7704A Vertical Amplifier Issues


n4buq
 

A few weeks ago, I posted a message regarding something that overheated from in/around the Vertical Amplifier board in my 7704A. Night before last, I was trying to determine what might be causing the traces to occupy only about 1/10 of the vertical space on the CRT and the vertical position has very little effect. I thought that the two connections to the vertical plates might be touching something between the vertical amplifier board and where they plug in to the pins on the CRT neck. When I powered it back on, I noticed smoke again and observed that R4419 was the culprit.

In my previous post, I noted that there appeared to be a "flying wire" mod on the vertical amplifier board but after noticing which component was overheating, I investigated the schematic and found R4419 is part of what appears to be a factory upgrade to include Q4420 and its supporting components. I also discovered that Q4420 is fully open so I plan to rebuild that entire circuit as the one I have isn't done as neatly as I've seen other examples.

One of the components is CR4419 which was originally a 2N4152 and I'm not finding much in the way of cross-references for that. I think the original part is still good but has fairly short leads and I would like to have a diode with longer leads to work with. Does anyone know of a good sub? I have 1N400x and 1N5819. Would either of those be suitable? I prefer the 1N5819 due to the slightly smaller package but should be able to work with either one.

Also, from what I've seen, the problem with my vertical amplifier may be U4413. I'm holding out hope it isn't that but I have a bad feeling that it is.

Thanks,
Barry - N4BUQ


Mark Vincent
 

Barry,

You can test from the two bases of the outputs to the collectors. That may indicate the output IC is bad. Q4420 is a 2N3906 and CR4419 is a 1N4152. A 2N2222A and 1N4148 or 1N4937 will work as subs for these two. At the emitter of Q4420 should be 15,6V. The 33 and 5600 ohm resistors should be 1W. The bias voltage at R4416 should be 10,1V. The output pins should be 36V and 37V when working. Since the 33 ohm resistor burned, it is likely the IC is bad. If the 50V supply went too high, that will short the output IC. There is a seller on "that site" that has some known working boards from this model. Ask if he has the vertical output board. There is a NOS IC on there also. See if the voltages at the pins are correct. These can be done at ends of resistors. The four 200 ohm resistors should have 9,3V at the IC. Voltages that are way off, will also mean the IC is bad. It looks like the parts you ordered made the power supply work.

Mark


n4buq
 

Hi Mark,

Okay on the parts. Apparently I was searching for a 2N4152 for CR4419 instead of a 1N4152 (which is readily available).

Is it acceptable to check the voltages at the IC without Q4420 connected? It would be good if I could go ahead check that now and get something ordered if necessary.

You mentioned a 2N2222A as a sub for Q4420. Isn't Q4420 an NPN? I have some 2N3906s on order (thought I had a few in my parts bin but apparently not).

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: TekScopes@groups.io
Sent: Tuesday, September 21, 2021 10:34:55 PM
Subject: Re: [TekScopes] 7704A Vertical Amplifier Issues

Barry,

You can test from the two bases of the outputs to the collectors. That may
indicate the output IC is bad. Q4420 is a 2N3906 and CR4419 is a 1N4152. A
2N2222A and 1N4148 or 1N4937 will work as subs for these two. At the emitter
of Q4420 should be 15,6V. The 33 and 5600 ohm resistors should be 1W. The
bias voltage at R4416 should be 10,1V. The output pins should be 36V and 37V
when working. Since the 33 ohm resistor burned, it is likely the IC is bad.
If the 50V supply went too high, that will short the output IC. There is a
seller on "that site" that has some known working boards from this model.
Ask if he has the vertical output board. There is a NOS IC on there also.
See if the voltages at the pins are correct. These can be done at ends of
resistors. The four 200 ohm resistors should have 9,3V at the IC. Voltages
that are way off, will also mean the IC is bad. It looks like the parts you
ordered made the power supply work.

Mark






Mark Vincent
 

Barry,

I was wrong about the 2N2222A being a sub. The original is a PNP. My mistake.

The voltages would be best checked with the transistor and resistors replaced. Testing the output IC from the base to collectors can be done now. One end of the meter to where the 33 ohm connects and the other end of the meter to each plate. That will tell if that part of the output transistors will be good. That is not a conclusive test for the output IC. Replace the parts. See if the 33 ohm resistor gets warm. If it gets hot/burns again, the IC is the problem.

Mark


n4buq
 

Okay, Mark. Will do.

BTW, can you explain the function of CR4419? I don't find the part of the circuit surrounding Q4420 outlined in the Circuit Description section of the manual for the Vertical Amplifier.

Thanks again,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: TekScopes@groups.io
Sent: Wednesday, September 22, 2021 8:46:02 AM
Subject: Re: [TekScopes] 7704A Vertical Amplifier Issues

Barry,

I was wrong about the 2N2222A being a sub. The original is a PNP. My mistake.

