Yet another butchered 250

[goldswimmerb]

I think so - at lower signal levels the lack of bias is more obvious I think. If you are getting a clear, well centered waveform that's a good sign. I also go for the square wave to check any issues with bass/treble. A good squarewave at 20kHz gives me peace of mind.

I was playing with square waves on my 250, it looks rather "ringy" but the oscillations are damped out fast.

 

[goldswimmerb]

I dont know if its my imagination or not, but when i dialed the bias back to 12mV or so the predrivers were a lot cooler than when it was up to 15-18 mV. I left it on the lower end thinking cooler is better. I need to learn how to interpret the output waveform of the distortion analyzer to see what the crossover distortion is. 12mV might be too low for that. Any thoughts on that would be appreciated.

The power Transformer runs hotter than the heatsinks in my 250 and 250M. I generally don't worry unless I can feel a difference between the channels or they're too hot to comfortably touch. They had those heatsinks exposed for a reason...

 

[ocdecio]

I just wanted to write for the record that a 1200B that I just got has 2N5087 in the tone boards instead of 2N4250 and they looked very much factory. So that to me indicates a safe sub.

 

[rickl]

get 20 on cut tape (a bit more than bulk) and write hFE on the tape to help match 'em.

https://mou.sr/3pCNTdH

 

[quadmatrix82]

So onto my conclusion post I guess, unless the unit nukes itself...
Things to note:
When rebuilding these amps always verify that you have +-.5-.7v on your predriver collector/cans, this is the bias voltage and should adjust with the pot, if not it is not safe to install outputs.

The bias voltage is directly affected by your DC offset in early revisions of this amp. Not entirely sure why but I believe it's because the DC adjustment interacts with your +-.7v. regardless, if you see a huge difference between the +.7 and -.7 then chances are your DC offset is adjusted incorrectly, or your diff pair is too low gain, or potential issues elsewhere.

ALWAYS test the drivers in the Amplifier, the "good channel" in my amp had one with a normal HFE of 60 and the other with a damaged HFE of 13, was just waiting to blow. That being said, predriver matching seems to be much more important than driver matching as this is the 2nd unit I've done that's been stable with one driver at 90 or so HFE and one driver at 170 or so HFE, I think Marantz may have some tolerance on the gain ranges provided.

If you have no bias movement on a 100w bulb with both channels (I ran into this) try a 150 or 200w bulb, the Vdrop from the 100w bulb can be high enough to prevent the amp from biasing.

MJE180G worked just fine for my bias Transistor once I installed the higher HFE diff pair, but ymmv since Marantz made so many mid production changes.

Check your resistors, especially solid carbon ones, they like to drift.

Anyway, as for my parts list:
2N4250 - PN4250
SPS1951 - BC547A
SPS 1952 - BC560C
2N4125 - 2N3906
2N4123 - 2N3904
MPSA20 - Central Semi MPSA20 (Still made)
Differential pair:
SPS1928 - 2N5089 matched with HFEs over 800. Gain is critical here due to how the DC circuit affects bias. BC550C would probably work too, but the pinouts are different.
Predrivers:
(gain match critical between NPN and PNP)
SJ2582 - 2N5415
SJ2583 - 2N3440
Drivers: (MJE parts should work, however you may have to order a few of each P/N of the PNP due to gain)
Using the below parts I was able to reasonably match the requirements set by Marantz. If you use MJE15031 you may wind up with 300+ HFE which wouldn't work, unfortunately it is luck of the draw as they don't bin these parts by gain.
SJ2585 - MJE15033
SJ2586 - MJE15032
Outputs:
SJ2520/2519 - used MJ21195/96 here due to availability, luckily these rarely differ by much in HFE.
MPSA93 or SPS2324 - used modern production MPSA93
MPSA43 or SPS2325 - used NTE287 due to availability
Bias diode replaced with NOS original part. Both did test okay however.
---------------------------------------
Relay board:
Rectifier Diodes:
MR752 - HER605G
1N4003 - UF4004
Relay:
OMRON MY2-DC24 - modified in by swapping pins 13 and 9 as well as 12 and 14. Check that your DC supply is within +-10% of 24V, mine was not (was closer to 30V) so a dropper resistor was added to extend coil life.
Transistors:
SS47 - MJ182G
SPS438 - BC546B

All that's left now is to modify in some better RCA connectors (if I feel like it) and this one should be good to go!

