Brenell Mk5 Recapping Advice

[dillweed4]

Hi all,

I've just picked up a very cheap (£10) Brenell Mk5 - needs a bit of work done and recapping. I just wanted a bit of advice - see my pictures. I think I've identified the ones to replace (both of the multi caps, the electrolytics and the metalised black ones) - I'm not sure about the flat waxy brown ones or the gold Wima ones - should I change those too? Most of them are the notorious Hunts caps.

Also some of the resistors look a bit knackered - is this a cause for concern?

Thanks,

Will.

[AC/HL]

The flat Mica ones will be OK, as probably are those Wima's, it's paper and electrolytic capacitors that are the problem ones. Unless the resistors are markedly out of tolerance they are probably OK too.

[dillweed4]

Thanks Bill - I'm not so much of an expert do you know what type are the black Hunts capacitors seen in first picture? Are they electrolytic? They gave no clue as to which side is polarised.

Thanks!

[Lucien Nunes]

The black Hunts Moldseals are paper, hence not polarised. As a rule anything under 1uF will not be electrolytic. If there is no polarity indication and it is not in an aluminium can with a bung at one end, then again it typically won't be electrolytic. Electrolytic caps are a (very) necessary evil and are only used to enable large values of capacitance to be fitted in a small space at low cost. While it is possible to make an electrolytic 0.1uf, no-one would normally want to do this.

FWIW high voltage Hunts electrolytic cans do sometimes reform well, they don't have the disastrous failure rate that made the Moldseals infamous. Also BTW, the date codes on those Hunts you are about to chuck point to late 1960 manufacture, the last one I can see is week 40.

[Diabolical Artificer]

I'd advise you take lots of pics from different angles but before you do that, go round and mark each electrolytic's + with a pen, then take pics. You then have a good reference to work to. Ah, just saw you've removed them.

What condition is the top and box in? I have a Mk5 here that needs a good clean and restore.

Andy.

[dillweed4]

Originally Posted by Lucien Nunes

FWIW high voltage Hunts electrolytic cans do sometimes reform well, they don't have the disastrous failure rate that made the Moldseals infamous. Also BTW, the date codes on those Hunts you are about to chuck point to late 1960 manufacture, the last one I can see is week 40.

Thanks for this very helpful answer! Interesting to know the date of manufacture!

[dillweed4]

Originally Posted by Dr Wobble

I'd advise you take lots of pics from different angles but before you do that, go round and mark each electrolytic's + with a pen, then take pics. You then have a good reference to work to. Ah, just saw you've removed them.
What condition is the top and box in? I have a Mk5 here that needs a good clean and restore.

Hi Andy, I wrote down a diagram and took photos of everything before I removed it... don't want to make any errors! Also have the schematic so I feel I have all bases covered

The top is ok - gonna give the heads a good clean and demagnetise and I think the band for the counter needs replacing - not sure how i'm gonna sort that. Motors are all fine though and perfectly functional which is great. Box is a bit dusty and the outer plastic layer that sticks to the wood needs to be stuck back down in places. I've already sorted most of this though.


I have one question to anyone, which is concerning those two electrolytic multi cans in my first post pics - how do I wire individual capacitors together to replace them? The big one is from the power section and has 2 caps in and the small is from the amp section and has 3 caps in.

Thanks!

[Stevie342000]

The simple answer is you stuff the new capacitors into the old cans as usually most people want to keep these items looking original.

I think the standard method is to carefully use a craft knife to cut out the old capacitors. By cutting the black seal without cutting yourself and it should all come out.

Not tried but sure it has been asked many times before, some one will give better details than me on how to remove the old capacitors.

If you do it carefully enough I think you can re-use the old terminals soldering the new capacitors to the old tags so nobody knows you've re-stuffed it.

[Goodizzy]

I have restored one of these- Brenell Mk 5, the gold deck.

You could reform, restuff or replace the filter caps. I prefer to replace all electrolytics in any electronics device this old. if the old ones fail, they could wind up costing you much more than the pittance a cap costs.

I have never restuffed caps but there are several forum posts and websites around the internet on the particulars. The electrolytic can has a shared ground between the two caps in it. That means when you wire them, you should solder the negative leads of the caps together. This will be your ground for the entire can.

