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Old 21st Dec 2021, 9:24 am   #1
mikes08
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Default Quad FM3 issues

I have been reading through the forum recently as i have a Spare FM3 that i am trying to resurrect - i knew it was in the loft somewhere - thank you Christmas decorations.
I have an output in mono I think but no stereo yet!
When i delved inside all lamps were blown apart from left tuning lamp so have replaced all. On powering up i can get the stereo light to illuminate tuning in and off stations. Right tuning lamp remains resolutely off but left lamp does dim as you move through stations. More investigations needed but i noted that voltage of left lamp is around 6v and that of right is only three ish 3.2 i seem to recall. Tuning beacon is also around 6v voltage and at lamp 4 - illumination for the facia is 14v.
I am reading up around the decoder and Tr101 (mono route) issues in a couple of threads on here. No obvious signs of burning or damaged components inside.
Odd thing is having replaced the bulbs when plugged into my 33 i got no output but hum!
I do have a fully working FM3 to compare against. Serial number is 10705 but does appear to have the later decoder. Is it worth replacing this?
I have really dragged this out as a project to do - could send back to Quad but I do like to solve a puzzle but no electronics expert.
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Old 21st Dec 2021, 6:38 pm   #2
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Default RE: Quad FM3 issues.

Progress....all working now in stereo but right tuning lamp refuses to light up...when I checked 3.2v...New bulb fitted...I presume it should be around 6/7v appears to be driven by TR 11?...any ideas please?

Moving forwards, the tuner is now working - the only issue i have is that the right tuning lamp will not illuminate - new bulb was fitted and when i checked the voltage was only 3.2v to that lamp as opposed to 6+ volts to the left side so i am wondering about the two feed transistors TR's 11 & 13 - BC109B's - a Darlington series which supply the lamp.
I can replace these but is the anything else i should be looking at as a potential fault.
Both my 303's were upgraded with Dada kits and looking to to do the same with my spare 33 also.
I am using the 33/303/FM3 into Spendor BC1's. I also have have a pair of LS3/5a's built from a Falcon electronics kit and Pink Triangle turntable - i worked for them many years back after college when the firm started as Arthur was a school friend which is where my interest in Hi Fi started but i am still very much stuck in the '70's
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Old 22nd Dec 2021, 7:50 pm   #3
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Default Re: Quad FM3 issues

Have a look on the DADA site as they have most cct diagrams . The early tuners ( sn 5885 ) have a chip that is no longer around and use the 2N5306 NPN Darlington Transistor for the lamps with a +/- 14v HT
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Old 22nd Dec 2021, 8:53 pm   #4
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Default Re: Quad FM3 issues

Firstly, check that the +14V and -14V supplies are both correct.

If they are, try measuring the voltages on the collectors of TR7 and TR9 as you tune through a station. The voltage on one should drop and on other the voltage should rise as you tune across the station. At some point, the voltages should both be the same.

If that's not what is happening, check the voltage across C29. That should also change as you tune across the signal, it looks like it should swing from a small positive voltage to a small negative voltage.

You could compare the results with those from one of your working FM3s.

Paula
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Old 23rd Dec 2021, 8:55 am   #5
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Default Re: Quad FM3 issues.

Thanks both and Paula - i noticed you name in some other threads on the FM3 when searching. I will delve deeper and report back. Is it worth also replacing the smoothing capacitors on the power supply whilst i am in there as will be original?

I have the original manual from new in the box which has the full circuit diagram in the back though am using the Dada site for parts etc.
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Old 23rd Dec 2021, 1:40 pm   #6
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Default Re: Quad FM3 issues

Changing the power supply capacitors is a decision you will need to make yourself.

Unless they are faulty, showing signs of leaking electrolyte or bulging, replacement is unlikely to make a difference to the performance of the tuner. I don't subscribe to the change them on sight philosophy in this type of equipment, but others will disagree.

One capacitor that I would consider changing, because of the way that it is being used in this circuit, is C29.

Paula
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Old 24th Dec 2021, 8:58 am   #7
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Default Re: Quad FM3 issues

Thanks Paula
I will take that on board and check the essentials first to get the lamp working properly. Will give me something to do over the bank holiday. Visibly they look ok at present so will leave those for now. Thanks for the pointers.
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Old 25th Dec 2021, 3:15 am   #8
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Default Re: Quad FM3 issues

I'm reading this thread with interest.
Paula, perhaps you could expand on your comment and tell us exactly why the way in which C29 is being used suggests it is due for replacement?

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Old 25th Dec 2021, 1:21 pm   #9
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Default Re: Quad FM3 issues

There are two versions of the circuit which use BC109s to drive the tuning lamps. They use different component reference numbers for the capacitors. The capacitor I was referring to is the one connected between the base of TR7 and the chassis. This is either C29 or C36 and is a 68µF 35V component and is drawn as a polarised type with the negative to chassis.

The circuit driving the indication lamps comprises a differential amplifier, formed from TR7 and TR9, with each transistor feeding a Darlington pair (TR10/12 and TR11/13) to drive the lamps. Such an arrangement is symmetrical, so the voltage on the base of TR7 will be the same as that on the base of TR9, when a station is correctly tuned.

