FM3 Power Supply Problem

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29-Apr-2017 02:27 PM

Sonny Boy

Sonny Boy

Posts: 9

Anyone help with a power supply problem or to be more precise lack of power please? Recently bought FM3 Tuner ser 38335, repalced two bulbs, worked fine for several hours then died. Smell of burning but no flashes/blue smoke etc!!! No signs of obvious component failure causing smell. Primary winding of transformer measures some 460R (okay?). Replaced D102, bridge rect and C113/114, smoothing caps. Problem still persisted, neg supply registering around 6 volts,  pos supply less (both should be +/-14V). Noticed TR102 (BC461) connected directly across the pos supply rail from bridge rect (e to bridge, c to supply, b to c of TR103, E5270) was very hot. Further investigation revealed old residual flux around b,c and e of TR102 had blackened (should have noticed earlier. Been a problem for a while?) and removal of transistor (sits directly on pcb) confirmed this was getting very very hot, pcb scorched underneath, not visibly obvious because of attached heat sink also sitting directly on pcb. Replaced TR102 but new transistor still gets very very hot. Secondary windings of transformer both measure about 6R at point on board where they feed the bridge rect so okay(?). Could the scorched board short circuit TR102 causing the problem or is it something else? TR105(BC142) in the same position but in the neg supply rail from the bridge rect seems okay. I am an enthusiastic amateur so electronics knowledge limited. Any help would be most welcome.
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30-Apr-2017 12:34 PM

EJP

EJP

Posts: 1342

I always replace both Zener diodes in these PSUs, and I occasionally have to replace one or both driver transistors as well. The transformer measurements seem OK. I don't think the overheated PCB will be short-circuiting, but you should certainly check it. You should also check the resistance of the -14V rail to earth at various points. This rail powers the RF and IF sections and you might have a rare failure in there which might be causing all this.
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01-May-2017 11:43 PM

Sonny Boy

Sonny Boy

Posts: 9

Thanks for your response, EJP. Have ordered some 1W 15V Zeners so will replace when they arrive. I measured resistance as suggested between -ve supply and earth and got 173R at several different points. Interestingly for the +ve supply I got 2R6 but not sure what this is telling me. I also measured the resistance between the b and e of the two power transistors (TR102/BC461, +ve supply,  my apparent problem transistor and TR105/BC142, -ve supply) and got the same result, namely 3K3. Again not sure what this is saying!
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02-May-2017 01:21 AM

EJP

EJP

Posts: 1342

The 173R is the lamp and its series resistor R36.

The 2R6 is a fault, either in the PSU itself or the MPX/audio section, which is the only thing the positive rail drives, starting at the 220R resistor R120.

The 3k3 comes from the resistor R109 or R127. You need to test transistors in diode mode, in this case with the positive terminal to the base. You should see about 600mV from base to collector and from base to emitter, and about 1.2V from collector to emitter. All the reverse cases should read open circuit unless there is a parallel resistance.

When changing the zeners you should also check R106/7 and R109/127.
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02-May-2017 06:11 PM

Sonny Boy

Sonny Boy

Posts: 9

Thanks EJP. From my transistor measurements I think you can appreciate my limited capabilities! The in-situ diode mode readings as follows:

TR105 (BC142):
b to c, 666: b to e, 671: c to e, >2V (pos lead to b,b and c)
b to c, open circ: b to e, 1.7V: c to e 710 (neg lead to b,b and c)

By comparison the equivalent readings for TR102 (BC461), my 'problem transistor' were:

b to c, 656: b to e, 657: c to e, >2V (neg lead to b,b and c)
b to c, open circ: b to e, 1.7V: c to e, 547 (pos lead to b.b and c)

So there seems to be some consistency there.

Some numbering on my schematic is dificult to read but I think the 220R resistor feeding the positive supply that you refer to is actually R128 and not R120. Anyway, measured in-situ this returns 218R so would appear to be okay.

