Monday, 31 May 2021

Technics ST-CH7L (CH-7 system). Won't come out of standby.

There I was, minding my own business having a walk at lunchtime along the side of the Avon river, and I bump into a guy I haven't seen for the best part of 15 years. Mark the aerial guy, which is very helpful as I need some aerials fitted on the top of the workshop. Excellent timing.

A couple of weeks later, Mark arrives and sort the new aerials as arranged. He does a superb job as always.

"I've got this Technics hi-fi which won't stay on... could you have a look for me?"

Yeah .... why not?

It's a CH-7 system. Sort of faux-separates where each unit is connected by a ribbon cable to the main amplifier unit. This one refuses to come out of standby. Press the power button, it comes on and instantly off again... 

Eventually it does come on, but the tuner is displaying rubbish... 

A couple more power cycles and it settles down, and works correctly.

You may also notice the display on the amp is all foggy..

The tuner contains all the control circuitry, so I'm going to focus on that first. 

Five screws (two on each side, one on the back) allow the top to be removed. We also need to get the front panel off, held on by two screws underneath.

The screws are removed that secure the PCB to the base... 

The more observant of you will notice the transformer.... surely technics didn't put a mains supply on the ribbon cable? Panic not. This transformer has a 24V primary!

...and the back panel removed to we can remove the main PCB...

R704 looks to have had a hard life. It checks out ok though...

... unlike some of the soldering on the board, which has started to crack up, it's re-worked.

The thing is temporarily reassembled to see if the poor solder is the cause of our issue...

Hopes of a quick and easy fix now fade, it's still the same.

A quick look at the diagram shows there are two main rails, 14.8V , regulated by Q703, and 6.2V , regulated by Q701. 

The 14.8 volts reads good, but not so the 6.2V... it's low at around 4V during the fault condition, but rises up after we have a few goes at getting the thing out of standby.  R701 & 703 check out ok, C704 looks ok on initial inspection, but when removed for testing smells very nasty ! It's job is to stabilise the base voltage on our pass transistor. It's electrically and physically leaky.. C702 measured a little high on ESR, so it's swapped out...

Bingo, the unit is now functioning reliably.

Now to sort out the foggy display on the amp.. 

The top cover is removed, and the front panel in the same manner as the other unit. 

The visible screws are removed from the rear of the front panel PCB. The front panel is made up from two PCB's, we only need to remove the top one.

Pull off the knobs, and remove the 12mm back nuts on both controls.

The pcb is pushed out from the front by pushing on the control shafts. It's got a sort of haze over the vacuum florescent display...

... and the filter in front of it...

... which is easily removed with a spot of window cleaner and a microfibre cloth... 

... and once reassembled, a nice bright display is restored :)

Another saved from landfill !

The guilty parties...

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
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...

Sunday, 30 May 2021

Leak Stereo 30 Plus

A friend of a friend, Ian, got in touch ...

"I've got some Leak kit that could do with servicing. Could this be of interest?"

I like Leak kit.

Yeah.. why not?

He's dropped off a couple of amplifiers (this one, and a Stereo 20), and a pre-amp (a Varislope 2, to match the Stereo 20).

First up , the Leak Stereo 30.

This was Leak's upgrade to the Stereo 30, which was an all germanium transistor affair, this is all Silicon. There's a pair of 2N3055's or equivalent in the output stage, developing around 15 watts per channel into 8 Ohms.

Opening it up , and it's obviously had a life! Lots of sad old electrolytics, in desperate need of replacement. Many leaking.

The main smoothing capacitor, has obviously been replaced in times gone by... The replacement was  somewhat smaller than the clamp would take, so was wrapped in tape and stuffed into a cardboard tube! Even then it wasn't tight, and was wobbling about!


The mains switch has failed, and has been bypassed with a bit of choc-block. 


The amplifier is divided into a chassis, which contains the bigger capacitors and mains transformer and output transistors, and 4 plug in PCB's... two pre-amps and two "power amp" boards, although the output transistors are mounted externally.

Here are the power amp modules. The left hand board is recapped and ready to go.. on with the right!

 ... and the pre-amp boards, this time the one on the right is done.

As the amplifier is a single-rail design, the output stage sits at "half-rail" when idle, the speakers are coupled to the output stage via a 1000uF coupling capacitor. It's imperative they are in tip top condition to prevent damage to the amplifier or speakers. They are both physically and electrically leaking.
The first one is removed, and has it's gizzards removed! 

