FG502 doesn't start at some specific settings


durechenew@...
 

The following post was initially in tek500 group; it seems that group is not as active as this one and I thought it may help if it's posted here (I see other posts for 500):

Have an FG502 for while; repaired some issues (if someone interested, I can provide details). For the specific item I have the issue is it doesn't start right away (triangular form generator) on rages 10^2, 10^3, 10^4 with the dial set at 7 and above (to 11); that happens when device is cold. It starts after 40 to 90 seconds (I believe it's about how cold it is). If worm, starts normal. In all other settings starts normal.
Digging around I found that the triangular generator stays in a state where is happy (I can provide all voltages in significant points of the schematic) until it decides to start working. I'm attaching two pictures of a normal start and the delayed one. In the second one (4018-1) note the Ch4 (blue) baseline is at the top of the display screen.
Question: has someone encountered this behavior (and a solution; or suggestions)?

P.S. it may be that the issue is larger (doesn't start rightaway above 10^2 in any condition when very cold); hope someone can suggest something, as I exhausted all (first) thoughts about how to deal with it. Two pictures in album with same name.


Mark Vincent
 

Since you say it needs to be warm to work, it suggests the electrolytics are bad. The schematic parts says they are electrolytic, not tantalum. Tantalums can go bad also. If you have or get access to an ESR meter, that will tell you about the condition of the electrolytics. The 1mfd ones can be replaced with film of 50V or 63V type. Film types do not suffer ESR change with temperature and the ESR is lower. The 10mfd ones could be replaced with tantalum rated at 35V or electrolytics at 35V or 50V. The 22mfd that are the output filters from the series pass transistors can be the same value or higher, e.g. 47mfd. The mail filters of 750mfd could be 1000mfd 50V (Nichicon UHE as an example). Leave the timing caps alone. They are precision types. For the electrolytics, Nichicon ULD types are the best for ESR and life I have found. The films can be Kemet, Arcotronics, Wima, etc.. If you want to really confirm that temperature is the problem on start up, spray the capacitors with component cooler when warm. If it stops working, that confirms that one is bad. Since all of the electrolytics are the same age, it is better to replace all of them. Use high temperature, low ESR and long life types for the electrolytics.

It is also possible a transistor/IC is bad when cold. The cooler will confirm that. Spray enough to cool. Maybe 1/2 second.

If other plug-ins in the TM500 frame have problems when cold, that means they and/or the capacitors in the mainframe need replacing. If it is only this plug-in, then work on this one. I hope this helps.

Mark


Ozan
 

I don't have this equipment but here is my feedback in addition to Mark's suggestions:

Digging around I found that the triangular generator stays in a state where is
happy (I can provide all voltages in significant points of the schematic)
Are +17V, "+17V (DCPL 1)", -17V, "-17V (DCPL 1)" on sheet <1> stable when cold (not working)?

In the happy non-working state what are the voltages (partial list is OK if you don't have them all):
1) at the gate of Q200, and collector of Q292.
2) bases of Q230A and Q230B
3) bases of Q290 and Q292
4) emitters of Q140 and Q175

In case gate circuit is acting funny: Voltage at collectors of Q325 and Q315

until it decides to start working. I'm attaching two pictures of a normal
start and the delayed one. In the second one (4018-1) note the Ch4 (blue)
baseline is at the top of the display screen.
What are the test points for the different traces?

Ozan


durechenew@...
 

For Mark,
I do not say it's not possible, but I believe no capacitor will warm up in a minute. Plus, there are conditions when it starts right-away (lower/higher frequencies).
There are (usually) two other plugins (DC502 and DM501A, great instruments - after some improvements) that always start and work without problem.
There is no IC in that stage.
A transistor with leakage current - one of the ideas I have in mind and didn't verify yet (specially Q204, Q210).

For Ozan,
Are +17V, "+17V (DCPL 1)", -17V, "-17V (DCPL 1)" on sheet <1> stable when cold (not working)?
Always stable (working or not) and (well) within spec, including ripple (I did a full calibration)

In the happy non-working state what are the voltages (partial list is OK if you don't have them all):
1) at the gate of Q200, and collector of Q292. As following: 1.124V at Q200-G, 1.116V at Q292-C (might be higher at gate of Q200, as I'm introducing in the circuit the input resistance of MM - 10 mohms; MM is a Fluke 179)
2) bases of Q230A and Q230B: -0.045V both
3) bases of Q290 and Q292: -9.01V both
4) emitters of Q140 and Q175: -6.47V at Q175-Base and 6.54 at Q140-Base (I don't have measurements for E, worth checking; but why would start at higher frequencies, same devices)

In case gate circuit is acting funny: Voltage at collectors of Q325 and Q315: I didn't verify if it's acting funny, but that circuit should have no influence as long as that input is without signal; and, again, why it starts at other ranges of frequencies?

