The HP 8600A Problem has Returned

[Chris K]

Have the top of the 8600 open and it is operating normally. Could be it's not getting hot in this configuration but I'll keep it on for as long as it takes. It's held steady for about 30 minutes. The difference in voltage across the R12 resistor is 0.357V DC in normal operation right now.

[Chris K]

2 hours and 45 minutes and we’re still working. As I said, all of this is with the top metal panel off so any heat can vent. There’s some significant warmth coming off the chassis mounted power transistor for this circuit and physically it is almost directly under the 723 and R12. My intent tonight was to let the 8600 fail and then freeze either the transistor or R12 but no failure so far. We’ll see what running overnight does and I’ll check it in the AM.

[kvflyer]

What about putting something like this on it?

[Chris K]

That's a possibility. It was holding steady at 5.003V DC this morning after about 10 hours of running. I put the top panel back on and left for work so we'll see what's going on when I get home.

[Kevin Kuehn]

Originally Posted by Chris K

That's a possibility. It was holding steady at 5.003V DC this morning after about 10 hours of running. I put the top panel back on and left for work so we'll see what's going on when I get home.

How long did it usually take to act up with the cover on? Hopefully the 723's current limiting will keep things in check until you return.

[Chris K]

Originally Posted by Kevin Kuehn

How long did it usually take to act up with the cover on? Hopefully the 723's current limiting will keep things in check until you return.

It took maybe 10-15 minutes to lock up. The symptom was/is a frozen frequency readout. When I cooled the 723, I monitored the voltage rise from around 4.6V to a bit over 5.0. Function returned but as the voltage on the test point dropped below 4.8 or so, the readout froze again. I could repeat this over and over. My plan was to freeze the 3055 and see what that did but, as stated, the 8600 never lost function last night. During lockups, I did turn the trimmer so it would maintain 5V DC. Last night with the trimmer maxed and normal function maintained, the voltage was around 5.3V. That scared me so I adjusted it to 5.003V DC. We'll see what I find in a few hours with the cover on all day. My attempts at doing a comprehensive alignment on the RCA, probably one of, if not the, most challenging early era TVs to get just right, even with an enormous amount of help and guidance from Penthode, were so compromised by faulty equipment and then issues with this piece, I had to abandon that project. I'm hoping I can get this working reliably again so I can finish the RCA.

[Kevin Kuehn]

When you freeze the 723, is it possible you're also cooling a few surrounding passive components? I remember initially you were able to randomly fix this by banging the case or bending the circuit board. So don't rule out the possibility there's still an intermittent passive component or connection in the mix.

[Chris K]

Originally Posted by Kevin Kuehn

When you freeze the 723, is it possible you're also cooling a few surrounding passive components? I remember initially you were able to randomly fix this by banging the case or bending the circuit board. So don't rule out the possibility there's still an intermittent passive component or connection in the mix.

Yes you are correct and I was thinking about that last night. The readout freezing is the only issue now. Back then, there were additional problems of frequency readouts all over the place etc. I replaced a burned out resistor, a shorted film capacitor and cleaned all of the board connection traces on all of the boards I could easily remove. That seemed to resolve everything but this persistent issue. Another thing I noticed is there's evidence of many IC chips being replaced on one particular board. When you look at the unit from the front, the board is on the left almost up front (at work and do not have the unit or schematic in front of me). It's almost exclusively 14 pin ICs and there's a lot of variation in manufacturers and lots of flux around the pin solder points on the back or bottom if it was horizontal. I've been looking for a replacement for around 6 months now but nothing has shown up. That's funny because Penthode suggested in the RCA thread I look at finding an 8600/8601 sweep generator combo and I bought one on eBay in about an hour!..Since then, no more 8600s.

[Chris K]

Home and still OK. This kind of intermittent issue drives me crazy. Voltage rock solid at 5V. Now, I did have the top of the cabinet just laying on top. I just installed it correctly and screwed it down so the unit is sealed. Let's give it a few hours tonight and see what happens.

[Penthode]

Did the voltage across the current limit resistor rise when it got hot?

[Chris K]

Originally Posted by Penthode

Did the voltage across the current limit resistor rise when it got hot?

I don’t understand the question my friend. I did measure the voltage at 0.36V last night when it was running for several hours and not failing in fact, 24 hours since I started this and it hasn’t failed. Now that the cabinet is sealed up, maybe it will fail and when it does I’ll measure the resistor

[Kevin Kuehn]

R12 is within the voltage regulation feedback loop. If it's resistance increases as it warms up, the regulator will compensate until the point of current limiting kicks in. So I think the question is, is the voltage drop across R12 still the same after running 24 hours?

[Chris K]

Originally Posted by Kevin Kuehn

R12 is within the voltage regulation feedback loop. If it's resistance increases as it warms up, the regulator will compensate until the point of current limiting kicks in. So I think the question is, is the voltage drop across R12 still the same after running 24 hours?

No it isn't. It was 0.357V or 357mV and right now it's 328mV. The unit is still holding and reading frequencies actively however, since the cover has been on, the voltage has dropped from 5.0V to 4.96V. Probably insignificant but I haven't seen anything other than 5.0V on the nose until I just measured it after taking off the top.

[Kevin Kuehn]

I think we were expecting it to increase with temp.
I'm not sure how to interpret that.

[Chris K]

Yeah I think I effed up. The difference yesterday was measured when the trimmer was maxed and the voltage was 5.3V and not the 5V it is now. So I don’t think the voltage difference on the resistor yesterday vs today really provides any information. If it’s still working fine by the time I go to bed I’ll turn it off and then back on tomorrow morning and test the resistor for a startup baseline. Then we’ll see what’s up when I get home from work