7704A (et. al.) Inverter Regulator Question


n4buq
 

The SMPS LVPS in my 7704A is in tick mode. I've found a couple of bad transistors and have ordered the last of them as well as resistors to make a load array somewhat like the one Dennis built. With those, I hope to finally get this PS working; however, until all of those arrive, I got to wondering about something in the inverter regulator.

The custom Tektronix chip, U3105 (in the 7704A but may have a different designation in other models of the series) regulates the inverter by changing the point in the cycle where a set of windings in T3030 are shorted to ground via Q3102/Q3104.

I notice there is also a diode, a 15V zener diode, and an RC network all connected in series between the collectors of both Q3102/Q3104 and ground. Is this a fail-safe overvoltage protection circuit for the inverter should Q3102/Q3104 or U3105 fail or is this a snubbing circuit to clip noise spikes (which I don't quite understand would be necessary), or something else? The manual doesn't seem to mention it - at least not that I've found.

Thanks,
Barry - N4BUQ


Albert Otten
 

On Mon, Sep 13, 2021 at 04:09 AM, n4buq wrote:

I notice there is also a diode, a 15V zener diode, and an RC network all
connected in series between the collectors of both Q3102/Q3104 and ground. Is
this a fail-safe overvoltage protection circuit for the inverter should
Q3102/Q3104 or U3105 fail or is this a snubbing circuit to clip noise spikes
(which I don't quite understand would be necessary), or something else? The
manual doesn't seem to mention it - at least not that I've found.
Barry,
Here is my interpretation. Waveform picture 3 shown at the node of C3101/C3102 and the mean voltage +9.9 V there suggests that the voltage is clamped at about +15 V immediately each cycle. The higher "hat" would be due to R3105/C3105. The open peak voltage of the 30T windings would be much higher than 15 V.
Albert


n4buq
 

Okay, Albert. Thanks for the reply. BTW, I presume you meant "waveform picture 3 shown at the node of
Q3101/Q3102", correct?
The reason I'm asking about this is I was trying to force the inverter out of tick mode and I thought that by removing Q3102 and/or Q3104 and using a variable AC input and monitoring the output voltages, I could do some better checking of voltages in the LV regulator; however, that doesn't stop it from ticking and I suspect CR3103, VR3105, and R3105/C3105 still cause it to tick - just at possibly a different voltage point.

Hopefully replacing the final bad transistor and getting proper loads will get this PS going but, if not, at least it will allow me to better troubleshoot it.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 2:50:11 AM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

On Mon, Sep 13, 2021 at 04:09 AM, n4buq wrote:

I notice there is also a diode, a 15V zener diode, and an RC network all
connected in series between the collectors of both Q3102/Q3104 and ground.
Is
this a fail-safe overvoltage protection circuit for the inverter should
Q3102/Q3104 or U3105 fail or is this a snubbing circuit to clip noise
spikes
(which I don't quite understand would be necessary), or something else? The
manual doesn't seem to mention it - at least not that I've found.
Barry,
Here is my interpretation. Waveform picture 3 shown at the node of
C3101/C3102 and the mean voltage +9.9 V there suggests that the voltage is
clamped at about +15 V immediately each cycle. The higher "hat" would be due
to R3105/C3105. The open peak voltage of the 30T windings would be much
higher than 15 V.
Albert






Albert Otten
 

Okay, Albert. Thanks for the reply. BTW, I presume you meant "waveform picture
3 shown at the node of

Q3101/Q3102", correct?
Hi Barry,
Aha, silly typing mistake, I wrote C3101/C3102, should be CR3101/CR3102 (and 3 other CRs!).
That's where waveform 3 at diagram 8 in my 070-1260-00 manual points to.
Note the the scale is 10 V/div.
Albert


n4buq
 

Then I followed suit with "Q" instead of "CR"! :(

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 8:35:00 AM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

Okay, Albert. Thanks for the reply. BTW, I presume you meant "waveform
picture
3 shown at the node of

Q3101/Q3102", correct?
Hi Barry,
Aha, silly typing mistake, I wrote C3101/C3102, should be CR3101/CR3102 (and
3 other CRs!).
That's where waveform 3 at diagram 8 in my 070-1260-00 manual points to.
Note the the scale is 10 V/div.
Albert






Albert Otten
 

Hi Barry, just in case it escaped your attention: for instance the 7904A manual contains a more extensive fault finding procedure for the similar SMPS.
Albert


n4buq
 

Hi Albert,

No, I hadn't seen that. I've downloaded that manual and am reviewing those sections. That should be a big help.

Thanks!
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 10:13:56 AM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

Hi Barry, just in case it escaped your attention: for instance the 7904A
manual contains a more extensive fault finding procedure for the similar
SMPS.
Albert






Ozan
 

Following link also has debug information on 7904 PSU which may contain useful information for 7704A PSU:
https://w140.com/Tek_7904_Power_Supply.pdf

Ozan


Göran Krusell
 

Hi, I have not followed this discussion very closely but my guess is that you are looking in the wrong direction.

