Chapman VHF-FM tuner-amplifier

[Phil G4SPZ]

My colleague Len Timbrell and I have just finished overhauling this system, which belongs to a local museum recently created within a disused industrial building. It consists of two chassis housed in a free-standing 19” rack cabinet, the upper chassis being a valved mono VHF-FM receiver with manual tuning covering 88-100 MHz and a separate mains PSU, and the lower chassis housing a 35-watt solid-state audio amplifier with 100 volt line output. It arrived with a switched microphone for voice announcements, together with several lengths of chopped-off multi-core cabling, and one public address loudspeaker. A search for service information drew a blank. The subsequent repair process has been lengthy, but very much a collaborative effort on a relatively unusual hybrid system.

On first inspection, the mains transformer in the tuner power supply was obviously burnt out, due to the 32+32uF reservoir/smoother being physically and electrically leaky plus the EZ80 rectifier having an internal anode to cathode short. Len’s junk box yielded a suitable transformer with adequate heater winding rating (it provides around 2.6 amps for the seven valves) and I came up with an NOS electrolytic and a new EZ80. After a little metalwork and component replacement, the power supply was back in working order.

The FM tuner seemed to be a conventional variable-capacitor tuned VHF superhet, the valve line-up being EF85 (RF amplifier), ECF80 (frequency changer), EF80 (IF amplifier), ECF80 (audio triode preamp and pentode output), EB91 (discriminator diodes) and EM84 (tuning indicator). A glance below the chassis suggested that I needed to replace a couple of cracked 0.1uF black Hunts Mouldseal capacitors, a leaking 16uF 350V decoupling electrolytic, and the 8uF electrolytic in the discriminator circuit which would probably be dried out. I replaced the four capacitors, connected up a pair of high-resistance headphones to the output and a length of wire as an aerial, and switched on... nothing. The valves all lit up, the magic eye worked, and the HT and LT voltages seemed OK at 260 volts and 6.3 volts, so I resorted to valve swapping. I changed the ECF80 frequency changer and the tuner burst into life. A second new ECF80 improved the audio output. The tuner now seemed pretty sensitive, pulling in stations all across the band in roughly the right places on the dial with just a bit of wire for an aerial. The audio quality sounded OK on the headphones, but I rigged up a decent test amplifier and speaker which confirmed that the audio quality was excellent. I didn’t attempt any realignment but left the tuner on soak test. The frequency changer ECF80 seemed to run a bit hot after a while, so I had another good look inside the chassis and took voltage readings on anodes and grids, but all seemed well within ratings and the associated components all checked out OK, so I left it on test for the rest of the day and it performed perfectly. The new power supply transformer ran just warm, HT was stable at around 265 volts and the total HT current was around 35mA.

Flushed with success, I then had a go at tracing out some of the multi-core wiring from the amplifier section to the microphone, and found another problem. Behind the mains on-off switch there was a separate little rectifier PSU housing a bridge rectifier, dropper, Zener diode and smoothing capacitors. This PSU board, its outputs helpfully marked “+12” and “+50”, seemed to power the microphone preamp and the volume/tone control boards at the front of the lower chassis, and was fed with AC from a separate, low-voltage single-ended winding on the main audio amplifier’s mains transformer. Both the 500uF low voltage smoothing capacitors were leaking their innards, so they were replaced and the correct voltage supply rails were restored. Len noticed that the neon in the illuminated rocker switch had failed, but I managed to find a neon small enough to fit in the space, and Len persevered and replaced the neon lamp and series resistor whilst retaining the original switch.

I then had a closer look at the amplifier. I pulled out all the plug-in modules and had a good look at the interconnections, and after a lot of fiddling around it all began to make sense. The microphone input multi-core cable went to the mic preamp board, and its switching cores to the volume/tone control board, on which there was a relay with a 48 volt coil and two pairs of change-over contacts. I assumed that this relay was used to change over between programme source and mic announcements. The mic preamp board amplifies the signal arriving along a balanced screened cable from the low impedance mic, and passes it to the relay on the tone control board. The input from the FM tuner comes into the volume/tone control board and passes to the other contact on the relay. The common output from the relay goes to the main audio amplifier input on the rear of the chassis. The mic preamp and volume/tone control boards are powered by the secondary 12 volt DC supply, and the relay has a 48 volt coil fed from the secondary 50 volt DC supply. The relay coil is switched by grounding the negative side, by the push-to-talk switch on the desk mic. A separate pair of contacts on the relay send 50 volts to the desk mic to light the lamp. The existing bulb was a 12 volt one which had, of course, blown, fed with 50 volts. I presume the manufacturer used such a high voltage because the mic could be situated hundreds of yards away from the amplifier rack. As 48 volt bulbs are rare and expensive, a 560 ohm 5 watt resistor was wired in series and a standard 6 volt 100mA lamp fitted instead. I traced the multi-core wiring coming from the amplifier and identified the core colours, and how they mated up with the cores from the desk mic. I also found that the monitor board was just a simple passive monitor with 2” speaker, audio transformer and volume control fed direct from the main amp speaker output.

