7B50 - No Sweep


n4buq
 

I'm attempting to resuscitate a 7B50 but, so far, have not been able to determine why there's no sweep. It's triggering (the lamp comes on appropriately depending on the LEVEL/SLOPE setting) and I can switch the DISPLAY MODE to AMPLIFIER and can get an XY display but, still, no sweep.

The waveform at TP262 appears normal; however, I don't see that waveform at the collector of Q266 and I presume I should(?). The base of Q305 is at -0.01V which, I presume, is not normal and seeing the tunnel diode (CR299) there makes me nervous. TP525 is also not showing the waveform (#8).

Any suggestions as to what to look for here?

Thanks,
Barry - N4BUQ


Mark Vincent
 

Barry,

I strongly suspect CR262 and CR299 are bad by your description. I have a 454 that had the same type of problem you have. I had to replace the four tunnel diodes for it to work because the 454 has two time bases. These were in the same places in the circuit as your plugin. This schematic is close to a 454 in this area of the circuit.

Mark


n4buq
 

Mark,

That's what I was suspecting as well. The block diagram page includes those diodes specifically in the TRIGGER PULSE GENERATOR and SWEEP GATE MULTIVIBRATOR blocks as does the circuit description paragraphs.

Not sure what I'll do for this. They're both a bit on the rare side but I think one more-so than the other.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, October 14, 2021 11:19:27 AM
Subject: Re: [TekScopes] 7B50 - No Sweep
Barry,

I strongly suspect CR262 and CR299 are bad by your description. I have a 454
that had the same type of problem you have. I had to replace the four tunnel
diodes for it to work because the 454 has two time bases. These were in the
same places in the circuit as your plugin. This schematic is close to a 454 in
this area of the circuit.

Mark



Albert Otten
 

On Thu, Oct 14, 2021 at 04:56 PM, n4buq wrote:
The waveform at TP262 appears normal; however, I don't see that waveform at
the collector of Q266 and I presume I should(?).
Barry, the collector of Q266 should show just a weak signal because Q273 is driven common base.
What is TP275 showing?
Albert


n4buq
 

Albert,

I'll have to check but I think I recall that TP275 is correct - or, at least, shows a waveform similar to what's shown on the schematic. I presume that's what's allowing the TRIG'D lamp to work, correct?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, October 14, 2021 12:25:13 PM
Subject: Re: [TekScopes] 7B50 - No Sweep
On Thu, Oct 14, 2021 at 04:56 PM, n4buq wrote:
The waveform at TP262 appears normal; however, I don't see that waveform at
the collector of Q266 and I presume I should(?).
Barry, the collector of Q266 should show just a weak signal because Q273 is
driven common base.
What is TP275 showing?
Albert



Albert Otten
 

Hi Barry,
Yes, this activates the trigger lights, but just to be sure I asked you about TP275. So TD CR262 is no longer suspect.
The reason for TD CR299 not firing can also be that Lockout is active via Q296 and Q293.
Albert

On Thu, Oct 14, 2021 at 08:22 PM, n4buq wrote:


Albert,

I'll have to check but I think I recall that TP275 is correct - or, at least,
shows a waveform similar to what's shown on the schematic. I presume that's
what's allowing the TRIG'D lamp to work, correct?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, October 14, 2021 12:25:13 PM
Subject: Re: [TekScopes] 7B50 - No Sweep
On Thu, Oct 14, 2021 at 04:56 PM, n4buq wrote:
The waveform at TP262 appears normal; however, I don't see that waveform at
the collector of Q266 and I presume I should(?).
Barry, the collector of Q266 should show just a weak signal because Q273 is
driven common base.
What is TP275 showing?
Albert


Roger Evans
 

Barry,

Following from Albert's suggestion that Lockout may be preventing the sweep from starting, can you try the following.

Select a slow sweep speed eg 0.5 seconds /div.

With a suitable trigger signal present set the trigger level so that the trigger light is off.

Set the sweep mode to Single Sweep and press the Reset button, the reset light should stay on. Single sweep reset 'should' reset all the stages of the lockout / holdoff circuitry.

Turn the trigger level to the point where the sweep should trigger. Do you see a single slow sweep?

Just for clarity, with Auto sweep enabled do you see anything at all on screen?

Regards,

Roger


n4buq
 

Hi Roger,

I tried the things you mention and I get no hint of a trace. The RESET button's light is never on.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Roger Evans via groups.io" <very_fuzzy_logic=yahoo.com@groups.io>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, October 15, 2021 10:26:54 AM
Subject: Re: [TekScopes] 7B50 - No Sweep
Barry,

Following from Albert's suggestion that Lockout may be preventing the sweep from
starting, can you try the following.

Select a slow sweep speed eg 0.5 seconds /div.

With a suitable trigger signal present set the trigger level so that the trigger
light is off.

Set the sweep mode to Single Sweep and press the Reset button, the reset light
should stay on. Single sweep reset 'should' reset all the stages of the
lockout / holdoff circuitry.

Turn the trigger level to the point where the sweep should trigger. Do you see
a single slow sweep?

Just for clarity, with Auto sweep enabled do you see anything at all on screen?

Regards,

Roger