Marantz 2250B rebuild thread

[catrafter]

@rBuckner I did something similar once. Fortunately it was on a 5V current limited device!

Alright, so I just finished up the PSU and protection circuit.
I did notice that my 14v rail is settling in at around 13.6V, will this be a problem? Seems strange that it is so low (was this way before rebuild too).

I posted my 'thesis' on what I think is going on in a PM to you.
I will repost it here just in case I am way off base.
Readers please let me know if I screwed up somewhere.

Tom

13.6V is fine. In fact, every model I have seen that uses this power supply scheme is around that voltage.
Increasing the zener voltage slightly would increase the output voltage but I don't think it will make any difference in the overall operation of the receiver.

H810 is a 14V Zener diode (with a tolerance of course, so it could vary slightly).
R809 is 10Ω (and has a tolerance as well). As H804 conducts, it's base current causes a slight voltage increase across R809 so the base of H804 is equal to the H810 Zener voltage plus whatever the voltage is across R809.
The emitter voltage of H804 is about the base voltage minus the B to E drop (assumed to be 0.6V) of H804 .

To get even more technical, lets assume that all the boards that are connected to J806 (the '14V' output) draw 100mA and H804 has a current gain of 50. (I'm assuming these values!)
That means that the base current of H804 is (roughly) the emitter current divided by the current gain or 100mA/50 = 0.5Ma.
0.5mA current flowing through R809(10Ω) causes a voltage of 10 times 0.5mA or 0.5Volt, so with a 14V zener, plus 0.5V across R809 makes 14.5V
14.5V minus the base to emitter voltage drop of H804 (assumed to be 0.6V) equals the output at J806, 13.9V
Or, zener voltage plus voltage drop across R810 minus the B to E drop of H804 equals the output voltage.

(Vz+VR810) - H804BEV= VJ806
(14 +0.5) - 0.6 = 13.9V

 

[goldswimmerb]

Alright, so this post is probably the last board I get to have fun with, the preamp/tone board. I love these models because the board is able to be completely removed fairly easily and comes all the way out.

@rBuckner this was why I was asking for the SparkOS install guide.

Photo of completed board ofc:

Here I went with the SparkOS BA312 replacements, since while they don't offer a huge improvement they do measure better.

I upped the main filter here to a 220UF nichicon UPW, had to angle it since I accidentally ordered the extra tall ones...

The small value capacitors here were wer replaced with equivalent value WIMA films, and the two film capacitors on the input were also replaced with WIMA films (I happen to be a WIMA fan).

As per the SparkOS guide I installed the two 47UF UKT caps that came with the kit, and for the 10UF caps I went with some bipolar UES capacitors.

I also swapped out the transistors on this board.

I replaced the 2SC1327s with KSC1845s, my unit seemed to use 2SA842s rather than the 712s the schematic called for. For these I went with KSA992s.

So far the amp sounds great. I will be doing the tuner boards soon, but they'll just get replacements for the electrolytic caps since I don't want to have to realign a tuner.

I did notice, both before and after the rebuild, that when starting the amp from cold with audio playing the relay would close (passing audio), the audio would play for a second or two, fade out for a second, then come back as if nothing had happened. Any ideas why it would fade out on me like that?

 

[catrafter]

After thinking about my above post for a while, I may be wrong about the voltage across R809, I believe it will be opposite in polarity of what I stated.
The voltage across R809 is very small (I measured 6mV) and has little impact on the operation.
Where's the embarrassed emoticon when you need it?
Some service manual have the output listed as 13.4V for the exact same circuit.

Tom

 

[highvoltage_]

...Where's the embarrassed emoticon when you need it?...

Sorry Tom, couldn't resist.

 

[goldswimmerb]

That +13 supply depends on a sloppy ole zener for the reference. Yours is just fine. The +13 pass transistor H804 has more voltage from collector to emitter so for the same amount of current being supplied as the +35 supply it will run hotter.

