Audio Innovations Series 300 repair

Dave the Quad phoned...

"Got an Audio Innovations here, it's cracking in one channel. Can you take a look?"

Yeah .... why not?

It's a Series 300 amplifier from the late 80's early 90's. Not sure I care for it's styling to be honest ... anyway...

Off with the top, and there's a nicely laid out PCB, featuring solid state rectification, an ECC83 phono pre-amp, ECC83 pre-amps, and four triode-pentode ECL86's (the two triodes in each output pair forming the phase-splitter)

Connecting a set of speakers, and waiting for everything to warm up, soon reveals the fault. 

The third ECL86's tube base is flashing over... This can be indicative of output transformer failure, a failure in the valve itself, or the amplifier having been run with the speakers disconnected. Once a tube base has started to flash over, no amount of cleaning will sort it reliably. We'll change all four to be on the safe side. Let's hope the transformer is OK. Try as I might to get a photo of it... I couldn't.

Disassembly is a bit of a pain ... 

First off, check that reservoir capacitor has discharged to a low voltage. This particular amp rapidly sinks to a safe 14V or so after a few minutes off... but check it anyway!

Remove the knobs. They're secured to the shafts of the controls by a small grub screw with a hex head.

Unsolder the wiring to the gain control. A red, white and black wire. I wonder why this network is suspended like this? Added after to remove a bit of DC from the pot perhaps?

Undo the nuts securing the controls to the front panel, and gently lift clear. 

There are three M3 nuts, and a few plastic stand offs which must be released to allow access to the print side of the PCB. I removed the valves. Keep the ECL86's in the same order that they were removed.

I gave the PCB a blanket re-solder as a good few of the joints looked suspect, and replaced the four output valve holders.

Once everything's reassembled, the amp is tested, and, thankfully, the output transformer is OK. 

A lovely sounding amp, and absolutely silent.. no hum and almost no blow. Superb. 

It's soak tested with Matt Berry - The Blue Elephant.

Another saved from landfill! 

Leak Varislope II.

Ian's last piece of kit is a Varislope 2 (technically a late Varislope II Stereo) to match the Stereo 20.

On thing I find a little odd about the Varislope, is it's use of EF86 pentodes instead of, say ECC83's.... anyway. 

One EF86 has been changed in the past. If Ian can detect one side souds a little "different" to the other, I'll find 2 new matching valves.

It's chock full of horrible, sticky visconal capacitors. On with the gloves!

One cap has been replaced in the past, and it's the same vintage to those in the Stereo 20, 1979.

The preamp consists of an EF86 gain stage, followed by a Baxandall tone control, and another EF86 stage, where you can vary the response of the tone controls, and negative feedback.

After the capacitors are evicted, I check through all the resistors. Unbelievably, only two cathode resistors are outside of specification. 

Studying the circuit further, it appears the EF86's are run close to starvation mode at around 250uA, great for gain, not so great for distortion. 

Does this matter? Nah, on test it sounds great... :)

Leak Stereo 20 Repairs and restoration.

Next up, Ian's Stereo 20

It's a slightly later "chocolate" coloured version (earlier ones where gold) and develops 20 watts from a pair of EL84's (6BQ5) driven from an all ECC-83 (12AX7) layout of preamp and phase-splitters. Power is provided from a GZ32 rectifier.

Dating from 1962, it's nicely laid out, and easy to work on. Looks like it's had some repairs in it's life, especially to the left hand channel (top in this picture) ...

Every capacitor is changed out for new, even though the two cathode bypass capacitors in the pre-amp had been changed some time in '97 accroding to the date codes. The main coupling capacitors were dated 1979. 

A good number of resistors were outside of tolerance, or not to the schematic, and were replaced. If something was out on the left channel (of which there were many), the corresponding part was replaced on the right, so as to avoid upsetting the balance between channels. 

The main cans are re-stuffed with modern 32uF components, and are mounted back using a 3D printed plastic cap.

Whilst the actual foil and (dried out) electrolytic matting were very easy to remove, not so the black sealing goo, which I presume is some kind of bitumen. Leaving the empty cans in the freezer for a few hours (whilst Mrs. Doz was absent!) hardened it, and it was easy to chip off. 

The modern components were a tight fit.

Finished of with a new Cap-cap TM ;)

You can get the files from https://www.thingiverse.com/thing:4872632

Once everything is done, the unit is powered up, and gives reasonable results, albeit down on power. HT is good, but one side has mismatched valves, and the other side, well, they are just tired. Ian told me that he'd borrowed a valve out of his other amp, as one had gone to air. It really does deserve a new set of output valves. 

The guilty parties...

Heathkit Stereo control unit USC-1

Happy New Year! 

Having sorted the MA-12's, time to move onto to the USC-1 preamp...

It's designed to be mounted into the front of a cabinet of some description. 

They keen-eyed amongst you may notice the absence of mains transformer. The power supply is derived from the MA12 amplifier via a multi-pin lead. 

It's a reasonable layout, but the chassis edges are sharp. There's two PCBs, each containing Two EF86's and an ECC83. The EF86's produce two diferent levels of gain, one setting for TAPE1/2, GRAM and MIC, and the second stage conntecting the inputs from RADIO and AUX. There's no real RIAA equalisation on the GRAM but a little bit of loading. The second EF86 feeds a tone stack, which is then amplified by one half of the ECC83, followed by the other. There's also a low pass filter implemented here. It's full of horrible Hunts... 

The base is removed to gain access to the solder side.

... and a good while spent changing all those nasty caps out...

