HMV 2152 Restoration

[OldTechFan96]

Hello,

I've recently started work on the HMV 2152 which was kindly offered by 'richard_in_edin'.

The tuning capacitor was seized and was freed off with some WD40 and some gentle rocking with a pair of Mole grips. I think that this is a common problem with this particular German made TC. I had a similar issues with a Dynatron Atlantis which used the same part.

The ferrite rod antenna was broken in three places. It has been removed and super glued back together.

Before testing started I did all of the usual cleaning and switch lubricating. The mains smoother was gently reformed.

Initial testing showed that there was no output from the right channel, the left channel was working well. VHF and SW worked. There was no MW and LW, presumably due to the antenna being clipped out.

I fed an audio signal into the 'gram' input and traced it through the RHC until it disappeared at the C of VT11 (audio driver). There was a signal on the B so I though VT11 might have an open B-C junction. The usual checks with a DMM showed that this was not the case and the transistor was reinstalled. I decided to stop at this point.

While looking at the service data I heard a buzzing from the RH speaker. I measured across the speaker terminals suspecting DC. There was 2V across the speaker tags. With the speaker unpluged the voltage increased to 24V. I suspect that coupling electrolytic C83 (Callins) has went bad.

I tested the RH speaker with a mini 12W amplifier and it sounds fine at low volume. When the volume is increased the speaker makes a crunching sound. I think the voice coil is rubbing against some debris. You can feel it when the cone is depressed.

When it comes to radio repair, amplifiers are my weak spot. When studying theory books I'd usually skip sections on Hi-Fi amplification. I thought it would be a good idea to stop and ask the forum for help.

Issues so far:

No O/P in RH amplifier.
DC on RH speaker terminals.
'Crunchy' RH speaker.

Any help would be much appreciated!

[crackle]

Regarding the tuning capacitor, it is likely to seize again when the WD40 dries out. You may find that working some decent clock oil in there whilst it is still free will have a better long lasting result.

I cant find any information on the HMV 2152 but if there is DC on the output it may indicate the failure of any DC blocking capacitor that may be fitted.
I would also check the final output transistors.
Mike

[OldTechFan96]

I have replaced coupling capacitor C83 and there is still DC on RH speaker terminals. I have removed and checked the final output transistors and they thankfully test good.

I might be making incorrect measurements as I forgot that they need to be with respect to positive chassis. 25V is the HT for this set so I might be unwittingly measuring it via the speaker terminals? I've been caught out by this before.

I do have a full service manual, but I don't think I'm able to reproduce any portion of it.

[crackle]

I am sorry "coupling capacitor C83" means nothing to me as the service manual is not available in any of my normal sources. Where is this in the circuit?
You can reproduce sections of the service manual as you wish on this forum as it is not available on Paul's site.
But to help clarity I would keep it to just the relevant sections. Or if you can, scan it and upload it as a pdf file. It may be useful to someone else in future.
Are you checking for DC on the speaker terminals with a load connected or open circuit.
Mike

[OldTechFan96]

I have uploaded part of the circuit.

C83 is listed as a 'loudspeaker coupling capacitor'.

When I'm measuring across the speaker terminals, I'm doing so without the speaker connected.

[frsimen]

You should measure 2V DC across the speaker terminals with this amplifier when the speaker is disconnected. The amplifier uses a bootstrap arrangement via R79 and R114/the loudspeaker, which will result in some DC appearing on the terminals. With the speaker connected, the voltage will drop to more like 200mV.

The important voltage to measure is the midpoint voltage at the junction of R80, R81 and C83. That should be close to 10.7v, using the chassis (positive supply) as the reference point. If it is, the DC conditions in the amplifier are correct. If not, check that the heatsink isn't getting hot before doing anything else. If you can, check the voltage on the emitter of VT9 as well, it should be 14.75 volts.

With DC coupled amplifiers, a fault anywhere in the circuit will throw the voltages out everywhere which makes them difficult to repair. The safest approach, is to check resistances with the power off. Compare them with the values in the good channel. That should allow you to narrow down on the area of trouble.

Paula

[crackle]

OK then, yes C83 was the capacitor I was referring to earlier. So that is changed for a new one.
I may be wrong but you could get a more meaningful measurement if you put a 100R load on the speaker socket.
Voltage measurements are normally made between the common ground or chassis, and the test point. However there are voltage readings given on the diagram which are measured between the base and emitter of the final OP transistors.
I think my next stage would be measure the voltages and check against the values given on the diagram and report any anomalies.

Just a note of interest I have always found Callins Capacitors to be as unreliable as the old fashioned wax paper ones. I have often seen many of their plastic radial types where the plastic case has popped off, leaving the guts exposed.

crossed with Paula's post.
Mike

[OldTechFan96]

Thanks both for the replies! I have been working through your questions.

frsimen:

With the speaker disconnected 14.5V was measured across the RH speaker terminals. Other measurements suggest that this figure is fluctuating.

The mid-point voltage is 15V.

14.6V at the E of VT9.

crackle:

I have made a spreadsheet of the voltages present at each transistor.

Things are OK until VT10. The B and E voltages are off. Also, only 0.5V is measured across R75. This should be 5V.

How should I proceed? What conclusions can be made from these voltage measurements? I've brushed up on transistor theory recently but no book seems to go into interpreting voltage measurements!

[crackle]

To me it looks like VT11 is faulty, with the power off and meter on diode setting, check the readings between all its pins, and compare them to the other channel.
I am not too good at analysing transistor voltages but maybe someone with more experience can advise better.