The voltages would be best checked with the transistor and resistors
replaced. Testing the output IC from the base to collectors can be done now.
One end of the meter to where the 33 ohm connects and the other end of the
meter to each plate. That will tell if that part of the output transistors
will be good. That is not a conclusive test for the output IC. Replace the
parts. See if the 33 ohm resistor gets warm. If it gets hot/burns again, the
IC is the problem.

Mark






Mark Vincent
 

Barry,

CR4419 is a clamp. Voltage at the emitter and one end of the 33 and 5600 ohm resistors can go lower than 15,6-15,7V, not higher. It also sets the common base voltage and current limit for the output stages. The voltage divider on the base of Q4420 sets that voltage at the base and emitter. If either/both output transistors in the IC short from C to B, that will cause the 33 ohm resistor to burn. Testing those two junctions will determine if the output transistors are good or bad between those two junctions. One thing that can cause the output to go bad is someone turning the bias control too high, R4415 at too low resistance making the voltage higher than 10,1V at one end of R4416. If that pot was not touched, it should be at the proper voltage. Once you get it working, check that voltage and adjust as necessary. If you want, you can turn the pot a little so the voltage will be bit low at first then increase. Starting at a lower current will not hurt. You reduce the chance of any damage with lower current. When set right, the vertical will be correct. The centering control may need adjusting. That is done with no vertical plug-ins installed. Any other controls for things may need to be touched up slightly with another IC installed. See if the controls need adjustment after it is working and a known type of signal is applied. If the trace is the way it should be, leave the other controls alone. The manual will say how to calibrate.

Mark


Albert Otten
 

On Wed, Sep 22, 2021 at 04:53 PM, Mark Vincent wrote:
If either/both output transistors in the IC short from C to B, that will cause
the 33 ohm resistor to burn.
Perhaps the later on added R4426 2k between Q4420 collector and GND is intended to prevent this.
Albert


n4buq
 

Yes, I plan to include that when I rebuild the circuit.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: TekScopes@groups.io
Sent: Wednesday, September 22, 2021 11:11:31 AM
Subject: Re: [TekScopes] 7704A Vertical Amplifier Issues

On Wed, Sep 22, 2021 at 04:53 PM, Mark Vincent wrote:
If either/both output transistors in the IC short from C to B, that will
cause
the 33 ohm resistor to burn.
Perhaps the later on added R4426 2k between Q4420 collector and GND is
intended to prevent this.
Albert






n4buq
 

Mark,

Thanks for the explanation. That really helps.

I checked C to B for the output transistors in the IC and those appear to be good. I also checked E to B for the input transistors and those also appear to be good (although the junction voltage showed to be about 700 mV which wasn't low enough to cause the Fluke diode check function to "beep").

When I was seeing a trace and, apparently, R4419 was not overheating at that time, it was "squished" vertically to about 1 cm high and off center. The POSITION knob on the plugin would only move the trace up and down about 2 or 3 mm. I tried the centering pot (R4493) and that had little to no effect and that still makes me worry that the IC is bad but I won't know for sure until I get the Q4420 circuit reinstalled.

The parts for the Q4420 circuit are due in today so I should have that together soon and can test the overall functionality.

Thanks again,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: TekScopes@groups.io
Sent: Wednesday, September 22, 2021 9:53:12 AM
Subject: Re: [TekScopes] 7704A Vertical Amplifier Issues

Barry,

CR4419 is a clamp. Voltage at the emitter and one end of the 33 and 5600 ohm
resistors can go lower than 15,6-15,7V, not higher. It also sets the common
base voltage and current limit for the output stages. The voltage divider on
the base of Q4420 sets that voltage at the base and emitter. If either/both
output transistors in the IC short from C to B, that will cause the 33 ohm
resistor to burn. Testing those two junctions will determine if the output
transistors are good or bad between those two junctions. One thing that can
cause the output to go bad is someone turning the bias control too high,
R4415 at too low resistance making the voltage higher than 10,1V at one end
of R4416. If that pot was not touched, it should be at the proper voltage.
Once you get it working, check that voltage and adjust as necessary. If you
want, you can turn the pot a little so the voltage will be bit low at first
then increase. Starting at a lower current will not hurt. You reduce the
chance of any damage with lower current. When set right, the vertical will
be correct. The centering control may need adjusting. That is done with no
vertical plug-ins installed. Any other controls for things may need to be
touched up slightly with another IC installed. See if the controls need
adjustment after it is working and a known type of signal is applied. If the
trace is the way it should be, leave the other controls alone. The manual
will say how to calibrate.

