Quad FM3. - UK Vintage Radio Repair and Restoration Discussion Forum

kriss777 · 21st Oct 2021, 7:22 pm

I have an unusual fault with my quad fm3 tuner.
My quad tuner is working ok, after trying another decoder to try and rectify the problem I still have the same issue, I am unable to to get a steady 19khz reading on pin 10 of the decoder the reading moves all over the place from about 18.80khz to 19.20khz this reading is checked with the aerial removed.
Any ideas/pointers to where I should be looking.
Many thanks in anticipation
Chris

david winter · 22nd Oct 2021, 12:21 pm

It depends on what version of FM3 you have.
It may be the CA3089 IF amplifier just before the multiplex.
I just had a silent FM3 and thought it was the dual gate transistors, until I discovered that the CA3089 IC still delivered some audio on a test pin, but nothing at the output.
This chip is widely available and cheap, so not hard to replace.
Also, I found that some of the ceramic capacitors in the multiplex (before the decoder chip) drifted in value. I'll try to picture them for you just in case.

frsimen · 22nd Oct 2021, 4:38 pm

Hello Chris,

The set up instructions suggest that you remove the audio feed to the decoder when trying to set up the free running frequency.

If a noise signal is fed in, the PLL will attempt to lock to the noise, which will result in the frequency jumping around as you are finding. Either applying a short across R102 or disconnecting one end of C104 should allow you to set the frequency more easily.

What you are observing is likely to be normal behaviour for the MC1310 if noise is present.

Paula

21st Oct 2021, 7:22 pm	#1
kriss777 Diode Join Date: Dec 2010 Location: Calne, Wiltshire, UK. Posts: 3	Quad FM3. I have an unusual fault with my quad fm3 tuner. My quad tuner is working ok, after trying another decoder to try and rectify the problem I still have the same issue, I am unable to to get a steady 19khz reading on pin 10 of the decoder the reading moves all over the place from about 18.80khz to 19.20khz this reading is checked with the aerial removed. Any ideas/pointers to where I should be looking. Many thanks in anticipation Chris

22nd Oct 2021, 12:21 pm	#2
david winter Hexode Join Date: May 2008 Location: Paris, France Posts: 336	Re: Quad FM3. It depends on what version of FM3 you have. It may be the CA3089 IF amplifier just before the multiplex. I just had a silent FM3 and thought it was the dual gate transistors, until I discovered that the CA3089 IC still delivered some audio on a test pin, but nothing at the output. This chip is widely available and cheap, so not hard to replace. Also, I found that some of the ceramic capacitors in the multiplex (before the decoder chip) drifted in value. I'll try to picture them for you just in case. Last edited by david winter; 22nd Oct 2021 at 12:42 pm.

22nd Oct 2021, 4:38 pm	#3
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: Quad FM3. Hello Chris, The set up instructions suggest that you remove the audio feed to the decoder when trying to set up the free running frequency. If a noise signal is fed in, the PLL will attempt to lock to the noise, which will result in the frequency jumping around as you are finding. Either applying a short across R102 or disconnecting one end of C104 should allow you to set the frequency more easily. What you are observing is likely to be normal behaviour for the MC1310 if noise is present. Paula