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Old 12th Oct 2020, 9:13 am   #1
Lef de art
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Default McMichael 381 Valve Radio project

Hello forum.

I just received my new antique radio
It is in good general conditions and never was opened or repaired before.
The seller message me he only managed to tune into MW stations with a very simple AM arial loop.
To be honest im afraid to plug it before check the capacitors so i haven't tried it yet.

Yesterday i have found on line the schematic and i removed the speaker / chassis.

A question now.
In the schematic it is mentioned only the uf capacitors value. Not the voltage . How i will replaced them?
Aby idea?
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Old 12th Oct 2020, 9:26 am   #2
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Default Re: McMichael 381 Valve Radio project

Just a reminder you must not publish information available on this site see rule B8

Cheers

Mike T
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Old 12th Oct 2020, 9:49 am   #3
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Default Re: McMichael 381 Valve Radio project

The service sheet available top right does give voltages around the valves,so that should give you a guide.Quickly downloadable.
Possibly depending where in the circuit put another 100 Volts or even a bit more to achieve the working voltage of the caps.
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Old 12th Oct 2020, 10:04 am   #4
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Default Re: McMichael 381 Valve Radio project

Electrolytic capacitors generally have a voltage written on the case.

If you intend to fix this radio by replacing all the capacitors, rather than doing tests, then I suggest that you do not replace any capacitors having a value less than 0.001uF. These capacitors are generally in the RF part of the circuit and normally reliable. If you replace them you may upset the alignment of the set.

Lots of useful information here:-

https://www.vintage-radio.com/repair...ion/index.html
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Old 12th Oct 2020, 12:56 pm   #5
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Default Re: McMichael 381 Valve Radio project

Very broadly, 630v DC for the axial wax ones on the underside of the chassis. You may sometimes come across one or two rated at 1000v, often with values of or around 0.01uf.

For the smoothing electrolytics, which do not always need to be replaced, 500v should be suitable for most sets.

The voltage rating is always on the capacitor somewhere. For the wax capacitors it may be hidden under a layer of dirt which needs to be gently scraped off.

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Old 12th Oct 2020, 1:29 pm   #6
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Default Re: McMichael 381 Valve Radio project

Hi. I have also one of these radios in my collection. Bought it from an Oxfam charity shop years ago. Will have to have an attempt to restore it.
regards
Mike
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Old 12th Oct 2020, 2:30 pm   #7
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Default Re: McMichael 381 Valve Radio project

Thanks for the information guys. All very helpful.
I will upload more and clear pictures from the chassis internal view later.
I will try to not change all the capacitors. I will make some tests before any chance.
The electrolytic may be in good condition because the seller didn't mention any hum sound..

One cable from down inside it is detached. I'm not that good to understand where it goes
I know my English are not very good also so I will add pictures to make all this more understandable.
Have a nice day everyone
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Old 26th Oct 2020, 12:22 am   #8
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Default Re: McMichael 381 Valve Radio project

And the project is completed !

I have check and change 11 paper capacitors (was all leaky) and unfortunately it was necessary to change and the electrolytics but without to removed them....

The radio has unbelievable powerful sound with nice bass.
It receive strong signal in all bands long, medium , short.
I used it already several hours and it is extremely stable.

I haven't meet any problem with the repair process. After capacitors replacement it worked immediately.

The only 2 minors problems I have are...

1) The radio it has 6 station button presets . This 6 buttons are adjusted each through one screw .
I have found stations through this but the signal is extremely week and the external antenna dose not change nothing.
It gets much louder only when i touch the 6 button chassis.
This happens only when i choose the presset buttons function.
Otherwise the Radio works perfectly and extremely powerful.

My second problem is in the first front left switch.
This switch has 4 selections
-high fidelity
-Normal
-Bass
-Foreign

When i chose the foreign nothing changes... I believe this happens because one wire of the switch it was detached and i can't configure where his position was

I will upload pictures with the cable and with the Radio .
Unfortunately i can't read the schematic ...

Sorry for my English....i try my best
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Old 26th Oct 2020, 12:29 am   #9
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Default Re: McMichael 381 Valve Radio project

Here you can see the detached cable.
I haven't moved it . It is in the same position as i have found it.
If someone can understand where it goes it will be very helpful.


In the next day's i will upload a video so you can see the radio better.
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Old 26th Oct 2020, 12:29 pm   #10
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Default Re: McMichael 381 Valve Radio project

It could be that the antenna isn't being connected to the push button unit properly or the RF section of the push button unit isn't being properly connected to the control grid of the mixer (V1) when the wave change switch is set to the Selector position, check that the switch contacts for S5 and S17 are making good contact and that L28 is ok.

