hello folks,

if you read the earlier post, i did some editing.

years ago i had a Philips Receiver Model 7841 that had a 200 mV DC offset on one channel and a very nice minimal offset on the other channel.

i gave it to my cousin for his shop and he recently returned it to me to have the pots and switches cleaned.

back in the day, i did the following:

i replaced ALL the transistors on the main amp board from the finals to the drivers and back to the signal input transistors from the tone controls.

i replaced all the resistors in and around the outputs, drivers, and signal input transistors.

i replaced all the electrolytics in the entire amp board and the power supply section.

i swapped the emitter resistors from each channel. the high DC offset stayed with the offending channel.

the idle current can be set very nicely on each emitter resistor, so there is no issue there.

i did NOT replace any of the few disc capacitors in the amp section.

i did NOT replace any diodes in the amp section.

so, where can i go from here ? i am not at all a pro with SS equip, but i really would like to learn something and fix this.

anything that i have done above has kept one channel with a 200 mV DC offset on the speaker terminals. the other channel is minimal.

the white dial pot on the far right is the stereo multiplex FM adjustment.

any help will be appreciated.

thanks,

steve

service manual here:

https://www.manualslib.com/manual/10209 ... -7841.html


http://akdatabase.org/AKview/albums/use ... Manual.pdf



steve

Could be an imbalance in the outputs or drivers. Are they matched complimentary pairs? One test (annoying but revealing) is swap the outputs in pairs from the good channel to the bad. Same with the drivers.

Of course, you can also end up with two blown channels. . Be careful.

Also, did you use exact part replacement transistors, or NTE/ECG/SK general purpose?

200mv is a lot of offset. You may be able to do some comparisons from the good channel to the bad. Like actual drops across transistor junctions... see where its coming from. DC coupled amps are sometimes a gigantic PITA. I know, I've mentioned this before. Any issue gets reflected just about everywhere.

I know it could possibly be anywhere. All transistors were replaced NTE back in the day. The 200 mv offset remained exactly the same. See above for other things that were done.

That 200mV can affect sound quality to a degree, but it isn't likely to damage most speakers.

Consider that the Sencore PA81 amplifier analyzer, which is pretty much the industry standard for amp service shops, has a DCV relay disconnect for its dummy loads, and its trigger threshold is 1VDC. This is meant to protect expensive output devices, and ultimately the customer's speakers.
I doubt they would design this costly piece of gear to allow an amp out of the shop after servicing that would end up frying the customer's speaker coils.

I mention that in case some readers may be concerned about their "dangerous" DCV on outputs.

As to reducing DCV, some people have done this by closely matching xistors alone. You could go on a wild goose chase, shotgun everything, and never solve this, if your outputs aren't well matched. I presume you verified the various voltages at nodes indicated in the schemo.

i think this will be a cat and mouse game.

steve

Output offset will almost always be an issue with the input differential pair. Looking at the schematic, that would be Q502/504 for the left channel and Q501/503 on the right. Ideally, these transistor pairs should be matched for Vbe and hFE characteristics.

There is no offset adjust in the unit. If the offset can't be tamed by using a matched pair on the input, then you can try adding an offset adjustment. In this case, I would suggest making R506 or R505 (depending on which channel) variable - replace the 4700 with a 4300 in series with a 1K pot. Then use the pot to adjust the offset. If you can't hit zero, then you may need to make the fixed resistor 3900 or 4700 depending on which end of the range you are running into.

excellent.

the RT channel is the one with 200 mV on it.

how about changing just a resistor(s) in the offending Right channel to bring the offset down ?

if so, which one or ones could i use a decade box on to "adjust" the offset, and then install a fixed value ?

would that be R505 or R506 depending which one changes the offending RT channel ?

or should i install a pot as described above ?

after shotgunning all the Qs on the amp board along with the resistors and lytics, thr DC offset is the SAME on both channels.

i find that to be stupid odd. lol.





steve

One thing I would look at, based on what you've done so far, is the base voltages on Q501 and Q502. They are both supposed to be at -0.1V.

In each channel, there is DC feedback from the output to the bases of the other side of the differential pairs (Q503 and Q504), and ideally there should be extremely little difference between the two inputs to each differential pair. In other words, you could measure the difference between the base of Q502 and Q504 directly (not with respect to ground, but with respect to each other). For a differential pair like this, it would be shocking if there were more than 10 or 20mV of difference; 200mV would basically have one of the transistors shut off almost completely. That would give you very distorted audio.

