2285B No BIAS (Driving me nuts)

[caesar56]

Long story short: I received a pretty chopped Marantz 2285B for Christmas. It had been worked on before in a bad manner (blew traces, weird parts, different resistor values, etc). Had to redo a lot of stuff here and there but the main amp circuit board (P700) has become a nightmare now. It had so many different parts changed that I almost ditched half of it.

After all the work I could lower DC offset to 0 (Rock solid, no variation) from the original 300mv, but I haven't been able to set BIAS voltage. No matter what I do, it stays at zero.

I am making a list of all the the parts that were changed in this board but in the meantime I would like to ask 1st)If there are known culprits for this no bias behaviour and 2nd)If the p800 power supply board could have anything to do with it, as I haven't done a lot to it since it's a nightmare to take out.


Thanks a lot in advance,
C.

 

[bryans12v]

Take bright, clear pics of the amp board and upload.

Have you confirmed power supply voltages and +/- rails?

Also would help to know what semiconductor subs you used and where.

 

[Halisson]

I had some problems with 2285B (including this one from my picture) due to improper maintenance and cold welds or broken tracks on the power amplifier board.
I know it may sound stupid what I'm going to say, but do not forget to compare the L-channel voltages to the R, so that you eliminate at least at what stage the polarization is not occurring.
Base and emitter Q001~004 or Q005 ~008 what voltage ?

 

[bktheking]

Zero bias - check to make sure your traces are all connected to the components you replaced. Guaranteed something isn’t connected.

 

[airtime]

I have to recheck the schematic, because it's been about 15 years, but check the power supply voltages to the output stages. Zero bias could be a PS issue on some designs. But again I have to check the schematic????

And I'm "assuming" this is both channels?

 

[caesar56]

Hello again and thanks a lot for all the answers, here are MANY bits of information, i'm sure they're talking to me loud, i'm just obnubilated now.

I noticed the outputs were attached with some kind of plastic washers thus the collectors were not conducting. Once i removed them all, DBT became bright, i tought maybe bias voltage was back but too high, but this was not the case. Now i have voltage between bias pins but it is as high as 0.5 volts. and the trimmers wont affect it. Also lost DC offset. So i think is safe to assume the amp is shorted now. Pulled out the outputs but they all measure well.

The amplifier comes out of protection if i remove the P700 main amp board, however some voltages are missing. According to SM im supposed to have 35.3V at diode Q813 in the p800 PS board but i only have 27.7V and none of the voltages at Q811 seem to match, neither voltages at Q810. I am guessing if both were shot, the relay wouldnt click and i would have a short, but the amp does come out of protection.
Relay at PS board was swapped with Z2634-ND
Main caps look original.

At pin J725, J724, J705 and J704 i have 52.3V in P700 main amp board wich means i do have enough voltage as shown in sm. (This is only if i either leave the plastic washers or remove the board completely).

Output transistors were mounted using silpads (No mica).

Here's a list of all the transistors present the P700 main amp board eight now

For the right side of the board:
Q723- C945
Q724- A733
Q705- A872
Q706-A1220 (original: A914)
Q703-C1775
Q707-C2690 (original: C1953
Q708-C3421 (original:C1568)
Q710-Mje15032 (original:C1913)
Q709-Mje15033 (original:A913)

For the left side of the board:
Q734-A893
Q733-C1845
Q736--A1220
Q735-A992 (original:A872)
Q737-C2690
Q738-C3421
Q740-Mje15032
Q754-A1015
Q753-C1815
Q739Mje15033



@bktheking I have extensively checked it but some traces are just... gone. It's been quite the work.
@Halisson Can't do this since basically the amp is shorted. (After removing the plastic washers).
@ airtime yes, both channels are misbehaving.
@ bryans12v if more, clearer, need better views on any place, let me know.

Basically, i had DC offset but no bias when the plastic washers were on, once i took them off, the amp shorted.

Here are some pics, are these on-semis in the right places?

Left side of the P700 board:

Right side of the P700 board:






Yes, this looks awful:

 

[bktheking]

Can you post a pic of these plastic pieces you speak of?

 

[bktheking]

And looking at the sm, your shorting cause you have the outputs installed backwards - pnp top npn bottom- you have npn top and pnp bottom. If the plastic washers were under the screw heads then they need to be discarded, i don’t see any reference in the sm.

 

[bktheking]

And I’ll explain a little tidbit- dc offset and bias are two separate circuits in an amplifier. Dc offset will work just fine with bias not working and vice versa. Your issue lies after the dc offset section of the amp if it was zeroing- I suspect once you reverse the outputs you’ll have more success.

 

[airtime]

i tought maybe bias voltage was back but too high, but this was not the case. Now i have voltage between bias pins but it is as high as 0.5 volts. and the trimmers wont affect it. Also lost DC offset. So i think is safe to assume the amp is shorted now.

@bktheking I have extensively checked it but some traces are just... gone. It's been quite the work.
@Halisson Can't do this since basically the amp is shorted. (After removing the plastic washers).
@ airtime yes, both channels are misbehaving.
@ bryans12v if more, clearer, need better views on any place, let me know.

Basically, i had DC offset but no bias when the plastic washers were on

There is something very wrong with the negative leg in the bias circuit. Someone really rewired this puppy.