Mark






n4buq
 

I am happy to report that rebuilding the circuitry around Q4420 resolved the issue I was having with the Vertical Amplifier. I can now, for the first time since I got this scope, see a trace and fully adjust its vertical position on the CRT. I am somewhat elated.

Unfortunately, some of the 1/4W metal film resistors I ordered were slightly larger than the others (almost as big as a 1/4W carbon comp) and I didn't think there would have been room to install that circuit quite like I wanted to so I built it on a very small square of project board, mounted that using an existing hole in the lower-left corner of the amplifier board, and ran jumper wires as needed. A bit ugly, but it works. Once the smaller parts arrive, I plan to build it in the spare, but small, open spot next to U4413 (which, I am relieved to find, is working).

Thanks so much for all the pointers!
Barry - N4BUQ

----- Original Message -----
From: "Barry" <n4buq@knology.net>
To: TekScopes@groups.io
Sent: Thursday, September 23, 2021 8:38:15 AM
Subject: Re: [TekScopes] 7704A Vertical Amplifier Issues

Mark,

Thanks for the explanation. That really helps.

I checked C to B for the output transistors in the IC and those appear to be
good. I also checked E to B for the input transistors and those also appear
to be good (although the junction voltage showed to be about 700 mV which
wasn't low enough to cause the Fluke diode check function to "beep").

When I was seeing a trace and, apparently, R4419 was not overheating at that
time, it was "squished" vertically to about 1 cm high and off center. The
POSITION knob on the plugin would only move the trace up and down about 2 or
3 mm. I tried the centering pot (R4493) and that had little to no effect
and that still makes me worry that the IC is bad but I won't know for sure
until I get the Q4420 circuit reinstalled.

The parts for the Q4420 circuit are due in today so I should have that
together soon and can test the overall functionality.

Thanks again,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: TekScopes@groups.io
Sent: Wednesday, September 22, 2021 9:53:12 AM
Subject: Re: [TekScopes] 7704A Vertical Amplifier Issues

Barry,

CR4419 is a clamp. Voltage at the emitter and one end of the 33 and 5600
ohm
resistors can go lower than 15,6-15,7V, not higher. It also sets the common
base voltage and current limit for the output stages. The voltage divider
on
the base of Q4420 sets that voltage at the base and emitter. If either/both
output transistors in the IC short from C to B, that will cause the 33 ohm
resistor to burn. Testing those two junctions will determine if the output
transistors are good or bad between those two junctions. One thing that can
cause the output to go bad is someone turning the bias control too high,
R4415 at too low resistance making the voltage higher than 10,1V at one end
of R4416. If that pot was not touched, it should be at the proper voltage.
Once you get it working, check that voltage and adjust as necessary. If you
want, you can turn the pot a little so the voltage will be bit low at first
then increase. Starting at a lower current will not hurt. You reduce the
chance of any damage with lower current. When set right, the vertical will
be correct. The centering control may need adjusting. That is done with no
vertical plug-ins installed. Any other controls for things may need to be
touched up slightly with another IC installed. See if the controls need
adjustment after it is working and a known type of signal is applied. If
the
trace is the way it should be, leave the other controls alone. The manual
will say how to calibrate.

Mark






Albert Otten
 

Congratulations Barry!
(Though I still don't understand how that 33R resistor could get burned when U4413 is OK.)
Albert


n4buq
 

I still think I may have shorted one of the output leads from U4413 to ground just before R4419 overheated. I'm still curious as to why the two long, output leads from U4413 are drawn out across and between the bottom of the board and the chassis on their way through the two insulated holes and then to the CRT. It seems like running those to a couple of pads on the board would have been cleaner and less prone to shorting but since those two leads are purposefully so much longer than the others, there must be a good reason. BTW, R4419 doesn't get warm enough to notice now. U4413 does get slightly warm but I think that's normal.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: TekScopes@groups.io
Sent: Friday, September 24, 2021 2:51:47 PM
Subject: Re: [TekScopes] 7704A Vertical Amplifier Issues

Congratulations Barry!
(Though I still don't understand how that 33R resistor could get burned when
U4413 is OK.)
Albert






Mark Vincent
 

Albert and Barry,

The reason I specified 1W for the 33 ohm resistor is because I measured 3,13V across the 33 ohm resistor. The base voltage is 14,4 and the emitter voltage is 17V. I measured this in mine that is working fine at the time of this post. I know the EB voltage is not standard ,6-,7V. The reason for the two longer leads on the out is to the load side of the distributed plates. That is what it is supposed to be. Having them go to pads would increase the C of the circuit.

The nonlinearity in the horizontal circuit may be due to bad parts. I can give you a list of parts that should be changed. If necessary, I do have a spare horizontal board that is known good if you need one. Check to see if the inputs are linear with another oscilloscope. I do know some resistors should be increased in wattage. I did add heatsinks with compound to the outputs to keep them cooler.

