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 Return to top of page · Post #: 1 · Written at 11:27:27 PM on 31 January 2022.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1693

I've been accumulating lots of things to either repair or discard. I've decided to spend one evening a week in a attempt to slowly clear the backlog.

So far I repaired 2 remote controls, a digital alarm clock and a valve radio. The rest of the pile is almost all valve radios that have broken down over the years.

Since there's not many of us left, and there might be someone out there that's interested in repairs, I'll document each week what I fixed and how. I'm not an expert like Fred or Marc, I just do what I can.

So, the remotes. First one is for a Sanyo combo unit. One day the buttons became flaky followed by nothing. Batteries ok. First thing to do is replace the tiny electrolytic that is wired across the batteries. This fixed it. So why does it make a difference? The reason is the batteries don't have enough power by themselves to operate the infra-red emitter. The power stored in the electro provides the extra oomf that is needed. When the electro dries out - the remote dies.

2nd remote, for a Bush DTV/DAB set-top box. Some buttons stopped working, eventually followed by nothing. When opened, liquid (similar to WD40 but odourless), was noticed on the buttons in the middle of the pad. Cleaned this off, remote works again. No idea where the liquid came from. This remote took a long time to get it open - they really don't want people repairing them.

Tandy digital clock. Time would go crazy or just show 12:00 by itself. The usual death of these units is once again dried-out electros in the power supply. This one has 2x 470uF/16V caps, so I thought it was a bit unusual for both to die, and only in a few years. I cheated and wired an extra 820μF 25V cap in parallel with the others. So far, it is working.

Last one is a Radiola 495MA, sound gradually goes low and distorted after a while. Valve line-up = 6BE6, 6AU6, 6BV7, 6X4. Measuring the grid bias gave only about -2 volts, when it should be at least -4. The radio still had wax caps - a mixture of Ducon and UCC. The Ducons seemed to be ok, but UCC caps are poison and must be replaced. Of course they were all leaky. This didn't fix anything though, so it was time for a modification. The main bias resistor is 100 ohm, I replaced it with 150 ohm. This doubled the bias voltage, and as a bonus the HT increased by 15 volts. All this meant the 6BV7, the 6X4 and the power transformer all have less load on them, meaning less watts wasted. This solved the distortion issue. Even though the radio has an inbuilt ferrite antenna, the thing produces hardly any volume. Connecting to an external antenna caused it to roar into life with lots of volume and it works well in that way.

That's all for this week.


 
 Return to top of page · Post #: 2 · Written at 12:42:37 AM on 8 February 2022.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1693

Today I fixed 2 more radios. First was a Ediswan Ekco RM204, otherwise known as the Brown Gondola. It uses the usual 7-pin lineup: 6BE6, 6BA6, 6AV6, 6AQ5, 6X4. The rather strange problem was that the plastic power plug was leaking a green liquid. I don't how that could happen, since it had been sitting in the cupboard for 15 years in a dry spot.

So, first thing was to replace the power cord. Then I noticed a few parts needed replacement too. The tiny 220k 6AV6 load resistor was open circuit, and I replaced the 2 ducon wax caps. The bias on the 6AQ5 should be 12.5 volts, but it was only 7, so modification time again. The 220 ohm cathode resistor was upgraded to 330 ohm, and a small 47μF cap wired across it. This increased the bias to about 10 volts. Next thing was the resistor for the RF screen grids was a sweaty pair of parallel 18K resistors. It was no surprise that the 6BE6 and 6BA6 were much too hot to touch. I replaced the 2 resistors with a single 22K resistor, giving a more reasonable 85 volts for the screens. This also allowed the main HT to increase to 286 volts, rather than 230 as specified in the circuit. The 6AQ5 bias increased to 12 volts. So a couple of mods increased the performance and reduced the power consumption.

