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SC502 Old Enough to Smoke?

DaveH52
 

I have a TM506 that came with a full load of plugins, all of which seem to work OK except the SC502 which emits a magic smoke smell. My uncalibrated nose tells me it's coming from the A1 main board on the bottom. My uncalibrated eyes did not detect anything looking discolored or burnt just an old spider nest that has since been removed. My uncalibrated nose says it's not coming from the HV supply section. Are there any "usual suspects" I should investigate first, before applying power again?

Dave Daniel
 

Is the ‘scope working to spec?

On Jun 2, 2020, at 15:40, DaveH52 <@DaveH52> wrote:

I have a TM506 that came with a full load of plugins, all of which seem to work OK except the SC502 which emits a magic smoke smell. My uncalibrated nose tells me it's coming from the A1 main board on the bottom. My uncalibrated eyes did not detect anything looking discolored or burnt just an old spider nest that has since been removed. My uncalibrated nose says it's not coming from the HV supply section. Are there any "usual suspects" I should investigate first, before applying power again?


DaveH52
 

No, it seems to be completely dead. I'm currently trying to locate the +/-20V and 5V test points to seem if one of them is overloaded.

 

Hi Dave,
Tantalum caps are the most obvious culprit. They usually have a small value (10 ohms for example) resistors in series with them. When the tantalum shorts the resistor overheats and disintegrated into a little black cylinder. Sometimes the area around the resistor is black too. Most of the time these tantalums are near the connectors on the rear where power comes into the plugin. The burned up resistors are a good thing to look for and usually easy to spot.

Next thing to do is check each tantalum capacitor for a short by putting an ohmmeter across it

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Dave Daniel
Sent: Tuesday, June 02, 2020 12:50 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] SC502 Old Enough to Smoke?

Is the ‘scope working to spec?

On Jun 2, 2020, at 15:40, DaveH52 <@DaveH52> wrote:

I have a TM506 that came with a full load of plugins, all of which seem to work OK except the SC502 which emits a magic smoke smell. My uncalibrated nose tells me it's coming from the A1 main board on the bottom. My uncalibrated eyes did not detect anything looking discolored or burnt just an old spider nest that has since been removed. My uncalibrated nose says it's not coming from the HV supply section. Are there any "usual suspects" I should investigate first, before applying power again?






--
Dennis Tillman W7pF
TekScopes Moderator

Larry McDavid
 

The Tek SC502 full manual is available on the TekWiki website here:

http://w140.com/tekwiki/wiki/SC502

I have a SC502 that works ok. I also have a SC504 that works ok.

Larry

On 6/2/2020 12:40 PM, DaveH52 wrote:
I have a TM506 that came with a full load of plugins, all of which seem to work OK except the SC502 which emits a magic smoke smell. My uncalibrated nose tells me it's coming from the A1 main board on the bottom. My uncalibrated eyes did not detect anything looking discolored or burnt just an old spider nest that has since been removed. My uncalibrated nose says it's not coming from the HV supply section. Are there any "usual suspects" I should investigate first, before applying power again?
--
Best wishes,

Larry McDavid W6FUB
Anaheim, California (SE of Los Angeles, near Disneyland)

DaveH52
 

Well I checked the onboard power regulators and the -20V is at -20.46, the +20V is at +16.64v and the +5V is at +4.25. Since the +5 supply gets its reference from the +20v supply I suppose I'll be looking at what's connected to the +20 rail first.

Harvey White
 

That's certainly where I'd look.  I'd look at all of the supplies that get their reference from the +20.   If that's on, then the problems go with the individual supplies.

One of the most common problems, as you know, are shorted or leaky tantalums.

Harvey

On 6/2/2020 9:03 PM, DaveH52 wrote:
Well I checked the onboard power regulators and the -20V is at -20.46, the +20V is at +16.64v and the +5V is at +4.25. Since the +5 supply gets its reference from the +20v supply I suppose I'll be looking at what's connected to the +20 rail first.


DaveH52
 

Interesting. Tantalum caps are generally very reliable. The solid type (CSR style) is preferable to the liquid (CLR style) because the liquid types contain sulfuric acid and make a real mess when they leak. The solid type usually specifies that a series resistor of about 20 ohms/volt is used to limit fault current. If this is done they can self heal. For a while, and it's been a while.

Harvey White
 

Has to do with how old they were, and that Tek (and likely everyone else) use a 6 volt Tantalum on a 5 volt supply, which was the recommended practice at the time.  I'd be happy using a modern 7 volt tantalum on a 3.3 volt supply, and perhaps iffy on a 5.0 volt supply.  The early ones were very underrated for voltage ratings.

The early ones tend to fail shorted.

Harvey

On 6/2/2020 10:22 PM, DaveH52 wrote:
Interesting. Tantalum caps are generally very reliable. The solid type (CSR style) is preferable to the liquid (CLR style) because the liquid types contain sulfuric acid and make a real mess when they leak. The solid type usually specifies that a series resistor of about 20 ohms/volt is used to limit fault current. If this is done they can self heal. For a while, and it's been a while.


DaveH52
 

Exploring that link sent me down a rabbit hole! Thanks!

DaveH52
 

I may have found a problem. C693 came in with a resistance of a whopping 1.3 ohms.

DaveH52
 

I was hoping I'd be able to remove the Main board without removing the CRT, but the only destructions start with "CRT Replacement" My efforts are currently thwarted by the rear screw holding the Horizontal Deflection board that was apparently installed by a 500 pound gorilla. I can't seem to find a Phillips screwdriver that will remove it. I've got it soaking in penetrating oil.

 

Please don't use a Philips screwdriver on any Tektronix equipment - they are all Pozidriv head screws. There's a massive difference: the geometry of the heads and a Philips screwdriver will cam out and wreck a Pozidriv screw.

The same comments apply to a lesser extent to using Philips screwdrivers on screws with JIS cross heads - similar to Philips, but *not* the same.

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of DaveH52
Sent: 04 June 2020 20:46
To: TekScopes@groups.io
Subject: Re: [TekScopes] SC502 Old Enough to Smoke?

I was hoping I'd be able to remove the Main board without removing the CRT, but the only destructions start with "CRT Replacement" My efforts are currently thwarted by the rear screw holding the Horizontal Deflection board that was apparently installed by a 500 pound gorilla. I can't seem to find a Phillips screwdriver that will remove it. I've got it soaking in penetrating oil.

DaveH52
 

So C693 has been replaced with an aluminum electrolytic so I can tell if it pukes, and because I had one leftover from the 2465 repair. So now the +/-20V and +5 are up and running. Unfortunately, there's no light on the screen at all. Someone has beaten me into the HV circuit where the filament supply also comes from, so I'm going to wait until morning before I go poking around in there.
Dave - AC2GL

DaveH52
 

My mistake. If it has a head with an + on it I call it a phillips, but you are correct they're really pozidrive.

DaveH52
 

So I've been able to probe a little in the HV section, some of the transistors that I could get to ohm out OK.
BUT! Hidden underneath the A5 AUX board there are some scorched components on the mainboard.
The A5 and the mainboard are connected cordwood style.
How on earth can they be separated so I can get to the components on the mainboard?
Dave - AC2GL

John Griessen
 

On 6/6/20 7:42 PM, DaveH52 wrote:
The A5 and the mainboard are connected cordwood style.
Are there pins and contacts like in 7xxx scopes? If so, just pull them apart.

DaveH52
 

That would be too easy. 5 capacitors and a resistor network have 8 leads soldered to the mainboard and 8 leads soldered to the aux board plus a diode and single turn winding. Not very servicable.