The base of TR9 is connected to chassis, so when correctly tuned, TR7’s base will also be at chassis potential, resulting in no potential difference across the 68uF capacitor. The differential amplifier itself is controlled by TR8. When no station is tuned in, the noise detection circuit (L4 and the transistors and diode to the right of it on the circuit diagram) will cause TR8 to conduct. When this happens, TR7 and TR9 conduct sufficiently that they cut off the Darlington stages, extinguishing the tuning indicator lamps. In this state, the voltage across the 68µF capacitor is several volts negative, making it reverse biased.

When tuning across a station, the noise circuit will not operate and the voltage across the 68µF capacitor will be somewhere between a small negative voltage and a small positive voltage.

The leakage current in a reverse biased polarised capacitor will be much greater than the normal leakage current. In this circuit the relatively high value resistors feeding the capacitor will prevent damage, but over time the capacitor may develop a higher leakage than is ideal. The type fitted may be a tantalum type, I don’t have an FM3, so I can’t say for sure. These have much the same behaviour as aluminium capacitors. Over a long period of time, the leakage of the capacitor may become high enough to stop the tuning indication working as intended, hence my comment.

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Old 29th Dec 2021, 9:18 am   #10
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Default Re: Quad FM3 issues

Hi
I read C29 as 0.68µF 35v according to the circuit diagram? I am going to remove from the board and check it along with the BC108/9's - on my model listed as 109's but actually 108's.
With regards soldering i have read about heat and old PCB's. I have a temp controlled iron so can adjust but presume as hot as needed to melt the solder but not too hot though you could argue that with more heat you don't have to linger too long.....
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Old 29th Dec 2021, 11:29 pm   #11
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Default Re: Quad FM3 issues

I think it will be better with a hotter iron so a quicker action for soldering can be made. The longer the iron is held in place, the greater the risk of the pad and track lifting as you probably well know.
Another thing to be careful of is the use of de-soldering tools. The common spring loaded plunger affair can jerk and cause a pad to lift off. Solder wick maybe better and with a swift action with a very hot iron, the solder can be removed without the pad.
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Old 30th Dec 2021, 9:32 am   #12
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Default Re: Quad FM3 issues

Thanks Andrew
Been in and out successfully and removed what i needed for now. Somewhere between hot and very hot but all ok - noted comment on suction de-solderer.
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Old 2nd Jan 2022, 10:18 am   #13
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Default Re: Quad FM3 issues

Just waiting for spares to arrive - checked the capacitor C29 and reading ok - also the resistor i thought might be a problem. Will check the transistors next when i get a delivery next week.
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Old 13th Jan 2022, 10:45 am   #14
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Default Re: Quad FM3 issues

Waiting for new cap to arrive but some odd things now - all lamps have gone out though have left original capacitor in line. Also checking voltages i get 24v + & - at the rectifier but when checking the voltages where i should be getting 14v +- I am only getting 1.3v on positive rail and 5.7v on negative? Puzzled now.
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Old 13th Jan 2022, 10:55 am   #15
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Default Re: Quad FM3 issues

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Old 13th Jan 2022, 6:16 pm   #16
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Default Re: Quad FM3 issues

That doesn't sound good. The best advice is to go over all your previous work carefully. Look for components wired incorrectly, solder splashes/bridged tracks, lifted tracks or anything else which may not be right. This highlights the danger of removing a lot of components at once. It makes finding a problem later on much more difficult.

If you disconnect the tuner from the power supply board, does the output voltage then return to what it should be? The supply should be able to work without an external load. If it does, the problem is on the tuner board. If it doesn't, there is a problem in the power supply, probably caused by a fault in the tuner. Don't connect the power supply back to the tuner until you have identified and rectified the fault on the tuner.

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Old 14th Jan 2022, 12:06 am   #17
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Default Re: Quad FM3 issues

Sorry to be a bit late but that was a superb answer to my question - thanks Paula

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Old 14th Jan 2022, 8:57 am   #18
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Default Re: Quad FM3 issues

Thanks Paula
I will double check and remove the connections to the RF board. All i have changed so far is the 0.68mf cap C29 and the two pairs of Darlington transistors that control the tuning lights.
I will remove the bridging wires and see if the issue is located on the power supply board or not and report back.
I can compare values against my working FM3.
I did go over the board with a magnifying glass to check. Did clean off old bits of flux with fibre glass brush and also some old off bits of solder left from when it was made - surprised that these were still there! Tracks all ok.
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Old 14th Jan 2022, 1:20 pm   #19
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Default Re: Quad FM3 issues

hi Paula
I did as suggested and uncoupled the tuner section and now get -14 volts on the negative side and +3v on the positive rail. I am suspecting TR 102 (BC461) one of the power supply transistors as i am getting a dead short across the base and collector using the diode test on my meter.
I will measure the voltages here also but wonder if this has blown?
Need to check it against readings of my working one.
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Old 14th Jan 2022, 2:36 pm   #20
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Default Re: Quad FM3 issues

- though that would also indicate something is amiss in the tuner section still....
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