I eventually lashed up a 100 volt line PA speaker into the top and bottom holes of the 4-pin Bulgin socket and switched on. The solid state amplifier powered up with a very loud "squawk" for half a second, which made me jump at first, but it settled down immediately and was virtually silent thereafter. Grounding the brown wire to the black wire changed over the relays and enabled the mic circuit... I got a nice hum when I touched the red or black mic input cores. Len later connected up the mic properly and was rewarded by a howl of deafening feedback, so we were confident that the mic and its associated switching were working correctly.

Today we assembled both chassis into the cabinet, reconnected the tuner, tidied up the wiring, fitted some proper earthing cables, Megger tested the whole thing and switched on. The main tone controls worked, but the volume control was very noisy and the audio from the tuner sounded strangled, whilst the signal from the mic was crystal clear. I cleaned the volume control, which reduced the crackle but the severe distortion persisted... the symptom sounded very much like incorrect biasing due to DC leakage through a capacitor somewhere, so I tried checking a few of the electrolytics on the volume and tone control board. The first one I found leaky was 10uF between the emitter of TR1 and the volume control, so I replaced that with the nearest I had to hand (22uF) but with no improvement. Rather than replace all the electrolytics, I tried tracing out the circuit looking for likely suspects.

There are four BC108 transistors on the board, and I identified two electrolytics that coupled between the collector of one transistor and the base of the next. I unhooked one end of each and, as luck would have it, one of them tested perfect and the other, 10uF between TR3 and TR4, measured leaky at 30k. In went another new 22uF and hey presto, lots of lovely loud and undistorted sound!

A full day’s soak test is scheduled for next week, after which the set will be returned to its owners and reinstalled for use and public display. Some sketches of the circuit and wiring connections have been placed inside the cabinet for the benefit of future repairers.

"Phil and Len, the Radio Men"

[Wellington]

Good job! Very good write up as well. What would such a thing have been used for - a factory PA system? I remember one at a company I worked for - they would page people across the site (seems rather quaint in light of the mobile communication we take for granted now). We didn't get music, though .

[Phil G4SPZ]

Many thanks Wellington, and yes indeed, that's exactly what it was used for. The exact details are lost in the mists of the recent past, but it seems that they played Radio 2 through the speakers all day long for background entertainment, and occasionally interrupted the programme with voice announcements, calling a certain member of staff to the telephone, for instance. This is widely known as a 'Tannoy system', although Tannoy was just one of several manufacturers.

Long after the advent of pagers, mobile phones etc, 'Tannoy' systems continued in use because they provided a service that other systems couldn't... they alerted everyone on site to the fact that a certain person was wanted. I worked for a time at hospitals which used such systems and they were excellent, although potentially annoying to everyone else, which is probably why they faded from the scene.

I expect many younger visitors to this newly-created museum will never have encountered such a system, which is why the owners were so keen to get it going again.

[jay_oldstuff]

That tuner looks very familiar to me, could it be a Jason kit Re-branded could it be from an earlier incarnation of the system that was then rebuilt with a solid state PA stage. very interesting and a fantastic wright up.

Jay

[Phil G4SPZ]

Hello Jay and thank you for the kind words. Do you know, that's exactly what I first thought - that the system had originally been all-valve, but a new transistorised amplifier had been fitted at a later date - but Len disagreed, and as he worked professionally for many years in sound distribution, I didn't like to argue! As far as we can tell, the entire upper valved chassis is of one make, Chapman. It looks like late 1950s to me. However, the lower chassis including the plug-in preamplifier boards is all one make, but different, and looks more like mid-1970s technology...

On the rear of the lower solid state amp chassis there's a label saying "35W AMPLIFIER MK IIA SUPPLIED BY ASTRONIC SER. NO. 3/964" and the modules and PCBs have "Astronic" and various part numbers on them. I could find nothing about Astronic other than they may have been an American company. Again, I'm not convinced that this was part of the original system. Perhaps we'll never know. At least it's working again now.

Phil

[vidjoman]

I built dozens of Jason Kits in the 60's and they were a horizontal chassis and not front to back. They also used an extra chassis as a power supply, but later versions had both on one chassis.
Chapman became part of another company that supplied factory systems - was it Derritron? As a rack system it could have been assembled by almost anybody and often was. I did several in that style, but not with Chapman tuners.