What are the test points for the different traces?
Ch1: output of plugin (scope it's AC coupled, but no difference if in DC)
Ch2: Q292-C
Ch3: Q230B-B (Q230A and Q230B are pair in the same capsule, in my plugin; seems other SN had separated ones)
Ch4: Q230A-C (AC coupled, DC level is around -9V)
Thanks for comments, got something to think about and do tomorrow
TT


Ozan
 

1) at the gate of Q200, and collector of Q292. As following: 1.124V at
Q200-G, 1.116V at Q292-C (might be higher at gate of Q200, as I'm introducing
in the circuit the input resistance of MM - 10 mohms; MM is a Fluke 179)
2) bases of Q230A and Q230B: -0.045V both
Q292C is connected to the diode bridge CR245/CR248/CR250/CR246. With 1.116V at CR245 cathode (and CR248 anode), CR245 should be off, CR248 should be on, CR246 should be on, CR250 should be off (this assumption can be confirmed using the Q230A base voltage from below).

In this condition all the current from CR248 is dumped to R248 and all the current from CR246 is dumped to R240. Expected base voltage of Q230A is:
97.6*(17-0.6)/(3.01k+97.6)=0.515V. However, you are measuring -0.045V. Most likely diode CR246 is bad or R245 could be bad (bad solder, bad resistor etc).

I would focus on the hysteresis bridge (bridge at the base of Q230a) first.

Ozan


Eric
 

I had an FG 507 and a FG 501A both had very bizarre issues with them and the waveforms were terrible and would some times collapse to DC. I would recommend before you go down the rabbit hole too far clean the cam switch contacts on the timing cam. Then clean them again. Then clean them again. Through the repair process I cleaned them a few times and checked every part on the board. Replace a few filter caps then did a moderately aggressive clean on the timing cam switches. The 4th moderately aggressive timing clean did the trick it needed less then steel wool but more then paper and alcohol.

Both units had the same issue with dirty switches, your milage may vary. As I only have a sample size of 2.

Eric

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of durechenew via groups.io
Sent: Sunday, March 21, 2021 7:45 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] FG502 doesn't start at some specific settings

For Mark,
I do not say it's not possible, but I believe no capacitor will warm up in a minute. Plus, there are conditions when it starts right-away (lower/higher frequencies).
There are (usually) two other plugins (DC502 and DM501A, great instruments - after some improvements) that always start and work without problem.
There is no IC in that stage.
A transistor with leakage current - one of the ideas I have in mind and didn't verify yet (specially Q204, Q210).

For Ozan,
Are +17V, "+17V (DCPL 1)", -17V, "-17V (DCPL 1)" on sheet <1> stable when cold (not working)?
Always stable (working or not) and (well) within spec, including ripple (I did a full calibration)

In the happy non-working state what are the voltages (partial list is OK if you don't have them all):
1) at the gate of Q200, and collector of Q292. As following: 1.124V at Q200-G, 1.116V at Q292-C (might be higher at gate of Q200, as I'm introducing in the circuit the input resistance of MM - 10 mohms; MM is a Fluke 179)
2) bases of Q230A and Q230B: -0.045V both
3) bases of Q290 and Q292: -9.01V both
4) emitters of Q140 and Q175: -6.47V at Q175-Base and 6.54 at Q140-Base (I don't have measurements for E, worth checking; but why would start at higher frequencies, same devices)

In case gate circuit is acting funny: Voltage at collectors of Q325 and Q315: I didn't verify if it's acting funny, but that circuit should have no influence as long as that input is without signal; and, again, why it starts at other ranges of frequencies?

What are the test points for the different traces?
Ch1: output of plugin (scope it's AC coupled, but no difference if in DC)
Ch2: Q292-C
Ch3: Q230B-B (Q230A and Q230B are pair in the same capsule, in my plugin; seems other SN had separated ones)
Ch4: Q230A-C (AC coupled, DC level is around -9V) Thanks for comments, got something to think about and do tomorrow TT


durechenew@...
 

Cleaned cam switch contacts twice, no difference between the first and second. Replaced both JFETs (constant current source one, lower part on schematic, was good when cold, stopped working in a minute or so); not a great pair when it comes to characteristics - at least according to my 7CT1N. Removed solder and re-soldered all components from triangle generator, checked all diodes for normal behavior (in circuit).
TT


Ozan
 

Removed solder and
re-soldered all components from triangle generator, checked all diodes for
normal behavior (in circuit).
Did you already check CR245/CR248/CR250/CR246 and resistors R245, R248, R250? The voltages you measured at two sides of the bridge are not consistent if all the components are good. Perhaps component tester failed to catch the intermittent behavior. Do you measure 1.116-0.6=0.516V at cathode of CR248 in stuck state? Or do you measure a negative voltage (bad CR248)?

Ozan


Ondrej Pavelka
 

I would in your case take a magnifying glass and really strong light source
and thoroughly check one and every solder joint. I known you said you
resoldered components around triangle generator but what about the rest?
I repair synthesisers for living and with intermittent faults like this
this thorough optical inspection shown the problem in most of the cases. I
would specifically look around where mechanical twisting / pushing of the
controls can introduce stress to the component connections.

