Causes of crazy DC offset but idle current in spec ...?

[Julian12345]

Would the STVs I pulled out of it serve an alternative purpose? They appeared to be glued on with something resembling a factory glue and screwed to a heatsink

Yep the output transistors and emitter resistors plus probably some other stuff are mounted externally to the main amp board, on the rear chassis

 

[Julian12345]

Here is one of the STVs as it stands once I unscrewed it and forced the glue away. They were really on there.. does this mean the circuit design as documented deviated in production?

 

[gadget73]

possible production change, but those do appear to be mounted in a place that makes sense if they are being used as bias compensation. As transistors get hotter they draw more current because the internal junction drop changes. Any silicon device changes at the same rate though so its pretty common to use a diode or diodes for that job.

 

[Julian12345]

service info here

JVC 5003 AM/FM Stereo Receiver with SEA System Manual | HiFi Engine

www.hifiengine.com

its a little hard to follow since the output transistors appear to be external from the rest of the board, and it doesn't show it all in one place. This is the driver board though

View attachment 3032570

and this is the output stage part of it all

View attachment 3032573

looks like fairly conventional quasi-comp single rail cap coupled stuff. Note the +40v spec on the + side of the output coupling cap and the 88v main supply voltage. If that voltage at the cap is way screwed up, the power output will be very limited and it will distort early so make sure that is correct. I got nearly double the output power from a Sherwood by fixing that and it sounded much better.

if I'm not way off here, R422 will be the centering voltage adjustment and R436 is your bias. I don't know what the bias ought to be set to, but it would be measured across one of the 0.5 ohm emitter resistors. If those adjustments are too screwed up it doesn't surprise me that the drivers get hot either.

This also calls for thermistors for bias temperature compensation, not triple diodes like an STV would be. An STV-03 should be 3 diodes in series inside that blob and if you measure it with a diode check function it ought to show about 1.8 volts of drop.

Hi there, I checked voltage at the coupling caps. I had previously replaced them with 100v 1000uf. One is showing 44v. The other was around 2 to 4 volts. And seemed to be changing a lot

Also I fitted 2n5416 to replace 537a. And left in the modern 2n3440's

The DC pots im sure are out of whack now but retuned to approximate positions (although release this means little without data). Didn't touch bias

So now wondering why one of what I believe are the coupling caps is tanked on only a handful of volts. What have I done I wonder.

How can I unpick this thing and take a big safe step back then return. At the moment I'm power cycling it on a dim bulb to take quick readings where I can as driver trans get super hot super fast

My dim bulb is not telling me theres a major issue. It behaves normally. No bright light.

 

[gadget73]

Cap is probaby not bad unless it was installed backwards, but fair guess you either have a dead output transistor or the fuse is blown.

check the fuse, and diode check the output transistors to see if they show opens or shorts.

 

[Julian12345]

Hey all, @gadget73 @avionic @GrisPato I'm about to pick this one up again
Would anyone mind giving me a process to follow for (let's assume all components ok and installed ok) resetting the DC balance pots ?

And for idle current would setting it to about 20mv across emitter resistors sound ok?

Thanks a tonne!!

 

[gadget73]

if the service manual doesn't have a specific procedure for it (I can't open it from here) then I'd say set the bias, adjust the balance control for +40v at the + side of the output cap, and re-check the bias to make sure nothing moved. If they interact, bounce back and forth until both are where they should be. They may drift a little as things warm up too so don't drive yourself crazy trying to get it dead nuts on, just make sure its close and stays in the neighborhood of where you set it.

I don't know what the bias should be set to. Hopefully its in the manual.

 

[Julian12345]

if the service manual doesn't have a specific procedure for it (I can't open it from here) then I'd say set the bias, adjust the balance control for +40v at the + side of the output cap, and re-check the bias to make sure nothing moved. If they interact, bounce back and forth until both are where they should be. They may drift a little as things warm up too so don't drive yourself crazy trying to get it dead nuts on, just make sure its close and stays in the neighborhood of where you set it.

I don't know what the bias should be set to. Hopefully its in the manual.

Thanks , yep no info in the service manual I have. When you mention output cap is that what sometimes people call coupling capacitors?

And adjusting balance controls are the trimmers for DC balance?

Thanks!

 

[gadget73]

Yeah, it'll be the big ones between the speaker terminals and the output transistors. C22 and C23, 1000uf / 80v.

