HMV 2340 low AC Voltage Operation Problem

[TimothyStuart]

I have just finished renovating the amp/tuner from an HMV 2340 radiogram. It had suffered a major eletrical incident including exploded capactors and not been used for over 20 years. I have replaced all the pre-amp and output transistors and most of the resistors, all the capacitors as well as the bridge rectifier. DC voltage checks are very close to the HMV circuit diagram.
It now operates but if I put 80V AC through it from my variac it sounds okay and then increasing the variac to 165V the sound quality remains unchanged. Anything over 165V and the sound distorts badly. I wonder if anybody has experienced anything similar with this unit as I am struggling to solve the problem. Thank you.

[ben]

Sounds like the o/p stage is still faulty. Check the biasing and voltages. Are both channels the same? What transistors does it use?

[TimothyStuart]

Hi Ben. The main output transistors are AD161 and AD162. I did replace them.
Both channels are the same. Thanks Tim

[SteveCG]

I'm not familiar with this model, but it is curious that the sound on both channels is alright at low mains voltage but deteriorates above a 165 volts.

As ben says the first thing to establish is whether the output transistor biasing is roughly correct. If these transistors are being biased on heavily then they will get hot. Can you safely feel them, with no signal input? They should be cool and remain cool under these no-signal conditions. If they are hot then turn off quickly and follow any advice on setting the bias given by the manufacturers. I would guess it will be in the region of 20 milliamps.

[TimothyStuart]

Thank you Steve. The output transistors are all running cool under no signal conditions and when up to 165V is being used. I have not run the unit on 240V for any length of time as clearly there is a something amiss at this voltage.
As a novice should I be checking the output transistor's resistors? I replaced them following the spec on the service manual. How critical is the wattage for these resistors please? I used a .5W. There is no value given for these resistor's wattage in the service manual. You mentioned a measurement of 20 millamps. Where should this 20 milliamps be measured please?

[Ed_Dinning]

Hi Tim, I think you need to be checking DC voltages around the output stages

Ed

[SteveCG]

TimothyStuart:

The 20 milliamps would be measured in the collector lead of one of the output transistors - your service info should say exactly where. However, if you have a digital voltmeter which can read DC millivolts then a way I use myself is to simply measure the voltage across one of the emitter resistors in the output transistor pair and applying Ohm's law, work out the current that way. For amplifiers where originality/least hacking about, is a goal it saves cutting into the leads.

As for the wattage of the resistors, I think I would be using a higher wattage whose value depends upon resistance and the currents you expect to be flowing through them; without more info on the amplifier I can't say further. One thing though is that the problem you have (distortion above 165 V) would not be caused by a correct resistor value but too low a wattage. The problem you get with too low a wattage is that the resistor would burn out though prolonged use at high out levels - and you have not got to that stage yet!

Thinking on the fault you describe further, do the amplifiers use an AC128 transistor in the driver stages? These can develop a fault known as 'tin-whisker syndrome'. This can cause all sorts of problems if the transistors are mounted on the amplifier chassis.

See this thread for more info.

https://www.vintage-radio.net/forum/...d.php?t=201719

[Michael Maurice]

Originally Posted by SteveCG

TimothyStuart:



Thinking on the fault you describe further, do the amplifiers use an AC128 transistor in the driver stages? These can develop a fault known as 'tin-whisker syndrome'. This can cause all sorts of problems if the transistors are mounted on the amplifier chassis.

See this thread for more info.

https://www.vintage-radio.net/forum/...d.php?t=201719

But i wouldn't expect it to cause distortion on both channels! Besides if they were faulty it would show up at any setting of the variac.

The original bridge rectifier was selenium, the one you fitted is almost certainly silicon which has a lower forward drop voltage and thus you get a higher HT,

[Jez1234]

If the OP has a schematic they could put up here it would make it rather easier for us to help.

[Michael Maurice]

The circuit diagram is available from this site.

The mains transformer is designed to give 22V which when rectified by a selenium rectifier will give around 25V, however you have almost certainly replaced it with a silicon type in which the DC output is 1.414 of the ac into it which is 31V.

That is around 22% higher than the circuit is designed for.

You may be best fitting the original rectifier and seeing what happens.

[ms660]

Originally Posted by Michael Maurice

The mains transformer is designed to give 22V which when rectified by a selenium rectifier will give around 25V, however you have almost certainly replaced it with a silicon type in which the DC output is 1.414 of the ac into it which is 31V.

It'll be less than 1.414*AC voltage in, you need to take into account the load current and the forward voltage drop across the diodes.

Lawrence.

[peter_sol]

Hello.

I have had the same problem turning up the variac.
It is possible that if you go past the point where you get distortion it will get better.
Best to monitor the centre point of the output stage which should be just under half the supply voltage.
Quoted as 10.7v for 25v HT.
Also note that the voltage regulator VT9 is common to both channels (and its associated components)

[TimothyStuart]

Thank you all for your help. Yes one AC128 tr is used but I replaced it as it tested below spec. I will try to source an original bridge rectifier as your assumptions are correct with the new silicon BR I have used. If I manage to solve the problem I will report back.

[Jez1234]

You can simply fit a suitable wirewound resistor to replicate the forward resistance of the selenium rectifier and thereby get the correct voltage plus the greater reliability of a silicon rectifier.

[TimothyStuart]

I have found out that D10 - which is a 15V zener diode - is faulty. The original should be either a BZX19 or BZX10. The diode removed reads TI 2150 A so I am not sure what replacement diode I should fit? I do not think I can get any BZX diodes (NOS) so will any 15V zener be a suitable replacement? What about the wattage value?
Thanks for your help.

[Jez1234]

Any 15V 500mW Zener should do the trick.

[TimothyStuart]

Thank you Jez