Lost trace line on 2700

Started by P3_Orion, September 24, 2024, 10:50:44 AM

Previous topic - Next topic

P3_Orion


Howdy,

New user here trying to get help with a failed 2700. The unit was working but the trace line was starting to go crazy with random zig zags and wild fluctuations in signals. It has now mostly stopped working although it does randomly show an inconsistent trace line that might span only an 1/8 of the screen width and then shoot across and retreat or just go away completely.

I asked Huntron for schematics or repair manual without success, although they must have them if they repaired their own products in the past. But I understand its all proprietary so here we are trying to figure this out from scratch.

Since the trace line from my understanding is an AC output signal, I have been starting from the connector trying to work back. I am only showing AC voltage at the probes in the mV range. I checked voltage at the 2w output power resistor. Again only showing erratic mV readings, this traced back to an output buffer (OpAmp, 634U) that I believe controls the output current. I again have erratic voltage in and out signals and the chip does have a proper DC supply voltage of I believe was 15v.

Since the input voltage is low and erratic I think the problem is further back, but I could spend forever trying to trace back to the transformer I am assuming. I do see there are 7 Voltage Regulators, 6 LM317 Mosfet style, all warm without to the touch and consistent so assuming they are all doing there job, then their are 2 'jellybean' BJT style voltage regulators that I can only assume work. There is no way to test them live as the risk of shorting is too high. I see test points all over the board, but again without a schematic I have no idea what any of them are tied to.

I didn't mention this before but the entire screen works normally and all the buttons appear to function correctly and the unit was working before the trace line become erratic and now mostly non existent.

Hopefully this might be a common failure and someone has a magic bullet to hand me :)

Thanks Again!
Team Member
Comploc/Digicomp Technologies

admin

Sometimes you have to talk with the right guy. :)

I think these have been posted to the forum before but here they are again.
Main board
Test procedure

The most important troubleshooting tool is your eyes.

P3_Orion

Absolutely Brilliant!

Thank You!  ;D

Quote from: admin on September 24, 2024, 03:56:42 PMSometimes you have to talk with the right guy. :)

I think these have been posted to the forum before but here they are again.
Main board
Test procedure


Team Member
Comploc/Digicomp Technologies

P3_Orion

Ok I have been testing and tracing for a few hours now and one thing stands out the most to me.

Everything DC seems to be working fine, and I have checked quite a few chips and I have the powers and grounds expected for the most part. Some outstanding issues I also encountered where the following:

On U46 the 5v VCC on pin16 was only 3.9, VEE on pin 7 read -4.9

On U49 5v VCC on pin 16 was 0.00V so I assume that is a problem and possibly tied to the lack of 5v on the following:

I am missing the 5v signal to all the source pins of Q5 through Q13. From the diagram these look like they should power the inductors which would then send a signal to the front banana jacks.

When I change the resistance selector 10,100,1k,10k,100k, that all triggers a 5v signal to the gate pins, but since there is no 5v signal on the source it never activates the inductors K3-k11.

When I run through the test procedure I fail on the voltage checks as there is no voltage present at the jacks. When I test for frequency all I get is 60hz, changing the frequency on the panel does not result in a change.

When I run it through the diagnostic procedure it passes on initial voltage checks but quickly fails on the offsets and will not proceed past this point.

Examples of this look like the following:
OFF = -32 -12800
OFF = -5 -3237
OFF = -11 -4257
OFF = -6 -2372

I can keep hitting proceed and I just get repeats or different figures but they all follow this same pattern forever. Actually, I didn't try forever, but probably about 8 times. Hold on now.....Ok so I can get past that but then it goes on to 'Set Compensation' and that just seems to go on forever without result.

Lets start with this. I have been on this for a few hours now so going to take a break and hopefully someone can guide me to the missing 5v signal?

Thanks in advance.
Team Member
Comploc/Digicomp Technologies

admin

Vcc on U46 and U49 should both read the same (as well as any IC attached to +5VA) since they attach to +5VA. Are you seeing +5V at TP18 (+5VA at the voltage regulator)?
The most important troubleshooting tool is your eyes.

P3_Orion

Yes, I have 5V on TP18.

Current Points Tested:

TP4 Oscillating 1.5 to 5.3
TP7 1.7 AC

TP14 0.0 ??
TP15 5

TP16 23 < 28
TP17 14.67 < 15
TP18 5 = 5
TP19 -23 < -28
TP20 -14.8 = -15
TP21 -4.9 = -5
TP22 4.99 = 5
TP23 3.3 = 3.3
TP24 31 > 30
TP25 9
Team Member
Comploc/Digicomp Technologies

fetron

#6
Hello!

Check R158, R162 1kOhm 1%.

Check CR5, CR9 1N914 diode.

Check C14, C24 25V 47uF and  C16, C19, C27, C72 0,1uF and C26, C70 25V 10uF.

P3_Orion

Quote from: fetron on September 25, 2024, 11:12:24 PMHello!

Check R158, R162 1kOhm 1%.

Check CR5, CR9 1N914 diode.

Check C14, C24 25V 47uF and  C16, C19, C27, C72 0,1uF and C26, C70 25V 10uF.

Ok so I am getting some weird reading on everything here and more. Not sure if my Fluke meter is not able to read things correctly but let me at least explain my findings.