On Mon, 22 Mar 2021, 05:40 Ozan, <ozan_g@erdogan.us> wrote:

Removed solder and
re-soldered all components from triangle generator, checked all diodes
for
normal behavior (in circuit).
Did you already check CR245/CR248/CR250/CR246 and resistors R245, R248,
R250? The voltages you measured at two sides of the bridge are not
consistent if all the components are good. Perhaps component tester failed
to catch the intermittent behavior. Do you measure 1.116-0.6=0.516V at
cathode of CR248 in stuck state? Or do you measure a negative voltage (bad
CR248)?

Ozan






durechenew@...
 

Triangle generator is stand alone, doesn't need anything else to work. I try to put all the relevant information in the post, sometimes I may miss somethings. Triangle generator starts (and works normal) at some different frequency ranges (se my original post).
I'll try all the ideas expressed here and see where it goes; unfortunately, after retiring, I find time available to do what you like is (much) less than desired... :)
TT


Eric
 

If it works like the FG 502 (making an assumption) that is the heart of the unit everything is derived from the triangle wave. If there is any setting where the triangle wave is not running that is why there is no signal. Even the sign is shaped from the triangle wave with a shaper network.

Eric

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of durechenew via groups.io
Sent: Monday, March 22, 2021 12:38 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] FG502 doesn't start at some specific settings

Triangle generator is stand alone, doesn't need anything else to work. I try to put all the relevant information in the post, sometimes I may miss somethings. Triangle generator starts (and works normal) at some different frequency ranges (se my original post).
I'll try all the ideas expressed here and see where it goes; unfortunately, after retiring, I find time available to do what you like is (much) less than desired... :) TT


durechenew@...
 

You're right, Eric!
But, by now, I believe everyone reading this post knows that. And it is what I said right from the beginning, triangle generator is not working (I mean, I didn't say there is no square, sin, that is obvious... pulse and ramp signal work based on the same stage but a little different). And It's starting and works normal on some frequency ranges (meaning - should we consider that there is no defective component in that stage? Nor cold soldering or switch mechanical functionality?).
And I believe you meant "If it works like the FG504", as discussion here is about an FG502...

Ozan, you were right and it was my fault when I wrote voltages for Q230A, B at the base(s): I misread the normal working voltages with those for 'happy state'.
Here's the last and, hopefully, correct list of readings; note, last digit might be different depending on temperature, position of sun in the sky or god knows what else
Q200- G: 1.122 V
Q204-B: 1.105 V
Q204-E:1.777 V
Q210-E: 1.025 V
Q290-C: -0.866 V (of no relevance)
Q230B-B: 0.503 V
Q230A-B: 0.504 V
Q230A&B-C both: -8.57 V
Q292-C: 1.118 V

Q175-B: -6.56 V
Q175-E: -7.25 V
Q175-C: -6.54 V
Q140-B: 6.60 V
Q140-E: 7.37 V
Q140-C: 4.165 V

CR140 and CR145 junction: 1.513 V
CR170A&CR175 junction: 0.720 V

CR248&CR250 junction: 0.313 V
CR245&CR246 junction: 1.317 V

All diodes reading like normal diodes when measured cold, and (as seen from voltages list) normal voltage drop for those in conduction; all resistors seem to be normal, with a note: R155 value is ~1k, R290 almost normal (both should be 820 ohms). I switched the two, thinking I may be able an unbalance the stage, but no difference.
TT


Ozan
 

to be normal, with a note: R155 value is ~1k, R290 almost normal (both should
be 820 ohms). I switched the two, thinking I may be able an unbalance the
stage, but no difference.
R155 should be 820-ohm, R290 changed from 1k-ohm to 820-ohm in later revisions. They were both upgraded to 0.5W from 0.25W. What value do you have for R155 and R290 now? It makes a difference in figuring out what the circuit does.

Ozan


durechenew@...
 

Both marked 820 ohms (grey-red-brown); initial measure in circuit R155 real value 1053 ohms (same value when removed), R290 real value 832 ohms and they look like 0.5W (if not more). Now the two have places switched, but, as I said, no difference in behavior of the stage
TT


Ozan
 


Both marked 820 ohms (grey-red-brown); initial measure in circuit R155 real
value 1053 ohms (same value when removed), R290 real value 832 ohms and they
look like 0.5W (if not more). Now the two have places switched, but, as I
said, no difference in behavior of the stage
It is better to place the proper value, 820-ohm nominal, in both places.

Voltages look OK ballpark. Perhaps some of the transistor gains decreased over time and now positive feedback coming back through C247 is not strong enough. I assume your serial number doesn't have the R247 resistor parallel to C247 given R155/R290 were 820-ohm nominal. You could try adding another 2.7pF in parallel with C247 to see if it helps. Note that C247 was selected so its value is critical to proper operation.

At the higher frequency settings (higher charge/discharge currents) slight mismatches in source and sink current may be flowing into R155/Q292 junction and place the operating point in the bad stable condition.

Looking at the min/max voltages at Q292 collector in normal operation in those high frequency settings may give other clues.

Ozan