[Nickthedentist]

These people made "Astronics" amplifiers, in Middlesex between 1942 and 1980 apparently:
http://www.radiomuseum.org/dsp_herst...mpany_id=14702

[Phil G4SPZ]

Thanks for that information, Nick. Based on what you found, I've done a bit more searching and found this entry from 1947 in Grace's Guide:

'Astronic' Electronic Equipment, Domestic and Industrial Products. Manufacturers of Broadcast and Television Receiving apparatus. High Fidelity Wire Recording and Transcription Equipment. Audio amplifiers, Audio and Power Transformers and Component Parts. Re-Broadcast Receivers and High Power Amplifiers for Re-Diffusion Service.

So that definitely appears to be the firm. Sadly they're no longer in business so they won't be honouring any guarantees But it has to be said that, following the replacement of a handful of leaky electrolytic capacitors, the amplifier is still working well at least 40 years after it was made. That's not bad by today's standards.

[AC/HL]

Does this bear any resemblance to the amplifier PCB?

[Synchrodyne]

The Chapman FM tuner at interest does not correspond to any of the regular models shown in the Chapman literature on hand from the later 1950s and 1960s. I wonder if it was a “special” made for only PA applications. It has a similar tuning scale to that of the solid-state FM1000, but otherwise it looks as if it might have been a variably-tuned adaptation of the valved FM90, which was a switch-tuned unit. Chapman did become part of Derritron by the early 1960s, although I do not the exact details of the amalgamation, takeover, or whatever. Jason was a competitor in the radio tuner business, with its Jason and Jasonkit lines, separate but overlapping. I think that some of its switch-tuned FM models did have a front-to-back layout.

Cheers,

[Phil G4SPZ]

Originally Posted by AC/HL

Does this bear any resemblance to the amplifier PCB?

It certainly does, Bill. I've printed out a copy of the PCB layout and will have a good look at the original when we soak test it this coming Thursday. Do you have any more information, such as the rest of the component list or a circuit diagram? We have not done any remedial work on this PCB.

Originally Posted by Synchrodyne

...it looks as if it might have been a variably-tuned adaptation of the valved FM90...

They certainly share a common parentage, and the chassis layout is virtually identical apart from the dual-gang variable capacitor occupying the space where the three presets sit on the FM90. The valve line-up is slightly different, 'ours' having two ECF80s and one EF80 (plus an EM84) as opposed to the FM90's single ECF80 and two EF80s. The small engraved Traffolite label below the tuner says "FM Radio Tuner SH491".

I am very grateful for all the information that Forum members have provided, which confirms if nothing else that the unit is relatively rare and, in combination with the solid state PA, probably unique.

Phil

[AC/HL]

As I said in the earlier thread, we used modules like those when I worked for Plessey many years ago. The attached is all I have about the 35W amp, but there was a 75W version which looked just the same as yours, hence the deduction. I'm afraid there are still no clues as to the Manufacturer, but there is a date. Sorry for the patchwork, I only have a domestic scanner.

[AC/HL]

Part two.

[Phil G4SPZ]

Bill,

That's fantastically useful, thank you very much for going to all the trouble to scan the original drawing. I have printed out the individual sheets and they have aligned perfectly.

I never cease to be amazed by the resources and helpfulness offered by the members of this Forum!

[Synchrodyne]

Originally Posted by Phil G4SPZ

They certainly share a common parentage, and the chassis layout is virtually identical apart from the dual-gang variable capacitor occupying the space where the three presets sit on the FM90. The valve line-up is slightly different, 'ours' having two ECF80s and one EF80 (plus an EM84) as opposed to the FM90's single ECF80 and two EF80s. The small engraved Traffolite label below the tuner says "FM Radio Tuner SH491".

Possibly Chapman swapped the 2nd EF80 of the FM90 for an ECF80, thereby adding a triode for use as an AF output/buffer stage. The addition of the EM84, which Chapman was using elsewhere in its range, was logical for a variably tuned unit. Also, it looks as if yours retained the AFC from the FM90 (using the triode in the 1st ECF80 as reactance modulator; the self-oscillating mixer used the pentode), again logical for a PA unit that would be expected to be tuned at start-up and then left to its own devices for long periods. Chapman did not include AFC on its variably tuned domestic tuners, though.

Cheers,

[Phil G4SPZ]

Yes, that sounds a reasonable supposition. I can confirm that the AFC is effective; on a four-hour soak test yesterday, drift was undetectable and no re-tuning was necessary.

For your information, Bill, the 35 watt amplifier appears identical to the one described on your drawings

The soak test was otherwise uneventful and the system will go back into service at the Coffin Works museum before Christmas. Thanks to everyone for their interest in this thread and also for information supplied