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Having trouble finding oscillation/bias problem - where do I look?

tusntuk2

tusntuk2

Active Member
Please help me find the cause of an apparent oscillation problem. It seems to be related to the bias circuit, or at least it's manifested there. This is a recently recapped Kenwood KR-5600. Great help from AK folks helped me spot a faulty diode and fix the relay, and the audio board and power supply were recapped. The amp sounds great on FM, but after warming up I noticed high bias readings in one channel. R output gets hot. When switched to Phono inputs or Aux the oscillation is heard and seen (very lo q) in the speakers. It changes speed with more volume or bass. I replaced the bias transistors Q7,8, and the trim pots. This helps tame the bias but it does not stop the heating up and high bias. When the volume is 0 the channel cools and the bias drops. I double checked the new caps, and all seems correct, but...I'm getting to the end of my ideas. Any body seen this before?
 
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Thanks for the reply! I do not have a scope. After the amp warms up i can see and hear the drumming. As the volume control increases the speaker cone moves wup-wup-wup maybe 90-120hz (guess, 3in2sec) which dramatically moves the cone. Oh, these are tiny 3" full range bench junk speakers, but it was on 3 way 12" boxes I first heard it. 8 ohms. As volume (or bass) increases, so does speed of wup-wup. There is a higher pitched buzz, too, which can be cut out with the treble control. The output trans and heat sink get hot, bias goes over 85-90 mv, and I shut it down. Looking around AK it seemed oscillation was the best description, especially with the thermal build up. Every voltage from the s/m has checked pretty close, so I'm stuck. The bias circuit on this side could be suspect, it varies more then the other side, but so far replacing parts hasn't fixed it. Wish I did have a scope to track this more.
 
What part of Arkansas do you live in? Perhaps there is a friend or fellow AKer in your neck of the woods who has a scope, and could offer some assistance.

Are you close to a university or technical school that might have an electronics lab?

Some retail stores that offer "repair" might be willing to take a quick look and offer a diagnosis, for a fee.
 
some SWAGs:
I suspect its in the power supply since the volume increases it. (could also be in the early
signal path - same symptoms).
in tube amps its called motorboating when there's insufficient/damaged resistance between RC stages
in the Power supply.

you could try putting a LARGE capacity cap with long leads and clips across the various spots
in the PS.

I can be wrong but I'm always more "correct" than the burn-in deniers. In fact I have corrected
some of their incorrect technical postings. so here is a proposition the climate deniers
are never able to make (I have an extra scope in original shipment packaging and am willing
to trade if you're interested) bought a second one so even though I'm smarter than the
burn-in deniers - I haven't found a use for a second scope.

best advice was earlier - you need a scope to "see" where the cycling occurs.
 
Hot Springs, about center state. Well honestly I don't know if it's oscillation, it just sounded like other descriptions. I'm not an EE, just a hobbiest and old musician who worked on tubes, so SS amps are not my familiar territory.

Thanks, Bob, I'll go back and check what I did. I recapped the PS and the Audio boards only, nothing else. I increased the main filters from 7500 to 10000, and upped voltage limits but kept microfarad values on the rest. Voltages around the PS board seemed reasonable, such as 19.1v for 18.5v on the s/m. Rails are running 39 - 40v while the s/m says 38.5v. Do they need to be tighter? I wondered about something breaking down with the heat, but don't know where to look. Doesn't happen at low volume.
 
good - you verified my guesses about the PS and audio circuit. voltages are within (reasonable
range and the cap values also.

it would seem that the motorboating is caused by something drawing more juice and something
else is competing and it's going back and forth like a tennis match.

your probably don't have to unsolder stuff, monitor heat (the symptoms may not cause more power
to be drawn and therefore more heat - you can confirm my lightly touching the heat sinks - shouldn't
be hot).

a low-probability SWAG: if your DMM/VOM is digital, you can see if the voltages (in the PSU
and going to the output stage) vary when you increase the volume. it will be hard to see
and may flicker or may not register (voltage differences is less than the threshold for updating
display).

another wild guess. try using/changing any switches that may exacerbate the issue
if they exist: loudness, rumble, and perhaps the bass controls. could be a way of
further isolating the issue (we're looking for a root cause). also rotate fast and slow
while listening - here you're looking for some control that might account for signal
loss/degradation/interference.

do check where you made changes - look at solder joints, places where there were cables
that either had to be pulled or unsoldered (for access), any changes since you started this
upgrade.

update: my apologies - I re-read your OP - your problem seems to be in the power
stage if the bias readings don't hold steady and high volume increases output stage
demand for juice and something heats up and draws more.

however, follow my earlier tips to ensure the issue wasn't introduced by earlier changes
and grab that schematic and DMM/VOM and put clips on the probes, attach clips
with unit off.
1. under low volume measure all the points the schematics have values for.
2. so same for when higher volume introduces the problem

wherever the voltages are out-of-printed-schematic range is where the locality of the problem
is.

several things to do. if possible get a can of freeze spray and during test#2 and some transistor
(and other component like resistor but not a capacitor) shows a different from spec voltage
then spray the puppy and if the motorboating goes away and then comes back, that's
the errant puppy.

