SBE Console V "speech spander" problem.

[TM86]

Title of the thread says most of it, but here's the details. Also, it might help to have schematic handy to see which components I'm talking about.

Speech spander board in a Console V, audio goes in, it doesn't come out. Kind of like a roach motel for voice. This happens in both CB and PA mode. Which, since this board is common to both, makes sense.

I have an audio signal at the base of Q1, so it's making it through IC1 OK. I have no audio signal at the collector of Q1, which is kind of a problem. I've checked Q1 with one of those multifunction testers, it tells me Q1 is good. I've checked C9 with the same tester, it shows the correct value. Checked R5 (47k Ohms), R6 (1k Ohms) and R7 (22 Ohms) in circuit, they're all within tolerance.

About the only thing that isn't matching what's in the Service Manual is the voltages at the collector and base of Q1. Book says 2.48V and 0.77 V respectively, I'm getting 6.11V and 4.99V. I did check this against Sam's CB-264 (the Sidebander V has an identical circuit) and it roughly agrees with the SBE produced manual.

I did try pulling C10 to make sure it wasn't shorted and feeding voltage in from another part of the circuit, made no difference. I have the checked the speech spander board ground to make sure that's connected to ground on the main board, it is.

So I'm left with what looks like a possible open circuit, except all the components read what they should. So I put the question to you. What obvious thing did I miss? Or non-obvious thing, for that matter.

 

[nomadradio]

voltages at the collector and base of Q1. Book says 2.48V and 0.77 V respectively, I'm getting 6.11V and 4.99V.

Don't have the schemo in front of me right now, but 5 Volts on that transistor's emitter is a hint. The collector voltage is high because the transistor is not pulling normal current. The more collector current, the lower the collector voltage. But the emitter voltage is way, way too high. If it were a true reading it would mean the transistor is pulling excess current. But the collector voltage reading tells is this can't be so. The resistor going from this transistor's emitter to ground must be bad, or just not connected.

You said the transistor tested okay, so that's what we're left with.

73

 

[NZ8N]

You forgot to put the Emitter voltage in your original post. What are you getting there?

Broken trace at R7 (22) or Emitter of Q1 (2SC710)? Sounds like the Emitter is not being pulled down, and the Base is "following" this voltage trend. If Q1 were testing bad, you could substitute a 2SC945, but the "firing order" is backwards from the 710, so turn it around. C is still in the middle.

I have similar problems at my work when testing the amplifier boards (uses BFR91A at each stage), we use 18 Ohm SMD resistors (most transistors have 2 from Emitter to Ground, 1 on each side of the E trace, some have 1, though), when I see a high voltage at the Collector and/or Base, it is usually the Emitter resistor that opened (cold solder joint, ROHS solder, or lifted end pad of the 18 itself (overheated from assembler before me)). Sometimes, though, the B-E diode opened in the transistor. Heating the Base tab up with the soldering iron sometimes causes it to function correctly for a few seconds, then open back up.

 

[Staybolt]

I've rebuilt 2 or 3 of these little boards by recapping them and I replace the chip. You can buy the chips off ebay. Good luck!

 

[TM86]

Emitter is at about 1.2 millivolts. I hadn't originally included that because it didn't stand out as "wrong", since the docs have it at 0V.

 

[DoctorAudio]

There my be DC coming from pin 8 of the IC riding with the audio signal (which might be normal if the output of the IC is floating). C8 is the blocking cap for the base. Try lifting C8 and see if the 4.99V on the base is coming from there. If so, replace C8.

 

[TM86]

There my be DC coming from pin 8 of the IC riding with the audio signal (which might be normal if the output of the IC is floating). C8 is the blocking cap for the base. Try lifting C8 and see if the 4.99V on the base is coming from there. If so, replace C8.

Sorry, this is sort of stream of consciousness as I go through what I did.

Pulled C8, voltage on base of Q1 went up to 6V. Checked C8 in the multifunction tester, it came out pretty close to 1 uF. So that's probably not it. Put C8 back in, voltage dropped to where it had been earlier.

For giggles, pulled R5, R6, and R7 to check out of circuit, since I had only checked them in circuit previously. R5 and R6 were good, R7 shows the 22 Ohms it should, but the tester showed it as a .01 uH inductor which seemed a bit odd. The meter of Ohms confirmed the 22 Ohm reading so I'm going to chalk that up to the tester getting weird with low resistances.

Out of desperation I pulled Q1 again and threw it in the tester. Still checked good, but I noticed the on the tester that Base and Emitter seemed to not be where they should be with respect to how Q1 was installed in the board. Which is odd since this is supposed to be an original 2SC710. I happened to have a 2SC945 laying around, so I threw that in the tester and it's leads had the Base and Emitter position swapped from the 2SC710, which is how it should be. So what I think happened here is a previous owner or their tech tried to swap out Q1 and just followed the component outline on the board for orientation. Bent the collector pin to allow me to insert Q1 "backwards" and soldered it back in. Voltages are now good around Q1 and it amplifies again.

So, yeah, that's what I missed on that one.

 

[nomadradio]

Way cool! I do remember the 2SC710, kinda the 23-channel precursor to the 2SC945. We used to keep both versions of the 2SC710 on hand. One of them had the collector in the center. Other one had the base lead in the center. No way to tell from the type markings alone which one is in your hand.

Testers that identify each lead by name can be incredibly useful!

73

 

[TM86]

Just to add insult to injury. Been spending some time on and off with the rest of this radio tracking down a problem with extremely weak receive (all modes) and AM transmit audio. Finally found it tonight, a solder bridge between the junction of R609 and R610, and a nearby ground. It was killing the collector voltage for Q601. Wasn't a dead short as far as the regulator was concerned, so nothing got extra hot to draw attention.

She desperately needs an alignment, but she seems to be working fine otherwise as of now.