SANSUI model 250 valved tuner-amp (Hum and stereo decoder issues)

[yestertech]

Does anyone have any experience of working on one of these ?
I have one on the bench at present and first impressions are its quite a beast
PP ECL82 outputs and an integral valve stereo decoder.
Plus a very unusual octal based tuning indicator which includes mono/stereo indication.
They've squeezed this lot into a solid state sized case !

It powers up, but all I get is a faint 100Hz hum, no signals, so time to start digging...


Andy

[Edward Huggins]

This Receiver has had many enthusiastic reviews. When, and if, you get this fixed, make sure you use sensitive speakers of 91db or over. You will be well rewarded.

[TowerRadio]

I have one of these,bought in my favourite Wiltshire charity for a very low price,in mint condition.Only thing wrong was the function switch was very stiff.A spot of light oil and it was perfect.Trouble is my Leak speakers are in Sawston and I need to check it out on headphones(it hasn't been powered on for a while).Maybe i'll be in for a surprise.

[yestertech]

A quick update ...
Well, we have some audio section signals at last. Lots of VV dirty valve pins, in one case on an ECL82 whose heater wasn't even on !
Some work has been done previously, denoted by the white plastic RS caps fitted around the output stages.
One of the IF valves had gone to air. This was the first in the IF strip so swapped for one in a following stage. Replacing this (6BA6)and the low emission Freq. Changer (6BE6) restored AM radio which only utilises the first of the three IF stages
A quick rummage through the valve store found another 6BA6 to replace the now missing one, which in turn brought about nice clear FM signals, albeit in mono.
The function switch has two positions 'FM' and 'FM STEREO' Although I had the switch in the 'FM' position, changing it to stereo made absolutely no difference.
I've been back through the service manual and performed all the recommended voltage checks. Nothing obvious there. There are a number of helpful test points listed, but I don't have the right test gear to perform the stereo checks and alignment.
My recollections of the principals of stereo decoding using 19K pilot tone and subcarriers have faded over time, so I will need to revisit these to try and work out what's gone missing in the decoder section !!

Andy

[paulsherwin]

There were quite a few integrated valve amps and receivers made in Japan at that time. Some of them had distinctly odd features by later standards, like dual AM tuners to support the experimental stereo stuff being trialled in the US. I always felt they were designed for the American market. That doesn't mean they're junk of course.

[Ted Kendall]

It appears to be a conventional multiplex receiver, with a valve decoder. The more bizarre stereo arrangements had disappeared by this time.

[Radio Wrangler]

If it's a stranger to these shores and intended for the American market, it might have the wrong time-constant of FM de-emphasis.

David

[Synchrodyne]

For the US market the FM de-emphasis would (or should have) been 75 µs. For Europe (and many other geographies) it would have been 50 µs. The latter was also used in Japan, although the (at the time) 76 to 100 MHz band used for FM would have required some differences anyway. Thus, the Japanese makers were accustomed to working with two de-emphasis numbers, so it is reasonably likely that exported equipment was destination correct as built. (Although that might not be so for “grey market” exports.)

The FCC decided upon the so-called Zenith-GE FM stereo system in 1961. (The FCC combined two competing and broadly similar systems; GE and Zenith did not co-operate on this, notwithstanding that they had been involved in earlier collaborative efforts.) After that the earlier US experimental systems, such as FM-AM dual transmitter, disappeared failry quickly.

In Japan, AM-AM dual transmitter stereo was in use approximately from 1954 to 1964, hence the existence of tuners and tuner-amplifiers with two AM tuner sections. Japan adopted the Zenith-GE FM-MPX system in 1963, I think. There was some overlap, and so for a short while a need for equipment that covered AM-AM and FM MPX, but as Ted has said, the Sansui 250 appears to have postdated this era, being FM-MPX only on the stereo radio side.


Cheers,

[Ted Kendall]

As I understand it, Zenith and GE arrived at the same answer by different routes, one developing a time division multiplex system and the other frequency division multiplex. Perhaps it was the FCC that twigged the equivalence? TDM is more popular as a decoding method, as sync with the subcarrier is easier to maintain, to the benefit of stereo separation.

[Synchrodyne]

Yes, that is a good summary. There was an earlier thread on this matter:

https://www.vintage-radio.net/forum/...ad.php?t=56858

I have since found some additional information, but I think that would be outside of the remit of this thread.