But if there is, for some reason, a DC offset at the base of Q501 or Q502, that will be replicated in the output. At DC, the gain of the entire output stage following the differential pair is 1. It's much higher for audio, but at DC all capacitors are open-circuit and can be ignored.

you said "not with respect to ground" when measuring the bases.

is that base to base and look for offset ?
steve

the offset on both left channel q502-q504 base to base is 12 mv.

it is 12 mv on the right channel base to base on q501-q503.

steve

OK, so far so good. The differential amplifiers are behaving normally.

So what's the base voltage relative to ground on Q501 and Q502? If they both are at the specified -0.1V, then I'd suspect leakage in a capacitor. Recall that I said the "gain" of the output stage is supposed to be 1 at DC, meaning the DC offset at the output should be almost exactly equal to the base voltage at Q501 and Q502. But that's only true if the capacitors are really open-circuit at DC. If C511 or C517 is "leaky" (or their equivalent on the other channel) it could introduce some "gain" into the output stage at DC, multiplying that 0.1V into 0.2V. Or maybe there's some other unintended leakage path somewhere.

Fixed

i removed 4.7k R506 and installed a 5.1k resistor with a 1k pot.

the range is -70 mV to 80 mV and it zeroes at mid pot setting.

THANKS MUCH !!

steve

Congrats! I am going to send you a Pioneer M-90 that has had several shops "fix" it with no success. I subsequently went through both amp boards, replaced a bunch of parts that were actually bad, and had it working pretty well. One side was overheating slightly, which I figured was just a bias issue. No big deal, easy to fix later. I set it aside for a few days.

Back on the bench, turned it on, and BAM ... all 4 power supply fuses blew. This thing is HAUNTED. Just because I am in denial, I replaced the fuses and tried again. 4 more fuses down the drain. This amp has some huge electrolytic caps on the B+/B- rails... with a relay system that applies half voltage to them till they charge up, and then click in for full voltage. I haven't gotten around to looking at it yet. I'm thinking either the caps are done for, or the relays are stuck on, or there's something turning the relays on too soon. They get powered when the B+/B- rails reach a certain threshold. But for BOTH + and - to suddenly start doing the same thing?? ...... haunted, I tell ya.

The thing is 200W per channel. Weighs a ton and a half. Isn't this stuff fun???? Next up is a dim bulb, and disconnect both power amps from the power rails. And probably blow more fuses anyway.

I wonder how far I can throw it without putting myself in the hospital.

Sounds like you have a plan. Using the DBT I mean, not throwing it. Really sounds like a classic case of blown output transistors.

Adjustable current trips/breakers are helpful. Can save money in the long run, and certainly saves on frustration.

The outputs are fine. All 16 of them. One of the previous shops changed 2 shorted ones but missed 2 leaky ones. And both bias networks. It’s been like that.....

hi barry,

thanks, but the congratulations must go to Retire Jay and Ylli.

those fellows told me exactly what to do.

i am absolutely NO solid state man. i can get by with general power supply, RF, and amplification, but i have no idea what the "tricks", circuit design, circuit principles, and "how it flows" like i do with tube electronics.

i would have had absolutely NO idea how to solve that problem.

many thanks for Jay and Ylli.

my fortee, knowledge base, and full understanding is in tube radio, amplification, and RF, not SS.

if this situation would have been with a tube setup and i was down to my last two 6L6gc tubes, i certainly could add circuitry for tube bias, idle current, and matching the cathode flow.

all is well with the philipd receiver.

what i can not understand in all of this is that the DC offsets stayed the absolute SAME when ALL of the transistors were shotgun replaced about 10 years ago. i got an envelope of radomly pulled NTEs.

if it were a transistor matching problem, would not the offset have changed when i replaced them all ? they were random pulls off the shelf.

i think it could be a poor design or something, but for what it is worth, the DC offsets are just perfeclty set at 0-5 mV with the added pot and resistor combo.

thanks again, very much

steve

The designers of that circuit, and in fact all, if they are any good take into consideration that there is a variation from one solid state device to another, sometimes a fairly large one. I think in your case, that is compensated for with feedback to some extent. It seems perhaps it was just time for a resistor or two to be tweaked. Usually, and always in higher power or higher end SS equipment, there is both a bias, and a quiescent DC setting for each channel of power amplification. Usually, because some tried with varying success to make it automatic, with the addition of huge and mysterious complexity. Troubleshooting one of these is ..... an experience. Here's the one in question, using an IC as the bias chip. What could possibly go wrong. AND it's of course this chip is virtual unobtanium today, and there's no NTE substitute. When you can find them, they are very expensive and you never really know what you're getting.

This is just one channel. One channel board has the low current power supplies and some other stuff, the other has the protection circuitry and some other stuff.

Weeks of non profitable fun here..........

That is why I love to work on tube amps, even the very old ones.