indicated that you bought the TT for the transformer, is it a 220V 50Hz or a 240V 50Hz model?

I'd use the C27 as the ground point , you could clip the com lead onto the light blue jumper lead.

Can you re-do your measurements?

Regards

Lonnie

ALF
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ALF replied on Fri, May 5 2017 6:05 AM

hi Lonnie,

working on the new mat now....!!

the transformer is for 240V 50Hz

new groundpoint in use delivered the following:

R75 in

C27+  23.4V

C24+ 13.4V

C28+  -0.6V

D26 anode -14.3V

P6/1 unsteady jumping between 0.8V / 6.4V

 

R75 out

C27+ 23.3V

C24+ 13.15V

C28+ -0.6V

D26 anode -14.5V

P6/1 6.5V steady

is that leading to 0IC1 as a suspect ??

ALF

Lonnie
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Hi ALF,

The +15V measurement at C28 should be independent of the 5V power supply so I think you need to work out why you are only getting -0.6V. 

A couple of basic things to check:

Make sure you measure the positive end of the cap.

Make sure connector P2 is properly inserted. Confirm that there is no shorts between the pins of P2 since this has been re-flowed.

Confirm you have ~zero resistance between the following: the poitive end of C27 and the emiter of 0TR1, the junction between R77 and R78 and the base of 0TR1, the collector of 0TR1 and the positive end of C28.

 

Regards

Lonnie

ALF
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ALF replied on Fri, May 5 2017 1:28 PM

hi Lonnie,

the resistance between

C27+ and  E of 0TR1  is about 0.5 Ohm

junction R77/78 and  B of 0TR1 is about 0.5 Ohm

C28+  and  C of 0TR1  is about 0.4 Ohm

P2 is definitely connected properly, there are NO ! shorts between its pins.

are these measurements of concern to you ?

BTW it wasn't 0IC1 but 0TR1, the TIP-32 chip as a suspect ?!

putting everything back together with the proper fuse delivered exactly what I feared: blown fuse, meaning the processor protection works

I'd rather see around 5V at 1P6/1

ALF

Lonnie
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Lonnie replied on Fri, May 5 2017 11:41 PM

Hi ALF,

It's not surprising that the fuse still blows, you have made any changes to address the issue.

You have two issues that need to be resolved, the missing 15V and the blowing fuse.

Lets stick with the 15V first.

Can you measure the voltage at the emitter of TR18 / R79 and the voltage at the junction of R77 & R78?

Regards

Lonnie

ALF
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ALF replied on Sat, May 6 2017 4:21 AM

hi Lonnie,

no, I hadn't changed anything - that is why it came as no surprise

junction E of TR18/R79 showed close to zero V

junction R77/78 showed about 23.6V steady

I like to understand the role of 0TR1 (voltage switching?) and TR18

cheers

ALF

Lonnie
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Hi ALF,

I think I may have been leading you up the proverbial garden path, the +15V coming out of 0TR1 is controlled by the POWER ON/OFF section of the circuit. This is controlled by the CPU and given that you don't have 5V at the CPU to turn the ON/OFF section on you also have no +15V.

0TR1 is a voltage regulator that provides the stable +15V. TR18 helps control the base of 0TR1. Google discrete voltage regulators for plenty of data and descriptions.

Back to the issue with 5V can you test the voltage at c,b & e of TR17 against ground. You will need to put a fuse back in and pull the P6 connector.

Regards

Lonnie

 

 

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