Cheers,

[PaulR]

I have the slightly earlier Sansui 500. It has incredibly complex switching to allow AM and FM to be received at the same time. FM works fine although AM doesn't any more as I had to rewire one of the switches slightly to move the HT switching to another part of the switch.

The problems I had with it were every paper in oil cap needed to be replaced (I think they were made by Suzuki) and all the electrolytics leaked fluid either when it was received or soon after I started to use it.

It lost FM at one point but a new ECC85 in the front end cured it.

[Radio Wrangler]

If the radio section is receiving FM reasonably and it's the decoder that's giving trouble, in lieu of a stereo encoder to test the decoder, a lot can be done with off-air signals. A valve based decoder is likely to be a 19kHz filter (tuned circuit) into a frequency doubler then a 38kHz filter (tuned circuit). Using an off-air signal, you can use a scope to check that 38kHz is being created, and twiddle the tuned circuits at 19kHz and at 38kHz to peak the 38kHz amplitude. It's likely that the modulators which apply 38kHz to the incoming mono audio from the discriminator are nothing more complicated than several switching diodes, feeding into a resistor matrix which adds and subtracts the difference signal to the left+right mono audio.

The decoder relies on getting the phase of the 38kHz right, but it'll be designed that the phase will come out right when the 19kHz and 38kHz 'side chain' is peaked.

Before digging into the decoder it may be wise to check the FM tuner IF alignment is OK and that the discriminator is set up properly. FM sets of this era are likely to use the Foster-Seeley discriminator as the ratio detector really came to prominence a little later.

A scope and sweeper/wobbulator make life easier. But without the wobbulator a plain mono FM sig gen can be used with the discriminator being twiddled to give the least distorted and large sinewave at audio. Fancy gear lets you get further towards an optimal result, and makes things easier to understand because you can see all of what's happening - making the thinking bit less burdensome. Without it, a bit of care can get you as close as you may find necessary.

Mis-aligned IF strips can give time delay variation over frequency. This is the same thing as non-linear phase. Why worry about this? Well non-linear phase variation applied to an FM signal means that ell the different sideband components of an FM signal (and there are lots) don't arrive correctly timed at the discriminator and various additions and cancellations miss each other. You get distortion coming out of even a perfectly adjusted discriminator. In an AM set, amplitude distortion in the IF gives distorted audio. In an FM set, it's distortion of TIME in the IF which spoils the audio. As FM IFs are supposed to be limiting, they are already distorting a lot in amplitude and it gets ignored in the discriminator, so it's a different sort of distortion which spoils things.

I'm suffering distortion of time in my hifi tuner, pass me my sonic screwdriver!

David

[steve ss]

Ok.

This takes me back a bit!

Around 1980 to 1985 I rebuilt quite a number of the Sansui 220 receivers,
they were if you like the mono version of the Sansui 250 although they
had the provision for a stereo decoder to be fitted.
Australia at the time of the sansui 220/250 release (1966-1967) had no FM transmissions so only the Sansui 220 was imported /distributed by Simon Gray Pty/Ltd.

So first a couple of warnings!!!

As previously mentioned there are a number of PIO Suzuki capacitors these contain PCB oil which over time sweats out, it is clear/yellowish sticky oil VERY TOXIC!!!!!.

First port of call is to remove and replace all of these for person safety reasons and they also had a habit of failing unexpectedly.

Note:

1 Wear disposable gloves while removing these components.

2 then discard gloves and capacitors safely.

3 Clip capacitors off close to the capacitor body leaving the pig tail available to re solder the replacement capacitor to.

4 Take your time and double check your work!


As already mentioned the selector switch is often stiff DON'T force it!

lubricate the shaft and switch contacts and wait for the oil to penetrate!

All the smaller electro's need to be replaced.

The larger "can type" capacitors may respond to reforming if not leaking electrolyte otherwise replace also.

The disc type (Se) rectifiers need replacement.

Ok

That's most of the general stuff!


The Stereo decoders across the range Sansui 250 to sansui 1000A

had a problem in that they would cease their stereo function unexpectedly and the fault would remain un diagnosed! as no amount of slug adjustment or component checks were productive.

As it turned out in the base of the decoder "cans" were "fixed" compression type siver mica capacitors often transparent green/yellow in colour in which the silver had migrate causing the capacitor to fail.

Disabling this capacitor and replacing it is a fiddly but doable job!

So if you have these symptoms this may be the cause/solution.


The good news is these units are a good look and surprisingly good listen when rebuilt.


I have included some pics of the Sansui 250 and Sansui 220 for the train spotters!



enjoy Steve ss.