transistor stripping ??

hi guys
i have a rotel RB-850 that i have owned for 20 years. i purchased another 850 from ebay, after it had arrived i tested it and found it to be "lacking" in sound quality. i opened both of them up and found the new amp was exactly the same apart form that it had different transistors. my original amp has (B817 D OG3) while the ebay amp has (B817 E ??? cant remember the last bit).
were the good transistors stripped ??

i have had a quick google and cannot seem to find any B817 D OG3's for sale.
does anyone know what transistors are better than my original one's ?
i have a few rotel amps I'd like to upgrade, before i take on the task of building an amp from scratch.

any info will gratefully appreciated
gaz
 
The data sheet for those transistors and their compliments is attached.

Generally when you see D817 ... there is an implied 2S in front of it, so 2SD817...

Judging from this discrepancy I would speculate one of two things... in manufacturing they moved from one 2SD817D to 2SD817E as a matter of availability -or- the amp has had prior service and "upgrading".

I doubt that changing the output transistors would have much affect on the sound quality... but if someone has been messing about trying to upgrade it the amp will warrant a thorough check out. Especially get the biases on the outputs checked and have a look at the capacitors and smaller transistors.

If the new one shows much sign of tampering, you probably have an explanation for the discrepancy in sound quality...
 

Attachments

  • 2SB817.pdf
    125.5 KB · Views: 34
thanks for the reply Douglas

as you can probably tell im new to this, but after downloading the service manual i found that two different transistors were used in the production run for the 850.
so i think your right about the move for D to E.
Douglas please tell me how you were able to find that it was a sanyo, as my google search listed loads of transistors.

thank you
gaz
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
The essential difference between the power transistors is their range of hFE or current gain. As you can see from the parts list found in the service manual, D or E grade are both acceptable so there is nothing wrong in having either type installed in original or repaired product. It would be unwise to mix grades in the same amplifier though. The remaining figures are batch/date codes.
Rotel RB-850 - Manual - Stereo Power Amplifier with bridging option - HiFi Engine

Electrolytic capacitors are notorious for age issues that affect sound markedly so at 20+ years I'd look there first, as others may suggest.
 

Attachments

  • Rotel RB850 parts.PNG
    Rotel RB850 parts.PNG
    36.3 KB · Views: 205

mbz

Member
2013-07-05 1:12 am
Agree the D/E hfe rank/classification is not going to cause your sound quality issues.
Need to look elsewhere. Rightly so, caps are an easy target however I'd start with a basic
health check, verify power supply, check bias, etc as per service manual. I'd also open it
up and have a good look for any heat stressed parts then move onto caps starting with
the power supply (if both channels affected)
 
found it to be "lacking" in sound quality

Seems like unlike the vast majority here you have been cursed with audiophile ears :)

Nothing harder than to get 2 amps of the same type but different vintages to sound identical. Apart from the transistors there are unavoidable changes in components over the production run and even worse - inconsistencies in component quality. Caps from different batches often sound different.

There are also a bunch of smaller things that can be responsible: power transformer phasing, different state of oxidation/sulphidation of contact surfaces and especially of relays, use of different hookup wire, etc.

Perhaps the only amp i have been able to compare among different production runs was the old NAD3020A. At the time it came out most of my friends owned a sample and once we made a comparison. No two would sound identical, but perhaps the biggest difference was caused by the production change of output transistors from Motorolla to RCA (iirc).

A basic electrical check is also mandatory but don't expect to reveal much.