[vwyf11]

Hey,

I’ve gotten to the testing point and I’m trying to adjust tr2 to get 3v between tp2 and tp3.  The voltage when I initially tested at these points gave me 3v steady with good consistency when changing tr2.  I moved on and could not get past step 6.  I remeasured between tp2 and tp3 and was no longer getting 3v but instead bouncing around 2.7 to 3.1v sporadically and tr2 no longer seemed to make the same consistent change when turning.  

I was checking for shorts anywhere and did find continuity between two pins on U3 either between output and adjust or output and input.  I will have to double check which pins exactly.

I’m not sure where to go from here or what may need checking

Thanks

[JPK]

I moved on and could not get past step 6
What happened at step 6?

Check the solder joints on R26, Tr2, R31.
Check the wire connections to the transformer, look for a thread that might be causing a short circuit.

I was checking for shorts anywhere and did find continuity between two pins on U3
Pins 1 and 2 are connected by a 7R5 resistor, whick way look like a short.

[vwyf11]

Sorry, step 7 was where I was hung up.  I could not match bypass on and off level.  I was reading 1VAC between pins 2 and 3 when bypass was up and 0VAC when depressed regardless of how much I turned tr1

[vwyf11]

R26, TR2 and R31 all look fine and no bridged pins etc.  The resistance on the 5k pots don’t test the way I would expect.  If I measure from the wiper to another pin on TR2 I only get resistance from 0.3ohms to about 1.6Kohms.  Not sure if there’s a problem there.  I tested between tp2 and tp3 again and the voltage swings from 2.7V to 3.1V and will never settle

[JPK]

I only get resistance from 0.3ohms to about 1.6Kohms
Normal because the trimmer is in parallel with R31+R32.

I have added some DC voltage checks to do here. Please compare with your values.

[vwyf11]

I did notice now that I forgot to trim the leads of T2 and that may have shorted to the case.  Is there a test while that is still soldered to check for issues there?

[JPK]

You could look for a continuity between ground and C8, C9, C10. But the best thing to do will be to shorten the pins.

[vwyf11]

I couldn’t get consistent voltage on a couple of the readings so I just decided to pull everything apart.  I made an incredibly stupid error and skipped over the last two diodes on the front panel pcb.  I couldn’t find the 1n914 diodes included but I had some 1n4148 diodes so I put them in their place at d10 and d11.  Testing seems to be going much better so far.  I know these diodes are almost identical so shouldn’t be an issue.  Will these be any problem?

[JPK]

No problem, these diodes are very close.

[vwyf11]

When I installed the diodes I was able to get a solid 3v between tp2 and tp3.  After this step I had double checked the next day and am back to not being able to get a steady 3v from tp2 to tp3.  The meter bounces around from 2.4 or so volts to 2.8v.  I cannot even get to 3v anymore. I’ve ripped everything apart and double checked everything now.  Tried reflowing parts, nothing seems to be working.  

The voltage from 0v to V+ and V- both give me steady correct voltage.  

All the other tests on the page you link to are giving me results in the ballpark for the required voltages but all fluctuate around 0.25v up or down from the target voltage

[JPK]

The diodes D10, D11 on the front PCB have no effect on the the output stage bias setting.
Is the voltage on TP3, against 0V stable, around -11.5V ?
Is the bias voltage bouncing when you tap the PCB or specific components with a pen ?

[vwyf11]

No, the voltage from tp3 to 0v is not stable.  It bounces around from -10 to -12.5 very erratically.  I’ve tried tapping around and checking at each component or the pcb but nothing seems to directly affect the erratic nature of the voltage readings.  It seems it’s getting even worse.  The readings are jumping instantaneously from 0v to 4.5v when measuring between tp2 and tp3.  Measuring the voltage at the card extender is a steady 15.5 volts + and -

[JPK]

Check the solder joint on D20.
If solder joints are OK try to replace D20 (Zener 4V7-1.3W).

[vwyf11]

I’ve replaced D20, Q7, U3, TR2.  Before replacing anything I checked between TP2 and TP3 and was getting 0.001V.  After replacing all the components listed, I’m getting the same values. 

Here are the values from the DC voltage checks in the link you gave from before:

A: 1.14V this value bounces around a lot
B: 0.561V
C: 0.525V this value bounces around a lot
D: 28.1V
E: 28.1V
F:0.001V

TP3 to 0V measure -12.8V

[JPK]

Is it time to send it to our workshop for a check?

[vwyf11]

As a last ditch effort I would guess, but would like to avoid sending it overseas if it’s possible.  

The voltage at the transformer was always the correct 24 volt reading before, now suddenly it’s 28.  I was atleast getting voltage between tp2 and tp3 now I’m suddenly seeing nothing.  Seems pretty drastic and like there should be something blatantly obvious.  I’ve looked at the schematic but it’s a little over my head with the amount of paths.  Is there a good straight shot to test voltage where the issue may be originating?  I obviously know I’m getting the correct voltage to the unit but having a tough time pinpointing the dc voltage path order.

[JPK]

E: 28.1V
F:0.001V

This can only happen if Q7 is dead or disconnected.
Can you measure the DC voltages directly on the pins of Q7?

[vwyf11]

I’m gonna double check tonight.  I know Q7 was cold where normally it’s pretty warm.  

Is this the correct replacement for Q7?

https://www.mouser.com/ProductDetail/863-2N6488G

[vwyf11]

Q7 collector - emitter 28.05V
Collector -base 27.93V

I was looking at where the test points are for tp2 and tp3 and tried testing from tp2 to the anode of d20 and I do get around 3V at that point.  Testing in this way still gives no change in voltage when turning tr2.  If I test from tp2 to the cathode of d20 I get 0V.

[JPK]

TP3 is at the cathode of D20, not the anode.

Collector -base 27.93V
What do you mean?
- Collector and base @ 27.93V or
- Collector to base is minus 27.93V

I know Q7 was cold where normally it’s pretty warm. 
 It confirms that is is dead or disconnected (check the solder joints).

Is this the correct replacement for Q7?
Yes, correct.

[vwyf11]

Yeah sorry i understand the cathode of d20 is tp3. So measuring from cathode of d20 to tp3 should give the same measurement as tp2 to tp3.  Is this correct?

When I measure from the anode of d20 to tp2 I do see voltage but not from the cathode of d20 to tp2.  I’m sometimes confused by zener diodes in how they operate but this doesn’t seem correct.  As if d20 is blocking from Anode to cathode.  

Sorry, so (+probe) on collector (-probe) on base gives (+27.93V)

[JPK]

(+probe) on collector (-probe) on base gives (+27.93V)
It shows that Q7 is dead.

So measuring from cathode of d20 to tp3 should give the same measurement as tp2 to tp3.  Is this correct?
No. Measuring from cathode of d20 to tp2 should give the same measurement as tp3 to tp2.

When I measure from the anode of d20 to tp2 I do see voltage but not from the cathode of d20 to tp2.
It just shows that there is no current flowing through Q7. The voltage you see is the voltage drop in the Zener, normally a little above 4V.

[vwyf11]

Ok thanks for clearing all that up!  Definitely gives me a better understanding