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Old 17th Feb 2020, 1:46 pm   #41
SteveCG
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Default Re: Another Quad FM4 problem

Err, I may be wrong but I am not sure the FM4 has AFC as you can smoothly tune and watch the tuning indicators change accordingly - unless, that is, AFC is disabled in 'manual tune' operation.
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Old 17th Feb 2020, 2:00 pm   #42
NikPrice
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Default Re: Another Quad FM4 problem

That's correct, it's disabled when manual tuning is selected.
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Old 17th Feb 2020, 4:13 pm   #43
chartz2
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Default Re: Another Quad FM4 problem

I was wondering about the AFC cap. Does it need to be a tantalum?
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Old 17th Feb 2020, 9:47 pm   #44
NikPrice
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Default Re: Another Quad FM4 problem

It might be an idea to disable the AFC completely and observe what happens. Linking pins 6 & 10 of IC10 (the 4441 discriminator) looks like a way to do this (pins 7 & 10 on ISS 5 PCB's.
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Old 17th Feb 2020, 10:03 pm   #45
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Default Re: Another Quad FM4 problem

AFC cap?
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Old 17th Feb 2020, 10:26 pm   #46
chartz2
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Default Re: Another Quad FM4 problem

C19.
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Old 17th Feb 2020, 11:13 pm   #47
NikPrice
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Default Re: Another Quad FM4 problem

I thought C19 was part of the control circuit for the muting funtion when changing presets?

C19 is a 1u5 tantalum though.


From the service manual:

"Muting

During the execution of any selection commands, pin 14 of IC1 carries a short logic "high" pulse. The "high" is sustained by D25, C19, R17, R18 and applied to the muting pin on IC10."

Last edited by NikPrice; 17th Feb 2020 at 11:27 pm.
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Old 18th Feb 2020, 6:47 am   #48
chartz2
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Default Re: Another Quad FM4 problem

Oh yes. My mistake.
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Old 19th Feb 2020, 12:54 am   #49
NikPrice
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Default Re: Another Quad FM4 problem

Well I changed D24 out for a 1N825 today, the difference between REF L and REF H is now almost exactly 6.2V (a couple of millivolts over.

No difference to the warm up drift though.
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Old 19th Feb 2020, 5:17 pm   #50
NikPrice
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Default Re: Another Quad FM4 problem

This morning I swapped out the TL092, as this appears to control the AFC tuning circuit. The tuning bars now work correctly, and there is no longer a jump of .05MHz when assigning a memory slot.

The frequency drift is still there, although it seems much reduced on memory channels where the AFC is operational.

Going to put the receiver under test for a couple of days and see where to go next.

Nik.
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Old 19th Feb 2020, 7:44 pm   #51
chartz2
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Default Re: Another Quad FM4 problem

Thanks for the update Nik
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Old 21st Feb 2020, 7:56 pm   #52
NikPrice
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Default Re: Another Quad FM4 problem

Aaaaannnndddd......I've come to a grinding halt. Managed to break the slug in L6. I bet Quad don't stock these any more. Nothing I can do until I've managed to source another one. Blast.
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Old 21st Feb 2020, 11:35 pm   #53
chartz2
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Default Re: Another Quad FM4 problem

Drop Rob Flain at Quad an email.
You can use the contact form on their website.
I’m sure he can help.
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Old 22nd Feb 2020, 6:34 am   #54
NikPrice
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Default Re: Another Quad FM4 problem

I will. Fingers crossed. It’s just the slug I need.
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Old 26th Feb 2020, 6:02 pm   #55
NikPrice
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Default Re: Another Quad FM4 problem

Well I managed to get L6 back in tune again by using 2 slugs from a couple of variable inductors I got hold of.

Going to wait now until my D24 replacement board arrives from Oz.
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Old 26th Feb 2020, 9:30 pm   #56
chartz2
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Default Re: Another Quad FM4 problem

Excellent news, and well done Nik. Do you have any photos then?
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Old 26th Feb 2020, 11:12 pm   #57
NikPrice
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Default Re: Another Quad FM4 problem

Not as yet, but I'll get the camera out when I'm done.
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Old 1st Mar 2020, 9:16 pm   #58
NikPrice
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Default Re: Another Quad FM4 problem

I'm wondering if the drift is heat related. Even with the heatsink the 7812 gets very warm.

The unit I'm working on doesn't have the lid to the PSU enclosure, or the plastic cover for the reverse side. So, in effect the heatsink is acting like a heater for the whole receiver.

By this thinking the radio should stabilize when the temperature inside the radio matches that of the 7812.

The only way to test this theory is to move the regulator outboard and test it. It's shame that there is no cooler running equivalent to the 7812.
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Old 1st Mar 2020, 10:43 pm   #59
chartz2
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Default Re: Another Quad FM4 problem

There was a TO3 version many moons ago though.
I too left the cover out and fitted a heat sink to the 7812.
The drift stops when the tuner is hot!
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Old 2nd Mar 2020, 1:30 pm   #60
NikPrice
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Default Re: Another Quad FM4 problem

A very good point. I'm going to fabricate a foil coated cardboard cover and see the difference between enclosing the psu and regulator, and leaving it open.

Nik.
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