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Help Troubleshoot Main Amp Board on Marantz 2216

Lewis Biggs

Lewis Biggs

New Member
Last night, I successfully (to a certain point) recapped my 2216. It sounded good, but I could not leave well enough alone. I decided to check/adjust the voltage across resistors R735 and R735 per the manual. I got a little careless with the probe (easy to do) and, while the unit was powered on, grounded the leg of R735 that connects to C721.

The left speaker immediately popped and began to hiss. Although music will play out of both channels at normal volume, the left channel sounds mildly degraded and hisses. Changing the volume does not affect the hiss at all.

When I switched R and L from the preamp board, the hiss stayed on the left channel and is unaffected. Thus, I believe the problem to be limited to the L side of the main amp board. I do not see any visibly cooked/bulged components, and I was not able to find any obviously bad components using a multimeter. In comparing ohm readings between transistors on the right and those on the left, some of the the readings are substantially different, leading me to think that I cooked one or more transistors.

Any ideas what I have done? Any tips for troubleshooting without a scope or is any further hackery by me just a waste of time? I would be very grateful for any advice.
 
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First up, you did right by checking the bias, otherwise what's the point of the recap.
Secondly, good observations on probable impact and the symptoms.

The candidates are H711, H713 and maybe H707 and H709. These TR's will probably
test OK, they kinda work, you just don't need the hiss etc...

My service manual has,
H707 2SC1384... maybe KSC2690
H709 2SA684... maybe KSA1220
H711 2SD313... maybe MJE15032
H713 2SD507

The next step is to find modern day equivalents for these TR's.
I've plagerized a fellow AK's work with the maybe replacements above.
I'm not strong in this area so I will leave it to others to suggest
replacements.

H711, 713 took the brunt of the impact but they are built to take it.
H707 and H709 are somewhat shielded from the short but more fragile.

Suggest you buy all 4 replacements and insert. You will need to recheck the bias.
 
Update on replacement transistors. The MJE's are way over spec for this application. Maybe others can chime in.

H707 2SC1384... KSC2690A
H709 2SA684... KSA1220A
H711 2SD313... maybe MJE15028, way over spec
H713 2SD507(2SB507 typo) MJE15029
 
Thanks for the help. When I turned on the amp again, the h711 power transistor started releasing magical smoke. Not good.

I pulled and tested all transistors on that board. H707, h711, and h713 are all nonworking. H703 gave a HFE reading under specs. Maybe h707 went down and took the others. I am replacing those 4 on both channels. $14 from digikey plus ship.

I will let you know how it goes... Learning experience. Now I really want a scope.
 
I blew out an output transistor that way. :yikes:
You can easily figure it out by looking at base voltages on your transistors. since your schematic shows power flow from left to right, you can go backwards through it.
write down your voltages.
i'm sure you now know to power the unit down before each test;)

If you do change any of the transistors before your drivers, buy about twenty each and match them.
The wrapped mated duo set (diff pair) MUST be matched, but i'm sure you know that.
 
Hmmm... you are giving me WAY too much credit by assuming I ordered a matched pair of power transistors. I did consider balancing HFE, but (i) I was not able to find any option on either Mouser or Digikey to select HFE classification classes (much less even closer matching), and (ii) I noticed that On Semiconductor listed the MJE15032 and MJE15033 as "complementary" and "designed for use as high frequency drivers in audio amplifiers", concluding that they are meant to be paired just like this.

I did order 3 of each. Maybe several pairs will be fairly close. How close do they need to be on a little 16w amp like this? Isn't the trim adjustment there to bring the darlington pairs reasonably close in gain? Is there a chance of cooking the board (yet again) if I cannot match them perfectly?

Sorry for so many beginner questions. I am extremely grateful for input.
 
I'd bet you'll never get a match on those drivers. Great transistors though! It won't cook your amp. Not finding all shorted out transistors and diodes will do that!

Honestly, I don't think Marantz matched the diff pair transistors for production but it's always a very good idea.
 
That is a relief because the drivers are a bit pricey to buy 10 or 20 of them (at least relative to the value of this amp). I was not planning to replace H709 and 710 since they tested good (and extremely close to one another in gain). I am planning to replace H707 and H708, and I ordered 10 of those. I was planning to match them relating to one another. But, I think you are telling me instead to try to match (i) H707's gain to H709 and (ii) H708's gain to H710. Correct? Thank you
 
Ok, based on wisdom and experience of others in this community, I have placed another digikey order for 20ea. of KSC2390A and the KSA1220A compliment for H707 through H710. That, with the other order, will permit me to replace all 4 drivers and each driver's respective darlington pair (H707 through H710), and (with any luck) should allow me to match up the darlington pairs fairly closely. I am also going to put together a simple dim bulb tester in an effort to avoid further wreckage.
 
I have an update and a request.

I have now replaced all of the cooked and/or wonky transistors on the applicable channel of the main amp board with reasonably equivalent transistors. The amp passes the DBT and plays, BUT.... there is ugly distortion on the channel and, when I try to adjust the bias on that channel, I am not getting any voltage reading over R735 (the manual calls for adjusting a pot to get 4.5mv).

I have spent numerous nights trying to track down the problem with a multimeter, but I not making headway. Yes, I am longing for a decent bench setup, but it is currently out of my reach reach.

Can someone please help me understand this main amp circuit better? Here is what I think I know and don't know ->

I think I understand the basic drivers -- that is, H707/H711 and H709/H713 are darlington pairs in push/pull configuration, and H705 and R727 (and associated resistors) adjust the biasing between the push and pull sides. That looks fairly typical to me.

However, I am having trouble understanding how H701 and H703 function in this circuit. They appear to function together as a PNP Sziklai pair, with the collector of H703 serving as the emitter of the PNP pair, and serving as the driver for the main darlington pairs. But, in a normal a PNP Sziklai pair, I would expect the emitter of H701 to be connected to the collector of H703. Instead, the emitter of H701 is connected to the emitter of H711 (which itself seems to be driven by H701, which seems to be driven by H701, and around and around we go). I suspect that this configuration is for feedback and somehow makes a differential amplifier, but I cannot figure out how (or recreate it in modeling software).

Can someone please explain this configuration in reasonably basic terms? Many thanks!
 
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