Sherwood S-7650CP: a receiver or a chameleon?

Dr.Turner

Active Member
I recently picked up a McLaren RK-745 in good shape.

McLaren_RK-745.jpg

Despite his unknown name, I was intrigued by the classy late 70s look, the weight (12 Kg) and the fact that I own another McLaren piece of gear which *sounds* great and is really well built, the TK-600 tuner (it's a rebranded Sherwood S-32CP).

McLaren_TK-600.jpg


After a few hours of googling I gathered some informations that may be of general interest for the AK community and particularly for the growing cult-like Sherwood followers.

The receiver is indeed a “clone” of the Sherwood S-7650CP, apparently rebranded for the italian market as you may see from this picture (courtesy from deadphish).

Sherwood_S7650-CP.jpg

But there’s more...the same Sherwood S-7650CP receiver was rebranded for at least four additional markets! Hence the title of my post. There are:

Inkel RK-745 (for South East market)

Inkel_RK-745.jpg


Blat RK-745 (for South American market)

Blat_RK-745.jpg


Eagle R7500 (for UK market)

Eagle_R7500.jpg


BASF D-6160 (for German market)

BASF_D-6160.jpg


I’m performing the basic cleanig procedure as outlined by dr*audio.
I will then hook up a DBT and check for problems. Report in a few hours with more pictures and nudies.

Later,
Dr.Turner
 
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More likely that Sherwood speced product from Inkel that was the same as ones they built for others. Could be differences inside though. Doesn't necessarily mean anything bad. Companies did the same with Japan's Foster and Standard Radio.
 
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Since there was no Sherwood S-7650CP Service Manual nor in AK neither in HFE I've bought one for reference.
As far as I can see the only difference is de-emphasis board that lacks 75us North American standard. Transistors, ICs, caps, resistors are all the same.
I've also made a visual comparison with some nudies pictures found online and everything seems just the same!
Only the tuning scale lacks the inferior fine graduation and the transformer is obviously not Sherwood branded. So far so good.

But something in the power supply board doesn't win me over...

More later,
Dr.Turner
 
Cleaned and DB tested. Everything is fine except:
  • with mute on picks only a couple FM stations
  • no stereo led (which is working...) and no stereo reception
It's well built with ceramic socks on most critical resistors, generous transformer and heatsink, and three ICs:

McLaren_RK-745_inside.jpg

After a couple of hours of testing, bias alignment and DC offset, the power supply transistor TIP31A (TO-220) failed leaving a visible burnt area on the board.

McLaren_RK-745_power_supply_BD.jpg

I've tested all the resistors to find other failing/failed components and six of them were out of specs or tested bad on Atlas LCR40. Replaced with new resistors, replaced all 4 electrolytic caps (increased value for the big caps from 6800uF to Kemet ELH 10.000 uF), replaced the IN757A diode and of course the TIP31A transistor adding a big aluminium heatsink. The only original components left are: 4 rectifiers, 6 ceramics, 4 resitors and the two smaller transistors (2SC734BL) as you can see from the schematics:

McLaren_RK-745_power_supply_schematics.jpg

Before replacing the components voltages were a bit out of range and I was hoping that after the replacement the situation would get a better. But it wasn't the case...
These are all the voltages: measured vs. (expected)

power_supply_BD+.jpg

Any idea of what is going on?
In the meanwhile I will check for misplaced resistors and look for a good 2SC734BL substitute. I already have all the other parts, so I could eventually finish the complete replacement of this (simple) board.

Later,
Dr.Turner
 
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My 7650 is in line to get worked over. Purchased in Ann Arbor, MI, when I was in school, 1978. Was a great combo with my Polk 7's! Eagerly waiting on your progress.
 
Not much of a progress, yet...
Waiting for suggestions why the voltages drift so much. I guess supplying 18% lower voltages to is bad for radio/preamp board...

In the meanwhile here's a picture of the oversized heatsink (cut, filed end tweaked from a broken 400W switching PSU).
The temperature on TIP31A is now 45°C, before was 98°C with the stock tiny heatsink. This is also strange, why so much heat on that transistor?

McLaren_RK-745_power_supply_heatsink.jpg

Of course my knowledge of EE is quite basic, but my will to understand how things work is huge, instead!


Later,
Dr.Turner
 
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A little update, but still puzzling alot!
One of the two 2SC734 transistors failed. I've replaced both with KSC1845 based on evidences suggested in 2-3 posts on AK.

This board suffered major damage for some reason: two electrolytic capacitors out of specs, five resistor out of spec, one power transistor failed (TIP31B), one small transistor failed (2SC734). :idea:

I'm performing another voltage test and report soon.

Later,
Dr.Turner
 
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With new KSC1845 the board finally works!

McLaren_RK-745_KSC1845++.jpg

PS board output voltages now drift 2-3%, can I consider it satisfactory?
Next I'm going on to understand three obvious problems:
  • with "FM mute on" the receiver picks only a couple of stations
  • no stereo led (which is working...) and no stereo reception
  • signal and tuning meters barely moving
Later,
Dr.Turner
 
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I've found another clone of the Sherwood, for the german market. This is the BASF D-6160 with ugly knobs.
The number of clones is update to 5, and still counting...:)

BASF_D-6160.jpg
 
[QUOTE = "Dr.Turner, post: 10260116, member: 114142"] Como no había ningún Manual de servicio del Sherwood S-7650CP ni en AK ni en HFE, compré uno como referencia.
Por lo que puedo ver, la única diferencia es la placa de énfasis que carece del estándar norteamericano 75us. Los transistores, circuitos integrados, tapas, resistencias son todos iguales.
También hice una comparación visual con algunas fotos de desnudos que se encuentran en línea y ¡todo parece igual!
Solo la escala de sintonización carece de la graduación fina inferior y el transformador obviamente no es de la marca Sherwood. Hasta aquí todo bien.

Pero algo en la placa de la fuente de alimentación no me convence ...

Más tarde,
Dr. Turner [/ QUOTE]

Zenith mc 7030 /7040 /7050?
 
Both have in common Inkel Corporation of Korea. Who then owned Sherwood, and built a good amount of OEM electronics for other brands in the era. This period was the last of the really good Inkel electronics before cost cutting began more prevalent in the industry.
 
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