The second radio was a Stromberg-Carlson 5A29 fitted with 6BE6, 6BA6, 6AV6, 6V6GTA, 6X5GT. The original 6V6GT had been replaced by the baseless 6V6GTA at some time. The problem with this radio was distortion, and in the dark it could be seen that the glass of the 6V6 was glowing blue. I interpreted this to mean the valve was being driven much too hard.

The fix for this was what you'd expect - the wax capacitor on the 6V6 was leaky as a sieve, putting over +6 volts on the grid. After replacement, the voltage became 0, and the bias was 12 volts - close enough. The blue glow was gone, and no doubt the 6X5 and the power transformer would run much cooler. The radio really should have the remaining wax caps, all the old-fashioned resistors, and the rubber wiring replaced - but I couldn't be bothered.

More next week.


 
 Return to top of page · Post #: 3 · Written at 11:21:31 AM on 8 February 2022.
BringBackTheValve's Gravatar
 Location: Linton, VIC
 Member since 30 December 2016
 Member #: 2028
 Postcount: 352

I have also encountered this green fluid in cables near pin terminals, the last just a few days ago.

Three common factors, so far, are;

a) Plastic flex from the late 50's early 60's (never appears with rubber)

b) Always near outer terminal ends i.e. closer to the plug end rather than the chassis, i.e. outside the equipment environment

c) Copper conductors surface often blackened near offending end.

I'm guessing then that the green ooze is copper oxide formed slowly over time where moist air gradually seeped in between insulation and conductor.

Also wondering if a certain batch of plastics used in that era released corrosive compounds which slowly reacted with copper and air, perhaps acting as a catalyst?


 
 Return to top of page · Post #: 4 · Written at 6:50:18 PM on 8 February 2022.
Kakadumh's Gravatar
 Location: Darlington, WA
 Member since 30 March 2016
 Member #: 1897
 Postcount: 162

I too have experienced that same "ooze" on plastic sheathed cables and almost always on quite OLD PVC cables and I am thinking that it is possibly due to the problems plastic cable makers had back in the early 1980's when some of the plasticisers were somewhat unstable.

Telcom had quite a few issues with the white sheathed cables used to interconnect exchange equipment racks in cables up to 100 pair size and there was fear that the "oozy" stuff was carcinogenic and there was a big push to identify and as much as possible keep the "oozy" cables away from other similar white sheathed cables in the buildings. Never did hear as to whether there were any health issues with the cables but they were quite slimy and if the ooze ever got close to conductors they all turned green.

So most likely is the same problem and possibly it is the PVC around the actual conductor of the cables that is causing the green stuff to start oozing out. Over a period I have encountered about 10-15 cases of 3 core mains cable doing that and it happens at either end and in one case inside an amplifier the green ooze corroded away one of the two rectifier diode legs so it only had a half wave input with one diode effectively open circuit.


 
 Return to top of page · Post #: 5 · Written at 9:23:33 PM on 8 February 2022.
BringBackTheValve's Gravatar
 Location: Linton, VIC
 Member since 30 December 2016
 Member #: 2028
 Postcount: 352

Around about the same time, early 80's, Telecom also had problems with silicon rubber boots fitted over MDF links in exchanges.

The MDF (Main Distribution Frame) links in exchanges were normally exposed, but the then new PCM digital transmission equipment required a relatively high DC voltage to power feed the underground regenerators between exchanges.

In the interest of safety for the MDF staff, rubber boots were placed over links which carried high voltage for the regenerators. Very soon we noticed an increase in spurious noise, and the source was traced to the links supplying power to the regens. And, as time went by, the noise levels increased.

Investigations revealed that the silicon in the rubber boot was attracted to the high potential and began to form a bridge across the link.

The technical term was (wait for it) Molecular Migration. Yep, silicon ions and electrons fell in love with their polar counterparts at the link bases and would move across the insulated MDF block surface, forming semiconducting bridges. (Wow, who needs expensive sputtering)

The offending boots, black in colour, were replaced with purple and clear boots, presumably with no amorous silicon compounds.