DC balance is the balance control, yes. The trimmer on the board that isn't the bias adjustment.

 

[Julian12345]

Hey @gadget73 + all, good and bad news. So two issues I fixed, one DC trimmer was whacky, it wouldn't behave normally. The other was OK. But nipped down to the local electronics parts store just before they closed and picked up a couple of 250k trimmers and installed. After finding a short (a tiny single strand of solder braid bridging a couple of traces on the driver board, I was then able to set voltage at the coupling caps to 40v.

I've checked my output transistors, there are no shorts. But I am running 2n3773 at the moment as I did short one of the original 2SD84's a while back.

I measured mV and mA across emitter resistors and got next to nothing at around 0.2mA and almost nothing in terms of mV. I haven't touched the idle trimmers, they are still with the glue seal from factory in place.

Biggest thing that's driving me crazy is trying to work out why the driver board transistors X408 X406 X404 and X407 X405 X403 are running very hot to varying degrees and get almost too hot to touch so I turn off the receiver and let it cool off. These transistors are no longer the originals (they mostly went up in smoke a few months ago) and while I did find some new old stock of the original types and do have them in my stash, I decided instead to install what I thought were equivalent 2n5416, 2n3440 (hope this is OK)... I'm reluctant to install the new old stock 2SC708 and 2SA537 as I don't want to burn them up again.

I've also replaced the 458 transistors on the driver board with 2SC945 which I have heaps of. I also have a tonne of KSC1845 if that's a better solution.

With an indicated almost nothing idle current I'm getting decent quality sound to halfway on both channels after which heaps of distortion.

Just have no idea whats going on with the driver transistors...

Any suggestions and help greatly appreciated because I'm so keen to use this receiver - it's been sitting there a while now. I'm at a loss

Thanks again for guidance so far

J

 

[gadget73]

I'm finding some conflicting datasheets on the original parts, a few sources say they are 100mhz parts, another says 15 mhz. Inclined to believe the 15 since its an official Toshiba datasheet. If so that sounds like the new parts are a reasonable sub. They exceed the voltage, current, and power ratings by a sizable margin and the gain numbers are similar.

Any chance you have a scope?

 

[Julian12345]

I'm finding some conflicting datasheets on the original parts, a few sources say they are 100mhz parts, another says 15 mhz. Inclined to believe the 15 since its an official Toshiba datasheet. If so that sounds like the new parts are a reasonable sub. They exceed the voltage, current, and power ratings by a sizable margin and the gain numbers are similar.

Any chance you have a scope?

I actually have two scopes! But I haven't learnt to use them yet. Now a good time?
One is 4 channel 70mhz from memory. The other is 2 channel
When matching I rarely look at the mhz component of tranisistors. What's the impact if the sub is much higher or lower speed over the original?
Fyi running on 100watt dimbulb atm but I did bypass briefly to see if behaviour changed with heat and distortion. Not really any change.

 

[gadget73]

If its much higher it can possibly allow for oscillation that wouldn't happen with a slower part. Thats kind of why I'm wondering if something fishy is happening. Oscillation causes high current draw = heat. I'd expect it to show up as high voltage across the emitter resistors too though. Either way if its oscillating it should show up as something weird on the output of the amp.


Any chance you've checked voltages against the schematic on the driver board to see if anything looks completely weird ?

Do both channels have the same transistors replaced or does one happen to still have original parts?

 

[Julian12345]

Hey @gadget73 , both channels have the newer transistors. I replaced both channels because it was frying both sets of original transistors...

I checked all the voltages.. well maybe not many of the ceramic caps but everything else... initial handful of readings I did on current limiter then redid them as felt it would be more accurate with full current.

I noticed R433 434 had been chopped out. I installed one with new 680ohm resistor. No change. Still gets hot so I didnt bother doing the other channel for now.

Here's a link to a high res version of the mark up attached https://www.dropbox.com/scl/fi/al83...2447.jpg?rlkey=v5iq75sz8jzrn5tg5nl8f3qju&dl=0

FYI The 43v across C411 and 412 is probably 39 to 40v now as I've made another adjustment to DC

Also the voltages for X401 402 were accidentally done with current limiter on so maybe 20% less than reality.