I will change the Zeners and associated resistors when they arrive but I think this is starting to look not so good. Would it help if I temporarily removed the decoder chip and powered up the receiver? If everything stayed cool would it indicate the chip was defunct?
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03-May-2017 01:00 AM

EJP

EJP

Posts: 1342

Those transistors seem OK. The 220R resistor is isolating the MC1310 from the PSU so that won't be the site of the short. More likely the PSU itself. Also have a look on the back of the board for soldering shorts.
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06-May-2017 10:44 PM

Sonny Boy

Sonny Boy

Posts: 9

Thanks for your continued assistance and useful advice EJP. Cleaned a lot of old dried flux from the pcb and checked for 'whisker' shorts through a magnifying glass. All appears ok. Replaced both Zeners and their 3K3 resistors. On power up unfortunately.the same problem persisted. I tested the two transistors connected to the power transistors, namely TR103 (E5270) and TR104 (BC214) and got similar readings to the two power transistors detailed in an earlier reply so I am assuming they are ok. I measured the ac voltage from both secondaries at the point they attach to the pcb and got 17.5VAC for each. At the bridge rectifier I got +19.4VDC and -23.3VDC respectively (theoretical about 25VDC?). Is this an acceptable difference? At the red and black link wires from the two power transistors on the track side of the pcb I got +0.6VDC at the red wire and -7.5VDC at the black wire, similar to what I got at the very start with TR102 perhaps not surprisingly continuing to get very hot. A little bit stumped as to what to do next!
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07-May-2017 04:24 AM

EJP

EJP

Posts: 1342

Me too. I don't think the asymmetry between the windings is anything to worry about: the tuner should still work in that state. You need to trace the short circuit. Unsolder one end of the red lead, at the decoder end. Now the s/c is either on that loose end, which means upstream towards the PSU, or at the pin you just disconnected, which means downstream on the decoder PCB. Now whichever it is, measure the actual resistance at whichever of these points is identified, as accurately as you can, and measure it again at either the top of R128 if the s/c is in that direction, or the top of D100 if the other direction, again as accurately as you can. The lower reading is nearer the site of the s/c. This is difficult as you are measuring 2 ohms and below.

If all that gets you nowhere, (counsel of despair here), check/change the 220uF capacitor on the other side of the 220R resistor, C121. Check that you have C113 the right way around: C113 and C114 face opposite ways. In fact it would be useful to know the voltages at (a) top of C113, (b) base of Tr102, (c) junction of D100 and R103. If you get this far my money would be on R103 s/c or D100 the wrong way round, or maybe R127 s/c. Also remove C101 and see if that helps.
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09-May-2017 09:06 PM

Sonny Boy

Sonny Boy

Posts: 9

Thanks EJP. Not sure I understand your comments about disconnecting a pin. The measurements in my last post all related to in-situ ones. Anyway, I measured resistance at top of R128 and got 2R7, and exactly the same at the top of D100 namely 2R7. I replaced C121 with 470u (value change recommended by DADA) and checked polarity of C113 and C114 as you suggested. I replaced these initiailly and thankfully I did get them the correct way round! The three voltage measurements you suggested I make are as follows (all with respect to the neg lead of the voltmeter connected to ground):

a) top of C113= +21.8VDC
b) base of Tr102= +21.2VDC
c) junction of D100 and R103= -0.61VDC

I replaced the two resistors(R103/R106 associated with the Zeners when I changed them so double checked soldering around R103 and as far as I can tell with a magnifying glass it is okay (so no s/c?). I measured the resistance of R127 in-situ and got 3K20 (is measuring in-situ valid?). I have not yet removed C101 but will do so and let you know what happens. Thanks again



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10-May-2017 01:11 AM

EJP

EJP

Posts: 1342

Oops, somehow I left out the sentence 'Unsolder one end of the red lead, at the decoder end.' I fixed it in the post. Make sense now?
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11-May-2017 01:13 AM