 
 Return to top of page · Post #: 6 · Written at 9:45:29 PM on 10 February 2022.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6367

Investigations revealed that the silicon in the rubber boot was attracted to the high potential and began to form a bridge across the link.

Telecom/Telstra seems to have a history of such things, probably the worst being the infamous 'Seal the CAN' management initiative debacle which resulted in what turned out to be corrosive gel being poured into the copper network, eventually causing countless thousands of faults as the insulation rotted on the pairs and leading to corrosion and breaks. I recall technicians talking about having to 'ring-bark' cables to find uncorroded copper to make new joints.

Worse, some cables were so badly affected that, any time a tech touched it to repair a fault, it was odds on he'd inadvertently create some new faults in the process, generating a vicious circle of repair calls.


 
 Return to top of page · Post #: 7 · Written at 11:40:26 PM on 10 February 2022.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4724

It is interesting to note that at one stage I did write an article for the Radio Club, which I will likely still have albeit I did start a major purge to clean out masses of duplication absolutely wasting Giga bytes. Some of it dating to the first of the DOS & UNIX based units.

A program that would annihilate zero content folders would be a boon. I already have an old one to annihilate duplicates.

Now! Plastic was the wonder material of the fifties that has proven not to be and there has been a radical compositional change due to its failure. I have noted: Coil formers and cabinets pillars and such forth, crumbling. Barbie Dolls and many other "plastic" items are a conservators night mare.

Even with Bakelite the Wheatstone Bridge here was developing what I considered too much Verdigris and I was also suspicious of the AVO-7X. This attributed to acid derived from the Formaldehyde Plastic. It has been confirmed in conservation that many plastics are as I thought, decomposing. Some are going hard & cracking & shattering. I note some plastic wiring in tractors & mains cable are sent rigid over time by, oils Diesel and heat. Several items in the Utes engine bay have suffered the oil & heat issue & been replaced, as has happened in the neighbours tractor as well. Mine (1951) had "Cloth Rubber" wire & I made it a new, & slightly modified, new loom in the eighties.

The decomposition of the PVC will generate Hydrochloric Acid which in the presence of water will discolour wire. It should be noted that some Acids, need to be diluted to work. Concentrated Hydrochloric & Sulphuric Acid will store in an Iron tank, but eat it if the concentration falls. Hydrochloric Acid is also the catalyst for further plastic decomposition. One often overlooks "Lewis Acids" as being a problem as well.

To counter the Acidic decomposition vapour, AVO & The Wheatstone Bridge & a couple of others have porous sachets containing Soda Ash, in them.

Marc


 
 Return to top of page · Post #: 8 · Written at 12:40:40 AM on 11 February 2022.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1693

A program that would annihilate zero content folders would be a boon

If you're using windows, use the command prompt and go to the root of the folder structure you want to work on.
Then type this.

FOR /D /R %g IN (*) DO .RD "%g" 2> nul

It checks every folder so could take a while. It works by trying to RD all folders, but RD only works on empty folders. If a folder only contained empty subfolders, the subfolders will be removed but not the original folder. You can run it several times until all empty folders are gone.


 
 Return to top of page · Post #: 9 · Written at 1:06:32 AM on 15 February 2022.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1693

Today I worked on 2 more radios.

First one, a Kriesler 11-7, with 6AN7, 6SK7, 6SQ7, EL33, 6X5GT. The stamp at the back said the output valve is a 6M5, but obviously not. The radio was at one time infested by mice, and the smell is still there. It's still full of ancient components, only a few critical caps have been replaced. The radio works fine, but the dial indicator had come off the fishing-line dial cord. It was quite a bother to get it back on, and eventually I discovered the cord had been misrouted over the bandswitch shaft, so tuning to the top of the dial would cause the pointer to jam and come off again. It's quite difficult to get fingers in there, so I decided in the end not to bother doing any work on it. It's a pity, because the case is nice, as is the dial glass, and the knobs are all there. But inside, everything is rusted out and smells of mouse pee.

