Bush EBM 60 Restoration tips

[Radiocruncher]

I have acquired a Bush EBM 60 from a recent amateur radio rally. I have not yet tackled a valve radio and due to this being a bit of a rare model I do not want to jump in feet first and mess anything up. I intend to take this very slowly and learn as much as I can on the way. What I'm after is some must do's before I apply power to this. I have a circuit diagram and have read the other threads out there on this version so I would be very grateful for any hints and tips as to how to best go about this and also best ways to clean the case, trim etc.

Thanks in advance

Graham

[Mike. Watterson]

Replace Capacitors at any valve grids if hunts or wax coated, only mica likely OK
Check capacitors on HT rails and decoupling screen grid. (Leakage)
Reform any electrolytics / check current draw.

Nice find

[Radiocruncher]

Thanks for that Mike. I've spent the afternoon releasing it from its case which was pretty scary when it came to removing the dial cover. All the low value caps have already been done and very neatly as well. I've tested some of the electrolytics and ESR is good but they have all gone high so will have to be replaced. Started a video on it as well https://youtu.be/p232XP98l6w

Thanks again

Graham

[Mike. Watterson]

"ESR is good but they have all gone high"
Eh? What does that mean.
Capacitance High? +100% even is no problem.
Leakage high? Then feed a supply via resistor so there is maybe 2mA, and watch see if current falls to zero after an hour or two.

Stills are FAR more useful than videos.

[Nickthedentist]

Don't forget that even decent cap testers like the Peak Atlas ESR can be fooled, and will give a good ESR + increased capacitance reading if electrolytic caps are leaky.

You really need to do what Mike suggests, i.e. power them at their working voltage via a suitable R, and measure the leakage current.

[Radiocruncher]

Originally Posted by Mike. Watterson

"ESR is good but they have all gone high"
Eh? What does that mean.
Capacitance High? +100% even is no problem.
Leakage high? Then feed a supply via resistor so there is maybe 2mA, and watch see if current falls to zero after an hour or two.

Mike I have got an old Heathkit capacitance bridge but I haven't got around to testing and restoring it yet. I was using my peak ESR meter in circuit as a 'quick' test. The 3000's were reading just under 10000 and the 1000 was 9000uf so I am assuming that this is ott. As I explained I am not used to these circuits so appreciate any help I can get. I won't mess with these and will simply change them. I may consider fitting new caps in the old aluminium cans to preserve the look of the radio.

Thanks

Graham

[Radiocruncher]

Originally Posted by Nickthedentist

You really need to do what Mike suggests, i.e. power them at their working voltage via a suitable R, and measure the leakage current.

Thanks Nick. Sounds like I will have to dig the Heathkit bridge out and fix that first.

Thanks

Graham

[Radiocruncher]

Originally Posted by Mike. Watterson

Stills are FAR more useful than videos.

Mike

I have taken the trouble to do a close up video of the back of the chassis. I have also photographed it so if there is a part you feel would be good to post then let me know.

Thanks

Graham

[tim.norris]

Graham, just watched the video, it looks to be in excellent condition even the dial glass looks clear ! member Mark Pirate is currently restoring a similar valved Bush MB 60 (MW & LW ) that might give you some tips , he`s going to upload some pictures of the restoration of the rear of the chassis soon.
Regards, Tim.

[Radiocruncher]

Thanks Tim. The dial glass is pretty good except for a couple of burn marks but I'm hoping to polish them out. I will have a look at Mark's thread as they are similar sets. I will post some pics of the front and rear chassis tomorrow.

Graham

[Thyristor]

That's a really good score !

The potted transformers are a giveaway; that its a bit special; an export model, the MB60 does not have potted transformers.
Interesting to note; the aerial's similarities to that on the 'repro' TR82.

There was an article in the Bulletin about all the different variants of the MB60; I think I've seen it online too.

The orange cap; is the bulk cap for the HT; reservoir and smoothing; the 50uF cap in front of it, for the OP valve's bias. The array of caps to the left, by the valves, are the LT bulk caps.
The component you werent sure of, is the rectifier for the HT.

Presumably, it was in use until the late 70s; did I spy one of those dreadful blue stickers on the tuning dial?

Its worth changing the HT MR (MR1 on the MB60 trader sheet) I've never needed to cahnge the MRs on the LT, on an MB60. I've found the HT does fall off due to MR1 going h/r.

A couple of options; remove all the valves, carry out basic safety checks (shorted / damaged caps, etc) then apply power via a lamp-limiter.
The battery valves are becoming harder to get (read - expansive!) HT can droop a bit, it doesnt matter, after all, batteries will expire, but you want the LT right, definately not high.

