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Old 1st Feb 2020, 5:29 pm   #1
NikPrice
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Default Another Quad FM4 problem

I have an old yellow and brown button FM4 in the 11xxx serial number range which I am trying to repair after a battery meltdown.

The problem I am stuck on is on power up the tune volts shoot up to the roof!

In about a second the voltage rises to 8.38V on REF L and 11.35V on REF H.

I've changed the CPU, all electrolytic caps, D24, T11 and from there I'm stumped.

Does anyone have any ideas or theories?

Thanks,
Nik.
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Old 2nd Feb 2020, 3:22 pm   #2
NikPrice
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Default Re: Another Quad FM4 problem

OK I've connected the battery neg terminal to the battery pos terminal via a 1R resistor as the specs for the original battery is 0.8R. This has brought REF L down to around 6V, (the specs are 2V for REF L and 8.2V for ref H - 6.2V difference across D24 -6.2V zener).


I'm not sure how to test the zener is in spec, and I'm considering removing the diode and measuring the voltage on the pads. Although I'm not sure what the results will tell me without correct references to go by.
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Old 3rd Feb 2020, 10:52 pm   #3
chartz2
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Default Re: Another Quad FM4 problem

Where did you source a new Quad TMS1000NLC ? I can't find any, anywhere. The zener can't be right anyway. If it says 6.2 V on it then it should be that, 6.2 V.
Testing a zener is easy, with a 9 V battery and a drop resistor (say 1kΩ).

Last edited by chartz2; 3rd Feb 2020 at 10:59 pm.
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Old 4th Feb 2020, 2:27 pm   #4
NikPrice
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Default Re: Another Quad FM4 problem

I managed to get the TMS1000NLC from ebay, the seller was in Germany.

I'm going to remove the zener and test it. I'm not sure that I understand the way it is supposed to work in this circuit. I thought that it should create a 6.2V difference from one side to the other. It's not doing this, and nor does D26, which is a 7.5V zener.

If REF H is supposed to be 8.2V then I think I need to find the source of REF H and follow this through to D24. Changing the op-amp chip IC5 made no difference so I'm guessing that REF H is out of spec before that.
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Old 4th Feb 2020, 4:13 pm   #5
vidjoman
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Default Re: Another Quad FM4 problem

Quote:
Originally Posted by NikPrice View Post
OK I've connected the battery neg terminal to the battery pos terminal via a 1R resistor.
Surely that make it almost a short circuit across the battery. Or have you described that wrongly?
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Old 4th Feb 2020, 4:26 pm   #6
NikPrice
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Default Re: Another Quad FM4 problem

Sorry, there's an M missing there. I've now replaced the link with a new battery.
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Old 4th Feb 2020, 6:45 pm   #7
John_BS
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Default Re: Another Quad FM4 problem

The derivation of VL is a bit convoluted.

Assuming the op-amp is working OK, you should have Vlow at about 1.9V at ambient temperature. It results from the addition of two currents flowing through R23 (22k): about 45uA from the LM334 and 41uA through R22. If Vlow is wrong, check those resistors and especially the LM334z. A quick n dirty check would be to just replace that current source with another 150k resistor and see if the voltages look about right. If that seems to work, replace the LM334.

John
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Old 4th Feb 2020, 7:39 pm   #8
NikPrice
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Default Re: Another Quad FM4 problem

Vlow is around 2.3V and I've just replaced the LM334, R22 and R23. I've removed and tested the replacement zener D24 and it appears to have blown.

When I power on the FM4 the display now starts briefly at around 90MHz and rises rapidly to 112MHz - as it does so the random station preset led that lit on power on goes dim and all the other leds come on at the same brightness level. At the same time I can see REF L rise from 2.3V all the way to 8.8V. REF H appears to be stable at 11.1V.
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Old 4th Feb 2020, 9:14 pm   #9
John_BS
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Default Re: Another Quad FM4 problem

There must be strange fault present to cause D24 to fail. It requires close to 100mA thro the zener to cause damage, and there's normally about 8mA flowing.

Vh is derived by adding the 6.2V to Vlow, so Vlow is the "base" for setting the voltages, not the other way around. I assume the 12V rail is correct?? It sounds like the TL094 can't output enough current to keep Vlow down to the correct voltage due to excess current flowing into the ouput from somewhere. You may have to measure DC voltages all around that area and post the results.

