Advice on restoring/repairing a Mullard E7600/3

[AmplifyAudio]

Hi everyone!

I bought a Mullard E7600/3 and a box of cards just over a month ago. I have a stack of various new and old tubes that I could do with going through at some point so I can sell the working ones I don't need. An AVO would probably be a bit more useful, but they are a considerable amount of money, this Mullard wasn't too expensive.

It hasn't been used in over 20 years and seems to be in fairly good and original condition inside. Someone has replaced the wax caps with polys, which also look quite aged themselves (which would tie in with the not used for over 20 years thing) but the electrolytics and everything else is untouched. Obviously, it's full of carbon resistors. Obviously, every single one I've metered is off! So, I think it might be better to replace all 102 of them! Arrrgh!

Any recommendations on what resistors to replace the old ones with? There are a few odd (by today's standards) values that are only available as metal film, so is it worth replacing them all with metal film? Or would carbon film be OK for the majority of them?

The E7600/3 and E7600/4 seem to be very similar machines with few differences. The schematic and parts list for the /3 shows tolerances but not ratings, so if I follow the resistor ratings from the /4, that should work, right?

I haven't done so yet, but before I start on the long and winding path of resistorgeddon, I'll check all the windings of the transformers. But, any advice and/or comments will be gratefully received.

Thanks in advance!

[SteveCG]

I don't know this unit, but as regards replacing the resistors can I suggest that you check the replacements can work at the typical voltages encountered with valves.

[Chris55000]

Hi!

I don't have any old valve–era component data–sheets to refer to, but the modern type of ½W carbon–film resistors are rated to have an element voltage rating of 350V continuous or 450V under surge conditions, whilst the 1W and 2W types are each rated at 500V continuous or 750V surge, so you should be OK with these!

I have an E7600/4 to restore myself but space & domestic limitations mean I've not got round to beginning on it yet!

Can I recommend you get the CT80 handbook for dealing with these as it had a much better circuit description in it compared to the Mullard books, and for refurbishment of chassis built/wired equipment in general, I recommend adding 1 mm p.t.f.e. sleeving over the leads of all replacement components to help prevent unforseen short circuits!

Chris Williams

[Lucien Nunes]

Are all the high-stabs out of tolerance as well as the carbon comps, or do you mean just the comps are all out of spec? Obviously, the makers used resistors with different construction and tolerances ranging from 20% to 1% according to how much impact they have on performance, and this can guide your approach to whether something needs replacing.

As mentioned above, matching power rating from old parts to new can result in the selection of new resistors that are not rated for the necessary voltage. It is sometimes necessary in the higher values to replace with a higher-power part in order to get the appropriate voltage rating. There is also the point that smaller modern parts of high power ratings achieve those ratings by being able to run at a higher surface temperature which might not be desirable. For a resistor to run at the same temperature it must have the same surface area, so there is much to be said for using one of the same physical size (which also looks a whole lot better).

I tend to standardise on 2W metal film for many valve circuit applications, usually rated at 500V and a good physical match for many 1/2 and 1W resistors being replaced. Unlike the original manufacturers, we do not have to shave every penny off the materials cost to make our restorations competitive as they did during manufacture.

[AmplifyAudio]

Originally Posted by SteveCG

can I suggest that you check the replacements can work at the typical voltages encountered with valves.

Oh yes, most definitely! IIRC I've seen 625v in scribbled font on at least 2 of the schematics which is almost Ampeg SVT B+ voltage. Which is pretty much all of the volts!

Originally Posted by Chris55000

...the modern type of ½W carbon–film resistors are rated to have an element voltage rating of 350V continuous or 450V under surge conditions, whilst the 1W and 2W types are each rated at 500V continuous or 750V surge, so you should be OK with these!

I have an E7600/4 to restore myself but space & domestic limitations mean I've not got round to beginning on it yet!

Yeah... I was hoping to have made a start on mine, but I have several guitar amp chassissis, 2 turntables and a few vintage HiFi units in the queue before it!

Originally Posted by Chris55000

Can I recommend you get the CT80 handbook for dealing with these as it had a much better circuit description in it compared to the Mullard books, and for refurbishment of chassis built/wired equipment in general, I recommend adding 1 mm p.t.f.e. sleeving over the leads of all replacement components to help prevent unforseen short circuits!

Got. And also, got!

Originally Posted by Lucien Nunes

Are all the high-stabs out of tolerance as well as the carbon comps, or do you mean just the comps are all out of spec? Obviously, the makers used resistors with different construction and tolerances ranging from 20% to 1% according to how much impact they have on performance, and this can guide your approach to whether something needs replacing.

