Kenwood TS440S Too high voltage in VCO 5 (7.8v versus 5v)

[Ghetto]

Working through a TS440S that I bought off ebay that had some of the glue problems in the VCOs and radio presented with DOTS problem.

Previous owner had scraped out glue and touched up back of VCO 5 solder joints but residue ate through some components so I replaced/fixed where necessary.

DOT problem now gone!! AM and FM receive now working but SSB not receiving right, lower audio and high pitched hiss/squeal.

However the whole time VCO 5 was showing ~7.8v instead of the 5v as per the service manual. I replaced Q36 for fun... and it changed to ~7.6v. At no point have I tried adjusting T20 to 5v as my online research shows the 7.8v is not a misadjustment problem but usually circuit/component. I could mark its current location and attempt adjustment I guess.....

I've checked L41 out of circuit (OK) - heard that can cause 7.8v thing. Replaced the two electrolytic capacitors in VCO 5, C185 and C191 still the same. Thought I'd throw it out there if anyone has any ideas or seen this problem before, and heres a video of current state:

 

[brandon7861]

If TP11 is at 7.8v, it means that Q35 is not pulling it down. This can be caused by a bad PLL (check for 3v on pin 8 of IC18), a bad emitter follower (Q34), a bad inverter (Q35) or a blown level-adjusting diode (D13). I assume D13 is a 3-diode package like the KB363 because of the 2v across it.

Check these voltages, they will narrow down the issue.

I don't think you need to be messing around with the VCO/JFET side of C187. The problem is almost certainly to the left of that since your varactor control voltage is wrong. You can see if the oscillator is at least working. If it is working, even at the wrong frequency, it hints that the problem is elsewhere.

Edit: thinking about it more, since the control loop is broken (wrong frequency), these circled voltages may be different, but the levels between them will still hint at a problem so report those voltages whatever they may be, We care about junction voltage drops, not the service manual voltages.

 

[Ghetto]

Thanks Brandon for that post - awesome detail for me trying to learn this stuff - appreciate it as always!. I put my measurements in blue back on the diagram below:

 

[Ghetto]

I wonder if I should take out Q34 and test base to emitter as they are showing the same value...

 

[brandon7861]

In your picture, where you wrote 4.8v, did you mean 7.8?

Yes, test that transistor

 

[Ghetto]

I just rechecked, definitely getting 4.8v on the collector of Q35 (2sc2459 middle pin) as per the diagram.

Tomorrow I'll get a chance to test Q34 out of circuit.

Forgot to mention, I think your right re: D13, it has a high forward voltage (~1.9) when I tested it out of circuit (OK) while soldering a new leg back on one side that had been corroded.

 

[brandon7861]

If TP11 has 7.8v and the collector of Q35 is only 4.8v (3v across R146), there has to be a solder bridge somewhere.

3v across R146 is 0.13666mA. The current flowing in R146 has to go somewhere. If it goes toward the VCO, it encounters two ceramic caps and should get blocked, If it goes up toward TP11 and across R147, it encounters an electrolytic cap (actually, two).

Considering it would take .00013666*100k=13.64v across R147 to account for that current (which would add to the 7.8 and be higher than the supply voltage, I think we can rule out the electrolytic caps being a source of voltage.

It is very likely that you have a solder bridge somewhere between Q34s collector trace and the trace of TP11. Inspect the area carefully.

 

[Ghetto]

Thanks for your help as always Brandon.

I've spent hours since you posted that message and not much luck unfortunately.

Q34 tested out of circuit (OK).
Q35 tested in circuit while Q34 out (OK).

Inspected Q34,Q35, TP11 traces think I found really small bridge (half diameter of hair) on Q35 trace that I removed.

Decided to resolder all joints in VCO 5 entirely as R146 and few other solder joints looked average under 30x loupe. This was me removing all solder from previous owners work. I'm really confident there are no bridges now.

Result, still getting the 7.8v on TP11, only thing changed is Q35 base voltage looks closer to 2.6v (was 3v now 2.56v) and collector of Q35 from 4.8v to 4.68v (Got worse!).

Updated diagram below after resolder. I really am lost (does not take much as a beginner..) to what could do this - I checked the resistors in the area all test OK. I wonder if c178 tantalum or something is weird.

Had to rebuild D13 as other leg fell off (check pics below - stuck on a loaf of bread tag!!), rebuild was a stop gap measure as I need to find a MV203 substitute. It was a nightmare...

 

[NODENS]

Assuming there is no bridge in that area, I'd check D13. I suspect that if Q35 isn't conducting, the bias from the Q34 collector would appear on TP11, more or less as you've described. Checking to see if D13 is open could explain the issue, provided there isn't a bridge.

 

[brandon7861]

3v across R146 is a little over a tenth of a mA and it is soaked in flux. I would try cleaning the flux off.

