HMV 501 Radiogram issues

[mancunian69]

Hi all,
I have been restoring this set for a customer for the past 5 months now and I can not get the gramophone to work. There is no audio output.

The original HMV headshell was open circuit and could not be repaired. I have replaced that with a Collaro headshell, which fits on the same.
I have measured the resistance of the output wires and it measures about 2.6k Ohm. Is this about right?
The headshell has new twin core coax wiring. Also new coax from gram connections to amplifier.

I have connected a 1kHz tone to the input of the amplifier and I can hear a tone, albeit up to a certain volume level, then it goes quiet.

I have connected the gram ouputs to my scope and I can see a signal. The level of this signal I am not sure what it should be on a gramophone like this. I haven't worked on 78rpm items before.

The radio works fine, but it is starting to become infuriating as to why the gramophone will not work. I've spent weeks and weeks repairing, restoring and French polishing this unit, but now, I just feel like grabbing the biggest hammer I can find and smashing it to pieces. If this was my set I would have done it by now!

I have noticed that one of the gram wires is grounded. Is there a reason for that?

I need help with this one to get it over the line.

Rgds

Michael.

[Cobaltblue]

That looks amazing puts my still unrestored 501 to shame.

I don't recall either of the Gram connections being grounded.

But to be fair its been 3 decades since I played with my example.

It's a horrible circuit ISTR connecting into the second valve which is configured as a grid leak detector.

Definitely do not hit it with a Hammer

Cheers

Mike T

[Techman]

I agree it's a bit of an odd circuit (having just looked at it), which is often the case with this really ancient stuff. One side of the pickup (screen) is grounded and the volume control is twin ganged with a separate one for the TRF radio. The first valve is biased to cut off to mute the radio when in gram mode. With you saying you've injected a tone and as the volume is increased it goes quiet - perhaps something strange is going on with the biasing. I would perhaps be looking at some of the capacitors and that 1 meg resistor going to the grid of that triode.

There doesn't seem to be any information on the gram deck and pickup head, unless it's in other service information for the particular record deck, but old magnetic pickup heads like this can range from just a few ohms with a matching transformer to several thousand ohms - I would suspect that yours is OK, but be aware that virtually all these old heads need the rubber parts replacing to stop them from chewing up records and producing a very weak sound output.

It looks like a very nice, and I would say rare set!

[vinrads]

Hi Michael that is looking lovely ,I have the same model I will try and dig it out it is somewhat out of reach ,and check the pick up etc. Mick.

[PJL]

The cartridge output needs to be as high as the SG output you used for testing.

Was the volume with the SG acceptably loud? It sounds like the signal is being rectified on the V2 grid which implies V2 is not biasing correctly. What is the voltage on V2 anode and cathode?

[Nuvistor]

The manual has a nice test, in fact it goes through step by step fault finding for the gram. It suggests using a pair of high impeadance headphones to test the output from the pickup.

Hope this is the same model.
https://www.service-data.com/product...57/3615/s12757

[Gridiron]

Looking at the information on the 501 in Newnes Complete Wireless, which directs you to the data on another HMV gram for details of the type 15 pickup, this pickup has an output of between 1 and 5 volts, which sounds high to me for a magnetic pickup.
Forum member PsychMan was restoring a 501 not long ago, he had some bits from me from a scrap one I have. He was going to have his pickup coil rewound (they always seem to be o/c ) so may be able to help.

[mancunian69]

Thanks for the advice. I need all the help I can get with this set. If any member would like to either lend or donate me an original headshell, or anything pertinent for this job for testing it would be greatly appreciated.
I will return any item if necessary.

I have taken the gram/chassis out again now for the 20th time. When I get it set up on the my bench I will take more measurements, and get back to you, particulary the tube pins. This is something else I am not used to either - the pinouts on these old tubes, which are not the same as the regular 9 pin items that I work with. Something else I have to get my head around.
To make life even more difficult, the chassis and gramophone can not be tested together on the bench due to the way they sit in the cab. Testing separately is not a problem.

Combine all that with my cramped, higgldey piggledy, smoke filled garage where at best space is at a premium you can see what I'm up against, and why I get so frustrated.

Rgds
Michael

[Heatercathodeshort]

Working on one of these, especially for a customer in such terrible conditions must be a nightmare!
It might be a good idea to split the deck from the main unit, extend the pick up and mains connections and support the deck on some wood blocks. I have a box of various sizes and they are very useful to prop up chassis etc.

I suspect that the HMV pickup employed in your gram has a very high impedance coil with a resistance of around 5-6K. I had this problem with a mid 30's HMV AC/DC radiogram that fed the pickup output direct to the N30 output valve. The original coil was O/C and after stripping down realized that the original gauge of wire was almost impossible to obtain and even more difficult to wind. It was thinner than hair!