John
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Old 4th Feb 2020, 9:36 pm   #10
John_BS
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Default Re: Another Quad FM4 problem

PS you could try replacing R21 with 2k2 whilst trouble-shooting and see if that changes anything.
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Old 5th Feb 2020, 12:02 am   #11
NikPrice
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Default Re: Another Quad FM4 problem

OK. Here's where I'm at:

Changed R21 with 2k2. High side (12v) measures 11.9v, low side 8.2v.
R22 8.2v and 1.9v.
R23 1.9v and 0v.

I'm assuming this is a voltage divider network to give REF-L and REF-H.

IC5 Pins 1+2 1.77v
pin 3 1.9v

D22 annode 8.3v cathode 7.7v
D21 annode 6.3v cathode 7.7v

D24 pads (no diode in place) 7.5v and 9v

These voltages do seem to change a bit suggesting there is some instability in the circuit.

Now, when I power the FM4 up the display reads 86.x briefly and then rises rapidly to 105.x MHz.

Any specific suggestions on where to look around IC5?

Also, the green LED D23 starts of dim and grows dimmer as the frequency rises on the display.

Thanks,
Nik.

Last edited by NikPrice; 5th Feb 2020 at 12:04 am. Reason: Addition
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Old 5th Feb 2020, 2:47 pm   #12
John_BS
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Default Re: Another Quad FM4 problem

Quote:
Changed R21 with 2k2. High side (12v) measures 11.9v, low side 8.2v.
R22 8.2v and 1.9v.
R23 1.9v and 0v.
Those voltages are correct.

Quote:

IC5 Pins 1+2 1.77v
pin 3 1.9v
On IC5, pin 3 should be identical to 1+2. Do you have an oscilloscope?

John
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Old 5th Feb 2020, 3:52 pm   #13
NikPrice
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Default Re: Another Quad FM4 problem

Yes indeed.
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Old 5th Feb 2020, 4:47 pm   #14
John_BS
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Default Re: Another Quad FM4 problem

You've already changed the TL094?

Just check the voltage between 1+2 and 3 in case the voltages were on the move when you measured them both relative to ground.

Then have a look with a scope to make sure there's no ac/ripple on the +12V rail, and ditto any ac on the other end of R21. If IC5 is OK you should be able to put the 470 ohms back. Has C12 been changed?

John
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Old 5th Feb 2020, 6:01 pm   #15
NikPrice
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Default Re: Another Quad FM4 problem

I changed the TL094 for an equiv TL084, both seemed to give the same results. Checking with the scope there appears to be little or no ripple on T9, although I changed it out just to be on the safe side. The readings on IC5 pins 1+2 show 1.84V 160mV ripple and pin 3 show 2.00V and 240mV ripple.

How accurate this is I can't be sure as my scope is a cheap and cheerful Hantek DSO5102P.

Something else I have noticed is that the voltages, when they settle after power on, appear to not be exactly as they were when last powered on. Rail volts appear to be the same, but REF-L and REF-H differ slightly every time the unit is powered up.

C12 has been changed and I have swapped back in the 470R R21.
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Old 5th Feb 2020, 7:45 pm   #16
John_BS
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Default Re: Another Quad FM4 problem

NB Just check the voltage between 1+2 and 3, so -ve to 1+2 and +ve to 3.

John
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Old 5th Feb 2020, 8:46 pm   #17
NikPrice
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Default Re: Another Quad FM4 problem

Apologies for misreading that. The difference is 600mV.
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Old 6th Feb 2020, 3:43 pm   #18
chartz2
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Default Re: Another Quad FM4 problem

Quote:
Originally Posted by NikPrice View Post
I managed to get the TMS1000NLC from ebay, the seller was in Germany.

I'm going to remove the zener and test it. I'm not sure that I understand the way it is supposed to work in this circuit. I thought that it should create a 6.2V difference from one side to the other. It's not doing this, and nor does D26, which is a 7.5V zener.

If REF H is supposed to be 8.2V then I think I need to find the source of REF H and follow this through to D24. Changing the op-amp chip IC5 made no difference so I'm guessing that REF H is out of spec before that.
Yes but it’s the wrong type for the FM4. I’ve seen it too. The source of your problems? What happens if you pull it out?
Have you tested you 6.2 V zener then?

Last edited by chartz2; 6th Feb 2020 at 3:50 pm.
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Old 6th Feb 2020, 3:53 pm   #19
NikPrice
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Default Re: Another Quad FM4 problem

I did not realise there were several different versions of the TMS1000NLC.

I've removed it and if I power up the display still starts up around 90MHz, pauses for a second or two and then races up to around 112MHz.
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Old 6th Feb 2020, 7:07 pm   #20
chartz2
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Default Re: Another Quad FM4 problem

Right. Try that:

https://www.diyaudio.com/forums/soli...anual-mod.html
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