I think I might check all my transistor orientations (pins)

*** update ***

According to schematic X405 has a different pin out to all the other transistors. So I rearranged the pins and installed E C B. This caused all driver transistors on that side of the board to go full meltdown mode but I caught it straight away. I believe I had installed 405 correctly the first time and the SM is incorrect. I'd think the voltages I took prior to this would also suggest its original orientation was correct when comparing to the other channel. I removed the three driver transistors after this happened, tested them, they were fine, reinstalled the way it was, back to the casually overheating from before.

Starting to wonder if the problem is external to the driver board. As both channels appear to be behaving the same way.

As mentioned before, currently have a set of 2n3773s in, plus I've replaced emitter resistors and the STV 3's with 3x 1004 diodes (I know the SM says it has thermistors but this one does have STV 3.. or used to until I broke one of the legs). I've seen what a dim bulb will do when biasing diodes go bad so I don't think its that at all.

 

[gadget73]

any change if you ground the inputs to the driver board ? that would rule out anything upstream. Looks lik 415 and 417 are the inputs, 416 is ground.

So far as I can see vs the schematic none of the voltages look crazy. They also look fairly consistent channel to channel. Really don't see any obvious reason why the small transistors would be getting hot to the touch. The original parts are all rated for less than 1 watt dissipation which basically means they don't flow any kind of current. Your voltages don't scream high current flow either so I'm at a bit of a loss here.

The 680 ohm not being there would change how the bias adjustment works. Do you get anything now?

 

[Julian12345]

For a second I get about 3mV across two (one on each channel I think) of the four emitter resistors then within a couple of seconds it goes down to about 0.3mV. The other two resistors show nothing.
With the grounding from input to amp board how might I go about that? I have plenty of those alligator clip test leads. Would I just connect test lead to 415/417 then to 416? What should I look for when I do it? less heat?
Update:
I connected 415/417 to 416 at the same time. Still get hot

 

[Julian12345]

I did notice reference to paired transistors in the SM but I thought earlier in the thread we established there are no complementary pairs needed in this design? Even if there is wouldn't the two types I have installed work?

 

[gadget73]

415 to 416 and 417 to 416. Gator clips are fine for that. If it stops getting hot, then its something from upstream of the amp board. If no change, its not that.


Those transistors are in different channels, but I'm guessing they want them reasonably matched so the two channels perform the same. I don't see why that would cause any sort of heating problems though. You should be able to take all of the parts out of one channel and the other would still work without any fuss.


do you have a set of good original transistors, enough to populate one channel with them?

 

[Julian12345]

Hey @gadget73 , just so to confirm where should I be testing for idle current? Maybe I'm in the wrong ballpark.. can u point out for me on attached images?

Also how hot do u think those transistors should really get, originally they had these little heatsinks also in attached images.

Update:

I put the heatsinks on and installed new idle current pots but as I can't work out how to read idle on this yet I have set them to 58ohms which is what the old ones that came out were.

I've run it for about a half hour

I'm just not sure how hot is too hot... those heatsinks are pretty warm but I wouldn't say they are burning me when I touch them UNTIL I turn up to maybe 1/3rd volume or more - they seem to react quickly when I turn the volume up however the output transistors stay dead cold. Ambient temp in my workshop tonight is about 75f (I checked your location, converted from c ) I still have distortion at half or more volume even with current limiter bypassed

Interested in your thoughts? Would you expect it to be able to run for a half hour without going on fire even if a little warm on driver transistors if it were truly messed up?

For the output transistors I have reverted to the remaining original 2sd84 on one channel and bux41 on the other.

Super keen to work out this idle current delema ... I feel I'm not testing correctly

P.s. having some strange noises come and go like sound of wind ... left channel only. Isn't impacted by changing balance left to right ... I think its some of those 2sc458's. Will deal with them after amp is sorted. Also I have changed the 2sc945s I installed on the driver board to BC550 for the closer match in transition frequency to the original 2sc458's

Cheers!!

 

[gadget73]

idle current in the output stage is measured by voltage across the 0.5 ohm emitter resistors tied to the outputs. I don't know what it should be set to though.

the original transistors are rated for 3/4 watt of power dissipation



that would be very little heat output. If you want to calculate th dissipation to confirm its somewhere within the limits, voltage from emitter to collector times the current flow. Voltage drop across the collector resistor divided by the resistor value will get you the current. Current * volts = watts

how much power output is it capable of into a dummy load? Is the clipping symmetrical? Is there any visible crossover distortion on the scope?