I chose not to re-stuff the can on this occasion, as there's a nice bit of tag strip that forms the power supply filter, so they could just be mounted there ...

At some stage in the past, the volume control has been replaced, for one without a switch. It's also been wired up backwards. It's now wired up correctly, so as not to deafen the unsuspecting user.

How's it sound? I thought there would be more "blow" due to the EF86's cascaded, but it's reasonably quiet. Obviously the TAPE setting is nothing more than a head amplifier, so not suitable for a line level output from a modern deck.  Pity about the RIAA, or lack thereof...*  Tone controls are actually very nice, and don't offer too much cut or boost. 

The switch-gear is lovely, and has stood the test of time really well. 

*upon reading the manual, it transpires the input stage was designed for a Decca FFSS cartridge, the manual does give some suggestions about use with other carts. 

Heathkit MA-12 Hifi amplifier restoration.

Neil called ...

"Got a load of old 'eathkit gear here , could use the once-over... Care to have a look?"

Yeah... why not?

There's four MA-12 amplifiers, two USC-1 "Stereo control units" (that's a pre-amp to you and I), and an AMF1 tuner ...

The amps are in various states of repair ...

Including one with a valve down to air ...

It's a simple push-pull amp, with an EF86 front end, an ECC83 (12AX7) phase splitter and a pair of EL84's wired ultra-linear. Hmmm , we've seen that before, and looking at the schematic, it's a Mullard 5-10 in disguise..

Off with the bottom ... 

Nice tag-strip construction, and, as these were sold as kits, the assembly looks OK.. some of these can be a disaster area.

Obvious faults... it's full of Hunts capacitors (evict on sight) , and the electrolytics are, well.. past it.

First things first, get that can out, and re-stuff it with nice modern parts ....

The cap is simply removed by gently prying it out with a screwdriver, and saving it.

Next drop the whole can into a container of boiling water for about 15 mins...

Wind a screw into the capacitor's innards ... 

and extract! Probably best wearing some gloves whilst you're doing this bit, I don't know what chemicals are in there ... 

This can is a dual 250uF, 60uF , 350V device. I've got a modern 220uF and a 68uF to go in there, so this is assembled and wired... 

.... and slid back into the can. The cap is sealed on with a little RTV.

The other can is a dual 50+50uF. As it's rubber cap is visible, we want to leave that intact. 

So the bottom bracket is warmed up slightly, and pried off.. 

and the bottom but off with a small pipe cutter... 

It's stuffed with two 47uF caps. They're slightly different shapes so they fit inside the can. 

The base is then sealed up with a splodge of hot-melt. 

The Hunt's are evicted and replaced with some nice polys. 

The two cathode bypass electrolytics (22uF, 25V) are replaced.

Testing shows the output is weak (our valves may be old), and unacceptable distortion...

Attention is turned to the output stage ... 

DC conditions on the two valves should be closely matched... they're not.

The two cathode resistors R21 and R22 have risen significantly in value, one to more that 3 times it's value, they're swapped out. R23 and R24, the screen gird resistors, are also out of spec. Changing this lot puts matters right. 

Now to do the other three! ....

Ekco U353 - a quick restoration.

Ages ago Steve was good enough to bring up Sam's Queen Anne for some repairs. Also in the boot of Steve's car was a broken Ekco U353. Now, I have one of these, (see here!) which I repaired ages ago. It's a basket case, and remains unused as it's knobs are missing.  The knobs are an essential electrical safety item, as the chassis is connected directly to the neutral of our mains supply (common practice back then).

Anyway here's Steve's...

Much better nick than mine, although there's a crack to the right hand side of the cabinet... 

... and the back's a little distressed where the heat from the rectifier and output valve has made the back brittle, and damage has occurred. Some of the brackets are in poor shape too.










Removal of the chassis is straight forward, off with the knobs (there are two small grub screws to undo), and removal of four screws on the underside of the case.










It all looks very clean and unmolested.












Amongst the dust in the bottom of the cabinet, a tell-tale blob of wax shows something's been getting hot...











... and sure enough ...












... C61 has had a right sweat on! As usual, these waxy caps will all be shot, if not now, in the very near future, and will all be evicted. C61 is the mains input filter cap, so it's replaced with a brand new X2 class safety capacitor. It's 0.01uF.







Now in my example, the output valve had been cooked as the cathode bypass capacitor had failed short, so this is also replaced. In fact there's very few capacitors in the whole receiver, so every electrolytic, paper/wax and hunts capacitor is replaced, except the main multi-section smoothing capacitor.








The cathode bypass capacitor is C55. C50, C49, C46, C1 and C20 are all replaced. C1 is a horrible black hunts thing.






After the caps are replaced, it's time for action. The receiver is connected to the variac, and the voltage slowly increased for about an hour, monitoring the HT voltage, and periodically checking the temperature of the main multi-section smoothing capacitor to see if it's getting warm. It wasn't.

After a quick tune about, stations are coming in, and the FM seems to working too. The receiver is aligned in accordance with the service sheet, and it's performing well, except the exclamation mark magic-eye isn't illuminated :(

R14 (68K) has risen in value to many megohms, and crumbles as I remove it! After it's replaced the exclamation mark works as it should.

The guilty parties!












... and the elusive knobs ...












A piece of car body re-enforcing mesh is cut out to cover the damaged areas of the back...











... and it's filled, drilled and painted. Not the tidiest job in the world, but it'll render the set electrically safe, and allow the required ventilation.

The set's plug is fitted with a more suitable 3A fuse (there's another 13A fuse I've gained!!) and it's reassembled and given a nice long soak test.










Another saved from landfill :)