 

[caesar56]

Great news but still something amiss:

And looking at the sm, your shorting cause you have the outputs installed backwards - pnp top npn bottom- you have npn top and pnp bottom. If the plastic washers were under the screw heads then they need to be discarded, i don’t see any reference in the sm.

You were right, those were at the wrong places and the washers are definitely not needed, but hold on, there's more.....

There is something very wrong with the negative leg in the bias circuit. Someone really rewired this puppy.

Yes, i decided to dig furter and found out the original caps were restuffed but the psu was.... bridged somehow? I understand it has a 'dual' PSU but it was wired like a common one....

Anyways, today i worked a lot on it, remade a lot of wiring and bridged 4 caps to work as the original (Dual config). The amp finally woke up and now i have bias and dc offset with a problem right channel DC wont stay put. It oscillates between 0 and 6mv quickly which makes me think those caps are bad (what else could it be?), also those huge trimmers are not linear they have some OL zones and most likely will go open anytime soon. The issue here are the spaces between the leads of those huge trimmers.

I'm thinking to mount those new main caps in a wafer but this is still speculation as is not even clear to me if the space is enough. I'm also wondering if it's a good practice to add a poly cap between each main pair to reduce noise, i've seen this been done but can't tell clearly if there are real benefits to it and what would be the correct values.
On a side not: The part in the P700 main amp board corresponding to the right side of the amp seems to have had blown in the past (Reason why the amp was 'fixed' in the first place) and this side is the one exhibiting the DC oscillation.

I really want to get rid of the way the P700 board is wired (and most of the amp at that), most of those unifilar cables broke and had to resolder them temporally but this looks awful. Has anyone done this? I'm not sure if it would be good to solder new cabling directly to the holes or crimping the cables with solid terminals and soldering these to the board like sansui did. Also what would be a good/safe cable gauge for this?

Thanks a lot again,

C.

 

[bktheking]

DC offset issue- have you replaced the differential pair - either Q701/702 or Q731/732 of the offending channels? I also replace the one connected to the diff pair, it's the same transistor 703/733. Remember to gain match them.

 

[caesar56]

DC offset issue- have you replaced the differential pair - either Q701/702 or Q731/732 of the offending channels? I also replace the one connected to the diff pair, it's the same transistor 703/733. Remember to gain match them.

I gather C1845 are good subs for those A1775. Now, is there a 'Smart' way to Hfe match those or do i have to go to the electronics store and use my DCA Atlas on each single one? Also what would be a good match? how much deviance in gain is accepted?

 

[bktheking]

I think you mean c1775 and yes good sub. Usually people order a bunch of them from mouser and measure them till they find a close match. I try and find as close as possible, say 375 and 378 or 395 and 400. The closer the better.

 

[catrafter]

BK is correct. Japanese transistors and their Fairchild (now On-Semi) substitutes: 2SA, 2SB, KSA, KSB are PNP types. 2SC, 2SD, KSC, KSD are NPN types.
Tom

 

[rBuckner]

Did you have the volume pot all the way down when measuring the bias? Audio will also make that number dance around! I like to also put them in Aux mode with nothing hooked to those jacks when adjusting.

 

[tsd71]

I would check all those solders and jumpers for continuity, there's a lot going on there.

 

[caesar56]

I think you mean c1775 and yes good sub. Usually people order a bunch of them from mouser and measure them till they find a close match. I try and find as close as possible, say 375 and 378 or 395 and 400. The closer the better.

BK is correct. Japanese transistors and their Fairchild (now On-Semi) substitutes: 2SA, 2SB, KSA, KSB are PNP types. 2SC, 2SD, KSC, KSD are NPN types.
Tom

Hello again,
Wanted to report i did as suggested and found a couple of interesting things:
1)The variation in the right channel wont stop, HOWEVER, after running the amp for 10 to 15 minutes and letting it warm up, both channels settled at 0+/-2 mv which i think is pretty decent. This however required the amp to run and 'warm up'. Is this a normal condition? -I'm really starting to think those ouput caps are bad.

2)A new issue has appeared where the amp would drop one channel, sometimes is the left, sometimes is the right. My suspect is the preamp because using the 'main in' plugs instead of the 'aux in' made the channel work again (I only tried this once because had visitors). Whenever the issue appeared, wiggling the knobs a lot but mainly using the volume to give it more oomph, made the channel work again, and it would work for the whole session until i turned the amp off, let it gool and again, no sound in one channel.
Whenever this issue appeared, voltages in the outputs seemed okay. Had correct bias and dc offset values. Haven't scoped the amp yet however.
Important to notice the relay is brand new and all the transistors in the pre board are original.
I am considering changing those 1345 with 1845 again. Is this the right pick? What else could make the pre go silent on one channel?

Best regards, C.

 

[Steven Tate]

Have you cleaned all pots and switches thoroughly with something like Deoxit? Intermittent channels are often traced to dirty/oxidized contacts in controls.

 

[rBuckner]

Amps need at least 10 minutes to settle down, I let them go for at least 30 before adjusting the last time.

Good advice from Steven! Those switching contacts are often "iffy" acting though. There's nothing wrong with just leaving jumpers in as long as they're shielded. In other words, don't use any "U" bars. There are short RCA to RCA cables around that can be used. The relay, though new, could still be the issue. I know that member Catrafter got ahold of a few from a bad batch a couple of years ago of some model number although I can't remember which. I think they were an Omron MY2 variant.