Marantz 2230 tuner amp
 

Where to start. I do some repairs for this chap one day he came with a car full including the Marantz 2230 he didn't want to spend much on the repair, as one output stage had blown, it ended up being mine, quite a nice unit.

I sent to the USA for the 2SC897 output transistors he was selling five so I thought good plan in case I needed another pair not cheap but they were marked Toshiba, meanwhile the driver board was repaired checked and double checked, I didn't want to blow the expensive outputs. Some time later the transistors arrived they looked the part, so they were fitted with some trepidation I switched on, and was greeted with a loud pop , >(( we now have four duff transistors, lights out door locked and :beer:

Next day repaired the driver board, I didn't want to risk another pair of the expensive output transistors :idea: what have I got that would fit found a couple of BU208A from my T/V repairing days, these were fitte , the big switch on no pop or bang or smoke, the idle current set perfect and it sounded good too :) .

Fast forward to this mourning tried the amp again perfect, so I will fit the proper transistors it will be fine, eer no it went with a big pop again, keep carm and carry on I thought repaired the driver board again fitted the two BU208A now playing away lovely .

The question were the transistors inferior they did test all ok Any thoughts please. Mick.

Re: Marantz 2230 tuner amp
 

Fake transistors??

Re: Marantz 2230 tuner amp
 

It would be good to work out why they're failing. A quiescent amp shouldn't be dissipating much power in the output transistors, so it may be the switchon surge that's taking them out. You could try it with a lamp limiter in place but I imagine you're running out of transistors to try now.

If there are no other faults then I agree the transistors are suspect. I usually try to sub known types I have in stock rather than trying to chase up obscure originals which may be fakes. Transistors are much less variable than is often assumed and subbing usually works well.

Re: Marantz 2230 tuner amp
 

Thanks for the replies , A quick update I have selected two BU208A with a matching gain , and the amp is working perfect ,I am a bit loathed to fit any other transistors at this point . Mick.

Re: Marantz 2230 tuner amp
 

I think you're right. If all the circuit voltages are correct, you can set the quiescent current, and it's sounding as it should, then it's best to leave it alone and chalk it up as a good repair.

Re: Marantz 2230 tuner amp
 

And there is more! could not leave it alone, tried the only good 2SC897 along with one BU208 very bad distortion , so tried it in the good channel 8-o perfect . So I have come to the conclusion the driver transistors need to be as per circuit . no equivalent will do .Left if alone now phew! Mick.

Re: Marantz 2230 tuner amp
 

You could have received fake transistors. Possibly common with semiconductors sold on the Bay from China and other places. I've been there with 2N3055. Since you actually paid more than next to nothing, you should at least expect to receive the genuine article. My 2N3055's were CHEAP so I had it comming.
If you didn't maybe the problem is caused by instability or selfoscillation in the output amplifier. Substituting with BU208A will drastically lover the frequency response of the output transistors. The BU208A has a transition frequency (fT) of just 4-5MHz which is LOW as output transistors go. The 2SC897 manages a fT of 15MHz which may just be enough to set off the deadly oscillation. The current gain (hFE) of a BU208A is also very low compaired, below 5 which is about 5 times lower than that of the 2SC897 adding to the instability problem.
You should do a re-cap (replace all electrolytic capacitors) on both output amplifier boards before mounting any new output transistors imho, and while you're at it do the same to the low voltage regulator board P800.
Then there's the problem of a Marantz 2230 not using 2SC897 as output transistors at all. My servicemanual for a 2230 states 2SD217 as output transistors. How can that be ?

Re: Marantz 2230 tuner amp
 

Thank's for that , all the electrolytic capacitors were replaced all resistors checked and replaced if necessary adjustment pot's cleaned , the output's were all 2SC897's .you maybe able to help me regarding the device clamped to the heat sink , I get a very high reading both ways , but the good channel is the same .
I will csope the outputs today , Mick.

May I ask what is your preferred driver transistors

Re: Marantz 2230 tuner amp
 

Just checked on the schematic the devices in question are in fact two diodes in series ,I am happy to say the both read ok on the diode range , also the schematic shows the output transistors to be 2SC897's Mick.

Re: Marantz 2230 tuner amp
 

2SC897 and 2SD217 are pretty similar, except the 217 has higher Ft. BU208 has similar Ft to the 897 but otherwise is a very different beast. Its much lower Ptot may be an issue if the amp's driven hard!

Re: Marantz 2230 tuner amp
 

Hi fitting the BU208 was just to check that the unit didn't blow the 2SC897 again , I was surprised how good it sounded ,half way nearly blows my speakers . I aim to fit the correct ones ,if I can find some good ones at a reasonable price. Mick.

Re: Marantz 2230 tuner amp
 

Could it be instability causing them to fail? You need to work out a safe way of powering it up if you go for more transistors.

Re: Marantz 2230 tuner amp
 

Found an audio site in the USA some recommend using 2N3055 some don't so off I went to our local electrical retailer , now the amp is running with two 2N3055's mid point set ok quiescent set spot on , sounds good .I think that's it ..phew! thanks for all the suggestions .Mick.

Re: Marantz 2230 tuner amp
 

No trace of any rf in the output Mick