Hum in 1950s Truetone Record Player

[bennie8]

Greetings,

I am attempting to restore a 1950’s vintage Truetone record player model 4DC5908, manufactured I believe in the UK and sold here in the USA by Western Auto Supply. The unit is a two channel stereo record player. The problem I am having is constant 60 cycle hum from channel B only. The hum in channel B is present even when the cartridge is not connected to the amp. The volume of the hum increases and decreases with the volume control pot. Channel A is quiet even at full volume.

The cartridge is new old stock and when the cartridge is connected to the amp and the needle is wiped with the fingertip the appropriate scratching sound is heard through each speaker.

So far I have replaced all three paper capacitors with orange dip capacitors. I have cut the hot leads; red-yellow-green- and blue, coming from the metal can capacitor and replaced them with same capacitance individual electrolytics. I did not cut the black negative lead. I tied the negative terminal of the replacement capacitors to other locations with the black negative lead from the can. I have replaced the selenium rectifier with a three amp 1000 volt diode. The two 50EH5 tubes (valves) have been replaced with new old stock ones. Swapping tube locations does not change the channel location of the hum.

Can someone suggest what I can try next? I am out of ideas.

Thanks so much,

Bernie

[Edward Huggins]

It's unlikely this was ever made in the UK. The circuit, using 2 x hi-slope output valves in stereo, was almost exclsuive to the US. It will, of course, use a high-output stereo cartridge which are in themselves quite rare and almost unobtainable in the UK.

[Simon Gittins]

You could try disconnecting the cartridge from both inputs and see whether there is a similar hum from both channels; hum level can increase when the input is unloaded and there may be a problem with the new cartridge or the wiring. Perhaps swapping the inputs over temporarily would give a clue.

[Herald1360]

Volume control dependent one channel only hum suggests a faulty cartridge or a broken connection (ground or signal) between the cartridge and volume control.

The amp described likely has the cartridge signal connected pretty much straight to the "top" of the volume control with ground being the same to the "bottom". There may be some isolating capacitors though the 120V hazards are less than the UK 240V ones.

[Peter.N.]

You could bring it over here - you would get a 50 cycle hum then.

I assume you have tried the valves, you can get heater/cathode leakage. Try earthing the grids of each stage in turn, that should give you an idea of where the hum is getting in.

Peter

[TrevorG3VLF]

Earth the input of the volume control. If it still hums the fault is likely to be the valve which allows hum when the input circuit has high resistance.
If the hum stops, then the source is the input.

Swap the valves, see if the fault follows the valve.

[usradcoll1]

Originally Posted by bennie8

Can someone suggest what I can try next? I am out of ideas.

Being in the US and having experience working on this sort of equipment, there's one thing I would do first!
Cut the black lead from the old 'lytic can and connect all the negative leads from the replacement caps to this lead. You might've landed one of the negative leads to the wrong location. I've done it myself!
It seems you've tried all the things I would've tried.
Dave, USradcoll1 Midwest USA.

[bennie8]

Thanks to all of you that have responded. The hum is present even when the cartridge leads are not plugged into the input RCA jacks on the amplifier. The hum does not follow the position of either valve including the original old valves that came with the player or new old stock replacement valves. Hum is always in the B channel.

The resistance between the ground side of the RCA cartridge plug to the amplifier chassis is 0.3 ohm, Fluke DVDM.

This AM I cut the black negative lead from the original canned electrolytic capacitor and verified again that the negative terminals of the replacement electrolytic capacitors were grounded to the negative terminal at other locations. the hum remains the same.

Could someone take a look at the schematic? Is it correct that C4 is tied from the chassis to the black lead that C1-Red,Yellow, Blue and Green negative ends are connected to?

Also I do not understand the ground symbol shown at the B- end of C4. This amplifier has a non polarized two prong plug so it cannot be earth grounded. What am I missing? Added note. The hum is the same regardless how the 120 volt plug is plugged in to the 120 volt source.

[Herald1360]

Well it looks seriously bean counted! A common cathode resistor and decoupling capacitor for both channels?!

Maybe the fact that the B channel heater is at the hot end of the chain might be involved?

The "ground" symbol is simply used to denote B- (HT-) / signal ground connections. The "chassis" symbol denotes the amp metalwork which (as I believe is quite common across the pond unlike here) is not connected to B- etc. C4 commons them for signal frequencies without providing a galvanic connection. Is C4 in good fettle leakagewise?

It's hard to see how such a simple low gain circuit could have much of a hum problem- it would need hundreds of mV at the input to have much effect.

One possible thought..... one of the heater pins of the valves is adjacent to the g1 input pin. Is their any possibility of leakage across the B channel valveholder between pins?

[usradcoll1]

There is a lot of good points raised on this thread. I wonder if there is a solder splash between the heater pins 3&4 to the G1 pins 2&5, It happens!
Take a cliplead and connect it to the B- and touch it to the grid terminal and see if the hum disappears, then investigate further.
Dave, Midwest USA.

[bennie8]

Herald1360 - do I understand correctly that the ground symbol throughout the circuit schematic is a common connection point for all circuit wires showing the ground symbol? C4 is a new orange dip obtained from the local electronics store and I would assume is in "good fettle".

