7A16A with an unknown modification MOD515D


Mark
 

I came across this 7A16A on Ebay cheap so I bought it. Good entertainment!

It has a strange modification that I cannot identify. Looked everywhere I can think of but can't find any info on it...

https://groups.io/g/TekScopes/album?id=263432

It seems to work but the attenuator range is off and when you reposition the trace with it's control it also affects the signal level.

If I can't find any info on it I will most likely just remove the mod and return it to a normal 7A16A state.


unclebanjoman
 

I wouldn't change it.
MOD515D adds to basic 7A16A the ability to remotely operate the band width,polarity and position functions.
Very rare. I think was used in conjunction with a 7912 or 7612D.
I don't know if it's usable in a 7854, but I think it is worth investigating further.

Max


Holger Lübben
 

Hi!

One sticker says; "Configuration WP 1000". This means it was delvered with a DPO (7704A+P7001).

Holger


Mark
 

Thanks you guys. That's more to go on.

I'm looking at the additional connections to the card edge connector to see if I can figure it out. Also the shielded connections going to the top. I've never seen these used before but then again I haven't seen everything.


Mark
 


Mark
 

So I confirmed that this amplifier should work normally when in the local mode. I also confirmed that this mod would only work on a 7704A with MOD 515C.

Back to the problem... In general the unit had low gain and when you use the Position control it would simultaneously adjust the gain of the applied signal higher at both extreme ranges of the control.

Solution: R705, bias resistor for the signal splitter amp, was out of spec. Was supposed to be 80.6 ohms but measured about 119 ohms. It was slightly discolored, see my photos... https://groups.io/g/TekScopes/album?id=263432

All in all good entertainment I would say.


Mark Vincent
 

Mark,

Obviously this resistor is underrated in wattage. Something like 1 ow 2W would be better. If a resistor looks like this, it should be a higher wattage. I have seen this problem of a overstressed resistor go up in value or open causing the same problem you have/had. An easy way to get the power needed is to parallel or series resistors, when possible. Sometimes getting the precision resistor in the wattage needed cannot be done with one resistor. You can use an 82 ohm 1 or 2W resistor in parallel with a 4700 ohm. That will give you 80,5938.,. ohms. Well within the 1% tolerance. This is provided you use 1% resistors or measured ones. Your plug-in is worth getting right because it is a rare piece. It looks in excellent condition.

Mark


Mark
 

Well a half watt would be good enough.

I don't know the history of this unit. If you look closely you will see that someone was tweaking the pots. Could of been a slip of the probe.


Mark
 

Actually the manual says these are 0.125 watt. I'm not familiar enough with TRW resistors but that would indeed be under rated. So interesting to say the least.

The actual power dissipation would be around 1/4 watt so 1/2 watt is safe.


greenboxmaven
 

For many years test equipment had minimal safety requirements. Newer equipment often has "sacrificial" resistors in parts of the circuit that also function as fuses. They often run hot and fail without reason. Usually the prints will have a label on them, calling them a "Critical Safety Component". Could the resistor you see overheating be one of these?

  Bruce Gentry  KA2IVY

On 4/29/21 17:05, Mark Vincent wrote:
Mark,

Obviously this resistor is underrated in wattage. Something like 1 ow 2W would be better. If a resistor looks like this, it should be a higher wattage. I have seen this problem of a overstressed resistor go up in value or open causing the same problem you have/had. An easy way to get the power needed is to parallel or series resistors, when possible. Sometimes getting the precision resistor in the wattage needed cannot be done with one resistor. You can use an 82 ohm 1 or 2W resistor in parallel with a 4700 ohm. That will give you 80,5938.,. ohms. Well within the 1% tolerance. This is provided you use 1% resistors or measured ones. Your plug-in is worth getting right because it is a rare piece. It looks in excellent condition.

Mark




Mark
 

No, nothing labeled like that. I'm sure it's just a mistake in the parts list.