Sadly the fibre top broke up, limiting our options a bit for re-stuffing. I scratched by head for a couple of minutes, and then invented a solution!! 

Ladies and gentlemen (and non-binary persons), may I present ... the Cap-cap (TM)
Empty cap, sans lid, ready for re-stuffing.

New capacitor fitted to Cap-cap (TM) ...

... and the Cap-cap (TM) and cap, fitted to the old cap. I hope that's clear !

Get the model from . It currently only fits 1" capacitor cans, I may make other sizes as I need.

Very smart.

So, back to our "cardboard tube" capacitor. There seems little point in restuffing it, as it doesn't fit anyway. Back to the 3D printer, and an adaptor is made..

... which snugly holds the new capacitor into the clamp. Neat.

With the capacitor replacement complete, all the pots and switches are cleaned up, and it's time to apply some power and see what happens. 

The two red wires to the top transistors are disconnected and multimeters inserted into the circuit to measure the quiescent current. 

The bias controls, which set the quiescent current are located on the two power amp PCBs.
Well, the bias is all over the place, and isn't stable. The two pots are cleaned up, and ... hang on a minute... 

... that bias pot... why is there solder on it? Someone's obviously attempted to make the contact a bit better by soldering the top spring of the wiper. Nasty. Really nasty.

After replacement of the pots with some nice multi-turn jobs, the bias is set and is nice and steady.

It sounds good, but the balance is miles off, the balance pot needs to be set to the right for it to be central. Also as the balance control is rotated more to the right, the left channel starts coming up again, but horribly distorted. 

The balance control is disconnected and measured, and the resistance values horribly non-linear over the movement on one channel. It's a stereo 20K linear pot. Easily available, and duly replaced.

... not so the broken mains switch section of the volume control. After a fruitless search of the internet for a suitable replacement, a few telephone calls are made, and I manage to secure a new-old-stock leak delta control, which will do nicely.

The amp is now done, and performs wonderfully. Should be good for another 20 years :)

Sony TC-161SD repairs.

Colin phoned

"My mate Susie's got a Sony Amplifier only working on one channel, care to take a look?"

Yeah... why not.

It's a Sony TA-1055. 

It was the world's easiest fix. A noisy tape monitor switch, which, after a few operations put itself right.

Susie also has a Sony TC-161 SD cassette deck. 

It's a top loading deck dating from 1972. 

This one doesn't play or record... Let's have a look...

We need to get in from the bottom. There are 6 screws, some recessed, in the bottom panel. This enables us to remove (after a bit of wiggling) the plastic bottom, and wooden case in one go.

This machine is lacking take up, but rewind and fast-forward work after a fashion, although at a somewhat leisurely pace! The fault's are bound to be mechanical.

The machine is able to detect movement of the tape using a small hall sensor, located on the pulley which is fed from the reels, and eventually leads off to the tape counter. If the hall detector stops seeing tape movement, it engages a solenoid, which stops the mechanism. This is how the autostop mechanism works.

Whilst the belts aren't in great condition, they are working, but are replaced anyway. The small idlers (there are three, one take up, one rewind and one fast-forward) are cleaned up with rubber rejuvenator, but are in remarkable condition. 

Now we have great rewind and fast-forward, but still no take up, which the autostop mechanism detects and stops the deck before our cassette is a mangled mess. The take up idler is not engaging. 

Sadly, it's rather buried under one of the capstans (This is a two capstan deck). You can just see it beneath the large grey capstan flywheel. 

Some disassembly is required...  

Removal of the capstan and flywheel gives us good access. 

The idler swivels into position, and tension is applied by a spring to engage it between the capstan and the take up clutch.. except it can't as the joint in the arm has seized solid. A little application of localised hot air from the SMD hot air soldering station frees it up, and when it's cool, a spot of oil is added. It now moves freely.

With everything reassembled, we now have all functions working... 

... except listening to the output, we've got very weak audio, and occasionally oscillation. Putting a blank tape in the machine and working the record button several times clears the fault. It's a tarnished record/play switch. A quick squirt of servisol helps. OK, we've now got playback but no recording. I can *just* hear audio, but it's very faint. Just as I'm beginning to suspect the record-play switch again, I switch Dolby off. Up comes record audio. Switch it back on and it's a bit intermittent. Another quick squirt of servisol to the dolby switch and all is well. I've also done the limiter, and tape selector switches as well as the record volume controls, just to make sure.