One thing I would possibly consider is modifying in larger predriver heatsinks, with the stock heatsink they run very hot.

I’m doing a similar restoration on my 250 which has the same board as you do you know what the bias diode is?

 

[ocdecio]

I’m doing a similar restoration on my 250 which has the same board as you do you know what the bias diode is?

Yes -

http://smallbear-electronics.mybigcommerce.com/germanium-diode-1n542/

The epoxy transistor I have a strong suspicion it is MPSA20.

 

[quadmatrix82]

Yes -

http://smallbear-electronics.mybigcommerce.com/germanium-diode-1n542/

The epoxy transistor I have a strong suspicion it is MPSA20.

Thanks! I have mpsa20 here already will get the diodes! Cheers

 

[goldswimmerb]

Bringing back this thread since I have another early 250 on the bench.

This time I am going to try using KSC2073/KSA940 for my drivers as I can get them in the prescribed gain range that Marantz wants on these. They seem to be closer to the 2SC783/2SA483 pairs people like to use, being that the KSC2073 has a 4Mhz fT, and from my experience works better in driver applications than the MJs.

 

[goldswimmerb]

As far as oscillating goes, I'll have to play around with it, the square wave looks like it's tending towards oscillating on me, but honestly I can't remember if it's better of worse than the MJE parts were as drivers.

This is a 1Khz Square wave into an 8ohm dummy load. At 20khz the output doesn't even look like a square wave anymore.

 

[goldswimmerb]

As far as oscillating goes, I'll have to play around with it, the square wave looks like it's tending towards oscillating on me, but honestly I can't remember if it's better of worse than the MJE parts were as drivers.

This is a 1Khz Square wave into an 8ohm dummy load. At 20khz the output doesn't even look like a square wave anymore.

Just an update, the distortion on the square wave is coming from my generator (just a DAC on my phone, not the amp). It does look like immediately after turn on in some cases I get a 3Mhz oscillation for a split second and then it dies off well before the relay closes. I'll chalk that up to the amplifier stabilizing before the relay closes rather than an issue.

 

[jd3rdcoast]

Just an update, the distortion on the square wave is coming from my generator (just a DAC on my phone, not the amp). It does look like immediately after turn on in some cases I get a 3Mhz oscillation for a split second and then it dies off well before the relay closes. I'll chalk that up to the amplifier stabilizing before the relay closes rather than an issue.

I have an odd looking square waveform from signal on Right Rear Channel of a 4270 I’m currently working on. Primary issue is FM but saw this after recapping Amp boards and P/S. Is that my Outputs struggling or indicative of issue on the board? Haven’t swapped OT’s yet. Currently focusing on FM issues then will get back to this.

 

[goldswimmerb]

Just an update, the distortion on the square wave is coming from my generator (just a DAC on my phone, not the amp). It does look like immediately after turn on in some cases I get a 3Mhz oscillation for a split second and then it dies off well before the relay closes. I'll chalk that up to the amplifier stabilizing before the relay closes rather than an issue.

I actually can get the unit to sustain that 3mhz oscillation with some effort (rapid power cycling), not sure what's causing it as the same parts were used in the left channel and that one's all stable.

 

[goldswimmerb]

@rich l that is terrible. Welding the pins to the sockets is inconceivable.

What worked for me was the 2SA1011/2SC2344 pair from Sanyo with pretty close hFE and very high fT (which some people don't seem to like because it may lead to oscillations - I didn't experience that). Very hard to find though.

Other possibilities:

SC5171/SA1930 ;Toshiba
SC2238/SC4793
2SA1837/2SC4793 Toshiba

I also tried the MJE's but they don't work, and as you mention they are too far apart in hFE and caused the fluctuations you mention that in my case would show as hiss on the speakers and in some cases take it to protection with rails voltage at the output.

I would be curious about the 2SA1837/2SC4793, I have some here that are almost perfect gain matches, but they're 70MHz ( a bit fast) and above the top gain range Marantz prescribes.