For the wire that's disconnected.....where in the circuit does the connected end connect to ?

All the above refs. are as per the Trader service sheet 399.

Lawrence.
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Old 26th Oct 2020, 12:40 pm   #11
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Default Re: McMichael 381 Valve Radio project

Quote:
Originally Posted by ms660 View Post
It could be that the antenna isn't being connected to the push button unit properly or the RF section of the push button unit isn't being properly connected to the control grid of the mixer (V1) when the wave change switch is set to the Selector position, check that the switch contacts for S5 and S17 are making good contact and that L28 is ok.

For the wire that's disconnected.....where in the circuit does the connected end connect to ?

All the above refs. are as per the Trader service sheet 399.

Lawrence.
Thanks Lawrence. I will check the V1 , S5, S17 and L28 .

To be honest i don't understand what you mean with the disconnected cable .
The cable starts from the "fidelity, Normal,Bass,Foreign" switch .
I believe it has to do with the "Foreign" position because is the only position where nothing changes when i choose it.
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Old 26th Oct 2020, 12:48 pm   #12
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Default Re: McMichael 381 Valve Radio project

Quote:
Originally Posted by Lef de art View Post
Thanks Lawrence. I will check the V1 , S5, S17 and L28 .

To be honest i don't understand what you mean with the disconnected cable .
The cable starts from the "fidelity, Normal,Bass,Foreign" switch .
I believe it has to do with the "Foreign" position because is the only position where nothing changes when i choose it.
V1 should be OK so no need to check that as reception is ok with normal tuning.

In the Foreign position, 6 switch contacts are in circuit so it would be good if we know which contact the connected end of the wire is connected to.

Lawrence.
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Old 26th Oct 2020, 2:29 pm   #13
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Default Re: McMichael 381 Valve Radio project

The cable starts from here...
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Old 26th Oct 2020, 2:59 pm   #14
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Default Re: McMichael 381 Valve Radio project

Quote:
Originally Posted by Lef de art View Post
The cable starts from here...
S37?

Lawrence.
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Old 26th Oct 2020, 3:38 pm   #15
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Default Re: McMichael 381 Valve Radio project

Quote:
Originally Posted by ms660 View Post
Quote:
Originally Posted by Lef de art View Post
The cable starts from here...
S37?

Lawrence.
Yes. S37
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Old 26th Oct 2020, 3:46 pm   #16
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Default Re: McMichael 381 Valve Radio project

For the other problem with the low signal in the 6 preset buttons i have check the Selector position, the switch contacts for S5 and S17 and the L28.
Plus i rechecked all the connections in new capacitors, resistors and cables. Can't find any problem.

All seems to be in position.
Maybe somehow it has to do with the disconnected cable ..
I can't think something else.
At least the rest works amazing !
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Old 26th Oct 2020, 3:54 pm   #17
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Default Re: McMichael 381 Valve Radio project

S37....According to the Trader sheet schematic the wiper of S37 should connect to the tap on the volume control (R15) and the contacts should be connected to R30 so far as I can make out.

On the switch diagram, the wiper connection for S37 is the one nearest the centre of that diagram.

Lawrence.
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Old 26th Oct 2020, 6:16 pm   #18
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Default Re: McMichael 381 Valve Radio project

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Originally Posted by ms660 View Post
S37....According to the Trader sheet schematic the wiper of S37 should connect to the tap on the volume control (R15) and the contacts should be connected to R30 so far as I can make out.

On the switch diagram, the wiper connection for S37 is the one nearest the centre of that diagram.

Lawrence.
Indeed looks like it was connected to R15 switch but even after connected it still there is no sound difference in the foreign position. And no change in the pressets button signal. All as it was before

So i disconnected it again ....
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Old 26th Oct 2020, 6:28 pm   #19
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Default Re: McMichael 381 Valve Radio project

Looking at the last photo posted it looks like the volume control (R15) might have been replaced with a standard pot (no tap) hence nowhere for the disconnected wire to connect to, I would have thought that if it was connected to the "ground" end of the pot (the one you've highlighted as a possible connection) then the Foreign setting would have an effect if the rest of the negative feedback components and connections are ok.

The disconnected wire shouldn't affect the reception when using the press buttons.

Lawrence.

Last edited by ms660; 26th Oct 2020 at 6:43 pm. Reason: Extra info.
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Old 26th Oct 2020, 6:36 pm   #20
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Default Re: McMichael 381 Valve Radio project

So it is better to leave it disconnected ?
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