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BRC/Thorn 3000/3500 PSU Module Repairs

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crustytv
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In addition to the missing collector volts in the feedback amp a new fault has developed now, the PSU is tripping as soon as the 58-65V rail starts up. I found that if I remove W615 ( isolating the feedback amp) the new fault is isolated the 58-65V rail comes up and it does not trip. Something in the feedback amp is not happy causing the crowbar to fire, this must be where all my problems lie, as I say tomorrow, wood for trees at the mo.

cctpsu.jpg

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Posted : 25/09/2016 12:19 am
crustytv
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Fixed the 58-65V rail trip, it was an electrolytic in the feed back amp C621. Replaced and now I have the 58-65V rail up and not tripping, whilst at it I also replaced C622 & C620 for good measure. The rail is too high though at 71V and I still have the missing collector volts and 40V on the base with 33V on the emitter for the feedback amp. So that's the two problems for today.

c621.jpg

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Posted : 25/09/2016 9:27 am
Red_to_Black
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A quick question Chris,

Is the 30V rail correct now ? I noticed in an earlier post you mentioned problems around R607, is this bit now sorted ?

"Also noted smoke emanating from R607. The voltage at the junction of R607/C608 is correct-ish ( 2V up) at 47V, the other end of R607 at the junction of C607 is only 7.1V and should be 12-15V. It seems to be getting very hot."

 
Posted : 25/09/2016 9:31 am
crustytv
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The 30V rail is sitting at 32V

R607/C608 junction is sitting at 48V at the other end of R607/C607 its sitting too low at 7.4V but I've not been able to establish why as last night spun off into other faults such as the tripping.

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Posted : 25/09/2016 9:41 am
Red_to_Black
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Ah ok, it might be an idea to start at the beginning, try account for your why the 12V is low first.

Excess ripple on the Collector side of the chopper transistor also influences the dynamic trip via C610/R610 and associated components, normally excessive ripple here means an overcurrent condition but something like a faulty component associated with this part of the PSU could also lead to false tripping.

 
Posted : 25/09/2016 10:00 am
crustytv
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Yes the low volts at R607/C607 is a mystery at present. To document where things are here is the voltage map

Transistor

Base

Emitter

Collector

Expected

VT602

0.8

0

0.03

B=.7/ C=.4

VT603

1.2

0.67

9.9

B=.8/E=.4/C=23.5

VT604

71.9

72.4

348

 

VT605

1.5

0.8

4.1

B=1/E=.7/C=11

VT606

2.3

1.5

2.5

B=1.6/E=1/C=6.5

VT608

40

33.4

0

B=30/E=31/C=10

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Posted : 25/09/2016 10:40 am
Red_to_Black
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Looking at that table, VT608 is a PNP transistor as the base is 1V -Ve wrt the emitter that transistor should be turned on, and there should be a voltage drop across the 10K collector resistor, either the base is O/C, or something on the monostable side is pulling it down to deck, there is not much on the monostable side to do this as there is either a diode or a couple of capacitors that isolate that line DC wise after R621.

 

OOPs sorry that was me looking at the expected column embarassed

Right, as the base is about 7V above the emitter the transistor being a PNP device will be cut off, we need to see why the base is +ve above the emitter then, back in a mo lol

Is C620/C621 ok?

 

I keep looking at T602 as that is common to the missing 12V line too, does that check out okay ?

 

Ps. I need more coffee grin_gif

 
Posted : 25/09/2016 11:09 am
Red_to_Black
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As this problem appears to be the classic "chicken and egg" feedback/driver problem, I think you might stand a better chance of at least narrowing down the problem to a stage by severing the feedback link entirely by disconnecting R621 at the junction with R637 and feeding in about 10V  from an adjustable external supply to see if you can vary the output from the chopper, by doing this you will be able to determine if the problem is with the monostable/driver side or the feedback error amplifier stage.

 
Posted : 25/09/2016 11:49 am
crustytv
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Red_to_Black said

Is C620/C621 ok?