Usradcoll1- I have carefully checked the solder connections and the insulating wafer board of the valve mount for solder bridging and found no issues. The B- point is any of the ground symbol points shown on the schematic? The grid terminals are pins 3 and 4 on the valves. But pins 3 and 4, is that viewing from the bottom of the tube socket or top and do I count clockwise or counter clockwise from the missing pin location?

Thanks for all the suggestions.

[usradcoll1]

From the bottom, component side is clockwise from the missing pin, top side is counter clockwise from the missing pin.
I always use the negative lead from the existing 'lytic cap. Less of a chance of an error that way.
Dave, Midwest USA.

[bennie8]

usradcoll1, I touched a lead from pin 6 on the valve to the negative grounds of the electrolytic capacitors and got a nice spark and pop in the speaker. What did I do wrong. I rechecked my touched points, touched then again with the same results. Perhaps pin six is not the grid?

[TonyDuell]

From the circuit diagram posted earlier, pin 6 is g2 (we often call it the 'screen grid' in the UK). It's connected to the 110V supply, the right hand end of R5 in that diagram. Shorting that to ground will produce a spark.

I think the grid you want (g1, 'control grid') is pin 2 or pin 5 of the valve.

[Chris55000]

Hi!

These valves are a very high–slope (14.5mA) equivalent of the HL90/50C5 that has no direct UK made equivalent, they operates on a 110V supply with a neg GB of –3.6V

Pinout:–

1:– k/bp, 2:– g1, 3:– h, 4:– h, 5:– g1, 6:– g2 7:– a (B7G base)

Va 110V, Vg2 110V, Pa 5W, Ia 42mA, Ig2 14mA, Vg1 –3.6V, gm 14.5mA/V, ra 11k, Ra 3k, Pout 1.4W (10% Dtot)

. . .there is nothing remotely approaching this slope as an audio valve in the UK!

Almost certainly there is some sort of leakage between g1/htrs in the B channel, which may be in the valve itself or the valveholder!

Have a look at the valves themselves carefully at the bottom where the base–pins come through the glass button base – if there is any sort of deposits there, the valve should be discarded – it may be powdery or silvery in nature, and it comes from old getter material evaporated from the getter holder during manufacture, and also minute particles of metal can be knocked off the anode and intermediate electrodes over the years, and these minute deposits can also accumulate in the bottom to cause inter–electrode leakage!

If you are certain the valves are above suspicion, try reversing the two heater leads to the B channel (V2) holder – this may possibly help, or rearranging the heater circuit so V2 has one heater pin earthed to high–tension negative (B– US terminology!).

If the player is wired–chassis construction, try replacing the valveholders with ptfe or ceramic insulated types to ensure the greatest possible resistance between the tags – if the player is PCB constructed – there's the old bogey of PCB leakage!

In a really bad case, you might have to rectify and smooth the heater current – as these valves take 150mA this isn't too expensive an undertaking with modern components – try a 1N4006 diode wired AFTER the heater dropping resistor, with a 33uF 160V axial electrolytic connected to the cathode of the diode and HT–, + terminal to diode cathode, then add another resistor of about 120 ohm 5W from the diode cathode/electrolytic junction, the free end of the diode connecting to the top end of the two heaters, and a further 33uF axial electrolytic wired directly across both heaters (NOT one cap across one valve!) as a smoother.

If you're unsure about any of the above I can post a circuit sketch!

Chris Williams

[Boater Sam]

Is the earth end of the volume control on B connected or is the track in the pot open circuit?

[bennie8]

Again thanks to everyone for your help. Could this be progress? I connected a shunt between the electrolytic capacitors negative connections, first to pin 2 and then to pin 5. In both cases the hum totally disappeared.

Chris Williams; I inspected both valves under a 30X Bausch and Lomb stereo microscope that I just happen to have laying around here and could not see any deposits in the areas you suggested. I indeed do not understand completely your additional suggestions and would like to hold them for clarification later if they are going to be pursued.

Boater Sam; I am not sure what is the "earth end" of volume control B. I think you are talking about one of the solder/wire connection points of the volume control. On the volume control pot there are three wires. One wire goes to a cartridge input center pin. One wire goes to pin 5 of the valve and the third wire wire goes through R4 to the negative connection point of the electrolytic capacitors.

Does anyone think that reflowing all the solder connections would help? I did previously inspect them all and did not see any cold solder connections or partial solder connections. I am a little leery of reflowing them and causing more damage than help.

[usradcoll1]

Originally Posted by Boater Sam

Is the earth end of the volume control on B connected or is the track in the pot open circuit?

That is quite an observation!
The low end of the volume control also furnishes the grid leak for the output valve. The 50EH5 valve always seemed to be used in Japanese radios, etc. US made equipment, such as this one seemed to use this type for the entry level models.

[crackle]

It would seem that the only connection for the ground from the pickups is via the 0.05uF capacitor connected between chassis and B- .
The problem has to be in a ground connection somewhere, maybe the channel B phono socket, or as Sam said a broken track or ground connection on the volume pot.

Mike

[Station X]

Originally Posted by bennie8

I connected a shunt between the electrolytic capacitors negative connections, first to pin 2 and then to pin 5. In both cases the hum totally disappeared.

Is this one of those players where the can of the original multi-section smoothing capacitor serves to electrically link what would otherwise be isolated tracks on the printed circuit board?