BTW, I never turn on and test some newly rebuilt circuitry later in the evening. If it fails I'll either not get to sleep or think about it first thing when I wake up and I'm up for the day then, even if it's 5AM!

@catrafter - I've also dreamed of a solution to a circuit problem and it was the right fix! Better yet, I once was working on a plug in pc board on an extender card and found a bad IC. Desoldered it and replaced it and then noticed it was still powered on. It was then working though! Talk about lucky!

You have no idea how many times thus far, with more problematic units I've woken up at 4-5AM to immediately begin working on only to waste an entire day chasing something extremely simple that I would have probably thought of had I sat back and gave it a rest.

I had a 2252 a bit back where I was working on the tuner and didn't realize I had the unit plugged in and on, gave a very very nice spark and a good wake up call. I've had a few run-ins with the 120V from the unswitched outlets...

 

[rBuckner]

Yeah... done the spark thing. I usually forget that only the negative main filter caps in 2385/2500/2600's have bleed resistors. My guess is that it has to do with power off thump/noise avoidance. The negative voltage cap also powers the soft start circuit so that may have something to do with it.

@catrafter : With the R809 update I agree with your analysis. Then there's the other variables such as transistor beta change with heat and I bet the zener changes a bit with heat. I also thought that the supply voltage to the current limiting resistor R807 varies with line voltage changes so may minutely change the zener voltage. Probably more factors at play here like load current shifts depending on what function is selected. That's enough thinking for the day for me!

 

[catrafter]

@rBuckner , after scaring the crap out of myself and melting my screwdriver on units that don't have bleeders, I made a bleeder using two clip leads, a plug to socket adapter and a 40 watt incandescent lamp.

Thanks for your comments on the power supply, it made my head hurt, too!
Tom

 

[rBuckner]

I have a 120 ohm resistor, like 20 watts or so with clip leads. It's great when I use it!!

 

[goldswimmerb]

Got the FM IF board done, 100uf caps were swapped over to nichicon UPW, the 22UF was swapped to a Nichicon UKZ and the small 10UF ones were replaced with Nichicon UFGs. Not much to do on this board, or any of the remaining ones for that matter...

 

[goldswimmerb]

Alright, so I got the last boards done. Got the Dolby FM board (because I'm a perfectionist at heart), the AM tuner and the FM MPX board.

The AM and Dolby were pleasures to work on, plenty of wire length and enough space. The FM MPX had me swearing a few times.

I made sure to replace all tantalum capacitors with WIMA films, sub 10UF caps became WIMAs, 10UF was replaced using nichicon UKT here as I had a bunch of them, 47UF caps wound up being UKZ as it was all I had. Used UPWs for the 220UF and the two 22UF caps. Used a UKL for the 33UF as it was all I had that would fit. I actually moved C337 to the back of the board since the WIMA I had was a little big.

If you can't tell, especially with tuners, I tend to be less rigid in my type selection, I figure anything is better than the ancient caps in there. Testing after getting the board recapped I am able to pull in many more stations than before and the tuner seems more selective.

AM seems to be pretty much dead in my area, although that may just be a result of my testing location. I was able to get one station so I'll call it good enough.

Pics attached.

 

[fisherboy]

UPDATE ON THE PSU!

I was an idiot and had a cold solder joint.... Whoops...


Thanks for turning in!

I'm sorry for the stupid question, how did you find out that the issue is a cold solder? Was via continuity test?

Cheers

 

[goldswimmerb]

I'm sorry for the stupid question, how did you find out that the issue is a cold solder? Was via continuity test?

Cheers

If I recall, as it has been a while, I was able to find it by just looking at the board under a flashlight, could see the cracked solder. Usually when I do wind up finding bad joints, however, it is from tapping around on the board, or visual inspection, never had much luck with the meter for bad solders.

 

[fisherboy]

If I recall, as it has been a while, I was able to find it by just looking at the board under a flashlight, could see the cracked solder. Usually when I do wind up finding bad joints, however, it is from tapping around on the board, or visual inspection, never had much luck with the meter for bad solders.

Thank you this is very helpful