Marantz Model Twenty Eight Troubleshooting

[garytoosweet]

I found an electrolytic that was put in the wrong way, replaced with a new one and I now have audio on both sides Left and Right, and correct voltages at the first transistors of the power amp (H407, H408), but the rest are off. Double/triple checked the orientation of the other caps and they are all good.

@Enzo it's clipping hard asymmetrically, the wave begins to clip hard at anything over 50%. Listening to audio source on speakers it sounds bad/distorted at any volume.

I pulled H409 and H110 and both test OK out of circuit. Although one hfe is low 38, spec sheet calls for min 40, the other is hfe66

Attaching another schematic with current voltages.

[Enzo]

Though it is sideways, it just looks to me like the output cannot put out but one polarity. Set the scope to DC coupling. Is the flat side of the waveform on the zero line?

You have 8v instead of 22v at the bottom of R431. What voltage is on teh top end of R431? Should be +38 or so, is it?

[g1]

What is the peak to peak voltage of that clipped waveform? Is it maybe just being driven hard enough to hit the rails?

[garytoosweet]

Resistors are good, transistor is good, so maybe that cap on the collector of H407 is leaky- C419 I think (hard to read but the 10UF/35V). Try lifting one side of the cap and see if your voltages change.

Thanks Dude. Pulled one side, no change. tested for capacitance and leakage, tests OK. It is a new Nichicon I just installed.

I'm still hunting.

[The Dude]

Resistors are good, transistor is good, so maybe that cap on the collector of H407 is leaky- C419 I think (hard to read but the 10UF/35V). Try lifting one side of the cap and see if your voltages change.

[garytoosweet]

Right side (Channel starting at H408) is passing audio albeit slightly distorted.

Left side (Channel starting with H407) is not passing audio.

I put a sine wave through it, I have signal at the base of H407 but not passing through.

I can't figure out what's pulling down the voltage after R431. Every component in that area checks ok.

Sine wave is signal at base of H407 and 408 (exact same).

Clipped wave is signal at Collector of H407, nothing on 408.

Also attaching another schematic photo with voltages.

Any pointers are appreciated.

[garytoosweet]

R432 tested ok with one end lifted.

All switches and pots have been cleaned. Line out from preamp into another amp is good. Another pre into Model 28 Tape In is distorted. So the problem lies in the power section.

This amp looks like it's had a prior recap, but the caps are generic brands, some were beginning to swell at the top, so decided best to not leave those in.

I recapped the whole Pre/Power Amp board aside from C429 and C430 (the 1000UF caps before the power transistor). I did pull those and test for capacitance and leakage, and they test ok.

While recapping, I found one trace that was lifted with spotty continuity which I repaired.

We now have correct voltages at H408. H407 voltages seem to still be an issue, they are now B: .36V / E: 0V / C: 10V

I swapped H407 with a new 2SC1815, no change.

There is 8.7V at the junction of R431 and 427, when 22.5V is expected. All resistors in that area (R421, 431, 427, 423) test ok.

[olddawg]

Start with the basics. Clean the tape monitor switches. Take a line out from your preamp section (tape rec) and see if it’s clean into another amp. Run a source signal into the tape in. If both channels are distorted check the power supplies.

[Leo_Gnardo]

One problem I've found common to all old Marantz's - electrolytic caps gone bad. Very much including the smaller ones scattered all over the boards. So bad some of them, goop leaked out and totally dissolved one - or both - of the leads. I'd scan everywhere to see whether you have any like that, or even go whole hog and replace every single one. A unit like that has to be around 50 years old and electrolytics pretty well give up the ghost over that amount of time.

[The Dude]

For starters, it looks like R432 might have gone towards open if H408 is good. Check the resistor.

[garytoosweet]

I got this receiver in a trade, was supposed to be working. I didn't get around to plugging it in for a few months.

When I did, VERY distorted, low output. Same across AM/FM/AUX. I don't have a turntable to test Phono now.

Rail voltages all check OK. Voltages for Preamp check OK, when going into the power amp (H407) something is off.

H408 should be B:0.65 / E:0.12V / C: 17V, but I've got B: .2V / E: 0V / C: 5.8V

I tried swapping H408 with a 2SC1815, made no difference. Changed back to original.

There are also voltage issues with the last transistors right before the power transistors.

I've attached the schematic, a zoomed in version of the pre/power section, and a photo of a marked up copy that I was using today. Full service manual is HERE.

Thank you in advance for any help with this one