FM3, replace zener-based regulator with LDO alternative

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29-Jan-2020 11:24 AM

Grog

Grog

Posts: 7

I'm restoring a sickly FM3 (>20k late serial number)

it's obviously been 'serviced' before as one of the main smoothing caps is 680uf and the other is 1000uf!

I'm thinking to do either

A) replace like for like all the zenner, and transistors in the PSU/reg sections with available equivelents, 

or

B)

remove the current regulators and replace with a LDO adjustable regulator, hopefully for increased reliability and small benefit of lower noise in the main rails. 

I would certainly maintain the +/-14v for the rails.

it would mean removing D100/101, TR102/103/104/105, R103106/127/109

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29-Jan-2020 03:03 PM

camelopardalis

camelopardalis

Posts: 3

I noticed in my recently bought FM3 a 680uf and 1000uf as well (also late serial number). Perhaps not serviced (PCB doesn't look like that) so this might be on purpose!

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01-Feb-2020 12:15 PM

hummph

hummph

Posts: 38

Hello,

Sometimes Quad put 680uF and 1000uF in the 33 and FM3, so I think yours maybe original. Is there a problem with the psu? if so I would just replace any faulty components before doing any modifications.


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01-Feb-2020 11:12 PM

Grog

Grog

Posts: 7

There tuner was faulty when last used 10+years ago, so I'm  replacing all the electrolytic caps etc. 
Seems a bit of a waste to rebuild the original regulators, then to replace them with a regulator. 


But in terms of reliability, cant help but think that a Ldo regulator must be better than the tenner based discrete regulators.
When the tuner failed, the transformer got so hot some of the wax started to melt out. 

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02-Feb-2020 04:17 AM

EJP

EJP

Posts: 1712

You can do this fairly simply but there is little benefit, even to reliability. I replace the zeners as a matter of course but nothing else ever seems to go wrong, unless there has been a short-circuit, in which case the driver transistors can blow.

If you do it, don't bother with LDO or adjustability, just use LM7815/7915.

EJP

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02-Feb-2020 08:35 PM

Grog

Grog

Posts: 7

Thanks, I'll replace the zeners and leave well alone.

Tempted by the regulator for lower noise also, but in reality,  it's a radio and I expect makes no noticeable difference!

I replaced the electrolytic caps and it powered up ok with PSU voltages 13.8 both rails, so seems to be working ok.

I'll do the zeners to help future reliability, need to test it actually functions next.

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03-Feb-2020 02:28 AM

EJP

EJP

Posts: 1712

The theoretical best SNR for FM is -64dB in stereo and -80dB in mono. So the difference between say -90dB and -110dB in the regulator is futile.
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03-Feb-2020 02:09 PM

Grog

Grog

Posts: 7

good point!

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11-Feb-2020 02:23 PM

Grog

Grog

Posts: 7

so good and bad news!

I followed the Dada recommendations, refurbished the power supply and the audio output stage, i now have perfectly balanced +/- 14.2v power rails.

but.

I have the right side tuning lamp on full, the left flickers with tuning (no antenna connected) 

I haven't even tried hooking it up to an amplifier / antenna, but i assume it needs to go off for alignment?

anything i can check or do simply, i have a sig gen and scope etc, but no specific RF gear.

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12-Feb-2020 07:51 AM

EJP

EJP

Posts: 1712

First question is whether it sounds OK and has reasonable sensitivity. If so, you may just need to adjust the tuning lamps to balance when you think you have it centre-tuned to a station, by adjusting RV2.

You can't align this unit without RF FM generation equipment.

Dada Electronics Australia

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