2215B Troubleshooting Help, Please

fromnowon

fromnowon

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Hello and maybe someone will have a suggestion to help me wrap-up a project.

It's a 2215B that I found last year, cheap, "for parts" with a channel out. $60 and it's in nice shape! I've done a lot of work on P700, P400, PE01 and P800. It's the first Marantz I've attempted to fix/restore.

First I found that output transistor H712 tested bad and so did R736 and R738 - replaced those and that actually fixed it. I've been listening to it with headphones for months now as I worked on boards here and there as I could find time. The adjustments worked fine - no issues. I followed the patfont procedure.

It sounds GREAT with headphones and speakers off. Fantastic! I could hear the effect of the new caps; pretty cool. It's also very quiet with no hiss. Nice solid bass and clean, clear highs.

However . . . with speakers, it only sounds great up to about 10:00. Past that point, which is pretty loud but not not beyond reason, there is some distortion, mainly in one channel but I think I hear it in both at times. Once the distortion starts if I switch to headphones I can hear it there too so I know it's not the speakers themselves. It's kind of a "fuzz" or buzzing sound that follows lower frequencies and even remains at say 9:00 volumes with headphones once it starts. High frequencies still sound crystal clear and awesome.

So it's a problem that shows up at relatively high volume but not with headphones alone. When I start to push the amp.

I replaced everything on the power supply board except ceramic caps, the two big resistors and H801 which is a 2SC1407 - just left that alone. Tonight I pulled and tested R801 (330 ohms 3W) and R802 (220 ohms 2W) and they tested fine, but I'm planning to order replacements.

I could put them back in and do some more testing. I wonder how you guys would track this problem down. I'd really appreciate any suggestions. Thanks!

Attaching a few pictures:

These tested fine but maybe they should be replaced; I'll have to order:
IMG_0335.jpg

Just about perfect shape!
IMG_0332.jpg

PS board with those two resistors pulled at the moment:
IMG_0334.jpg

Here's my P700. I went cheap and used Amazon metal film resistors for the .47 ohm ones (4). Maybe I should get some better wire wounds for those spots. I also replaced the carbon composites with metal films but left the others.
IMG_0333.jpg
 
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If you haven't already done so, I would start by checking the DC voltages of all the transistors on the P700 PCB.
Does the distortion happen on all inputs (AM, FM, phono, aux, tape monitor)?
Does it happen on both Main and Remote speakers?

upload_2022-6-8_15-9-50.png
 
You should check the continuity of the current limiting lamps M701 and M702 in the base circuit of H712 and H714. I have seen them open before, after an output section failure.
 
AE Bob said:
You should check the continuity of the current limiting lamps M701 and M702 in the base circuit of H712 and H714.
Click to expand...
Thanks @AE Bob
I get continuity "beeps" for both when I manage to find the correct pads so I figured they were ok.
Actually I have these replacements as well so I could change them out if needed.
IMG_3897.JPG
 
I'll recheck those little lamps too.

About to go on a major voltage checking excursion . . . see what that might turn up. I really appreciate the ideas on this little 2215B project. Probably seems like kindergarten to you guys who fix the big, complicated amps.
Listening with headphones at the moment after putting those resistors back in and it sounds so nice and powerful and clean. That distortion under load is unacceptable though.
Maybe I'll try testing power output WPC to see how close it is to 15 with my poor old scope too if I can figure that out.
Obviously I'm still learning so I'm very appreciative of advice.
 
Look for open internal connections in those TO-220 output transistors. I have seen that style transistor fail open frequently, many times intermittently.
 
This kind of issue usually means there is an issue in the output stage of the power amp. I'd start by testing the outputs as suggested by AE Bob above.

If all tests good, test it on the scope if you have one with a load and post a picture of the waveform when it starts to show the issue.
I had a really weird distortion issue with one of these boards once in a superscope and had to tweak a ceramic cap value.
 
Thanks again for the ideas; really helps.
I checked voltages on P700 and P800 and they look really good. I uploaded them below.

In terms of the outputs - they are all new and the voltages checked good for ECB on all 4 - does that exonerate them?
H711: 2SD313D => MJE15032
H712: 2SD313D => MJE15032 <== ORIGINAL 2SD313 WAS BAD RIGHT CHANNEL
H713: 2SB507D => MJE15033
H714: 2SB507D => MJE15033

After more listening, the fuzz seems to be intermittent in the right channel which had the blown output.

I haven't checked the other 18 1/4W channel resistors in each channel on P700 (although I did replace all of the transistors and electrolytic caps on P700 and the 2W .47 ohm emitter resistors <= but I went cheap with those, which might have been a mistake). I guess I should pull and check resistors.