Mike

[frsimen]

You are only interested in VT9 to VT13 as VT7 and VT8 are not DC coupled to the output stage, so won’t affect the voltages. VT9 appears to be working correctly, so can be ignored too.

Of the other measurements, nothing much seems to be correct. I’m not sure how you are measuring some of these voltages, they should all be referenced to the chassis/positive rail. The collector of VT12 should be at around 23.8 volts, the same as VT11’s emitter, as they are connected together. The base of VT12 should be the same potential as the collector of VT11 as they too are connected together.
The voltages around VT11, while wrong don’t particularly indicate anything is wrong there.

The potential across the loudspeaker terminals is way off target, so something is clearly wrong there. Is R114 connected and the correct value and is C83 wired in the correct way around?

What is odd is that the emitter voltage of VT10 is lower than the midpoint voltage. It should be higher, and the only way that it can be lower is if current is flowing through C81. Bearing in mind Mike’s (Crackle’s) comments yesterday regarding Callins capacitors, C81 perhaps should be changed first of all.

If changing C81 doesn’t get things going again, I suggest that the safest way to proceed from here is to measure the resistance of the following resistors in circuit (don’t disconnect them to measure).

R67, R72, R73, R78, R79, R80, R81 and R114.

Measure them both ways around and record the results. Compare the measurements with the corresponding resistors in the other channel.

Do the same for the transistors, initially use the diode test range, then repeat on a higher range. Again, compare with the other channel. Don’t worry about a 10% difference in readings between channels but a much larger difference should ring alarm bells. Also, carry out all the measurements before disconnecting anything. There may be more than one fault.

Once all measurements have been made, home in on the area where there is a big difference between the working channel and the bad one.

Paula

[OldTechFan96]

I've changed C81 and this has brought about a change in DC conditions. The voltages have been recorded below. The mid-point voltage is now 20V.

R114 and R115 have been omitted in my HMV. They are just wire. C83 is fitted correctly.

R67, R72, R73, R78, R79, R80, R81 and R114 all measure OK and their LHS counterparts are close.

VT10, 11,12,13 have all been tested and are OK.

I think we are making a little bit of progress.

[dazzlevision]

In case it is of help/interest, here is my list of production mods to this chassis (the service manual for the HMV 2152 also covers the Ferguson 205STA).

I have three Ferguson 205STAs and they all have different "Schedule" letters (Ferguson/Thorn use the Schedule letter to indicate the mod state of the chassis).

[frsimen]

C81 must have been quite leaky for the voltages to change this much. They look even worse now.

The problem seems to be that if R114 isn't fitted, the amplifier won't work properly without the loudspeaker being connected. That could explain the very odd voltages. You need to add R114 to prevent future trouble when the loudspeaker is disconnected. That should restore voltages to nearer the correct level. Do the same for R115 in the other channel, while you are at it.

There is one concern here, though. VT10 has been running with the base junction reverse biased by 7.5 volts. That won't do it much good, so in the interests of reliability it needs to be replaced. Any small signal PNP transistor will work in that position. Something like a BC558 or a 2N3906 would be a good choice. You will need to change the corresponding transistor in the other channel too if you have run that channel with the loudspeaker disconnected.

Paula

[crackle]

Originally Posted by OldTechFan96

R114 and R115 have been omitted in my HMV. They are just wire.

If those 2 resistors have been replaced with wire then wont the speaker OP be shorted to ground.
Later in same post you mention R114 is OK.

Mike

[frsimen]

Dazzlevision - Thank you for the information on changes to the chassis used in this set. It's odd that R114/R115 aren't mentioned in that.

Crackle - Yes, if the resistors had been replaced with wire links then the speaker output will be shorted, as you say Mike, so that can't be right.

The wire links are not in place of R114/R115. I don't know what they are for, but they should be left alone. R114/R115 are additional resistors, not fitted in your unit. Looking at the RH channel, R114 completes the DC feed to the base of VT13, via R79, and the through the bias diode connected transistor to the collector of VT11. Without R114 or a loudspeaker, the driver transistor's collector doesn't connect to the positive side of the supply and the whole amplifier will tend to float towards the negative supply if the loudspeaker is disconnected.

You have been testing without the loudspeaker being connected and have been measuring high voltages on the loudspeaker side of C83, which are caused by R114 not being present. Add R114 and things should look a lot more sensible.

The same is true for the other channel too.



Paula

[Station X]

I don't know whether it has any bearing on the matter, but when I repaired a Cambridge P50 last year some low value resistors were simply wire links.

[crackle]

100 ohms is not that low and in the position shown it would be disastrous to replace it with a wire link.
I think it would be safer to add R114 and R115 back as per the circuit as Paula says .
Mike

[Station X]

I was thinking more or R80 and R81 which appear to be 1 ohm. They should be easy to spot.

[OldTechFan96]

R114 and R115 were not included in my model. It looks to be an early example from the middle of 1967. S/N 1276. I could retrofit them in due course.

I have fitted the RH speaker and there is now output from the right channel. The mid-point voltage is now 11.5V for the RHC. 160mV is measured across each speaker terminal.

It is hard to asses the performance of the amplifier as the right speaker makes a crunching sound a the cone moves. When I gently push down on the cone there is a crunching noise. Could be the voice coil rubbing some debris? How does the dust cap come off?

[dazzlevision]

The speaker (made by EMI?) looks to be date stamped Nov 1967.

If the speaker cone’s dust seal is still intact, then it’s likely the voice coil former is rubbing against the magnet’s pole piece. Alternatively, the outer flexible ring of soft plastic material could be disintegrating - a common problem with some makes/vintages of loudspeaker.