On Resistor R158 I am reading 854 Ohm, far below the 1% rating
On Resistor R162 I am reading 964 Ohm, far closer but still out of the 1% spec.

Most resistors are returning expected values but some are returning half values or unexpected values that could be from being in circuit with other components?

CR5 and CR9 are not 1N914 diodes, they are BAT54 style diode. Regardless of style they are passing voltage both ways, and not just CR5 and CR9, but all of them on the board of the same part number are acting the same way. A couple will allow the voltage passing through to climb to 2.3v before the meter goes OL.

On Capacitors, I have a huge amount that are reading OL, others are reading far outside the expected ranges. Examples would be C14 and C24 reading 230uF, C15, C25, C28 reading OL, C70 reads 231uF

Literally almost every capacitor measured reads either OL or some unexpected value. I put a new battery in the meter and results are the same.

I would think that if this was some kind of systemic failure throughout the board I would have much bigger issues then just losing the trace line, but I could be wrong?

On the capacitors, its possible my meter cannot measure some of the finer .01uF values, but larger values like the 47uF should be pretty obvious.

I would really like to verify these diodes and capacitors, I need a solid confirmation of what is good or bad so I don't feel like I am in the electronics Bermuda Triangle of lost souls  :o
Team Member
Comploc/Digicomp Technologies

fetron

Hello P3_Orion!

The components should be measured removed from the circuit. Otherwise, you will measure an incorrect value.
You could replace the capacitors and resistors named above without measuring.
If you still want to measure a capacitor, you must measure it with an ESR meter (e.g. U1733C or ESR70).

P3_Orion

Quick update, I have ordered a dozen BAT54 diodes and am just going to replace them all. Since they are a SOT23 component, desoldering and testing them is fine but I might as well replace them while I am in there.

On the capacitors, its very possible my meter will not measure them correctly in circuit. I do have a capacitor/inductor tester so I will pull a couple suspects out when I tackle the diodes and test them on the cap meter.

Shame huntron won't support these any more, we would gladly spend the money to either have it factory repaired or even purchase a known good main board. We are a small shop and purchasing a newer unit is too costly and our older 2000 units are fading fast.

2000's have been the foot soldiers of the shop and have been going to 30+ years strong, but they are showing signs of dying soon. We really would like to get this 2700 up and running reliably.

I want to say thanks to everyone who has helped so far, will post up as soon as I get some parts replaced. Hoping its just a couple simple diodes, that would be cherry!

Team Member
Comploc/Digicomp Technologies

P3_Orion

#10
Well, I replaced all 16 bat54 style diodes, the other axial diodes tested good. Well its not fixed but it did make some progress I guess. It again passes all the voltage diagnostic tests, but fails on the other tests as before.

In the diagnostic menu if I select wave test, I at least get a line, its not a wave but its a line none the less so I at least know its capable of outputting the line to the display. I was briefly getting a partial line but when I grounded the probes together it did nothing and eventually just went away.

I put it in scan mode and alternate mode and I do get an alternating 5v signal on all the output transistors Q5-13, Except, Q11 has 5v constant and Q10 seems to be doing nothing with no voltage output.

Lets see TP4 that had a huge oscillation from 1.5v to 5.3 is now stable at -4.5 volts, interesting it flipped polarity.

C62 exhibits some odd behavior, it shows -14v on one side and -11v on the other. All the other caps show approximately zero voltage on one side and signal voltage on the other side, typically 2.5, 5, 14, 26, positive or negative, etc.. All the caps seem to be working, none are dead shorts or open, except maybe C62??


Other than that haven't made any progress really. Not sure what to do?

If there are any experienced Huntron gurus that think they can fix this board for a fair price I would entertain sending the board out for repair.
Team Member
Comploc/Digicomp Technologies

fetron

#11
Okay!

On what side of the drawing is C62?

P3_Orion

Quote from: fetron on October 04, 2024, 02:09:11 AMOkay!

On what side of the drawing is C62?

Looks like C62 checks out. Its tied to U53 which is the IC for brightness control which works just fine. The C62 is tied directly -15v and is in a comparison circuit for the IC. So no issues found so far.

Problem is I am unsure where to be looking for the problem. I know this is directly related to the wave form signal output the screen but I am unsure what drives that signal output.

Perhaps someone could point me in the right direction?



Team Member
Comploc/Digicomp Technologies

admin

One thing I forgot to suggest you look at was F1 and F2. These are in the signal amp circuit located near the back edge of the PCB. Newer models had resetting fuses but older ones did not.
The most important troubleshooting tool is your eyes.

P3_Orion

Quote from: admin on October 07, 2024, 01:52:49 PMOne thing I forgot to suggest you look at was F1 and F2. These are in the signal amp circuit located near the back edge of the PCB. Newer models had resetting fuses but older ones did not.

Thank You, fuse F1 (100mA) is indeed open, I do not see a F2 fuse (50mA). But I am at least in the right ballpark I hope, I will start testing everything within the signal amplifier stage and see what I can come up with. As a note I had checked that fuse but the device diagnostics testing did not flag any incorrect voltages so I assumed I was perhaps testing the fuse incorrectly, as I mentioned I am in the Bermuda Triangle of electronics repair where absolutely nothing is as it seems  :(
Team Member
Comploc/Digicomp Technologies