From a brief 20 mins of prodding around with my old faithful cheapo Digitek (which I've always found surprisingly accurate) and my Avo 8 also agrees, it seem that quite a few of the high stabs are way off further than tolerance, as well as most of the carbons. In the next few days, I'll be waking up my HP 410B (hasn't seen power for 12 years) so I'll have a proper accurate prod around with that!

Originally Posted by Lucien Nunes

As mentioned above, matching power rating from old parts to new can result in the selection of new resistors that are not rated for the necessary voltage. It is sometimes necessary in the higher values to replace with a higher-power part in order to get the appropriate voltage rating. There is also the point that smaller modern parts of high power ratings achieve those ratings by being able to run at a higher surface temperature which might not be desirable. For a resistor to run at the same temperature it must have the same surface area, so there is much to be said for using one of the same physical size (which also looks a whole lot better).

This is all very good and true info, some of which I seem to have forgotten in the 12 years that I haven't been repairing valve equipment! (I'm a live sound engineer, took a 6 years out to repair guitar amps and other stuff, currently don't technically have a job so repairing stuff!)

One thing that may cause a bit of heated discussion: I don't care what the repair looks like. Well, no, it needs to be neat and tidy. But I'm certainly not one for hiding modern components. I feel a bit bad that I have to take an iron to the artwork that is the factory build of this machine, but it will (hopefully) end up being a fully working and useable device.

Originally Posted by Lucien Nunes

I tend to standardise on 2W metal film for many valve circuit applications, usually rated at 500V and a good physical match for many 1/2 and 1W resistors being replaced. Unlike the original manufacturers, we do not have to shave every penny off the materials cost to make our restorations competitive as they did during manufacture.

Yet another good point re: manufacturing and component costs.

Thanks everyone for the info/thoughts! I shall post updates as I continue this rather huge tas...

I should just job lot all the valves that I don't need, shouldn't I? And measure plate voltage and current of the ones I need in an amp chassis. Why am I doing this?!

[TowerRadio]

Before worrying about the resistors I would focus on the electrolytics as these probably need replacing first and with quality components.Les

[Electronpusher0]

I agree about the Electrolytic capacitors, especially the high voltage ones that supply the CRT, C3 and C4. In my experience these always need changing.
I found that the close tolerance resistors were all within spec but the rest needed changing.
The high voltage retifiers (EY51) often fail but can be replaced as a temporary measure with 1N4007 silicon rectifiers. (they can be permanently replaced if you wish, it depends how original you want to keep it)

The principle difference between the /4 and /3 is V4. It's an EF50 in the /3 and EF91 in the /4.

Send me a private message with an email address if you want a copy of the schematic and manual for the /4.

Peter

[AmplifyAudio]

Originally Posted by TowerRadio

Electrolytics

Originally Posted by Electronpusher0

Electrolytic capacitors

Oh, absolutely yes! Although, not just the electrolytics. I don't know the age or condition of the Polys that replaced the original waxies so for the sake of a few pence and a bit of soldering, they're going too! I have a great place for storing old capacitors. It's called the bin! I have no sentimentality for old and/or possibly failed components.

I have had a good poke around that there internet and have found a few different examples of fitting new caps including one using tea-bag board, so I'll figure a way of doing something neat and safe.

Originally Posted by Electronpusher0

I found that the close tolerance resistors were all within spec but the rest needed changing.
The high voltage retifiers (EY51) often fail but can be replaced as a temporary measure with 1N4007 silicon rectifiers. (they can be permanently replaced if you wish, it depends how original you want to keep it)

I haven't yet gone through every resistor, but I went through quite a few on the left and right side of the machine, and most seemed to be far off tolerance, including some of the high stability ones. The other thing is that carbon resistors can drift a fair bit in use, once the internal temperature of the unit starts changing. I know this from the vintage analogue synths I repaired many years ago, although most of the owners seemed to consider opening a door near it and the subsequent tuning drift, a feature. Not a feature, it wasn't designed or intended to do that!

Also not a desirable feature for test equipment, so I may end up just replacing all of them! Also, this could explain why I saw several of these machines on eBay with something like "has been restored/repaired, but doesn't complete the setup/calibration sequence".

Originally Posted by Electronpusher0

The principle difference between the /4 and /3 is V4. It's an EF50 in the /3 and EF91 in the /4.

Send me a private message with an email address if you want a copy of the schematic and manual for the /4.

Thanks for the kind offer Peter, but I think I have found and downloaded everything available off of here and the Air Ministries site including the original /4 and improved /4 schematic!