Marantz Model 27 troubleshooting

[mikepick]

Hi folks! I just came into a Marantz Model 27 for a very reasonable price, but it did come with some problems.

View media item 14555
The reason it was being sold was that the left channel begins to distort and eventually drops out after a certain amount of time. I've verified this twice now by running it for a while. Eventually the left channel begins to softly distort, starting with a kind of burping and farting on the louder sounds and then the channel volume begins to fade. The rear top of the unit then becomes quite warm. The problem exists across all the inputs in my testing.

The other problems are somewhat minor.

The signal meter does not seem to work, it is lit from the left side but does not move at all when tuning stations.

The tuner is a little misaligned as well, the local NPR station at 93.9 comes in at 92.75 or so on the dial. Looking through the service manual, it seems like adjusting the tuner is pretty tricky and so I'm willing to live with that particular issue.

On an initial interior inspection, the unit appears quite clean. I did notice some suspect goop on a transistor heat sink on the power board, so I'm wondering whether a bad cap might be the issue.

View media item 14553
View media item 14554
Any advice would be appreciated. I still need to do some tests to see if the signal is good from the preamp when the channel drops out.

 

[Jailtime]

I've never seen a Model 27, rare bird there. You might be able to slide the dial pointer on the string to get it to indicate the correct frequency. Most of them just have the pointer stuck to the string with a dab of glue.

I'd suspect transistors in the driver stage of the amplifier are the cause of your problems. Would be nice to get it narrowed down to amp or preamp though.

 

[mikepick]

I've never seen a Model 27, rare bird there. You might be able to slide the dial pointer on the string to get it to indicate the correct frequency. Most of them just have the pointer stuck to the string with a dab of glue.

I'd suspect transistors in the driver stage of the amplifier are the cause of your problems. Would be nice to get it narrowed down to amp or preamp though.

Yes, I've got the service manual from hifiengine and am going to test that as a next step.

I've cleaned up that suspect goo, and on closer look it seemed more like hot glue. I have had a closer look and it looks like there is more goop in between the big heatsink and the power board, and it looks like wax, which makes me wonder whether someone set a candle on it at some point.

According to the manual, there is a dynamic temperature sensing unit which regulates voltage to the big transistors. I'm wondering whether a failure in here might be a culprit.

WRT the dial pointer, that's a good tip, thanks. I think I might check some other stations locations and see if the offset is even across the spectrum, and in that case maybe move the pointer, although I'm scared to touch the thing. to be honest.

 

[mikepick]

An update. The trouble with testing this unit is that I need to run it long enough to induce the distortion. Ran it for a while this afternoon with leads coming off J404 and J405 of the pre-amp board to another amplifier. The distortion appeared there as well, which seems to indicate the problem is further back than the power amp.

 

[wlhd1610]

You could try a can of freeze spray and do one transistor at a time once it starts acting up.
I have a couple of 27's here awaiting restoration.
Bob

 

[mikepick]

Just to follow up: with the help of my father (it was his line of work in the day) we tracked the distortion down to originating somewhere in the first stage of the preamp board. There was a clear signal going in to the preamp board when the distortion was present, and you could hear it in the output to the tone controls.

H401 / H402 are the notorious 2SC458s, and H403 / H404 are 2SA494s, and measuring the voltages against the schematic resulted in very high readings when the channel was distorted.



I replaced those with KSC1845s and KSA992s, and so far, so good. The BCE voltages measure a touch lower than schematic values but consistently.

Fired it up with the dim bulb and no shorts, and so I did some listening last night and this morning. Seems to be a bit less boomy and also not as loud as before, but warm and clean and no distortion so far, although I need to listen to it for a few hours to be sure. No pop or snap on power-up or down either, which was an issue before.

Also, I removed the signal meter and first tested the attached capacitor. It tested much lower than spec, but then testing the meter itself with a resistor and Ohmmeter seemed to yield nothing. I found a replacement on eBay and snapped it up. When the replacement arrived, I decided to test the old one again for a laugh, and it worked! I've put it back in the unit and now it is working as it should, going to 85-90% on a good strong NPR signal. (Yes, I use the kids snap circuits for testing. )



I''ve set the DC balance to 0 and am setting the bias now, and I'll try to run it, hoping that I may have got the issue.

I suppose I ought to consider whether to recap it at some point, but I'm not going to right away, except for maybe the filter caps, which are 3 times the size of modern equivalents and should help with airflow and heat.

 

[mikepick]

So far so good. One of my issues with this amp is that it seems overly bass-y and resonant, although I thin part of that is that the better speakers I have are Paradigm Titans. I've mitigated it somewhat by stuffing paper towels in the speaker rear ports.

 

[mikepick]

Update: I've been running this Model 27 for a month of so now, and the repairs have held, so I get even levels in both channels and it's generally well behaved.

I am having some nagging doubts over the sound, though. It's a bit dull overall and too resonant in the low end, so much so that I tend to run it with the low filter on all the time. There is clarity and detail, but it's not bright or dynamic, if that makes sense.

I realize that this unit is relatively rare, but I'm wondering if anyone has impressions of how it ought to sound? It certainly compares poorly to my 1030, and the Kenwood KA-3500 I just rebuilt is also much more crisp.

Basically I'm mulling over doing a recap / transistor replacement on the pre / power / phono boards, and the filter caps etc, but I'm wondering how much that will improve the sound in the end.

 

[mikepick]

Update: Some success! My buddy lent me a 2325 and a pair of KEF LS50s to try out, and this spurred me on to do some work on the 27.

I recapped the power supply board and the phono board, and the improvement is very noticeable. It has much more clarity overall with a nice warm tone, and a lot less of the low end resonance that made it muddy sounding before.

Power board parts from Mouser:

C602: 10uF 35V Elect: Nichicon UFG1H100MDM
C603, C604, C605, C606, C611: 470uF 35V Elect: Nichicon TVX1H471MCD
C607: 470uF 16V Elect: Nichicon UFG1C471MPM1TD
C610: 47uF 35V Elect: Nichicon UFG1V470MPM

(C611 is on the underside of the board)

Phono board parts from Mouser:

C301, C302: 2.2uF 35V Elect: Nichicon UFG1H2R2MDM
C303: 22uF 35V Elect: Nichicon UFG1V220MEM
C308, C309: 10uF 16V Elect: Nichicon UFG1H100MDM
C310, C311: 3.3uF 25V Elect: WIMA MKS2B043301H00JSSD Film
C312: 47uF 35V Elect: Nichicon UFG1V470MPM

Next up is replacing the filter caps. Unfortunately, I order a pair of 30mm diameter Nichicon UFG caps instead of 35mm, the bracket size.

I have some big heat shrink tubing that I think I'll use to shim them into the brackets though, instead of waiting on a new order.