 

[brandon7861]

It has nothing to do with D13. Its out of circuit value checked out.

If you have 4.8v on the left side of R146 and 7.8v on the right side of it, 136uA of electrons are flowing from left to right. Who cares where they came from, the question is, where are they going? Since those electrons are looking for a positive home, ignore all components to ground from there on.

Looking to the right toward the VCO, two ceramic capacitors and a JFet would have to be bad to reach a positive voltage. Unlikely.

Looking up toward TP11, the only connection is to R147. That is a 100k resistor. To make 136uA of current flow through it, it would need 13.6v across it. Add that to the 7.8v already there and you are above the supply voltage. This rules out the entire R147 path.

There exists no other path, so no other explanation besides a solder bridge or some other unintentional path to a positive source. Considering there is a 7.8v source right there, I suggest you clean it really good and then take a closer look.

 

[Dr. Frankenstein]

The Missing Path:

After looking your pictures over, especially the trace side of the re-flow work you did. You have not noticed a few parts in that area that have not been printed in the schematic ..... on page 73 of the service manual.

You need to go to page 72 of the X-ray view of that same area, find tank T20
(surrounded by a square) located about half-way down the page and the area you want is surrounded by a rectangle.
For ease of viewing both pages, I have two copies of the service manual open on my computer; one for page 72 zoomed in to T20 area, one for page 73 zoomed in to the IC18 to T20 area.

From page 72; items R147, R148, R149, C180, C181, C182, and from there C180 goes to a harness connection that says "FMD" of the x-ray view ..... are all plainly missing from the schematic on page 73, but shown just outside the boxed in area (just like C179) of the T20 circuit rectangle of the x-ray view, all being part of the TP11 test point circuit.

Since the x-ray view, of page 72, is looking down at your component side of the radio, the trace side is a mirror view of the picture you posted of the bottom area of your radio. To your right of your picture, it can be seen that those parts are in your radio. How they play a part with your problem, I couldn't say, but you may want to first disconnect the FMD harness connection to see if it helps, then test C180 and C182, or follow the FMD harness.

This is what you have missed because of the schematic not showing it ............. it happens a lot. I hope it helps you fix the problem and grow your hair back.

73 and good luck...........

 

[brandon7861]

The Missing Path:

After looking your pictures over, especially the trace side of the re-flow work you did. You have not noticed a few parts in that area that have not been printed in the schematic ..... on page 73 of the service manual.

You need to go to page 72 of the X-ray view of that same area, find tank T20
(surrounded by a square) located about half-way down the page and the area you want is surrounded by a rectangle.
For ease of viewing both pages, I have two copies of the service manual open on my computer; one for page 72 zoomed in to T20 area, one for page 73 zoomed in to the IC18 to T20 area.

From page 72; items R147, R148, R149, C180, C181, C182, and from there C180 goes to a harness connection that says "FMD" of the x-ray view ..... are all plainly missing from the schematic on page 73, but shown just outside the boxed in area (just like C179) of the T20 circuit rectangle of the x-ray view, all being part of the TP11 test point circuit.

Since the x-ray view, of page 72, is looking down at your component side of the radio, the trace side is a mirror view of the picture you posted of the bottom area of your radio. To your right of your picture, it can be seen that those parts are in your radio. How they play a part with your problem, I couldn't say, but you may want to first disconnect the FMD harness connection to see if it helps, then test C180 and C182, or follow the FMD harness.

This is what you have missed because of the schematic not showing it ............. it happens a lot. I hope it helps you fix the problem and grow your hair back.

73 and good luck...........

The parts you think are missing from the schematic are right here. We don't need to consider them because there would need to be roughly 20v at these parts for the 136uA of current to flow through R147.

 

[brandon7861]

I flipped the xray view to match your picture and traced out where TP11 is (in red) and where the 7.8v source is (in blue). Any solder bridge would need to connect red and blue. Since they only get close in one spot and I see no bridge there, I am left to suspect that thick amount of flux circled in pink. Clean that off (clean the whole thing) and test it again. Green was what you measured at 4.8v and orange is ground.

 

[Dr. Frankenstein]

@brandon7861

They were right in front of me the entire time and I didn't see them in the schematic...... but they are still part of the circuit to TP11, and the components should be checked.
Like you say to" clean the flux", I have seen instances where a flux (dirty) area of a PLL just needed to be cleaned up to get loop running properly.

@Ghetto

You say you checked R146, is there a chance that it changed value (higher) or is open, but that still doesn't explain where the 7.8V is coming from.
The only other recourse would be to try to adjust T20 (which you state that you haven't done yet), just to see if it makes any difference (But make sue you know where it is before you turn the slug) .... being that it's what adjusts the voltage to TP11 to 5V ±0.2V.

good luck