For a while last night I thought it could be related to the balance slider - I wonder if it would be hard to temporarily jump the balance control to get it out of the picture. I cleaned the other switches so much I can't imagine cleaning them anymore but it was tough to get spray in the slider.

I do have an old scope that I can try to use to look at some waves with a dummy load. That could be interesting.

P800 vlt.JPG

P700 vlt.JPG
 
With a dummy load connected, I fed this signal to AUX (right channel)
IMG_3916.JPG

Also connected my scope to the speaker terminals,
at relatively low volume it looks good:
IMG_3917.JPG

Turned the volume up to around 10:00 which is where I think I start noticing the distortion:
IMG_3915.JPG
^^ maybe that's the "fuzz" that I'm hearing?
 
fromnowon said:
With a dummy load connected, I fed this signal to AUX (right channel)
Click to expand...
  • I think your signal generator is feeding in too large a signal into the AUX input and overdriving the amplifier output when you crank up the volume.
  • Your signal generator looks like it's putting out either 2V p (or 2V p-p), but the specification (from the Service Manual) for the AUX input is an input sensitivity of 150 mV.
  • Measuring the typical output of my tape deck (that I can input into tape monitor or aux inputs), the max voltage I can output (with output maxed out) is about 1V.
  • My guess is that it looks good at lower volume levels is because the volume control comes before the main amp so you are reducing the main amp input. But when you crank it up, you are putting a large input to the main amp.
  • Try to reduce the AUX input signal to less than 1V (e.g. 750mV) then look at the output on the scope and see if the distortion goes away.

upload_2022-6-12_22-40-8.png
 
Thanks @dankik that's interesting. I'll do some more experimenting. Possibly I'm sending too strong of a signal in, both when I listen and when I look at a sine wave.

I use a Topping D30 DAC which has a max output of 2.1 Vrms @1 kHz. In other words, computer USB => DAC => AUX in, and I've been leaving the computer volume at 100%.

I do this with other amps and haven't noticed distortion at the volumes I listen . . . maybe I need to turn down the input level though.

Thanks
 
Here's another couple of thoughts:
  • Do you hear the same on AM/FM or phono input?
  • The "fuzz" is only on the top-half of the sine wave. Since the power amp is class AB the noise is happening only on the positive side (looks like it could be some oscillation).
  • Does this occur on both channels, or only one (L/R)?
  • Did you perform this audio adjustment (below)?
upload_2022-6-13_7-50-9.png

Here's a post from another forum regarding overload on vintage receivers (maybe this applies here also). Try reducing your DAC output level and see if it helps.
upload_2022-6-13_8-6-18.png
 
Thanks @dankik and sorry for delay. I really appreciate your suggestions and will try them! Big storm here on Monday and we just got power back at 4AM today. It was terrible with record heat at the same time.

I hadn't seen that other thread - very similar to my issue, no solution found! I'll try a few other things and post back. This seems like a good chance to learn how to use my scope and signal generator to find a problem.

BTW, I saved and labeled all of the old transistors so I can put them back. Except for the outputs they all tested ok. It would be interesting to try with a 750 mV signal and if I can still see the problem on my scope, then I could try tracing it and put back original parts in select places to make it go away.

dankik said:
  • Did you perform this audio adjustment (below)?
Click to expand...

I followed this procedure and set to 20mV across both resistors:
adj.JPG
 
fromnowon said:
I followed this procedure and set to 20mV across both resistors:
Click to expand...
Why 20mV, the Audio Adjustment procedure (above) says to set it at 10mV DC?
Does the distortion occur on both channels?
Be careful with the scope probe ground when you are probing (connect to chassis ground only).
Do you have a "Dim Bulb Tester" so in case something does short, you don't take-out the whole amp?
 
dankik said:
When you hear the distortion on the speakers, do you have the receiver on the DBT, or not?
Click to expand...
Good question! It's not been on DBT during listening. Regular power.

I think over-driving the signal is a real possibility so I'll explore that and try listening to FM.

Also - I replaced every transistor on P700. I can list them out and compare hFE specs on the new ones to how the old ones I pulled out actually measure. I believe they are all the normal replacements. I didn't really have to replace them just decided to for long-term reliability.

Maybe the combination of these replacement transistors is causing some issues . . . ? I'd like to track it down so thanks for all the suggestions!

KSA992FBU H701 2SA564
KSA992FBU H702 2SA564
KSC2690AY H703 2SC1384
KSC2690AY H704 2SC1384
KSC1845FTA H705 2SC828
KSC1845FTA H706 2SC828
KSC2690AY H707 2SC1384
KSC2690AY H708 2SC1384
KSA1220AY H709 2SA684
KSA1220AY H710 2SA684
MJE15032 H711 2SD313D
MJE15032 H712 2SD313D
MJE15033 H713 2SB507
MJE15033 H714 2SB507
 
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