If Barry needs 33 ohm 1W and 5600 1% I can send them to him. The 33 ohm does not need to be 1%, 1W yes.

The output IC is operating normally at warm. I so happy you got the vertical problem fixed easily. I did add a 12V fan to cool the two circuits. It is running less than 12V. I like my items to be cooler than original to give longer life of parts.

Mark


Albert Otten
 

On Sat, Sep 25, 2021 at 05:15 AM, Mark Vincent wrote:
The reason I specified 1W for the 33 ohm resistor is because I measured 3,13V
across the 33 ohm resistor. The base voltage is 14,4 and the emitter voltage
is 17V. I measured this in mine that is working fine at the time of this post.
I know the EB voltage is not standard ,6-,7V.
Hi Mark,
Something must be wrong here. For comparison, in one of my 7704As, at Q20 I measured Ve = 15.45V, Vb = 14.86 V, Veb = 0.59 V.
Across R4419 I found merely 0.125 V. (BTW this resistor was 39R 1/8W, clearly a replacement soldered in later. Original 33R also burned??).
Is your A44 board an early version at which Q20 etc. have been added later on? With possible mistakes? All my boards are later versions.
Albert


Roger Evans
 

Mark,

I agree completely with Albert, if you look at the nominal voltages on the schematic the output transistors of U4413 should have a collector current of 36mA ( (50-37)/360R ) so with any reasonable current gain they should not draw more than a couple of mA of base current. Albert measures 4mA for the total base current into both output transistors which is eminently resonable.

Neglecting the base current into Q4420, its base should sit at 14.88V ( (5k / 16.8k) * 50V) and its emitter should be around 0.6V-0.7V more positive so 15.5V - 15.6V and Albert measures 15.45V. The function of Q20 is to provide a low source impedance to bias the output transistors of U4413 at 15.45V rather than the 15V whichTek could have obtained by connecting R33 directly to +15V. This also implies that R4419 should be a low inductance type to maintain the frequency response of the amplifier.

I think it is safe to say that any junction transistor which measures a forward Vbe of 2.6V is no longer a transistor! If you take Q4420 to be open circuit from base to emitter then R4420 and R4419 supply the base current to the output transistors without any voltage regulation which is why the bias voltage shoots up to 17V.

Regards,

Roger


n4buq
 

I checked the voltage across R4419 and that measured 32mV. I'm pretty sure either something shorted or Q4420 acted badly before losing one of its junctions.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Roger Evans via groups.io" <very_fuzzy_logic=yahoo.com@groups.io>
To: TekScopes@groups.io
Sent: Saturday, September 25, 2021 11:08:01 AM
Subject: Re: [TekScopes] 7704A Vertical Amplifier Issues

Mark,

I agree completely with Albert, if you look at the nominal voltages on the
schematic the output transistors of U4413 should have a collector current of
36mA ( (50-37)/360R ) so with any reasonable current gain they should not
draw more than a couple of mA of base current. Albert measures 4mA for the
total base current into both output transistors which is eminently
resonable.

Neglecting the base current into Q4420, its base should sit at 14.88V ( (5k /
16.8k) * 50V) and its emitter should be around 0.6V-0.7V more positive so
15.5V - 15.6V and Albert measures 15.45V. The function of Q20 is to provide
a low source impedance to bias the output transistors of U4413 at 15.45V
rather than the 15V whichTek could have obtained by connecting R33 directly
to +15V. This also implies that R4419 should be a low inductance type to
maintain the frequency response of the amplifier.

I think it is safe to say that any junction transistor which measures a
forward Vbe of 2.6V is no longer a transistor! If you take Q4420 to be open
circuit from base to emitter then R4420 and R4419 supply the base current to
the output transistors without any voltage regulation which is why the bias
voltage shoots up to 17V.

Regards,

Roger






Albert Otten
 

I still wasn't very confident about my results and now did measurements in another 7704A. The A44 board is the later version and R4419 is 33R as should be. But guess what ... the voltages were as if Q20 etc. did not exists (Ve < Vb) and as if CR4419 just conducted.
50 mV across R4419 (in stead of earlier 125 mV across 39R) which is closer to what Barry found.
I appeared that R4420 (5k6 to +50V) was open, but without any sign of overheating. Luckily this does no harm and explains the results.

Albert


Albert Otten
 

(continued) After a temporary replacement of the open R4420 I measured the following:
Q4420 Vb = 14.98 V, Ve = 15.64, Veb = 0.66 V and across R4419 again 50 mV (of coarse).
This means about 6.2 mA via Q4420, 1.5 mA via R4419 and 4.7 mA emitter current via Q4420. Altogether very reasonable values for that transistor.
Albert