Marconiphone 4214 Recording Issue

[DeKay01]

Good day, I'm new to tape recording altogether, and to working on these machines.

I recently bought a Marconiphone 4124 Mono machine that was in a state.
With a lot of assistance and advice from threads here and from Trevor of The Vintage Audio Club on Facebook, I have since disassembled it, regreased the two bearings and oiled everything that needed oiling.
I also cleaned and polished the two heads and cleaned everything I could, and replaced the belt.
After reassembling the machine, I found there to be a couple of problems.

One is that recording and erasing is not working. Since I bought this machine to make my own recordings, this is rather important to me. I believe it to be an issue with triggering, as the magic eye doesn't give me an idea of the levels, and the erase head is not erasing. Playback does work, for the most part.

The second is that when using the internal speaker, I need the tone dial turned completely anticlockwise for it to make a sound. This problem doesn't apply to when using external speakers, as the audio is fine on them. This isn't really something I care about, just thought it was worth mentioning.

I suppose I'm just looking for advice on how to mend these issues. I haven't replaced caps, though I could if it's likely to be the issue.

Thankyou all in advance,
Jack

[paulsherwin]

The first thing to do with any old tape recorder is clean the switchgear contacts very thoroughly. Recorders often have additional switch mechanisms controlled by rods or wires, so make sure you have cleaned everything, and that all mechanical linkages are working as they should.

[DeKay01]

Thankyou for the quick response - to the best of my knowledge, all mechanical linkages are working properly.
As for the switchgear contacts, I've cleaned them as best I can without desoldering the whole pcb, using electrical contact cleaner solvent. Though they still look a little dirty/black.

[DeKay01]

After I posted this, I set about trying to clean the contacts some more, I ended up using the rest of my contact cleaner on it, and now the record function works, many thanks, I'm really quite glad.

[DMcMahon]

Hello Jack,

If I read it correctly you are saying that you can get normal sound operation from the internal speaker when adjusting the volume control, but only if the tone control is at the fully CCW (counter clock wise) position, is that correct ?

If that is correct then it sounds like something on the tone control circuit is pulling down the signal, which would not be that common.

Looking at the electrical schematic there is a low value capacitor (C12) at the bottom end of the tone control (R18) which connects down to ground. If C12 is shorted then it may give this symptom. Also if C12 is electrically leaky then it may pass DC voltage which might also cause a problem.

There should not be any DC voltage present at C12 but if capacitors C10 and C11 are leaky then there could be DC voltage passed through.

The listed value of C11 is a little confusing, it is shown as 40mF. Technically the small "m" is "milli" so that would be 40milli Farad which would be a very large capacitor in this application. I think it is actually 40μF (40 micro Farad) because sometimes manufacturers used the m symbol rather than the correct μ (micro) symbol. Also seems a little odd that a 40μF (which would be an electrolytic capacitor) is in this part of the circuit.

If you have a multimeter I would first measure resistance across C12 (220pF) to make sure that it is not shorted or is a low resistance. Before doing this make sure the machine is unplugged from the mains voltage supply and allow at least 15 minutes for the internal capacitors to discharge.

David

PS. I forgot to say in the above post that I cannot see any obvious explanation that would explain it being OK on external speaker ?

[DeKay01]

Wow, thankyou so much for the response - I'll give it a check when I get back home.
A million thanks for such a detailed explanation, it's awfully kind of you.

With regards to the external speaker thing, In my concentration on the deck, I completely forgot that my external amp has its own built-in EQ, so that was the cause of that.

Thank you once again for the advice, I wholly appreciate it!

Jack

[Lucien Nunes]

What type of external speaker are you using and to which socket are you connecting it?

An unpowered / passive loudspeaker connected to the 'EXT. LS' socket would be driven by the tape recorder's internal amplifier via the tone control, and should behave like the internal speaker. A powered / active external speaker or sound system connected at line level to the 'RADIO' output will receive a signal regardless of whether there is a problem with either the output stage or the tone control.

