Some experiences in repairing quad FM4

[nickpot]

Quad FM4 Service manual does not tell detail spec. of all parts.

In return for getting valuable information regarding quad FM4 from here,
I would like to share my experiences in repairing FM4.

I have Old Quad fm4, bought it in 1983, serial: 012xxx
It operates in dual voltages 117/220V.

1. First failure (1992) : N1 suppressor failure, S/C somewhere in transformer

- replace N1 with X2 suppressor 0.047 uf/100 ohm/250 V
- Transformer oversea shipping cost is too much,
so rebuild transformer in local shop here.
- found bad TR12/TR13, replace with new trs (TR12: BC184, TR13:BC214C)

2. Unstable operation (2013):
Replace old Al. leaded CAPS and T9
C4 : 1K uf/25V (vishay/dale)
C5: 1K uf/25V (vishay/dale)
C6: 220 uf/50V (nichicon muse series)
C57: 470uf/50V
C36: 22uf/50V
T9: UA 7812 (12V/1.5A)

3. Second failure (2015) : battery leakage

Mine is old model, there was no basket for battery leakage.

This happening was serious disaster to me.

D21/D22/D24/C11/C12/C16, and parts of PCB were damaged by acid fluid.

Original D21/D22 is 1S920. but it was not available in market. So replace it
with 1S921. don't buy cheap 1S921. use good quality parts.

D24 is 1N823, 6.2V zener diode. this part affects on tuning voltage.
Bad quality 1N823 causes wrong setting in FM banding.
More accurate Zener diode such as 1N823a, 1N825/A(military spec.), is recommended.

In my case, 1N825 operates in exact 6.2V. Resulting FM band range was dead accurate, From 87.5 Mhz To 109Mhz. very happy with its accuracy.

4. Improvement (2016)

replace D5:BB204 (quality one)
C63 : 470nf/100V bipolar
C30, C37: 2.2uf/50V polar
T11 : LM334Z (quality one)

Resulting sound quality is wonderful.

But I found 0.05 Mhz frequency shifting at each preset.
Don't know the reason why. I would like to try alignment.

Parts of checking points in Sevice manual are not clear.
I can recognize checking point A, B, C.

Where is checking points D,E,F ?

I would appreciate if someone let me know those points.

Thanks for watching this posting

 

[ejp]

You need to look at the photograph on page 11. Quad were a bit naughty in reusing test point labels as schematic connection labels, especially in the case of C which is a test point at which to maximise the voltage but a Vcc node on the schematic. They should have used TP1, TP2, etc., and labelled them on the schematic and the PCB. I've been caught recently by exactly this issue even though last time I did it some years ago I was perfectly well aware of the photo.

 

[davidsrsb]

N1 often fails, but the transformer is unusual.
It looks like you have dirty mains supply. A VDR across the mains input would be helpful. Are you on 11V or 240/220V?

 

[Aniol1349]

Hi guys,



I'm repairing a FM4 (silver) and I replaced most of the components causing issues (IC1, LED drivers, IC6, D23 zener, D24 led and all caps) My unit is still stuck at L Ref voltage.



I can get it to tune and operate presets by replacing one LED drivier with BC183B (inserted backwards for pin compability, original ones are L series). In this scenario the unit works fine but not the LEDs - one LED is permanently stuck.


I'm not sure how is this affecting the V Refs but it is.


Any ideas would be much appriciated!


Thanks