FM 3 early Snr:

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30-Dec-2017 03:59 PM

PeterMisil

PeterMisil

Posts: 3

Hello everyone! My name is Peter, I live in the southern part of Sweden in Trelleborg. I'm new to Quad and this fantastic forum.

I have a 33 and 303 in great condition and working fine, I recently bought a FM3 with Snr:2870, a sticker on the underside indicates that it might have been serviced by Quad in 1978.

At first startup all the lights went on and station could be heard but with hum unless signal strength wasn't high. I have replaced the two PSU caps (1000uF/25V) and C113 (220uF/10V).

I have checked the voltage on the underside where it's supposed to be +-14V, I read 12V on my DVM! and after 30 to 45 seconds the tuner shuts off. It takes a couple of hours until you can power it on again and the same thing happends again. I have a pair of new zeners and a rectifier at hand, is this the right thing to replace them before anything else?

Fuses are ok, checked for bad solderings but can't find any and all out of ideas how to continue.

If anyone has a solution/idea about this please let me now.

Peter

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31-Dec-2017 10:38 AM

EJP

EJP

Posts: 1469

You're on the right track. Replace both Zener diodes in the PSU. The rectifier is probably OK but if the problem persists after changing the Zeners by all means replace it as next guess. Also check all the resistors in the PSU, and be prepared to change all the transistors in it as well.
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11-Mar-2018 02:26 AM

PeterMisil

PeterMisil

Posts: 3

Hello again, tnx for answer! :-)

The zeners and rectifier are replaced by now and everything is working so much better. My tuner is sensitive and I can tune in many stations only by using a simple t-shaped wire antenna.But unfortunately the hum is still there, much less than before but still way too much for listening. It's a very typical 50 Hz hum so my best guess is that theres still something wrong in the power supply section. I hope that someone has an idea what to do do/try next! :-)

Best Regards

Peter in Trelleborg, Sweden



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11-Mar-2018 02:31 AM

PeterMisil

PeterMisil

Posts: 3

I have uploaded a few pictures on Instagram about this project and vintage hifi, if you feel like it please follow ljunggrenpeter on Instagram.

//Peter


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11-Mar-2018 04:00 AM

EJP

EJP

Posts: 1469

I'm not a member of Instagram.

The key question about your hum is whether it persists if you remove the aerial? If it doesn't, you need a Jensen VRD-1FF as on this page: http://jensen-transformers.com/home-theater/video/.

If it does persist, you have an earth loop via one of your input cables.

EJP
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