Author Topic: Marantz model 26 receiver: Almost done (Read 4903 times)

David_AVD · « **Reply #25 on:** June 04, 2022, 11:07:33 pm »

You need to divide and conquer when troubleshooting. An example:

Is the his affected by the volume control? YES: issue is in whatever is before that. NO: issue is in the tone or power amp stages.

If not affected by the volume control, do the tone controls alter the hiss sound? YES: issue is in tone control stage. NO: see other tests.

If there are pre-main links on the back panel remove them. Is the hiss gone? YES: issue is before them. NO: issue is in power amp stages.

Is the hiss only when phono is selected? YES: issue is in phono preamp. NO: issue is in something common to all inputs.

This is a simplified example, but hopefully you get the idea.

Alex_Baker · « **Reply #26 on:** June 05, 2022, 12:28:09 am »

OMG, yes I did forget to attach the manual, I apologize

I have narrowed the hiss and hum issue down to after the volume control (mostly) and before the tone control switches and knobs, The volume largely does not affect the noise, I can disconnect the wire from J404(which is the left channel) and it does not change, whereas the treble tone controls do affect the noise.

Also of note, the hum mostly goes away if I touch the chassis with my hand, is that a ground problem?

andy3055 · « **Reply #27 on:** June 05, 2022, 03:12:18 am »

You should hit your head harder than that!

Alex_Baker · « **Reply #28 on:** June 05, 2022, 03:50:01 am »

OK, I just realized what is going on. Not with the amp, don't get your hopes up, its way more stupid than that

I have been trying to attach a PDF that is 11MB, and as you know that is way too large. Earlier I couldn't figure out why I would get booted to a "create new topic" page whenever I tried to post a response with the service manual for the model 26.

In my defense there really needs to be an error message for that thing.

Anyway, now that I look like an idiot, I attached just two PNGs of the schematic, and the information in my last post is still valid, obviously.

David_AVD · « **Reply #29 on:** June 05, 2022, 04:43:05 am »

From a quick look at the diagrams transistors H404, H402 are the potential source of the hiss.

Alex_Baker · « **Reply #30 on:** June 05, 2022, 04:48:41 am »

Is there any way to test that other than replacement? I don't have any replacements at the moment and it would take me a couple of days to get any.

andy3055 · « **Reply #31 on:** June 05, 2022, 05:00:58 am »

Does the hum and hiss present on both channels? If not you can interchange the transistors one at a time and see if the problem moves with it.
What input have you selected?

Alex_Baker · « **Reply #32 on:** June 05, 2022, 05:04:16 am »

Noise is the same in both channels and input selected does not matter.

I disconnected J404 on the left channel and it makes hardly any difference, however when I disconnected J412 on the L ch it is almost silent.

andy3055 · « **Reply #33 on:** June 05, 2022, 05:42:25 am »

I can't find the jumper you are referring to. It is hard looking at the diagram on the phone!
However check the circuitry before that jumper.

Edit: Found one end of the J412 but not the other end. It seems like some sort of a feedback loop. Check that C414-3,3uF cap. It may be noisy.

strawberry · « **Reply #34 on:** June 05, 2022, 06:34:08 am »

volume pot not making good contact
or is it just set to Tuner or Phono

Alex_Baker · « **Reply #35 on:** June 05, 2022, 01:24:18 pm »

I know a jumper can't cause noise I was just disconnecting signal wires to isolate different parts of the circuit.

The volume control has no effect on the hiss, and it works normally for actually playing audio. All electrolytics have been replaced and I have the input selector on "aux".

strawberry · « **Reply #36 on:** June 05, 2022, 02:15:11 pm »

could disconnect J408 to rule out gain stage
feedback resistors R412,R414 (gain ~36)
R402,R404,.. possible noise source (replaced with metal film resistors)
they use same transistors in phono stage so not much noise expected from them
power amp: R430,R438 (gain ~32)

Alex_Baker · « **Reply #37 on:** June 05, 2022, 04:24:16 pm »

Disconnecting J408 makes the hiss (and hum) much louder.

The hum is still affected by touching the chassis or ground connections.

Alex_Baker · « **Reply #38 on:** June 05, 2022, 06:10:35 pm »

With what I know, I believe that the majority of the hiss is coming from the second preamp stage(for example H404 and H406 on the right channel, I misspoke earlier and said left).

I think this because grounding H414 and H412 with a jumper wire reduces the hiss to nothing, however grounding J406, J408, and J410 does not change the hiss at all. Now the question is what can I do about it, I have seen that replacing carbon film resistors with metal film ones it supposed to reduce induced hiss.

I have ordered a kit of MF resistors as well as an assortment of small transistors just to have on hand, but those will not arrive until Tuesday the 7th.

james_s · « **Reply #39 on:** June 05, 2022, 06:14:30 pm »

I once fixed a loud intermittent hiss in a Montgomery Ward receiver by replacing a mosfet, but that was only affecting one channel. Semiconductors can become noisy, I know resistors can produce noise but I don't think I've ever seen a failure mode where the noise got worse.

Alex_Baker · « **Reply #40 on:** June 05, 2022, 06:25:57 pm »

Does anybody know the meaning of the letters stamped on top of some of the transistors? There are 4 different ones, GR, Y, O and R. I read somewhere that it was a code for a gain range, but I was unable to confirm this.

strawberry · « **Reply #41 on:** June 05, 2022, 08:49:19 pm »

grounding J408 will reduce hum
it is possible that it is how this device is made
higher current gain is better (GR) datasheet

Alex_Baker · « **Reply #42 on:** June 05, 2022, 09:09:20 pm »

I tried grounding J408 a different way and it does reduce the hiss by a bit as well as eliminate the hum, so that is good to know.

Alex_Baker · « **Reply #43 on:** June 07, 2022, 05:13:57 pm »

The metal film resistors that I ordered came in today, replacing R402 and R404 made no difference unfortunately.

strawberry · « **Reply #44 on:** June 07, 2022, 05:40:42 pm »

amplifier gain could be problem (design) picking up static noise
stage sensitivity ~60mV
power stage ~1V

Alex_Baker · « **Reply #45 on:** June 07, 2022, 05:45:07 pm »

So you are saying that there is nothing wrong with it? That is what I was about to ask.

The spec for noise floor was -60dB, is there any way to measure that to see if the amp is still within spec or not?

David_AVD · « **Reply #46 on:** June 07, 2022, 10:03:27 pm »

Why are you replacing resistors when transistors H402 and H404 are way more likely culprits. The number of noisy resistors I find is minuscule compared to noisy transistors.

Alex_Baker · « **Reply #47 on:** June 07, 2022, 10:44:11 pm »

Do you think all the transistors in the preamp could be noisy? Because the hiss is equal in both channels. I tried putting some transistors from a cheap amazon transistor kit in, they are S9014s which crosses to a decent Fairchild part, but I doubt these are them. Anyways my point is it made no difference with these particular transistors.

I just wanted to try putting some metal film resistors in the preamp because I had them and I was curious. Now I know that it does not improve anything.

andy3055 · « **Reply #48 on:** June 08, 2022, 12:30:00 am »

You said that there was a marked difference when you were poking around the tone or volume control. Spend your time in that area.

Take a .1 uF (may be around 50 volts or so) cap and start grounding the signal path to find which stage is generating the noise.

strawberry · « **Reply #49 on:** June 08, 2022, 06:17:59 am »

low noise preamplifier and oscilloscope. App Note 124 - 775 Nanovolt Noise Measurement
SNR = 10 log (Signal ÷ Noise)


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Author Topic: Marantz model 26 receiver: Almost done (Read 4903 times)

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