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Old 25th May 2022, 3:21 pm   #1
agardiner
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Default McMichael 137, low volume

Hi,

So this set arrived after the new owner bought it from an antique shop supposedly restored.

It had a massive loud hum. First, the cabinet had been beautifully restored and polished, really amazing so no wonder it caught the eye! Dismantling revealed that it had been recapped, and 2 silicon rectifiers in place of the rectifier valve ( no series resistor). A switched socket was fitted in place of the PU input for Bluetooth use etc.

Although good caps had been used to replace the old waxies, the paper smoothing cap (8 / 8 / 4) had been replaced with very old electrolytics. One of these was open circuit.

I removed the silicon rectifiers and refitted a valve rectifier, and replaced all 3 power supply electrolytics. The set now works on all bands, but is not very loud. Hardly any hum so that is good, but just quiet.

My question therefore is does anyone have one of these sets or is familiar with them, and how loud should they go? Bluetooth is just as quiet via the input, so that suggests amp. All valves test strong emission.

Any advice gratefully received.

Thanks, Adrian.
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Old 25th May 2022, 3:43 pm   #2
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Default Re: McMichael 137, low volume

Have you checked if any resistors have gone high,and the voltages at the valve bases?
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Old 25th May 2022, 3:45 pm   #3
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Default Re: McMichael 137, low volume

Suggests o/put stage and audio before that, indeed check voltages and as mentioned reses for going high.
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Old 25th May 2022, 3:51 pm   #4
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Default Re: McMichael 137, low volume

Check the resistors - old ones have a tendency to go way-highh with age; this being worse with high-value ones. My Eddystone 840A had low AF gain when acquired; the first-audio-stage's anode-feed resistor had gone high to the extent that the anode was working at a much lower voltage than the screen, which did nothing for the poor little EAF42's ability to amplify....
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Old 25th May 2022, 4:38 pm   #5
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Default Re: McMichael 137, low volume

Check how he has wired the BT switch. These perform very well with room filling volume. J.
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Old 25th May 2022, 5:09 pm   #6
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Default Re: McMichael 137, low volume

Thanks all. Resistors were my next port of call. The previous restorer has replaced many, but maybe a wrong value or missed one.

Good to know that they perform well. Just wanted to check before deep diving.

Cheers,

Adrian.
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Old 25th May 2022, 9:54 pm   #7
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Default Re: McMichael 137, low volume

Hi Adrian, as I'm sure you will know, measuring resistance values in-circuit will often result in misleading values owing to parallel circuit paths. Obviously you can overcome that problem by first desoldering one leg of each component but it's a faff. You may wish to consider first measuring the valve anode, grid2 and cathode voltages and comparing them to those quoted in the service sheet. High or low voltages help point towards bad resistors and valves. Cheers, Jerry
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Old 26th May 2022, 11:37 am   #8
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Default Re: McMichael 137, low volume

Although it does sound like an issue with the AF stage, my experience with 3.3V BT modules is they are low volume and need an additional amplifier.
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Old 26th May 2022, 12:08 pm   #9
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Default Re: McMichael 137, low volume

Thanks Jerry, will do.

PJL, thanks for your suggestion, but the low volume is present when the bluetooth module is not even in circuit.
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Old 31st May 2022, 1:44 pm   #10
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Default Re: McMichael 137, low volume

Fixed. All voltages were fine and potential problem resistors had already been replaced.

After a lot of poking about I found that the fixed tone correction capacitor, had been incorrectly replaced by the previous restorer. Instead of 2n, they had fitted a 470n! Basically shorting out the output signal! Replaced for the correct value and the set now sounds splendid.
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Old 31st May 2022, 3:31 pm   #11
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Default Re: McMichael 137, low volume

Worst thing out, faults that are self made. We have all been there!
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Old 27th Jun 2022, 12:51 pm   #12
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Default Re: McMichael 137, low volume

Hi all,

More help needed please with this set. Proving very difficult indeed to sort everything out; not helped by the difficulty assembling/disassembling.

So; thought this one had been nailed - working well on MW and SW, and bluetooth working too. However, the new magic eye arrived last week, and after fitting I found that although it glowed, no operation was occuring. Checking at this stage the grid to the eye, showed just above 0V. According to the eye specs it should be 0V for fully open, and around -4V for closed. Powering the grid from an external supply proved the eye to be OK.

