Analogue Electronic Repairs

Possibly the most expensive potentiometer I possess.

Tech tip for the less experienced. Most decent scopes have a 4mm ground terminal. With this attached to the ground on the equipment under test allows freedom to roam around with the scope probe without constantly having to attach the probe ground. You can also tape the ground clip out of the way to avoid accidental contact with supply rails, especially useful on valve equipment! ⚡️

Holiday time being a relative quiet period for repairs leaves space for construction. Dirty construction at that! FET active RF probe for the audio signal tracer. Now to find a tube to fit it into...

It's been a while since I worked on one of these. Bush VTR103, 3 band portable from the early 60's. All electrolytics and a few out of spec carbon resistors changed. Full realign and all is well. Unusually no faulty tin whisker transistors to deal with. Full strip down, hot water wash of case. Polish with Novus 3 stage plastic polish.

I first used a high volume solder sucker like this in the late 70's. In the UK they were considered a 'posh' USA made tool, but highly effective. Roll on 40+ years they are cheaply available from the far East. The one area where quality control is lacking is around the coupling between top and bottom sections. They can be loose, allowing a secondary air path leading to loss of suction and sometimes the sections will rotate apart during use. A simple fix is a couple of turns of PTFE tape around the top section insert. Also the occasional few drops of Dow 200/50 Silicone fluid or similar help the action.

A 50's vintage Marconi Instruments pulse generator provided these nice octal chassis sockets along with some cool 6L6G type tags and a pair of still strong matching valves. Project?

Levell TG66a audio oscillator. This is my go to unit for general use. I find the stepped attenuator very convenient. During use the 0.1 multiplier range died one me. The unit had the same fault when I bought it several years ago. My guess at that time was switch fault, the fault cleared whilst fault finding which included switch cleaning and moving interconnect wires around. No such luck this time. A bit of digging around revealed Q1 base near short to chassis on 0.1 range, 0.8k on other ranges. This led to the 3 parallel capacitors C8, C9 and C10 - forming the 222nF lower capacitor of the Wein bridge network. C9 turned out to be near short. Replaced, all good - and on frequency, got lucky with replacement capacitor actual value.

Back to the Yamaha MSR250 with the miniature PCB mounted pots. PCB shot due to mechanical damage. Root cause, no mechanical support for pots. Luckily there is enough space to fit conventional miniature pots and an intermediate support plate to provide mechanical strength. A bit fiddly feeding 18 jumper wires through PCB, made easier by cutting each one to slightly different lengths.

Fender Blues Deluxe Reissue. Apparently this valve just shattered. It has obviously suffered excess heating at some point judging by the discoloured print. Comparing the getter of the broken valve to the good it certainly looks like some impact damage. Amp plays with my test 6L6's, there is an occasional 'click' on the output - not present on the PI input. So I suspect something still lurks. In almost 50 year working on valve gear I can't remember a valve explosively breaking like this. Many cracked or overheated with envelope distortion and 'suck in' but never this. Fingers crossed it's not insulation breakdown on OT. Anyway it's 21:30 in the UK, time to call it a day!

In contrast to my recent post regarding a powered Yamaha speaker, this MAZ 8 recently arrived on the bench. Apparently not used for a while, it was making strange noises when last powered. Poor ECC83 has let the vacuum out. Replacement NOS Mullard ECC83, bias check, usual electrical safety checks following damaged mains input socket replacement and it's sounding real nice. Anyone have a circuit for these?

Yamaha MSR250 powered speaker. Ever wondered why consumer electronics are so cheap, here is part of the answer. Potentiometers with no other support than the solder connections to a PCB. All 6 pots have some damage to the frame solder points, 1 gain pots has bent and is now intermittent. The solution will be to fit panel mount pots to a new hidden panel behind the original front. Them wire new pots to PCB.

Sometimes a bit of traditional wax cord lacing helps with cable alignment within a body. Or you could just stuff it in and hope for the best!

A most useful test box. 600 Ohm 1:1 transformer with multiple inputs/outputs. Balanced to balanced, balanced to unbalanced. I use this way more than I thought I would when I built it.

Some of my best friends are guitarist's, just saying...

Ovation OP-PRO came in with the fault ‘runs battery down in less than a week’. Sure enough, current draw around 10mA in off, rising to 30mA when tune activated. Strip down, once again the thermal camera proved it’s worth. C46 (directly across battery) lighting up when power applied. Replaced, quiescent draw now uA. Must remember to take more photos during repairs!