I would soak test the PT before you give it a proper go to ensure no shorted or open windings. Is your PT the impregnated paper type? Mine was, and the 6.3V rectifier winding had shorted. I had to put in a separate filament transformer as a result. Mine had been stored in an attic, however, and had gone through years of moisture and hot/cold cycle.

I didn't have any resistors in my unit that were highly out of tolerance. I replaced all power and decoupling resistors prior to starting, however, so if they had drifted I wouldn't have known.

Finally, take a look at the cork brake linings. Mine had disintegrated into nothing and had to be replaced. (It was really very easy)

[Diabolical Artificer]

You can still get multi can caps with typical value's IE 16,32,50u from Audiocap or Watford. Try to use decent caps made for instance by F&T they do multi's and single HV types. If you have to get radials try to get 105 deg C caps made by decent manufacturers like Nippon Chemicon, Rubicon, Vishay or Panasonic. Oh and make sure the replacement cap has a good ripple value.

For motor run caps try Spitfire Generators for reliable cheap caps.

Andy.

[dillweed4]

Thanks all! I should be able to get cracking with all our your help. I'll report back when I've reassembled with the new caps in...

Best,
Will.

[vidjoman]

I had a Mk 5M that has separate record and replay amps and found that after about 5 years use from new that changing the resistors reduced the background noise by quite a lot. Well worth doing while you change the caps. Only cost a small amount compared to the cost of caps.

[dillweed4]

Hi guys,

I got some caps and replaced some things. The electrolytic multi can in the preamp section (3 x 16uF 350V) I replaced with 3 x Panasonic radial 22uF 350V ones, with the negatives wired together. The schematic says they are 16/20uF caps so I thought it ok to put these 22uF ones in… Am I right?

Then C2 and C13 (50uF x 12V) I replaced with 47uF 63V caps…. I think 63 V was the lowest value I could get, and I read that you can exceed the required voltage by a fair amount so I assumed it’s ok?!

I have also put in a replacement orange cap for the Hunts .5uF 250V black paper mould… It’s 250V .47uF

I’ve wired up some other stuff too in the preamp section, 2 x 500V 0.05uF ceramic disc caps in the place of the 0.04uF 500V Hunts paper caps.

For the multicap in the power section (2 x 50uF 350V) - I've put in 2 x 350V 47uF caps. The 0.5W 470 ohm resistor between the positives was cracked, so I replaced that with a 0.6W 470.

I also had to replace the mic in jack as it was pretty dodgy.

Well I double-checked my work and everything seems to be wired correctly and good solder joints. Turned the thing on and it works. One major problem though - when I plug something into the input source I get this kind of crackly signal, almost like a broken speaker (but it's not the speaker) - weird distortion and constant volume changing. I tried it on the monitor out to an external speaker and same thing occurs - so I'm not sure what the problem is.

If anyone has any suggestions?

Thanks
Will

[Lucien Nunes]

Variable volume with crackle often indicates poor contact and the R/P switch is a prime suspect as it switches so many points in the signal chain to repurpose most of the circuit between record and playback. If you have DeOxit I would apply that, or a non-oily switch cleaner, sparingly to the R/P switch and click it back and forth a few dozen times to start with. Check the volume control and valve holders for poor contact also.

Your description suggests the crackle occurs on Amplifier, but are either Record or Playback affected? How are the DC voltages looking around strategic points in the circuit e.g. HT, valve anodes and cathodes? What signal source are you using? Do you get normal deflection of the M/E on record?

The monitor output is not intended to drive a speaker - if you have connected it to an amplifier and you still hear the crackle, this more or less eliminates the output stage and the amp/oscillator switching. If you connected the speaker to the L/S output then nothing is eliminated. To eliminate each stage in turn you can try injecting a signal directly into each valve grid, obviously a very low signal is needed for the 1st stage, rising to many volts for the output.

Not relevant to the fault but:

Then C2 and C13 (50uF x 12V) I replaced with 47uF 63V caps…. I think 63 V was the lowest value I could get, and I read that you can exceed the required voltage by a fair amount so I assumed it’s ok?!

If these are the cathode bypass for the signal stages, then they normally only have a volt or two across them. Concensus is that an electrolytic should ideally be used at perhaps 20% of its rated voltage as a minimum, so the 63V parts are not the best choice. They will be fine for now and probably a long time hence, but technically a lower voltage rating might have been preferable.

other stuff too in the preamp section, 2 x 500V 0.05uF ceramic disc caps in the place of the 0.04uF 500V Hunts paper caps.