Or you can power up, with D cells, and a current limited HT supply.

I've found ESR meters not terribly useful on vintage equipment; check with the Ohms range of a DMM, or AVO, as rough guide.

Check voltage readings as you go, dont just change all the caps.

Btw, I do like your videos, keep up the good work.

[Radiocruncher]

Originally Posted by Thyristor

Or you can power up, with D cells, and a current limited HT supply.

I've found ESR meters not terribly useful on vintage equipment; check with the Ohms range of a DMM, or AVO, as rough guide.

Check voltage readings as you go, dont just change all the caps.

Btw, I do like your videos, keep up the good work.

Thanks so much for the advice and pointers as to what I'm looking at. Very helpful as I am a novice with this type of set. I will indeed test everything first before replacing. I think the dual (orange sleeved) cap could be okay as the reading is not far off. I can't quite reach 90v with my bench supply but I do have a battery eliminator but don't think it's got current limiting. I can hook up a bank of pp3's to give me the 90v but I understand that the valves are delicate and don't like being overvoltage. I will be having another look at this tomorrow now so hopefully I can progress a little further. Incidentally there is a missing part and thats the SW aerial link. It should be attached via the small aerial board to the brass trim but this is missing. Still trying to decypher the circuit diagram.

Yes it hasn't escaped the dreaded station marker stickers. Roberts bulletin article was very good and interesting reading.

https://www.google.com/url?q=https:/...zqHxaKK_IG5pPQ

Thanks again

Graham

[Radiocruncher]

Well after the comments I thought I would post some images of the front and rear chassis. It has had a fair bit of work but there are definitely some bits missing for the SW aerial. I will do my best tomorrow to check the electrolytics and I now have a more readable schematic thanks to Robert, so I will check the lower value caps against the schematic to make sure they are the correct values.

Thanks

Graham

[Thyristor]

Looks like its had quite alot of work done to it in the past; I wonder if this was 'in service' repairs, or work by another restorer; I guess we will never know.

Just worth mentioning, if you do initial light-up via external supplies; as oppposed to the on-board mains TX; you must switch on the LT supply first, then switch on the HT just slightly after.
When switching off, turn off HT first, then LT last.

Ofcourse, both together is OK, as via the on/off switch; just when fiddling with bench supplies, this isnt always easy; and the valves must never be allowed to see HT without LT present, as Cpl. Jones used to say; 'they don't like it up em!'

Lots of series batteries for the HT is ok ofcourse, but maybe start a bit lower than 90VDC, and introduce some series resistance, on initial light-up.
Batteries can deliver a lot of current.

[Radiocruncher]

Originally Posted by Thyristor

Just worth mentioning, if you do initial light-up via external supplies; as oppposed to the on-board mains TX; you must switch on the LT supply first, then switch on the HT just slightly after.
When switching off, turn off HT first, then LT

Thanks again as I wouldn't have known that the LT had to be on first. I think in this instance I will power it up with my isolation transformer and variac. I believe it's still preferable to use a lamp limiter so I will need to put one together first. I'm tempted to try to reform the 6v caps in the LT supply but I might have to put a circuit together to do that.

Thanks

Graham

[ms660]

I didn't know that either (heater/HT sequence)

Is that specifically for directly heated battery operated valves or all valves?

EDIT: What about a valved receiver with a silicon or metal type rectifier, that's HT available before heaters heated.

Lawrence.

[nebogipfel]

Originally Posted by Mike. Watterson

Stills are FAR more useful than videos.

Perhaps for extreme close-ups but I just wanted to say that I think Graham's videos up to now illustrating his restorations on transistorised sets have been very good and not only do I enjoy watching them but have learned a great deal.

Good luck with your adventures with "bottles" Graham. I look forward to learning along with you

[Radiocruncher]

I enjoy watching them but have learned a great deal.

Good luck with your adventures with "bottles" Graham. I look forward to learning along with you.

Thanks for the vote of confidence John. It will certainly be a learning curve. I learn something new every time I look at it and from the posts left on here. Got to put a lamp limiter together next!

Thanks

Graham

[Robert Darwent]

Originally Posted by Thyristor

There was an article in the Bulletin about all the different variants of the MB60; I think I've seen it online too.

The BVWS Bulletin article that I wrote appeared in the Winter 2010 issue - since that time I have found further export variants to add to the series.

For those interested: http://www.wavesintheair.co.uk/mb60tr82.html

Regards