Sonny Boy

Sonny Boy

Posts: 9

Yes it does make sense now, thanks EJP. I disconnected the red positive supply wire at the RF/IF board and measured resistance on the power supply board. 77R on the black neg supply, open circuit on the pos supply. Measuring resistance of the disconnected pos supply at the RF/IF board I got 2R7. Is this board the problem? Powered up tuner and hey presto -14.5V and +14.5V coming from power supply with no over heating power transistors. Panel lamp working okay now. I traced the pos power supply on the RF/IF board and had a look to see if any obvious problem- non that I could see. For some unknown reason I lifted slightly the black link wire connecting R100 to the TR11/12. Measured resistance of pos supply at RF/IF board and now got 77R3! Not sure if this is coincidence. Powered up tuner again and tentatively reconnected pos supply to RF/IF board and supply votages read okay. Tuner works!!! One problem remains however- the tuning lamp next to the tuning knob is lit all the time with +6V on centre contact. The other does not light at all and has +14V5 on the outside (coil) of the holder irespective of where the tuning dial is set. Apart from that the tuner does actually work now (I've heard it!) but not sure how I have apparently solved the initial problem, or to be more precise you have- thank you. Is there now a problem surrounding TR11/12 and the other two transistors TR10/13 that seem to be involved with the tuning lamps? Could this be the source of the original problem?
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11-May-2017 01:51 AM

EJP

EJP

Posts: 1342

You solved it, maybe only by resoldering.

It sounds as though one of the tuning lamps is open-circuit. You can test it with a multimeter in the 200 ohms position, should be a few dozen ohms. They are 12V LES types, readily available, although I generally replace them with yellow LEDs in series with 1k2. Watch the polarity, as it is different between the two lamps.

If it isn't that, you need to investigate the 4 transistors that control the tuning lamps. It's not unknown for one or more of these to fail. From memory they are all BC109s so for a few bucks you can just replace them all. Also check the associated resistors.
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11-May-2017 07:48 PM

Sonny Boy

Sonny Boy

Posts: 9

Thanks for your forebearance EJP. Measured resistance of tuning lamps. Bulbs out of holders about 37R. Returned to holders, LP1 about 38R, LP2 open circuit. Did your diode test on the four transistors (I'm learning!) TR10 to TR13. TR10 and TR12 connected to LP1 seemed okay, TR13 (LP2) appears okay but TR11 (LP2 again) gives s/c between the c and e so this appears to have failed. I have ordered some BC109 transistors and will replace TR11. Seeing as they are all grouped together on the pcb is it worthwhile replacing all four since collectors all connected together which in turn are connected to +14V supply?
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12-May-2017 03:14 AM

EJP

EJP

Posts: 1342

Sure, that's what I said, replace them all, and check the resistors.
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12-May-2017 01:28 PM

Sonny Boy

Sonny Boy

Posts: 9

Thanks EJP, my feeble brain didn't initially connect totally with your penultimate sentence until I just re-read it! I think I was just smug with myself for having identified one of the four that had failed.
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21-May-2017 09:25 PM

Sonny Boy

Sonny Boy

Posts: 9

Hi EJP, had delivery problems with transistors so update has been a while. Replaced the four transistors namely TR10 to 13 and checked resistors (they were okay). Happy to say the FM3 powered up as it should! Short circuit sorted, pos and neg rails now both reading +/-14.4V, power transistors nicely warm. Problem though with LP 2 (one of the two tuning lamps, the other is LP1) failing to light. LP1 fine. Odd since I changed all the lamps originally. The voltage at the two tuning lamps varied as the tuning dial was turned indicating something wrong with LP2. Changed it for another new one and bingo- unit working as it should. The lamp looks okay under a magnifying glass but measured resistance >1M0 so must be faulty. Thank you for your valued help without which the FM3 would be in HiFi Heaven or perhaps that should be Hell. Thanks again, well pleased.
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