I did try a few EMI pickups with a lower resistance but the results were disappointing.
I ended up winding as many turns on the tiny bobbin with 47 gauge wire until it was full. This measured around 2K but the results were just OK with no volume to spare.

I ended up making up a small preamp with an EF91 [Z77 etc] in a very small box. The EF91 has a 6.3v heater but will work very well with just 4v from the main units 4v supply. It just takes a little longer to warm up from cold.

Many EMI pickups are wired with twin core screened [uninsulated] wire with the screening earthed together with one of the pick up leads. It depends on the circuit and if an input matching /step up transformer is employed.

Hope this helps. John.

[Radio Wrangler]

You've said the gramophone doesn't work, but what about the radio? Is it OK, and does its volume control work normally across its full range?

If it's putting pick-up audio into the grid of a grid leak detector to use it as an audio gain stage, then I'd expect the bias to be changed for the gramophone mode to make that stage linear.

David

[mancunian69]

I have just pulled the MH4 tube (V2) and noticed that I can hear a rattling sound if shaken lightly. That doesn't sound good. I hope it's not a faulty tube, as an eBay search has revealed them to be very expensive.

Rgds
Michael.

[PJL]

Rattling sounds are OK, it is loose bits of glue in the base. It's the voltage measurements we need as it might be low emissions. MH4 are common types used as a detector and should not be expensive.

[mancunian69]

Ok, I've got some numbers which read as follows:

V2
Anode 49.5V
Cathode 6V (gram only) Zero on MW and LW

I've got about 158V HT out from PSU

It is extremely difficult to get the probe to the pins. I have wires with broken insulation in there too which I may rip out and replace as good practice.

Considering there are relatively few components in this set, but access to some of them is a real pain.

I wanted to ask about those black TCC caps. Will they need to go or do they hold out well?


Rgds

Michael.

[PJL]

Those TCC caps I believe are mica and should be OK.

Where are you measuring the HT? looking at Trader 563, it shows the MPT4 anode voltage should be 280V but that would imply R12 is dropping 130V. I then looked at the 'other' servicing notes and the circuit includes an additional resistor in V1 anode feed that sounds a bit better but it has no component value list.

The MH4 again looks strange. The auto-bias resistor is marked in the Trader as 1K so it's running at 6mA and according to the datasheet it should be completely cut-off with -6V bias. Are you sure it was 6V and not 0.6V?

So... we don't seem to have service data that can be trusted. I suggest you check the circuit and values against the trader and mark up any differences then take some voltage measurements.

[mancunian69]

Today, I have taken out and replaced as much of the old broken wiring as possible, including some wires that go inside the cans.
It has given me more room to probe V2.

In gram mode I have 165v at the anode and 6v at the cathode, not 0.6v
In MW mode, I have 50v anode and zero at cathode.

I'm not sure why, but when I connect my little sig gen to gram input the voulume now goes up all the way without fading. It is a cheap and cheerful 12v unit that goes from 0-3vac

Those TCC caps when taken out all measured low, so I have ordered some mica replacements. I can see myself replacing everything at this rate.

The service manual, taken from the Radio Museum is what I have been using. It's confusing because there are two different schematics on the document.

Michael.

[ms660]

6 volts at the cathode is too high, check the cathode bias resistor (R8 1k Trader) with the power off and switched to gram, if that's ok check the resistance between the cathode pin and ground.

Lawrence.

[PJL]

Start at the beginning and measure the raw HT volts on C16 (about 290) then the smoothed HT on C9 (about 270?).

[PJL]

The capacitors are 200 and 300pF and most likely mica. How confident are you that your meter can measure capacitors that low? The values are not very critical so I would not bother replacing them yet, you should focus on getting the valve voltages correct first.

Can you confirm that the valves are exactly as per the service sheet, V1=MS4B, V2=MH4, V3=MPT4, V4=U10.

[mancunian69]

I have a capacitance meter that measure caps down to very low values when it comes to the TCC's
I have done another round of voltage checks.

The rough DC from rectifier tube is 277v
At the junction of R14/C15 the voltage is 211v
At the junction of R12/C9 it is 162v

Please note that the original 4+4uF cap was not there when I got this set. I have replaced C16 and C15 with 4.7 and 10uF electrolytics respectively.

The schematic I am using is taken from Radio Museum, and is the larger of the two .

R14 is now a 2.2kOhm ceramic and R12 is a 4k7 Ohm ceramic.

Michael.

[PJL]

That is the 3rd different circuit diagram for the 501 and is the 2nd one with no component values. This one has additional HT smoothing. Where did you get the values for your replacement R14 and R12 from as we will not be able to provide much in the way of help without an accurate circuit with component values.