 

I keep looking at T602 as that is common to the missing 12V line too, does that check out okay ?

 

Ps. I need more coffee grin_gif  

Hi R2B,

as mentioned in post #43 C620, C621 and C622 have been replaced. I've not checked T602 as these are supposed to be bullet proof, the 12V  is not missing its just low

I've OD'd so much on coffee my eyes look like this. eek_gif My heads in a mess I might be wrong but the two faults I'm faced with are the missing 5Volts from the junction of R607/C608. The second one being the feedback circuit is non functional due to the collector volts missing. This means the time constant of the mono-stable is not working which in turn mean the mark space is not working so the chopper is not switching correctly. But .... Me brain has gone to porridge........waa_gif

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Posted : 25/09/2016 11:51 am
Red_to_Black
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see above post ^^^ this might bring a touch of sanity back. laugh

I have had another coffee rolf_gif

 
Posted : 25/09/2016 11:53 am
Jayceebee
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Chris said
Yes the low volts at R607/C607 is a mystery at present.

  

 Do you mean R607/C608 Chris? It's that the monostable mark space ratio is very low, VT605 is conducting for most of the time and R607 will be passing a lot of current. The reason for tripping intermitently is that the 60v rail is perilously close to the 72v threshold of the crowbar circuit. 

Have you changed R620, this causes some strange symptoms if it goes high or low.

John.

John.

 
Posted : 25/09/2016 12:00 pm
crustytv
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Jayceebee said

 Do you mean R607/C608 Chris?

Have you changed R620, this causes some strange symptoms if it goes high or low.

John.  

Hi John,

yes and yes

This PSU is giving me a run for my money just to remind me not to take them for granted.

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Posted : 25/09/2016 12:08 pm
Red_to_Black
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Hi JC,

I was wondering about the M/S ratio being at the extremes to account for the low 12V line, another line of thought I had was if the chopper transistor was being biased on a bit too far wrt problems either with the driver transformer T602 or associated components in the driver stage then the error feedback signal would be effectively bypassed by the fault, agree btw that the output is perilously close to the tripping threshold.

Hence the suggestion of breaking the link with an external voltage to see if the monostable/driver stage was actually capable of regulating or if the problem was in the feedback amplifier section, this would save chasing round in circles. laugh

 
Posted : 25/09/2016 12:12 pm
Red_to_Black
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In view of the strange faults on the LTB module for the 1974 Ultra 6714 caused by the decoupling capacitor has C607 and C623 been cleared as being ok ?

 
Posted : 25/09/2016 12:42 pm
crustytv
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lifting R621 to isolate the feedback amp I fed in 10V from a PSU. All the rails come up but the jig trips due to I imagine the crowbar firing, no time to take readings.

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Posted : 25/09/2016 12:48 pm
crustytv
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Actually I think that's coincidence and a red herring, the PSU has gone into trip mode again!

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Posted : 25/09/2016 12:52 pm
Red_to_Black
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So it sounds like the problem is in the driver/monostable section then and it is not regulating correctly, I take it you tried varying the external voltage up say by a volt or 2 to see if that would pull the PSU output back ?

 
Posted : 25/09/2016 12:55 pm
crustytv
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Its when R621 is lifted that the PSU trips, with it connected it does not.

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Posted : 25/09/2016 1:00 pm
crustytv
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Red_to_Black said
I take it you tried varying the external voltage up say by a volt or 2 to see if that would pull the PSU output back ?  

yes I took it to 15V and still it tripped.

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Posted : 25/09/2016 1:04 pm
crustytv
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Red_to_Black said
In view of the strange faults on the LTB module for the 1974 Ultra 6714 caused by the decoupling capacitor has C607 and C623 been cleared as being ok ?  

Yes C623 was removed and tested OK.

C607 is a pain in the wotsit to remove but was checked in circuit with the Peak ESR meter, the capacitance was almost perfect & the ESR was low.

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Posted : 25/09/2016 1:16 pm
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