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-   -   McMichael 381 Valve Radio project (https://www.vintage-radio.net/forum/showthread.php?t=171970)

Lef de art 12th Oct 2020 9:13 am

McMichael 381 Valve Radio project
 
5 Attachment(s)
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?

Cobaltblue 12th Oct 2020 9:26 am

Re: McMichael 381 Valve Radio project
 
Just a reminder you must not publish information available on this site see rule B8

Cheers

Mike T

HamishBoxer 12th Oct 2020 9:49 am

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.

Station X 12th Oct 2020 10:04 am

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

Gabe001 12th Oct 2020 12:56 pm

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.

Gabriel

telstar 12th Oct 2020 1:29 pm

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

Lef de art 12th Oct 2020 2:30 pm

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

Lef de art 26th Oct 2020 12:22 am

Re: McMichael 381 Valve Radio project
 
5 Attachment(s)
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 :)

Lef de art 26th Oct 2020 12:29 am

Re: McMichael 381 Valve Radio project
 
2 Attachment(s)
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.

ms660 26th Oct 2020 12:29 pm

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.

Lef de art 26th Oct 2020 12:40 pm

Re: McMichael 381 Valve Radio project
 
Quote:

Originally Posted by ms660 (Post 1303784)
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.

ms660 26th Oct 2020 12:48 pm

Re: McMichael 381 Valve Radio project
 
Quote:

Originally Posted by Lef de art (Post 1303790)
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.

Lef de art 26th Oct 2020 2:29 pm

Re: McMichael 381 Valve Radio project
 
1 Attachment(s)
The cable starts from here...

ms660 26th Oct 2020 2:59 pm

Re: McMichael 381 Valve Radio project
 
Quote:

Originally Posted by Lef de art (Post 1303820)
The cable starts from here...

S37?

Lawrence.

Lef de art 26th Oct 2020 3:38 pm

Re: McMichael 381 Valve Radio project
 
Quote:

Originally Posted by ms660 (Post 1303829)
Quote:

Originally Posted by Lef de art (Post 1303820)
The cable starts from here...

S37?

Lawrence.

Yes. S37

Lef de art 26th Oct 2020 3:46 pm

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 !

ms660 26th Oct 2020 3:54 pm

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.

Lef de art 26th Oct 2020 6:16 pm

Re: McMichael 381 Valve Radio project
 
1 Attachment(s)
Quote:

Originally Posted by ms660 (Post 1303843)
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 ....

ms660 26th Oct 2020 6:28 pm

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.

Lef de art 26th Oct 2020 6:36 pm

Re: McMichael 381 Valve Radio project
 
So it is better to leave it disconnected ?

ms660 26th Oct 2020 7:14 pm

Re: McMichael 381 Valve Radio project
 
Quote:

Originally Posted by Lef de art (Post 1303902)
So it is better to leave it disconnected ?

My apologies, I made an error in my previous post when I said to connect it to that lower tag on the pot in your last photo, if you were to connect it to there it would upset the AGC delay voltage so I would leave it disconnected and concentrate why there is poor reception when using the push buttons, the poor reception won't be anything to do with that wire being disconnected.

Lawrence.

Lef de art 26th Oct 2020 8:30 pm

Re: McMichael 381 Valve Radio project
 
Quote:

Originally Posted by ms660 (Post 1303917)
Quote:

Originally Posted by Lef de art (Post 1303902)
So it is better to leave it disconnected ?

My apologies, I made an error in my previous post when I said to connect it to that lower tag on the pot in your last photo, if you were to connect it to there it would upset the AGC delay voltage so I would leave it disconnected and concentrate why there is poor reception when using the push buttons, the poor reception won't be anything to do with that wire being disconnected.
Lawrence.

Thanks Lawrence.
I disconnected it. I think you are correct with guessing.
I believe to, the potentiometer it is replaced. It is shorter from the rest switches. Even the bakelite button stays difficult in his position (because the switch is shorter and the button screw can't fix the button correctly) ..

I don't know what else to check...for the presset buttons .
Maybe i will leave it as it is.

Lef de art 26th Oct 2020 11:39 pm

Re: McMichael 381 Valve Radio project
 
1 Attachment(s)
I have fix the preset buttons !!!!!

A million thanks to you Lawrence because you was the inspiration to fix it.

You mention that the potentiometer is replaced...

After that I thought that maybe the one who had opened it before me had confused the cables in the presset system.
So I put them upside down and immediately the preset buttons worked normally! Whenever again a big Thanks to Lawrence because from his own observation came the solution ...

ms660 27th Oct 2020 9:52 am

Re: McMichael 381 Valve Radio project
 
No problem.

Lawrence.

Lef de art 27th Oct 2020 5:24 pm

Re: McMichael 381 Valve Radio project
 
2 Attachment(s)
Good evening forum.
The original R15 volume control that appears in the manual was 500K and taped in 5K from earthy end. Can I get the same result with the new 3pin volume control by simply putting 5K resistance on the last leg?

Any idea?

ms660 27th Oct 2020 5:40 pm

Re: McMichael 381 Valve Radio project
 
No so far as I'm aware.

Lawrence

Lef de art 27th Oct 2020 6:49 pm

Re: McMichael 381 Valve Radio project
 
There is any way to create the 5K condition for the unplugged cable?

ms660 27th Oct 2020 7:00 pm

Re: McMichael 381 Valve Radio project
 
Not that I know of, except to find a pot that's similar to the original.

Lawrence.


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