McMichael M201R crackling

[JoshWard]

Hi all,
I have just got this set up and running again however I am having problems with a persistent crackle. I am working from the Trader sheet (1340)

The symptoms are:
Crackle present on all wavebands and gram. It is quieter when VHF or gram push buttons are selected.
The crackle is still present when no push buttons are selected
It can be altered with the volume control
It is still present with the ECC85, ECH81 and EF85 removed
It goes away when the EABC80 is removed

Work done so far:
I have checked with the lights off to see if there is any sparking coming from anywhere but I haven't spotted anything.
The on/off switch has been bypassed (it was faulty) so it's not that causing the problem
Working around the EABC80 I have replaced C43, C51, C52, C53, R19, R20, R21, R22, R26, R27
I have tried substituting the remaining valves (EABC80, EL84 and EZ80)
I have checked the valve holders for broken pins/dodgy contacts

Does anybody have any other suggestions? I wouldn't be surprised if it is the wavechange switch causing problems- it's a pretty flimsy paxolin affair- but I'm not sure which terminals I should try hardwiring to test that theory.

[cathoderay57]

Hi Josh, a dodgy contact on the switch bank is the most likely offender. referring to Trader Sheet #1340, check also the volume control for continuity. If that checks out OK then connect the output from C52 to a separate amplifier. If there are no crackles then you will have ruled out the switch bank. There is a remote possibility that a faulty magic eye could be injecting crackles via R23 so try the set with the magic eye removed. Cheers, Jerry

[JoshWard]

Thanks Jerry. I checked the volume control and all OK. I popped another control in just to be certain and same problem. I ran with the magic eye out and changed R23 (value had drifted anyway) but no luck sadly.
I rigged up another amp and sure enough the crackle was still there so it does look like the wavechange switch is the problem.

Can anybody suggest a way of working out which contacts could be causing the problem? Or is it a case of stripping it down and looking for further signs of arcing?

[JoshWard]

Working on the principle of nothing ventured nothing gained I have now removed the wavechange assembly and started to strip it down... I will report back with the findings!

[PaulR]

Brave man! I see you are still keen on McMichaels, Josh.

[cathoderay57]

Be careful to look for discoloration of the paxolin wafer. Apart from the possibility of a simple bad contact, where contacts carry HT sometimes the arcing after years of make/break causes carbonisation (indicated by the discoloration) and then conduction of HT onto an adjacent contact carrying small signals, hence crackling. I even found this on an Ever Ready set using only 90v HT. If you find this then cleaning might not fix it. Usually the wafer needs replacing or if you are extremely lucky you might find a spare contact set that you can use, or a hard-wired alternative. Cheers, Jerry

[JoshWard]

Thanks both. I have had issues with wavechange switches arcing in a couple of sets- in those cases it was the HT feed for the FM front end. I am surprised to hear of it in a battery set!
I have never removed a wavechange switch bank but with this it's either do or die. If I leave it as is then it's unusable. I might not be able to repair it but at least I will have tried! I may have to figure out a way of hardwiring it onto a particular waveband. I have partially dismantled the switch now and the paxolin is VERY thin. Still being stuck on furlough at least gives me the time to do it!

[JoshWard]

Hi all,
I have now managed to strip the wavechange switch down and gain access to the wafers but sadly the paxolin was so thin and brittle a lot of it fell apart in the process. Virtually all of the contacts are black so it looks like it's been arcing in a few places! It looks like I will either need to find a donor set or this one will become a donor! Shame as other than the switch it works beautifully.

[PaulR]

Such a shame, and a nice looking set. Pending a donor switch could you leave it permanently linked for FM? That is probably the most useful one and would make it usable?

[JoshWard]

Hi Paul,
I'm not too worried about making it usable quickly as I have more than enough sets to listen to! Instead it can wait patiently until something comes along. If I can get some better wafers I might be able to do something with it. Thinking about it further I could replace the broken paxolin panel with a thicker one and instead of cutting slots (tricky) I could drill holes for the terminals and give each hole a blob of Araldite to keep it all together.
This is all that remained of the old panel which fell to pieces!

[PaulR]

Hmm - unless it is very rough you would want to repair the donor of course! I have two McMichael 135s but the mains transformer is burnt out in one of them. The obvious thing to do would to make the one in somewhat poorer condition a donor buy I am seriously thinking of having a new transformer wound as they both actually work. Only one set of legs unfortunately!

Making new wafers sounds interesting - and challenging.

[Ed_Dinning]

Note that a capacitor could be bodged across the top of an IV transformer if it had gone o/c

Ed

[Sideband]

Originally Posted by JoshWard

Hi all,
I have now managed to strip the wavechange switch down and gain access to the wafers but sadly the paxolin was so thin and brittle a lot of it fell apart in the process. Virtually all of the contacts are black so it looks like it's been arcing in a few places! It looks like I will either need to find a donor set or this one will become a donor! Shame as other than the switch it works beautifully.

Paxolin can become conductive if there is persistent arcing. Not a lot you can do about it when it's a switch apart from linking contacts out but even then it doesn't always work.

[PaulR]

Thanks Ed.

I did that ages ago with a Cossor 501 until a kind forum member sent me a replacement. This is actually the mains transformer. I have two sets and am restoring the cabinet of the older, better one. Unfortunately that is the one whose transformer is burnt out. I have recapped both chassis and they both work. I could just transfer the chassis from one to the other whilst thinking about asking you do a rewind but I want to keep the correct chassis to keep the serial number right. I have a reason for this and am thinking about starting a success story thread about it but I realise I am going off topic for this thread so I will leave it until I have finished the cabinet and can take some photos for a new thread.

[JoshWard]

Look forwards to seeing that thread, Paul! I have recently acquired another 135 although lack of space to display it is putting me off doing it at the moment. I can recommend Ed's transformer rewinding service, he did one for a 1930s French set I own.

Thinking on this set some more it looks like I am going to have to make do with the original wafers. I will try and clean up the contacts and inspect more closely for blackened/potentially conductive areas of paxolin. I am going to have to make up a new retaining sheet out of something. Any suggestions of suitable materials are welcome. Tufnol maybe? It needs to be something that can easily be drilled/not too brittle!

[Freya]

I can recommend Unclad fibreglass Epoxy sheet, available in many thicknesses and sizes.
https://www.ebay.co.uk/sch/i.html?_f...Sheet&_sacat=0