Maybe - use a heatsink clip if you have one (or a transistor heatsink for those that are
encased in a metal can) and do the same - noting whether the volume needed to cause
the problem increases - more cooling means delaying the onset.

then perhaps both. this issue is strange because your unit does work so there's the baseline
and only when more juice is needed does it motorboat.

BTW. high frequency oscillations are not usually audible (maybe adds an edge to higher tone)
so in a manner of speaking it is better (only if degrees are included).

BTW2 - you could try using canned air - test with hand - try upside down. the propellent
follows one of the laws I learned a zillion years ago - pressurized release causes
temperature drop. use the straw to narrow the spray vectors.
 
2 thoughts
I once had my soldering iron plugged into same outlet amp I was testing was plugged in (tube integrated) the iron was causing fluctuations in the bias of one channel.
Other thought.
I never had my nose inside one so this is an educated guess ) some amps have diodes tied to transistors to help track bias with transistor temperature.
Maybe yours is faulty (loose, cold joint, bad contact):idea:
 
checking, checking...yeah, it's a weird problem alright. Never saw this problem tied to the volume control before.

Thanks, nj, there are diodes in the circuit, and a thermistor to feel the temp of the output heat sink. I figure the thermistor is working, because the bias increases when the output gets hot, and decreases as it cools. The diodes were my next guess, followed by the voltage limiter circuit transistors.
 
I've seen "motor boating" after a unit was serviced and the problem was a transistor backwards or mixed pnp with npn, I don't remember exactly, at the power supply feeding the preamp section. Double check your work if you did something in that section.
 
Lots of ideas here but I tend to think the PS is OK only because the issue is related to the right channel?
You can you run this in a split mode. Try running as an amp only using another as a pre. See if it's experiencing the same heating and speaker whoop-whoop.
Then operate it as a preamp with another amp if needed.
Without a scope:
Start there and see where it takes you.

When rebuilding many things can be missed. This issue might be as easy as a simple visual check of all the work performed. A missed solder or inverted cap.
 
I agree with elnado. The only time I've had motorboating (twice) it was related to transistors - in my case it was too high gain -however the trannies were being used to amplify sound - I don't think that's the case in your case. However check the trannie gain and orientation of the ones that you replaced.

When did the problem start, what was the last thing you did before it started?

Did you do the work in stages and keep checking in between?
 
Thank you Gentlemen et al! Quite forgot I could split this one, that's a good idea. It is noted that the volume and tone controls are on the Audio board though, not in the preamp section. Still worth a shot.

I had not swapped any transistors before noticing the problem, only electrolytics in PS and Audio boards. Double checked my work and many voltages and it was all OK (darn!). There was some funky soldering covering two torn traces, I tried to smooth that out but I don;t know if that was it. It was on two transistors in the V/I circuit, too, so next I'm going to try replacing the mess. Let you know when I'm done. Sure appreciate the help!
 
Eeeeyowww. Swapped out Qm10, Qm16 (KSA733c) and Qm12, 14 (Ksc945) to clean up solder blobs for missing traces and to make sure the V/I voltage limiter circuit was working. This suggestion from old Echowars thread, and with a voltage problem I hoped it would help. I noted higher gain in the newer transistors. Also swapped diodes Dm 1-4, double checked work, powered up with DBT and things seemed fine. Plugged in, took PS voltages and some on the new transistors, and everything was close to spec. Played radio quietly for 15 minutes, no problem. Checked bias, 41.7mv and 39.5, so I was getting happy! THEN -

switched selector to Phono 1, and turned up the volume and WUP-WUP-WUP. Suspected thermal runaway too, as it got hot in about a minute and bias went crazy high, like 1v ?!?. I shut down quickly, so I didnt get a bias reading on the other channel. Back to the drawing board....think I'll sleep on it.
 
OK. Now things are getting interesting. Split the Preamp and Power amp, stuck my Kindle in the Power amp and heard beautiful music. No signs of any motorboating or other oscillation. I havent tried the preamp alone, I need another amp. I did not replace anything in the Preamp, only the Power amp and PS. SO - if the problem originates in the Preamp, where might I look? This preamp has 2 IC's. Voltage from the PS board seems close enough (19.8v, spec is 18.5v). The volume and tone controls are on the Audio board I recapped, not the Preamp for the Phonos.
 