I'm not sure where these are in the circuit but if they are part of the equalisation networks, then you should stick to the original values or the response will be incorrect. Upping value is usually OK with bypassing and coupling where the the cap simply needs to have a 'low enough' reactance at all frequences of interest.

[dillweed4]

Hi Lucien,

Thanks for your detailed reply. I have taken a look at the R/P switch - there is no obvious culprit like a loose connection or solder joint, but it sure does sound like that when I plug something into the amp. I have no applied some DeOxit I'm just waiting for some more to arrive but I will do that to the R/P and the valve sockets.

Record and playback are also affected - I can't seem to get any sound at all from a tape I played. I tried output signal on three sources - the monitor out (to an externally powered speaker), the headphone out, and the built-in speaker - same deal with all. The monitor out was running to an amplifier yes, so that eliminates the output stage like you suggested.

I havent checked DC voltages yet either, but I will do when I find a moment!

Thanks for the tips on the other bits.

[Goodizzy]

I am still confused as to the symptoms. Do you get any sound other than crackling and volume swells? Does it happen with both MIC and RADIO 1/4" inputs? On playback, record and amplifier modes?

Also, do you have the correct schematic? There are many iterations of this Brenell model.

[dillweed4]

Hey there, sorry if I explained it badly.

If I set the machine to 'amplifier' and turn the volume up with nothing plugged into to any input, I get the normal background noise you'd expect with any valve amp, plus intermittent 'click' or 'pop' noises that are not very loud.

I have plugged a guitar into mic input, I can hear my guitar signal, but the volume constantly changes (edging on the side of crackle). You can hear it but like I said the level is always changing with weird phasy noises. It's not a bad jack input as I already changed that.

I can get very quiet signal on the radio input if I plug in an iPod or something, but its barely there.

On playback mode I get nothing at all, including when I play a tape.

In record mode also nothing.

Yes I have the correct schematic I am sure.

Thanks!

[dillweed4]

Hi guys,

Can someone tell me what the black component is connected to the ground wire near the jack socket? It looks pretty beat!

Thanks

[dillweed4]

Hi guys,

Well after a few months I decided to have another crack. I built an audio test probe (best thing I've done in a while) and I have been tracing the audio path today with it.

I'm working on finding the cause(s), and I have a few questions if someone is happy to help it would be great.

1) The 'crackle' in the signal cannot be heard until I monitor the audio probe off the headphone out/speaker jack at the back of the deck next to the power switch - so could it be the jack itself or something in between the output transformer and it that is causing that intermittency? Signal seems consistent up to that point.

2) The 'monitor out' jack has no audio signal at all going to it - there is a 220K resistor before it which looks fine, but when I test the side that leads to the monitor out, I get nothing. Before there is signal. Can a resistor go completely dud like that?

3) If I follow the audio signal after the first valve (EF86), I get full sonic bandwidth when monitoring before a Hunts 47pF silver mica cap, but after it, it's lost the low end and sounds much thinner. Perhaps a silly question but does that sound like its dud too?

4) My photo shows the output transformer - the solder keeps coming loose with the lug on top, and i'm wondering if it matters if it is connected or not? The solder does not want to stick well either after multiple goes trying.

Thanks!

[Radio Wrangler]

1) that sounds logical. Jack sockets also go noisy and some have switch contacts which go bad too.

2) a monitor output I'd expect to be driven via a DC blocking capacitor, and then to have a high-ish value resistor to ground to ensure the capacitor charges to the right voltage and any poor unsuspecting thing plugged in doesn't get a big surprise.

3) it depends on what impedance your audio probe is loading things with. A small value capacitor into a very high impedance (like a valve grid) might give perfectly fine audio bandwidth, but if your probe is lower impedance than the valve, the low frequency roll-off will be increased in frequency and the bass will vanish... possibly higher.

4) It looks like the transformer connections are to a tag strip and the tag strip ground lug has been soldered to the top of the mounting clamp of the transformer (Yuk!) There seems to be no solder has ever wet the clamp, and getting the clamp hot enough to get solder to wet it would be risky for the transformer. Does the mounting lug of the tagstrip get connected to anything? If so it'll need connecting properly to the chassis. Otherwise a very thorough de-greasing of the top of the clamp and the tagstrip followed by some epoxy might do the job.

David