You need to check U3105 pin 2, BAL SENSE, where the voltage should be 0V in normal operation. If it deviates more than 200mV (+ or -) then the PS should turn off.

Also check U3105 pin 13, I SENSE, where the voltage shall be positive, perhaps +0.23V, in normal operation. In an over-current situation the voltage drops below 0V and the PS should turn off.

If you hear a hissing and sparking sound you may have a problem with your voltage multiplier.

Carry on and good luck
Göran


n4buq
 

Very good! I appreciate the link.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Ozan" <ozan_g@erdogan.us>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 11:28:36 AM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

Following link also has debug information on 7904 PSU which may contain
useful information for 7704A PSU:
https://w140.com/Tek_7904_Power_Supply.pdf

Ozan






n4buq
 

Until I get that bad transistor replaced, then I don't plan on trying it connected to the display unit as a load. I tried that before I was aware of the bad transistor. Once I get that, I do plan to test it connected to the display unit. If it's still ticking, then it's either a problem with the display unit or the PS. In that case, the external load resistors will at least eliminate the display unit from the equation. I'll report back once I get the transistor replaced.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Göran Krusell" <mc1648pp@gmail.com>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 11:29:02 AM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

Hi, I have not followed this discussion very closely but my guess is that you
are looking in the wrong direction.

You need to check U3105 pin 2, BAL SENSE, where the voltage should be 0V in
normal operation. If it deviates more than 200mV (+ or -) then the PS should
turn off.

Also check U3105 pin 13, I SENSE, where the voltage shall be positive,
perhaps +0.23V, in normal operation. In an over-current situation the
voltage drops below 0V and the PS should turn off.

If you hear a hissing and sparking sound you may have a problem with your
voltage multiplier.

Carry on and good luck
Göran






Mark Vincent
 

Barry,

Goran is right about that area. VR3147 being open will throw the circuit off. C3127 and C3129 are marked ,1mfd. Some models use 1mfd in this place. I have replaced mine with the 1mfd 63V film types because the ESR of the originals was too high causing too much ripple and low B+. After replacing the parts, the B+ voltages at the pins on the 0067 IC were correct. These parts can be checked while the parts you have on order are being shipped. I also replaced the 1 and 2mfd electrolytics on the inverter board with film types of a higher voltage. The originals were bad in mine.

Mark


n4buq
 

I just checked those with my component checker (without U3105 plugged in as I wasn't sure what effect that might have) and, yes, I have several bad tantalums so those will be replaced. Oddly, C3150 is bad (measures about 32pF). I really didn't expect that one and, to be honest, thought the smaller tantalums would have been okay. I was wrong.

I wish I'd realized all this when I had the board out. Some of those can be replaced without having to remove the board but some, particularly C3150, may not be possible.

This poor power supply has had a rough time of it...

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 12:55:11 PM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

Barry,

Goran is right about that area. VR3147 being open will throw the circuit off.
C3127 and C3129 are marked ,1mfd. Some models use 1mfd in this place. I have
replaced mine with the 1mfd 63V film types because the ESR of the originals
was too high causing too much ripple and low B+. After replacing the parts,
the B+ voltages at the pins on the 0067 IC were correct. These parts can be
checked while the parts you have on order are being shipped. I also replaced
the 1 and 2mfd electrolytics on the inverter board with film types of a
higher voltage. The originals were bad in mine.

Mark






n4buq
 

In regards to C3150, Tek used a 1000pF, 1%, dipped silver mica. I don't know if they did that because of wanting to hold a tighter tolerance or better temperature compensation (or both). Since R3150 is a 49.9k 1% so there must be something rather critical going on with that circuit and I presume I should replace it with a dipped SM as well(?).

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "n4buq" <n4buq@knology.net>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 1:29:59 PM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

I just checked those with my component checker (without U3105 plugged in as I
wasn't sure what effect that might have) and, yes, I have several bad
tantalums so those will be replaced. Oddly, C3150 is bad (measures about
32pF). I really didn't expect that one and, to be honest, thought the
smaller tantalums would have been okay. I was wrong.

I wish I'd realized all this when I had the board out. Some of those can be
replaced without having to remove the board but some, particularly C3150,
may not be possible.

This poor power supply has had a rough time of it...

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 12:55:11 PM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

Barry,

Goran is right about that area. VR3147 being open will throw the circuit
off.
C3127 and C3129 are marked ,1mfd. Some models use 1mfd in this place. I
have
replaced mine with the 1mfd 63V film types because the ESR of the originals
was too high causing too much ripple and low B+. After replacing the parts,
the B+ voltages at the pins on the 0067 IC were correct. These parts can be
checked while the parts you have on order are being shipped. I also
replaced
the 1 and 2mfd electrolytics on the inverter board with film types of a
higher voltage. The originals were bad in mine.