Are you only getting this issue on phono?

Are you getting it in both channels?

I am almost convinced this is due to an issue with trannies that you have subbed into the phono section. If the problem is isolated to there of course.

Please re-read posts above. People are trying to assist you.

Look again at orientation and gain of any trannie subs you have made
 
If only in phono, of course focus on the phono preamp and power supply to that section. Don't forget the function selector switch.
 
Thanks again! I know everyone is trying to help, that's why we're here, it's a great place with great people. Please carefully review my posts, that way we can better focus on the cause. Thanks elnaldo, the selector switch is my next target.

Perhaps everyone is not familiar with the KR-5600. The receiver is laid out with the Preamp board mounted vertically, tightly crammed against the back wall by the RCA inputs. The Preamp board contains two IC's that handle the boost for all functions - FM, phonos, tapes, aux., and input and tape selector switches. The FM/AM board sits separately to one side, with a small PCB under it holding the muting circuit. The Audio board contains both tones, volume and balance controls, and circuits for the differential pair, pre-driver, driver and power out transistors, bias and voltage limiter. This I have learned from previous AK posts. The Preamp and Power amp are split and then re-coupled with shunts in back. The Power Supply board is of two halves, one with the main filters and one literally folded over it with the voltage regulator and various pin outs for rail voltages and supply for the other PCB's and the lamps.

With IC's in the Preamp, there are no discrete transistors there. It's also a bugger to disassemble! So I let it go, and did not even replace the 8 small electrolytics there. I probably will have to go do that, but up 'til now I have done nothing to the Preamp PCB.

I changed the flyback diode at the Relay to get the Relay to close. It works fine now, about 2 sec. delay from power on. That was the original problem to repair to get the amp to play. Fine. Done. I listened to the FM, it sounded good. I did NOT listen to the phono, etc at that time, which I know now was probably a mistake. The motorboating may have already been present, but I did not check those functions. My bad, lesson learned. Always check EVERYTHING.

Then I replaced all electrolytics in the Audio and the Power Supply PCB's. That was all, besides lots of De-Oxit, of course. This on everyone's advice and also because it's my fourth recap project. The audio and electrical improvements are worth it. The FM sounded wonderful in the recapped amp after this. I thought I might sell the machine, so I installed new lamps (LED's) and cleaned it up nice. Of course before selling it you check all the functions, and that is when I found the low freq. wup-wup-wup cycling the little speakers hard.

In my attempt to tame the wild bias swings as the amp got hot I changed the bias trimmers and the bias transistors. Better bias control, but still motorboating on phono and aux. Then I changed four diodes and four transistors for the right side of the voltage limiter circuit. I'm still guessing at this point, I don't have a scope, just a fair DMM. Replacing these semi's did not solve the issue. All old semi's tested OK, (NPN's had lower gain than the new ones). The oscillation remains on the right side.

Thanks to zebulon for the suggestion to split the preamp and power amp. I have tried to carefully inspect my work, triple check polarities, transistor pin outs, soldering and voltages. What I can see checks out OK. I even reflowed some suspect solder points. When I split the amp the Power amp seems to work fine, there is no evidence of the motorboating in the power amp, and bias stays calm, DC offset below 10mv on both sides. Sounded good. These two PCB's are where I did all of my work. Well, i also swapped the dial lamps. I'll be giving it a good cranking tomorrow to make sure of the power amp.

I'm getting another amp to test the Preamp section, and see if the problem might originate there. Actually, i hope so! Until then, I have apparent motorboating and potential thermal runaway in the right channel when the preamp and power amp are coupled together. Most AK posts about this suggest bad caps, which these are all new, and possible grounding problems, which I can't find. I don't know how to find any possible AC leakage. Can that be a cause? A scope, you say? Darn. I'll be hitting on that selector switch and the Preamp soon!

Thanks for taking the time to read all this wind and still try to help!
 
tusntuk2 said:
When I split the amp the Power amp seems to work fine, there is no evidence of the motorboating in the power amp, and bias stays calm
Click to expand...
Good write up.

Some clues in your section above. Have you replaced any trannies in the pre section at all, I note your reference to there only being IC's in the pre-section with no trannies there.....

Yes definitely test the pre section with another poweramp.

I'm not an expert but suspecting HF oscillation in the pre section being amplified in the power section. I note you have replaced all caps, this to me would address any stray DC from the pre section being amplified. Are you positive positive that all caps are the right way around?
 
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