QRX 6500 internals

[j4mmy]

OK hope you're not getting bored with me posting on this receiver but I thought some internal shots might be interesting.
I'm very impressed by the power supply in particular very large and high quality transformer and primary smoother caps. Tuner section looks to be 3-gang so presumably "midrange" rather than "high end". Unfortunately can't view the decoder pcb's as they're under the tuner and I don't want to dismantle it further at this stage.

I'd be very interested to hear people's comments.

 

[j4mmy]

from the front...

 

[j4mmy]

from above

 

[j4mmy]

tuner pcb's close-up

 

[j4mmy]

varicap/tuner close-up

 

[j4mmy]

just in case anyone's wondering I've decided to keep this for my own use (so not for sale,
please ignore my earlier post today asking
for a valuation)

 

[j4mmy]

The power output stage has 4-off toshiba
2SC793 per heatsink, presumably 2 devices per channel in a quasi-complementary configuration?
There are two vertically-mounted pcb's to either size of the transformer which I presume are the output driver amps, they appear to have some heatsinking unless it's some kind of fancy regulated power supply.

 

[j4mmy]

transformer close-up

 

[j4mmy]

varicap/power trannies closeup

 

[j4mmy]

last one

 

[j4mmy]

There are two 4700 uf capacitors on each side of the transformer and one taller 4700 cap, perhaps one is an amp output cap though I'm only guessing and the other two psu smoothing.

But I think that transformer is one of the secrets to this thing, I'm not surprised the sound quality is good, the regulation is probably excellent especially when used at normal listening levels eg output power perhaps a watt or two rms.

Does anyone know how good the tuner unit in this type of receiver is? It certainly sounds good but I haven't done any measurements.

I'd be very interested if people have any suggested tweaks or renovation hints since the performance must have fallen off somewhat with age eg supply voltages drifting etc. I haven't spotted any leaking electrolytics yet though!

All in all if this was 1972 "good midrange" kit then whatever was the top-end kit like??!!

 

[B3Nut]

Actually, that 3-gang tuning cap's the AM cap. Most receivers used 2 AM gangs, even at the high end. The FM tuning cap is right behind the AM cap, note the separate pulley. I'd bet you've got a 4-gang FM cap under that lid.

TP

 

[j4mmy]

Well I'll be pleased if you're right, it always used to be the "received wisdom" that manufacturers cut costs on the tuner in a receiver and that you'd get better performance from a separate tuner than the one that comes in a receiver, but it looks like this has quite a high quality tuner in as well. I found the manual and it's got a service guide in the back with all the schematics,
and you're right, the FM receiver section cap is a 4-gang one.

Here's a few of the specs:

Audio Section
Power output
Continuous 60W x 4 @ 4 ohms load
(1Khz, each channel driven)
37W x 4 @ 8 ohms load

Power bandwidth (IHF)
20 to 30 khz

THD < 0.5% at rated output

IMD < 0.5% at rated output

Load impedance 4-16 ohms

damping factor >30 at 8 ohms load

FM section
sensitivity 1.8 uV

THD < 0.8% (stereo)

SNR > 65db

selectivity > 70db

IF rejection > 100db

Which I imagine is quite good for 1972.

I dare say I'm not getting this spec after 30 years though; there are some detailed alignment procedures listed for the audio and radio sections, would it be worth getting these done? How much do these receivers drift off the factory alignment with age?