For those who decry the penny pinching construction techniques of today, the 1980s would like a word. HH 5 channel amp, all reverb switches failed due to plastic latching mechanism, all switches pop riveted in! Drill out, replace with modified new switches mounted using machine screws and nyloc nuts.

Icom IC-2725e dual band transceiver, "I think a transistor has gone". Certainly has!

If you are in the UK and need a 2SK3075 driver for this rig message me, I have genuine Toshiba in stock. Send PM.

Some faults are more obvious than others. Splinting up for a track repair tomorrow.

This Fender Deluxe Reverb came in with report of 'breaking up at lower volume than usual'. A quick bench test showed asymmetric distortion at all but the lowest output levels. Tracing through showed the distortion occurring in the output stage, R62 (screen grid) was quickly found to be open circuit. More investigation showed the root cause to be V7 control to screen grid short circuit. Replacing R62 and V7 restored a symmetrical output waveform. This is one of those situation where the often given advice to swap valves around would not really have helped diagnosis, but would most likely have resulted in the destruction of the other screen grid resistor.

Not a technically difficult repair, this post is more about sharing techniques and diagnostic ideas. A Dynacord Powermate 1000 came in with reports of 'noisy'. Once on the bench it quickly became apparent that the fault lay with the right most stereo channels. Advancing the faders on these channels sent the mixer into oscillation - at low fader settings random low frequency noise was present.

These channels share a number of dual and quad op-amps, checking the +/- rails to these proved the negative rail to be at -0.6V rather than -15V. A resistance measurement to ground gave the rather low value of around 5 Ohms, this along with a high resistance resistor points to a failed component. A check of the circuit gives 8 IC's and 4 capacitors as possible candidates, given my luck this could get tedious!

Enter the Fluke infra-red camera. Once a reference scan of the faulty board was made the suspect negative rail was connected to a -15V current limited supply (250mA). Then a second scan taken. This quickly reviled the culprit, a (almost) short circuit 100nF decoupling capacitor, small yellow in the PCB pictures..

This was quickly replaced along the a new 10 Ohm resistor. Noise cured with guesswork eliminated.

B+K 510 transistor tester. I've been after one of these for a while, mainly due to the in circuit capability - not perfect but usable. Cheap off ebay with leaking battery damage to battery holder. However, common 4 cell AA holder and all is well for < £20 (UK). I'm on the lookout for a circuit if anyone has a copy. Nothing detailed in user manual.

I am a big fan of using rechargeable batteries to reduce the risk of equipment damage. However, there are limits! Very old NiCad PP3, Crybaby GCB95. New battery clip and stomp switch.

This isn't Spinal Tap, early VOX T-60. These go to nine. It's one quieter, isn't it?

Mind the gap! The quality control of some low end components leaves a lot to be desired.

Battery backed power supply for a VESDA aspirating smoke detection unit. Thankfully no PCB damage.

Google nest heat links do not appreciate 240VAC into the 12V thermostat terminals! This is around the 10th unit I've had with this user error.

ISO standard frequency display, happy new year! My resolution is 10Hz.

So, after getting my Marconi 2019a signal generator working again, the obvious thing to do is test it against my Racal 9008m modulation meter. Which is obviously when I find it's not working, no lock. 70s Racal test equipment = one thing, tantalum capacitors. Sure enough one obvious failure on the PSU board, along with both reservoir caps. Replaced the blackened tant and smoke contaminated ceramics, still dead. Negative regulated rail is short circuit to ground. Disconnecting the measuring circuit board removes the short. There are a number of 47uF tants on the -ve supply rail, one found to be short, others with slight leakage. I don't often shotgun, but this is one of those cases. All tants removed and replaced. In addition to the short 47uF around 1/3rd are leaking on a quick bench multimeter test ( Solartron 7150 plus). This would possibly increase if tested at rated Voltage. Caps replaced with good quality electrolytics. Now fully stable and in good agreement with sig gen.

The Howling Clowns All Star Christmas Extravaganza Those minstrels of merriment The Howling Clowns present their annual Christmas spectacular. Merry Clownmas & to all, & to all a good Howling!

So this weekend has seen the successful completion of a couple of long standing repairs. 1st this Marconi 2019a signal generator, glad to have this back in my arsenal of test equipment. The 2nd, a Denon DRM800a with intermittent auto stop issues which turned out to be a design fault. More on that one later.