Marantz 1152dc high dc offset

[Rubbertoe81]

My 1152dc is reading very high dc offset in the left channel from the speaker terminal.
It climbs to almost 1vdc within a few minutes with the trimpot at minimum position, the right channel is ok.
So far I have replaced all 2sa872 & 2sc1775 transistors on the main amp board with closely matched pairs of ksa992 & ksc1845.
I have also checked every resistor and diode on this board in circuit but couldn’t find any suspect components.
Last night while looking over it I found some crusty residue on the bottom on one main filter capacitor, when I scraped it with a screwdriver I discovered a hole and electrolyte oozing out!
Could a leaky filter cap have any direct relation to dc offset, or just another issue altogether?

Any advice would be appreciated, cheers..

 

[Roger_88_K]

If the amplifier only reads high DC offset on one channel only, I assume the leaky capacitor should not be the problem to look for in the first place.

The amp is basically the same as the 1122dc and is a dual mono design. If it would affect the DC offset, and thus the auto biasing, the right channel would have problems too.

However, with that being said, the filter capacitors would still be my first go-to making sure the rectified DC voltage is stable at all.
There are only limited replacements for these ones.
The last time I ordered them, the Kemets were the only ones that fit the PCB lead spacings, and am very happy with them as they fitted perfectly.

You should also check the J-FETs on the input stage and the thermistors on the back of the pcb. There are four of them each.

I will try and find the replacements I bought for the filters.

Cheers, Roger

 

[Rubbertoe81]

Ok thanks for that, yeah I did suspect that if the leaky filter cap was to blame it would likely affect both channels but I wasn’t certain.
I did actually start looking online for replacement filter caps and they do indeed seem quite hard to find, most stores (mouser/ digikey etc) seems to be out of stock of the kemets even.

I will take another look at the thermistors, they appeared intact visually at least.
Are you able to advise suitable replacements for the j-fets at all?
As I’m sure you’re already aware there is no way of adjusting bias current on this amp either, however the outputs and the main heatsink don’t appear to be getting warm.
The drivers and small heatsinks they’re mounted to are getting quite warm though..

 

[Roger_88_K]

(Just curious, when measuring DC offset, is the amplifier used as integrated? Do you use the AC coupled or DC coupled mode.
Instead, unplug the pre amp / main in cable)

 

[Roger_88_K]

Are you able to advise suitable replacements for the j-fets at all?

Yes, the 2SK117 or 2SK170 are both replacements I used in these similar output stages. They worked great in the 1122DC as well as the 170DC power amp.
I've seen other posts of members having good results with an unknown SMD-type and another type J-FET uPA68h.
For more information you could search for 2SK109 replacement here on AK.
There are several excellent posts of members who had issues and had good results replacing it with the types I mentioned above.

As I’m sure you’re already aware there is no way of adjusting bias current on this amp either, however the outputs and the main heatsink don’t appear to be getting warm.
The drivers and small heatsinks they’re mounted to are getting quite warm though..

Correct, it's auto biasing.
In both amps I mentioned here, the drivers do get warm -not to say hot- when running idle.

 

[Rubbertoe81]

(Just curious, when measuring DC offset, is the amplifier used as integrated? Do you use the AC coupled or DC coupled mode.
Instead, unplug the pre amp / main in cable)

The ac/dc coupling switch made a very slight difference to the offset temporarily but not much.
I believe I did check it with the pre amp disconnected previously but I will check that again too.
Last time I was testing it was getting to around 300mv on the left channel, I put the amp aside for a month or so then when I started looking into it again yesterday it had seemed to be worse and was rising to nearly 1 volt dc so I turned it off and then noticed the leaking cap.

 

[Roger_88_K]

https://audiokarma.org/forums/index.php?threads/1152dc-dual-main-filter-caps.633177/

I found a thread on suitable replacements for your filter caps by @Leestereo
The second option is still available at digikey at the moment.

 

[Roger_88_K]

The ac/dc coupling switch made a very slight difference to the offset temporarily but not much.
I believe I did check it with the pre amp disconnected previously but I will check that again too.
Last time I was testing it was getting to around 300mv on the left channel, I put the amp aside for a month or so then when I started looking into it again yesterday it had seemed to be worse and was rising to nearly 1 volt dc so I turned it off and then noticed the leaking cap.

Not sure whether the pre out/main in has to be connected to adjust it correctly as the ground reference is split as discussed in several threads on AK.
Apart from that, I am able to set it around 0mV (+-5mV) on both amps, with or without the pre/main connected on the 1122DC.
As the 170DC is a power amp and DC offset adjustment is being made without any signal connected, I believe it doesn't need the ground reference to adjust the DC offset.

Have you changed any electrolytics on the P700 board yet?

 

[Rubbertoe81]

I don’t understand exactly what you mean about the ground being split? But I will read up about it.
I haven’t replaced any capacitors in the amp at all yet, just some transistors on that main amp board so far.

Thanks for the link to the filter caps, looks like they might be my only option currently.
I guess I’m unable to do any further fault finding with the power on until I have replaced those either way..