Mark










n4buq
 

Yesterday, I discovered that C3150 was not bad. I lifted one leg and it checked good which really didn't surprise me as I don't think dipped silver micas have a reputation for going bad like that. With U3105 out of the circuit, I didn't think there would be anything else that would have affected an in-circuit test; however, apparently that was a false assumption because the +7.5V "rail" goes to quite a bit more places than I had first thought. I notice that other caps also check very low (30pF or so vs their designated values) in circuit and they all may be related to the same sort of in-circuit testing issue.

I'm still waiting on parts but hopefully, once I get those, I can make better progress on this PS.

Thanks for listening...
Barry - N4BUQ

----- Original Message -----
From: "Barry" <n4buq@knology.net>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 1:29:59 PM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

I just checked those with my component checker (without U3105 plugged in as I
wasn't sure what effect that might have) and, yes, I have several bad
tantalums so those will be replaced. Oddly, C3150 is bad (measures about
32pF). I really didn't expect that one and, to be honest, thought the
smaller tantalums would have been okay. I was wrong.

I wish I'd realized all this when I had the board out. Some of those can be
replaced without having to remove the board but some, particularly C3150,
may not be possible.

This poor power supply has had a rough time of it...

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 12:55:11 PM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

Barry,

Goran is right about that area. VR3147 being open will throw the circuit
off.
C3127 and C3129 are marked ,1mfd. Some models use 1mfd in this place. I
have
replaced mine with the 1mfd 63V film types because the ESR of the originals
was too high causing too much ripple and low B+. After replacing the parts,
the B+ voltages at the pins on the 0067 IC were correct. These parts can be
checked while the parts you have on order are being shipped. I also
replaced
the 1 and 2mfd electrolytics on the inverter board with film types of a
higher voltage. The originals were bad in mine.

Mark






n4buq
 

To follow up on this, I built a load resistor array (similar to the one Dennis detailed). That helped me isolate a few more transistors that, while tested "good" out of circuit, weren't working as they should when plugged back in to their sockets.

One of the 15V rails (I don't remember if that is + or -) is off by perhaps a volt and I think one of the +5V rails is high by about a volt (that might be the rail for the lamps, though, so perhaps not a real issue) so I have a bit more troubleshooting to do but I now get a trace, the lamps and plug-ins light up and work (somewhat), and the heat sinks are not excessively warm anymore so I'm almost there.

Thanks for all the pointers!

Barry - N4BUQ

----- Original Message -----
From: "Barry" <n4buq@knology.net>
To: TekScopes@groups.io
Sent: Wednesday, September 15, 2021 9:08:05 AM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

Yesterday, I discovered that C3150 was not bad. I lifted one leg and it
checked good which really didn't surprise me as I don't think dipped silver
micas have a reputation for going bad like that. With U3105 out of the
circuit, I didn't think there would be anything else that would have
affected an in-circuit test; however, apparently that was a false assumption
because the +7.5V "rail" goes to quite a bit more places than I had first
thought. I notice that other caps also check very low (30pF or so vs their
designated values) in circuit and they all may be related to the same sort
of in-circuit testing issue.

I'm still waiting on parts but hopefully, once I get those, I can make better
progress on this PS.

Thanks for listening...
Barry - N4BUQ

----- Original Message -----
From: "Barry" <n4buq@knology.net>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 1:29:59 PM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

I just checked those with my component checker (without U3105 plugged in as
I
wasn't sure what effect that might have) and, yes, I have several bad
tantalums so those will be replaced. Oddly, C3150 is bad (measures about
32pF). I really didn't expect that one and, to be honest, thought the
smaller tantalums would have been okay. I was wrong.

I wish I'd realized all this when I had the board out. Some of those can
be
replaced without having to remove the board but some, particularly C3150,
may not be possible.

This poor power supply has had a rough time of it...

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 12:55:11 PM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

Barry,

Goran is right about that area. VR3147 being open will throw the circuit
off.
C3127 and C3129 are marked ,1mfd. Some models use 1mfd in this place. I
have
replaced mine with the 1mfd 63V film types because the ESR of the
originals
was too high causing too much ripple and low B+. After replacing the
parts,
the B+ voltages at the pins on the 0067 IC were correct. These parts can
be
checked while the parts you have on order are being shipped. I also
replaced
the 1 and 2mfd electrolytics on the inverter board with film types of a
higher voltage. The originals were bad in mine.

Mark






Albert Otten
 

... and I think one of the +5V rails is high by about a volt (that might be
the rail for the lamps, though, so perhaps not a real issue)
Barry, too high +5VL can be an issue since this is used in the balance sense against -17V. Albert.


n4buq
 

Hi Albert,

Oddly, I powered it up again this morning and the rails are much closer than they were last night. Lamps voltage is slightly low (tenths of a volt) but, otherwise, they look very close now.

The scope has vertical issues and will post that in a new thread but I'm pretty excited about getting things working to the level they are now.

Thanks again,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: TekScopes@groups.io
Sent: Monday, September 20, 2021 7:40:45 AM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

... and I think one of the +5V rails is high by about a volt (that might be
the rail for the lamps, though, so perhaps not a real issue)
Barry, too high +5VL can be an issue since this is used in the balance sense
against -17V. Albert.