So, back delving into the set. All resistors and capacitors in the AGC / eye circuit have already been replaced, and double checking them proved the correct values. So I turned my attention to the alignment. Careful checking, showed the IF was off, at around 470KHz as opposed to the correct 465KHz. Careful alignment corrects that, resulting in a good improvement in level. However, it effectively kills the radio; no amount of RF alignment will restore operation. Mal-adjust the IF and the set returns to life.

Any ideas? I am wondering if the RF/Oscillator valve is defective. When in the fault condition, any gentle tapping around or on this valve results in that typicaly valve 'oscillation sound'.
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Old 27th Jun 2022, 1:00 pm   #13
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Default Re: McMichael 137, low volume

Hi Adrian, sounds like either the mixer/oscillator or, perhaps more likely, the IF amp has gone into spurious oscillation. Check the metallising earth is good on the IF amp valve AC/VP2. I seem to remember the top cap of one of these valves was connected via a long brass threaded screw inside a vertical pillar sticking up on the chassis. The end connections can get dirty so a gentle clean at each end can help. Don't overtighten the end nuts otherwise the screw shears off (I did it). Also the end of the rod is very close to the back cover and if the chassis isn't quite in far enough it will foul the back cover. Jerry
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Old 27th Jun 2022, 1:49 pm   #14
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Default Re: McMichael 137, low volume

Hi

I can't see a change in IF frequency of only 1% having a significant effect on operation, but I agree that if the IF strip was marginally stable at the "wrong" setting you might just have changed the phase of any sneak feedback path enough to have pushed it over the brink into oscillation.

These prewar sets would have had AGC designed for the sort of RF input voltage you get from a fairly long aerial, so the AGC/eye may not do much with a short length of wire. If you can inject plenty of RF from a sig gen you should be able to get things showing, assuming any instability has been dealt with.

I have a soft spot for this design, and have a 138 on the "to do" list. Plenty seem to have survived. By the time the 138 came out McMichael had abandoned the fancy flip-up tuning scale for a more sober design, and added presets to keep up with trends. Mine has the matching stand with cabriole legs, I can imagine it dancing round the room given the right choice of music !
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Old 27th Jun 2022, 2:27 pm   #15
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Default Re: McMichael 137, low volume

I liked the M-shaped station legends on the 138 tuning dial. Jerry
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Old 27th Jun 2022, 2:52 pm   #16
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Default Re: McMichael 137, low volume

Quote:
Originally Posted by cathoderay57 View Post
Hi Adrian, sounds like either the mixer/oscillator or, perhaps more likely, the IF amp has gone into spurious oscillation. Check the metallising earth is good on the IF amp valve AC/VP2.
Yes I would tend to agree with this as well. You can probably tell if the I.F is oscillating by measuring the control grid volts on the AC/VP2. Maybe that's why the IF was off-tuned in the first place.

For what it's worth, I'm working on a Philips radio at the moment....certainly not in the same league as your McMichael but with a similar problem. I.F spot-on at 470Khz but a tendency to 'take off' when tuned in to a station. Off tune the I.F slightly and it works. Replace the I.F amp valve......complete cure and I.F tunes correctly to 470Khz. OK the valves in the little Philips are all-glass types so no separate outer screen but the internal screen in the UF41 may have become disconnected.

With your AC/VP2 you can try the 'silver foil' trick. Just wrap some kitchen foil around the valve and a few turns of tinned copper wire to act as an earth and take the copper wire to a convenient earth point or even just hold it against the chassis. If it cures the instability then you can either leave the foil in place if you are not too bothered about originality or look for a replacement valve.
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Old 27th Jun 2022, 3:04 pm   #17
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Default Re: McMichael 137, low volume

Many thanks for the latest suggestions. Will try replacement Mixer and IF valves and report back.
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Old 4th Jul 2022, 7:41 pm   #18
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Default Re: McMichael 137, low volume

OK, final report on this set.

Sadly the new RF and IF valves did not solve the problem. Indeed, they appeared to make the problem worse; of course they didn't, but the issue is getting worse. Further testing suggests that one of the front end coils is responsible and that the oscillator is both unstable, and prone to cutting out.

However, the set has now been solved by a different take given the customers ultimate requirements. One of David Winter's FM modules has been installed and the set converted to FM. AM is no longer a concern. Pity in some respects but at least the set is alive and the customer going to get a lot of enjoyment out of it.

It still has the valve amplifier, excellent speaker and cabinet and original valve power supply complete with NOS rectifier valve.

Customer happy, but will never know exact cause of the problem. Likely to be front end coils.
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