The output pentode doubles as the record bias / erase oscillator, so a single problem in this area could affect erase, record, internal speaker and passive external speakers connected specifically to the 'EXT LS' socket' only. I would not be surprised if it were related to the record/play switching. Much of the circuitry in a small tape recorders of this type is completely re-purposed according to whether the machine is recording or playing back, hence switch problems can be widespread and far-reaching. Or, the tone control capacitor may be short-circuit; disconnecting it should restore volume on the internal speaker if it is. Replace the coupling caps if they are suspect. If they leak, they can damage the output stage even if you are not using it during playback, rendering it useless as the bias oscillator when you want to record.

Surely the coupling cap would be 40nF, or 0.04μF in old money? I'm not familiar with any manufacturers other than crazy high-end computer and scientific people using mF to mean millifarad, although MF and more often MFD for μF is normal.

[dazzlevision]

I've restored several of these these DB21 and DB42 tape deck Thorn brands machines.

They all had some brown or black coloured "Hunts" Moldseal capacitors in the electronics.

These become electrically leaky and should be replaced, in order to avoid damage to the ECL86 and its output transformer - and the correct performance spec of the electronics.

[DMcMahon]

Reference Post 8.

Yes totally agree that 40nF/0.04uF would make sense, but it is listed clearly as 40mF.

The capacitor symbols on the Ultra 6214 / Marconiphone 4214 ERT schematic are all the same, irrespective of whether the capacitor is an electrolytic or non electrolytic, so one cannot tell from the schematic if C11 is an electrolytic (I would not expect it to be normally).

Yes agree it is very unusual to see mF meaning milli Farads.

Have definitely seen mF used (very occasionally on some old documentation) for micro Farad, the same as MF/MFD.

Jack - if you go inside and come across C11, it would be very interesting to know what value is shown on its case, C11 connects to one end of the tone control and also same connection also connects to one end of R17, a 220kohm resistor (red red yellow colour bands). The other end of the tone control connects to the 220pF capacitor C12 (this capacitor is shown physically at the tone control itself). The middle (wiper) connection of the tone control connects to a contact on the Play/Record switch.

R17 & C11 are near the middle of the board towards the bottom edge.

David

[DMcMahon]

C10 & C11 would be the prime candidates for electrical leakage potentially affecting the output valve/output transformer, there could also be other moldseal/paper capacitors further back in the electronics that could also be leaky.

[steve83]

Originally Posted by dazzlevision

I've restored several of these these DB21 and DB42 tape deck Thorn brands machines.

They all had some brown or black coloured "Hunts" Moldseal capacitors in the electronics.

These become electrically leaky and should be replaced, in order to avoid damage to the ECL86 and its output transformer - and the correct performance spec of the electronics.

I don't have any experience with this deck in particular but just to say that I am repairing another Brenell amp at the moment (separate to the one I started a thread here on) which uses these brown/black Hunts caps and all tested as electrically leaky and needed replacing!

[DMcMahon]

Yes Hunts Moldseal and several other brands/types of paper/waxy paper capacitors of similar vintage do not always hold up well after more than 50 years.

[DeKay01]

Hey, apologies for taking so long to give an update on this, it's been a busy week.
I checked the caps, and they seem to all be OK for now, though replacing them is on the agenda someday. After spending a lot of time disassembling the PCB to get access to the recording matrix?/slider, I cleaned that off with *a lot* of solvent and elbow grease to expose the contacts underneath the grime.

After that, I reassembled it and the recording works!
As for the treble, I believe this could have been an error on my part.

So I can safely say that the machine has now been mended, it sounds wonderful, with the help of all you fine folk, and special thanks to Trevor of The Vintage Audio Club on Facebook, who guided me through this project.

I have a funny feeling this won't be my last machine, haha.
Thanks again,
-Jack.

[DMcMahon]

Well done and thank you for update, always good to hear final outcomes.

David