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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 27th Jun 2017, 8:49 pm   #1
peterb3901
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Default Philips OM345

Having removed two faulty OM345 amplifier modules from a Marconi 2022 I was curious to find out why these two devices had failed. I discovered that it is possible to easily remove the brown resin coating covering the OM345 by soaking it in a solution of Sodium Hydroxide and Alcohol (Meths) for about half an hour (CAUTION- this mixture is very caustic) whereupon the resin coat can be completely removed by gentle brushing under running water.
Subsequent examination revealed that in both cases resistor R3 (please refer to OM345 data sheet) had failed thus depriving the amplifying semiconductor of its bias.
By soldering a 5.6K SMD resistor piggy back style across capacitor C1 (see data sheet) the bias was restored and the device resumed working in the test jig I have set up drawing a current of 12ma.
It may be that not all OM345 failures are due to R3 failure but it's worth a try and the really good news is that it is not necessary to remove all the resin coat to effect this repair because C1 is the "lump" that can be seen under the resin coat on the far right hand side, component side. The coating can simply be removed by careful scraping is away with a sharp blade to expose the SMD capacitor terminals.
I will continue to soak test the two repaired OM345s to verify their longevity and performance.
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Old 27th Jun 2017, 11:06 pm   #2
chriswood1900
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Default Re: Philips OM345

Well done that is a very useful tip for dealing with a very common problem.
Chris
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Old 28th Jun 2017, 12:11 am   #3
PJL
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Default Re: Philips OM345

Having a Marconi 2022 here which still works this is very useful information.

What is the view on the cause of the fault? Have OM345's always had a reliability problem or is this a fault that only occurs after many years of use? Is it a gradual drift in value or sudden failure of R3? Is it caused by faults in the equipment such as power rail issues or overheating? I see a thread here where 3 had failed in the same 2022 which seems to imply the failure was caused by the 2022 itself? Are there particular OM345's in the 2022 which are more prone to failure?

My 2022 is tired and sometimes reports an error 07 on power-up. Some time ago I ordered replacement electrolytics and switches with a plan to do a complete overhaul but is this wise given it currently works? From the circuit diagram it appears I can measure the OM345 input voltage on pin 1 and get a reasonable idea of the condition of R3? Anyone know the expected voltage with +12V supply?
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Old 28th Jun 2017, 8:16 am   #4
peterb3901
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Default Re: Philips OM345

In both cases R3 was open circuit. The resistors are formed from a thin metal film laid down on the ceramic substrate, R3 I measured (with a traveling microscope) at only 0.04mm wide the thinnest resistor by far on the board. Heating and cooling cycles might easily break this. Incidentally the OM345 schematic shown in the data sheet is not totally accurate, R1 although present is not connected at the transistor base end and there is an additional resistor(31R) inserted into the pin5 line.
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Old 28th Jun 2017, 10:44 pm   #5
Radio1950
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Default Re: Philips OM345

Thank you very much for the info.

What proportions for the mix please?
What quantities of each please?
How long to soak say an OM345?

And, how did you come by this method?
Are there any safety considerations?

There is also some more information over on the Yahoo Marconi Group, on both Marconi 20xx sig gens and OM345s.
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Old 28th Jun 2017, 11:17 pm   #6
G0HZU_JMR
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Default Re: Philips OM345

On the old thread linked to below I looked at a faulty OM345 back in Jan 2014 and found that it was possible to fix it by scraping away just a part of the coating and fit a SMD resistor across a cap.

https://www.vintage-radio.net/forum/...=100840&page=2

I also suggested an alternative 'quick and dirty' repair using an external 4K7 resistor if you need to do a temporary repair and this is described in the link.

Also I took some images of the inside on a camera and also a thermal camera and these are shown in the thread.
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Old 29th Jun 2017, 5:32 pm   #7
peterb3901
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Default Re: Philips OM345

To G0HZU I was completely unaware of your previous work on this topic but I will take a look. Your method of restoring these devices looks similar to the conclusions I reached ie scrape away the resin coating on C1 and fit a SMD resistor. Thanks for pointing this out.

To Radio1950 the mixture I used to remove the resin coating is very caustic, don't get it on your skin, eyes, clothing etc but it can however be safely disposed of down the drain. Take 10ml of water add 10ml of meths then dissolve half a teaspoon of sodium hydroxide in the mixture. It will warm up as the sodium hydroxide dissolves then drop in your OM345 and leave for half an hour or so. I first used this mixture many years ago when working as an industrial chemist for removing resin, paint and varnish from equipment. After soaking your 345 you might also like to drop it into a small quantity of vinegar before washing it to neutralise the sodium hydroxide.
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Old 1st Jul 2017, 11:19 pm   #8
Radio1950
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Default Re: Philips OM345

Thanks for that formulation information Peter.

I tried the method out on some old ceramic capacitors and only had partial success, but my components probably had a different coating.
Even with a three times concentration of the NaOH, I only had some success.

I am curious about the need for the alcohol; what part does it play in the process please?

Anyway, I have two Marconi 2022s and a 2018, so I probably will have to repair some OM345s etc at sometime.

Thanks again for the info in this thread and others on the subject.
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Old 2nd Jul 2017, 1:01 pm   #9
PJL
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Default Re: Philips OM345

Anyone know the normal pin 1 (bias) voltage with 12v supply? I would like to check the condition of mine when I give it it's 30 year service.

PS: Found what I need on page 5-84 of the service manual:
Pin 1 = 0.85 +/- 0.1V
Pin 5 = 6.5 +/- 0.5V

Last edited by PJL; 2nd Jul 2017 at 1:16 pm.
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