The KSC/KSA pair I am currently using is stable unless you sit around turning the power on and on rapidly by unplugging and plugging the unit back in, if you do that you'll get stuck with around 3MHz at 1v on the output, given that the oscillating is only in an edge case it's probably fine, but, it leads me to believe that faster and more gain may actually be better (contrary to popular belief).
Seems more like a ground-related oddity rather than a legitimate issue.

 

[rBuckner]

I have an odd looking square waveform from signal on Right Rear Channel of a 4270 I’m currently working on. Primary issue is FM but saw this after recapping Amp boards and P/S. Is that my Outputs struggling or indicative of issue on the board? Haven’t swapped OT’s yet. Currently focusing on FM issues then will get back to this.
View attachment 2678902

Is the lower trace's input coupling on "AC"? Kinda looks like it.

 

[jd3rdcoast]

Is the lower trace's input coupling on "AC"? Kinda looks like it.

No, I don't believe so. This was comparing Right Front to Right Rear output into an 8ohm load. The Left Front/Rear side had equal waveforms similar to the top waveform. Both were in 4ch mode.

 

[Stoddrob]

So, I'm using the safety trick I learned on my 250M, you can measure your voltage that is used for bias on the collectors of Q507 and Q508, you should have something reasonably close to +.6v and -.6v.
It allows for testing without outputs installed

goldswimmerb,

I am getting really to test the Maranatz 250 amp boards I just repaired. See images below. I was hoping to test before installing the output transistors as you suggested above. Can you help me out with some step-by-step test sequences that a sixth grader can understand?

When I received the amplifier back from a fellow AK member who worked on it, and it failed after a few hours of play (protection replay would not click-on), he told me to remove the output transistors in one channel at a time. I took out one set, and tried to power it on. it still failed. I then took the other set out of the other channel and tried again — and it powered-up and protection relay kick-in. So I know from experience that you can power the amp without the output transistors in. However, then I foolishly didn’t have a DBT. I didn’t do any damage. But now I do.

When I power this amplifier back on, I will do so with a new isolation transformer, a new Variac, and a DBT with either 100 or 150w bulb. I also have a new Siglent SDS120X-E Oscilloscope that I have never used before.
The main filter caps are fully discharged and all wires removed.

I was planning on connecting one channel amp board at a time but I’m not sure if that that’s right. I made identical changes to both amp boards. Powering both on and testing each channel for results against the other seems to make sense to me. Besides, both amp boards are only mounted to heat sinks and not to the chassis so they are easy to access when I power it up. At least I think it can be powered-up in this manner.

Everything has been updated on these boards. Everything is matched. I have researched every component and substitution. I feel pretty comfortable with the work that has been done. But now its time to power-it-up and see.

I turned both BIAS points all the way counterclockwise as directed by the Marantz Service Manual. So they will need to be set. I do not believe I can set BIAS until the output transistors are back in, however. Is that correct?

I would like to ask you EXACTLY where I should be placing my DMM probes in initial testing. I don’t want to make any mistakes. I worked too hard to get to this point. For instance, you mentioned the voltage in Q507 and Q508. I have installed new 2N5416 PNP and 2N3440 NPN in place of the S39613 and SJ2582. I assume you apply test probes to the back side of the board at the solder joints. See images. The Collector is the pin farthest from the tab. See image. So do you place one probe on Collector and one on Base or Emitter? And its different for NPN vs PNP, correct?

What about testing other voltages? Is there one or two other places to test to check? Do you ever test with the black probe to chassis ground? Or always between two places on the board?

And what about BIAS? Marantz gives different methods in different manually that seem strange. See the attached update method. Do you apply the red probe on one location and the black probe on the other? I am totally confused because I get so much conflicting information on how to check and set BIAS.

I thought I’d ask and pick your brain. But I need to be shown how. I don’t have enough experience. I should have been born in the “Show Me” State of Missouri!

Thank you for any reply in advance. The images is of the amp boards are as they are now after changes. One is labeled and one back is labeled with pin locations for Q507 & Q508.

