HMV 2328 FM radiogram.1966.

[FERNSEH]

I'm servicing an HMV 2318 "Stereomaster" radiogram.
The audio amplifiers are working but there is too much hiss coming from both left and right channels. The hiss is present irrespective of the position of the radio-tape-gram selector switch.
Also the radio is dead.
There is a lot of Callins capacitors to replace.

DFWB.

[dazzlevision]

Originally Posted by FERNSEH

I'm servicing an HMV 2318 "Stereomaster" radiogram.
The audio amplifiers are working but there is too much hiss coming from both left and right channels. The hiss is present irrespective of the position of the radio-tape-gram selector switch.
Also the radio is dead.
There is a lot of Callins capacitors to replace.

DFWB.

I restored my Ferguson 3332 stereogram a few years ago, which has similar/identical audio output stages and found that the Mazda/Thorn Germanium AC156 transistors can become very "hissy". They usually have a white outer plastic sleeve, with the markings on.

[FERNSEH]

Hi Dazzlevision,
the audio amplifier in your Ferguson radiogram looks identical to the types employed in the HMV 2328.
The Mazda AC156 transistors can be replaced with the Mullard AC126.
The IF amplifier in the FM tuner has AF114 transistors, likely one of those has the internal "whiskers" fault.

DFWB.

[dazzlevision]

Originally Posted by FERNSEH

Hi Dazzlevision,
the audio amplifier in your Ferguson radiogram looks identical to the types employed in the HMV 2328.
The Mazda AC156 transistors can be replaced with the Mullard AC126.
The IF amplifier in the FM tuner has AF114 transistors, likely one of those has the internal "whiskers" fault.

DFWB.

Yes, I'd agree about the AC126. I did have some good ex-equipment AC156 which I used as replacements - I hope they don't go hissy though!

I had to replace the AF11x transistors in the AM RF and common AM/FM IF stages of my stereogram, for the usual reason. Luckily, the VHF RF unit uses early Silicon types.

[jimbilton]

69 guineas when new! That was quite a lot of money in 1966, I imagine.

[dazzlevision]

Originally Posted by jimbilton

69 guineas when new! That was quite a lot of money in 1966, I imagine.

Yes, it was. It demonstrates how the cost of consumer electronics had beaten inflation over the past several decades, largely due to the advances in technology and production methods.

Even so, these HMV Stereomasters do sound rather good (albeit not High Fidelity in the true sense). I have a slightly later one myself that I fully restored.

[paulsherwin]

VT102 is a prime suspect.

Looking at the circuit, an Si replacement should be OK without other component changes if you don't have any AC126s to hand. A good old OC71 should be OK too, though it might be a bit noisy.

I agree that the AF11xs are likely to be bad. Again, you may be able to get away with Si replacements - BC558s, 2N3906s etc.

[FERNSEH]

This set fixes itself! When performing tests in the FM IF amplifier section the receiver burst into life. It's almost certain the AF114 transistors have suddenly healed themselves. The fault(s) in the transistors will return so it will be good policy to replace the lot!
The FM coverage needs extending to at least 102Mhz so that ClassicFM can be received.
Two BA110 vari-cap diodes are employed for tuning coverage between 88.5 to 100Mhz.

DFWB.

[Michael Maurice]

Originally Posted by paulsherwin

VT102 is a prime suspect.

Looking at the circuit, an Si replacement should be OK without other component changes if you don't have any AC126s to hand. A good old OC71 should be OK too, though it might be a bit noisy.

I agree that the AF11xs are likely to be bad. Again, you may be able to get away with Si replacements - BC558s, 2N3906s etc.

I have replaced AC156's with BC558's or even BC559's without any further component changes.

[FERNSEH]

Right side amplifier is now fairly hiss free but the left side amplifier continues to be very hissy. Transistor VT101 replaced with a silicon type and the 220K bias resistor R102 replaced.
The hiss is present irrespective of input switch position.
The FM radio continues to operate very well. AFC function excellent.

DFWB.

[stacman]

69 GNS, plus the purchase tax of 20/0d...

[FERNSEH]

One Guinea = twenty-one shillings, £1.05p in new money

DFWB.

[Radio Wrangler]

There's plenty of supply voltage, so lots of room to tolerate the higher Vbe of silicon.

I'd suspect you have some noisy transistors in addition to whiskery ones.

The place to be wary silicon replacements is in the circuitry around the outputs and drivers, with the diode/transistor series pair acting as bias compensation. It was still relatively early days and the ways to tame thermal runaway hadn't percolated through to consumer stuff.

David

[FERNSEH]

Reference to circuit diagram on post No.1.
Gave up with employing silicon transistors. Transistors VT101 and VT102 replaced with Mullard AC126, a near equivalent to the Mazda AC156.
220Kohm resistors R102 and R106 replaced.
Now the background hiss is barely audible.
Now, I must turn my attention to the right side amplifier.
FM radio receiver has stopped working again.

DFWB.