I Want to Fix this Oscilloscope but....

There are still distributors of thermionic valves to be found, though they are few and far between. An internet search engine may turn up something, as might the long-abandoned Yellow Pages section of the local telephone directory, perhaps?

Regards.

Good point to note but:

Nothing to alert with the DC instant-output. There is nothing in Valve-operated equipment which is vulnerable to instant application of DC supply to still not warmed-up remaining cicuit. And this rectifier is just for less than 500mA.

I have the experience of replacing the old rectifiers of 100s ampere with with semiconductor rectifiers. In circuits having Hi capacitors have already series connected Surge-Protectors like NTC thermisters, however in such circuit you have to look carefully for surges on switch-on. A valid point here.

Regards.

A GA for good answer.

It a good idea to replace not only Valve rectifier with latest Semiconductor rectifiers but also old Semiconductor rectifiers, or the under-rated ones with better & latest ones when needed.

Why not use 1/2 bridge rectifiers, 3 legs 2 diodes connected as Anode-common for

(-), & Cathode-Common for (+) output.

Hi Jepo,

Give MHz Electronics a call. 602-681-9100. If they don't have it, you don't need it. They may have 10 or 15 of these scopes sitting around.

While this may sound like an endorsement but it isn't. They just have tons of "old stuff".

If you've never been there, go take a look around. It's worth the drive. They're on E. Washington St.

If you still can't find the tube, let me know and I'll PM you a private source who may be able to help.

Good luck! And welcome to the party.

Google is your friend (you've got 3 days, 7 hours left to bid):

http://cgi.ebay.com/6AX6-(1)--NOS-TESTED-RECTIFIER-VACUUM-TUBE-(H16)_W0QQitemZ310192143685QQcmdZViewItemQQimsxZ20100104?IMSfp=TL100104015006r20639

Alright thanks for the information so far. I will definitely go to MHz Electronics and see what they have.

http://cgi.ebay.com/6AX5-VINTAGE-TUBE_W0QQitemZ270427296524QQcmdZViewItemQQptZVintage_Electronics_R2?hash=item3ef6b9130c

6AX5 - buy it now for $4.50 plus $5.15 shipping!!!

A Google search for vacuum tubes turns up quite a list of suppliers. Or a Global Spec search works as well...

Vacuum tubes should not be too difficult to find. They have a faithful, almost religious following among musicians, for use in guitar amplifiers. I think that there are even some companies in Russia (and one or two eastern block countries) that still manufacture them. Do a search for tube guitar amps and you should come up with quite a few suppliers.

Heres one:

http://thetubestore.com/nos-6ax5.html

And this place has about a bazillion, including a listing for yours. Its a 6AX5GT but it will work just as well:

http://www.vacuumtubes.net/prices.htm

These are just a few I found.

Try Antique Electronics in Phoenix, Arizona, they have a very good stock of tubes and parts. They also go by AES, a simple search will find them. I would suggest picking up a few of the other tubes while you are at it, they are only going to get more expensive unfortunately. Stick to NOS (new old stock) where possible.

Do NOT, under any circumstances, replace a tube rectifier with solid state diodes unless they were specifically made for the purpose, it can have dire consequences on the following circuitry. Solid state diodes have much lower voltage drops and can supply way too much voltage to the load.

Some years ago i got an osciloscope like that one :most of capacitors were destroyed:check them before turn on the power.The most (perhaps not) all of them are easy to get in the common market.-

Nice old device, I love it :-)

It's use will not be as convenient as the modern o'scopes because the sweep is only synchronized, not triggered but it's a good way to learn using an o'scope.

6AX5 is a full-wave rectifier, electronically very similar to 6X4 though the socket is different. If you cannot acquire one you can use 6X4 with a socket converter.

Before turning it on you should check the capacitors C25-C26-C27ABC

Perry hit it right on the head and a GA. The diodes are much cheaper than what it would cost in replacing the 6AX5, IF you could find one. As you might have already figured out, connect both cathodes (banded) to the left side as seen on the schematic, and the anode of each to the two connections (one is pin #3, can't make out the other) on the connection side of the socket. There is no issue in using this semiconductor replacement what was cited in other thread(s).

It would be a good idea to replace the associated electrolytic capacitors C27 A, B and C (where you could use separate caps - I would use a couple of mounted solder type terminal strips), and C10 as they may be original.

Good Luck!

I wouldn't recommend to replace the rectifier tube with semiconductor rectifier.

According to the Voltage chart the power transformer gives 400 V AC. It's peak value is about 560 V. C27, the filter capacitor is rated only to 450 V!!! When you use a tube rectifier it starts to conduct together with the other tubes so there will not be an unloaded situation, the rectifier will always be loaded so the capacitors can never reach the full peak Voltage. In addition, the Voltage drop on the tube is about 20 V.

If you substitute the tube with semiconductors the Voltage drop will only be a couple of 100 mV and the rectifier will start immediately, without the load of the other tubes. That can result the death of at least the filter capacitor C27.

So, if you would like to have a vintage oscilloscope in good condition try to buy a 6XA5...

Qqberci: You are only correct to say that the voltage across C27 will be higher, but only perhaps 20V DC. In this application it does not matter. Also, electrolytic capacitors are rated WVDC (Working Volts DC). So not to be so concerned.

You make some assumption that all vacuum tubes "come to life" at the same time? Never will occur.

Only if our inquirer should want a truly restored antique should he may desire to delve into it as a money pit. He didn't seem to suggest that.

Too: I have replaced a grunch (hundreds?) of such rectifiers over the years with semiconductor diodes and never=ever had a problem.

if our inquirer should want a truly restored antique

What else can be his purpose with a 4.5 MHz untriggerable o'scope?

Perhaps an entry level means for his ends? My first was similar obtained from a horse trade. Not literally.

When I was in college, didn't have two nickels to rub together. So, I appreciate where he may be coming from.

Should he convince me, I would sell him a dual trace Tektronix 465B scope for $1, but he'd have to pay shipping. It does have a HV problem though (trace died) that I don't opt to dig into.

Too, he'd likely become a returning CR4'r wanting suggestions on how to fix this one. I've fixed one or two previously by merely replacing the electrolytic caps (excuse the expression?) in the HV power supply.

Jepo,

I went to ebay and typed in 6AK5, and got a full page of them. Give them a try.

I bought this for a buck on e-bay about a year ago and thought I could try and fix it , but not realizing what I was getting into, or really realizing its abilities. I was hoping to get a cheap O-scope to use at home for labs. I would like it to work exactly as a refurbished antique and then go from there. I won't try and rebuild a power supply with diode rectification since that would void it's antique status and possibly make an easy fix harder. Thanks for all the replies, I got a lot of info from them. I will update you when I have it successfully operating.

Its a fun machine as an electronic antique, but for usage today, I personally would like an external sync, which I am reliably informed, it does not have.....

There are available on ebay really nice small and cheap scopes with a storage facility (once used, never to be missed!). Even if you are maybe not in a position financially to buy one now, that should be your aim....as its simply irrational to buy a good one secondhand and pay probably more than half the cost of this one (around $500 on German ebay):-

25MHz Digital Scope with color screen

I am sure that ebay where you are has the same one also available, I am not recomending buying in Europe if you live in the USA for example!!

Its small and light and has a 3 year guarantee......

There is nothing worse than faultfinding and having to faultfind first on the faultfinding equipment!!!