Rob

 

[goldswimmerb]

goldswimmerb,

I am getting really to test the Maranatz 250 amp boards I just repaired. See images below. I was hoping to test before installing the output transistors as you suggested above. Can you help me out with some step-by-step test sequences that a sixth grader can understand?

When I received the amplifier back from a fellow AK member who worked on it, and it failed after a few hours of play (protection replay would not click-on), he told me to remove the output transistors in one channel at a time. I took out one set, and tried to power it on. it still failed. I then took the other set out of the other channel and tried again — and it powered-up and protection relay kick-in. So I know from experience that you can power the amp without the output transistors in. However, then I foolishly didn’t have a DBT. I didn’t do any damage. But now I do.

When I power this amplifier back on, I will do so with a new isolation transformer, a new Variac, and a DBT with either 100 or 150w bulb. I also have a new Siglent SDS120X-E Oscilloscope that I have never used before.
The main filter caps are fully discharged and all wires removed.

I was planning on connecting one channel amp board at a time but I’m not sure if that that’s right. I made identical changes to both amp boards. Powering both on and testing each channel for results against the other seems to make sense to me. Besides, both amp boards are only mounted to heat sinks and not to the chassis so they are easy to access when I power it up. At least I think it can be powered-up in this manner.

Everything has been updated on these boards. Everything is matched. I have researched every component and substitution. I feel pretty comfortable with the work that has been done. But now its time to power-it-up and see.

I turned both BIAS points all the way counterclockwise as directed by the Marantz Service Manual. So they will need to be set. I do not believe I can set BIAS until the output transistors are back in, however. Is that correct?

I would like to ask you EXACTLY where I should be placing my DMM probes in initial testing. I don’t want to make any mistakes. I worked too hard to get to this point. For instance, you mentioned the voltage in Q507 and Q508. I have installed new 2N5416 PNP and 2N3440 NPN in place of the S39613 and SJ2582. I assume you apply test probes to the back side of the board at the solder joints. See images. The Collector is the pin farthest from the tab. See image. So do you place one probe on Collector and one on Base or Emitter? And its different for NPN vs PNP, correct?

What about testing other voltages? Is there one or two other places to test to check? Do you ever test with the black probe to chassis ground? Or always between two places on the board?

And what about BIAS? Marantz gives different methods in different manually that seem strange. See the attached update method. Do you apply the red probe on one location and the black probe on the other? I am totally confused because I get so much conflicting information on how to check and set BIAS.

I thought I’d ask and pick your brain. But I need to be shown how. I don’t have enough experience. I should have been born in the “Show Me” State of Missouri!

Thank you for any reply in advance. The images is of the amp boards are as they are now after changes. One is labeled and one back is labeled with pin locations for Q507 & Q508.

Rob

The metal cans are connected to the collector on the predrivers transistors, you should see between .4 and 1v depending where the bias pot is set. You can measure from the front of the board between the transistor case and chassis. If you see that voltage on the collectors before you install outputs you should be good to go.

As always, power up testing should be done on the DBT.

 

[Stoddrob]

The metal cans are connected to the collector on the predrivers transistors, you should see between .4 and 1v depending where the bias pot is set. You can measure from the front of the board between the transistor case and chassis. If you see that voltage on the collectors before you install outputs you should be good to go.

As always, power up testing should be done on the DBT.

So you are saying you can simply touch the 2N5416 or 2N3440 transistor case “metal can” with the red probe and the chassis with the black probe and look for between .4 and 1v? Would you want the heatsinks reattached to the chassis?

Regarding setting BIAS: I imagine the output transistors must be installed to test and set BIAS. From the Service Bulletin instructions, and as far as I understand, I would set the DMM probes at the emitter resistors R531 and R532, and as I turn R526 adjustment clockwise and look for between 12 and 15mV. Is this correct?

Thanks in advance. Rob

 

[Stoddrob]

Have you ever tried 2N6211 and 2N3583 in 240/250/1200 power amps that have TO-66 drivers?

Tom

I noticed no one gave a real response to this….are these compatible TO-66 replacements for a Marantz 250?

 

[ocdecio]

I noticed no one gave a real response to this….are these compatible TO-66 replacements for a Marantz 250?

I use 2N6211 and 2N3583 regularly with good results.