Second radio is a brown-and-white plastic Healing Golden Voice, no model number to be seen. Problem was intermittent low volume. It doesn't make much volume anyway, but it would drop right back to barely audible. Valves are 6BE6, 6AU6, 6BV7, 6X4. There was a mystery resistor connecting the G2/4 pin on the 6BE6 to the D2 pin on the 6BV7. This didn't seem right, so I removed it. The marked value is 4.7M, but it measured 7M. But it made no difference to the low volume. Replacing the 6AU6 with a 6BA6 seemed to stabilise it, but no matter what I tried, there simply isn't much volume to be had. The voltages seemed fine. This combination in theory should work ok-ish, but not this time. After fiddling for a while the intermittent seemed to be gone, but stuck with the low volume. By now it was 1am so I gave up, put the 6AU6 back in and put the radio back together. Someone in the past had got irritated by its shananigans and had stabbed the front, putting a hole in the speaker, and breaking 2 of the flimsy chassis mountpoints..

I had purchased both of these radios in Queensland.


 
 Return to top of page · Post #: 10 · Written at 8:59:29 PM on 21 February 2022.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2088

AWV, the developer and the only maker of the 6BV7, promoted it as making a non-reflexed 4 valve set practical.

It never did achieve the promised gain. AWA used it themselves in a couple of radios where they used a 6AU6 in the IF and a complex delayed AGC network to get some semblance of reasonable loudness from the thing - only possible with a brand new 6BV7!

They tried the same trick in an early TV, pushing the IF - limiter stage to the point where acceptable AM rejection was possible only by very critically tuning the ratio detector. Again, no spare gain for quiet audio passages.

Then the 6BM8 appeared and made all this unnecessary.


 
 Return to top of page · Post #: 11 · Written at 10:38:42 PM on 21 February 2022.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4724

One of the biggest causes of deaf sets is alignment. If you can get a circuit that helps. By now all waxed paper caps will be cactus and so will the electrolytics. The essential thing to measure on waxed paper caps is leakage which 99.99% will fail, so why bother testing them.

Leaking NP caps destroy bias & that will guarantee problems. Fault finding with them in place I consider time wasting. I have had a lot of issues with 6BA6's &12BA6's recently.

Mouse doo on wafer switches and in general, succumbs to Soda Ash solution with a bit of dish washing liquid as a wetter. Wash it off quickly as it will de-lustre Bakelite. Keep it well away from Bakelite cabinets.

Albeit perhaps paranoia, but I have damaged lungs. The major warning I put into the Radio Club newsletter does mention bio hazards & I will where there is evidence of occupancy spray it out with Glen-20.

Mono filament plastic cordage, like fishing line is useless for dial drives. The old nylon cordage for dials is allergic to solvent cleaner.

The original synthetic cordage was a mono filament with a woven sleeve over it. You can still get a modern replacement.


 
 Return to top of page · Post #: 12 · Written at 12:45:30 AM on 22 February 2022.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1693

Just did today's bunch. Firstly a nice-looking peach Fleetwood FL-1207A, which is a clone of a Philips model. Valves: 6AN7, 6BH5, 6BD7, 6M5, 6V4. I had plugged it in and it worked, unplugged it for a moment, and when it was plugged back in it just crackled and the 6M5's grid was glowing orange. So, straight to the repair bench. As expected, the speaker transformer had blown. I found a replacement but when measured it was also blown. So, 2 transformers went in the bin. A bit more searching brought up one that looked right, but it was marked as for a UL41. That valve only requires 3000 ohms, but 7000 was needed. So, more searching, and I found one for a ECL86 / 6GW8. It was installed, and success was mine. Like the Philips model, this radio makes a lot of volume.

Next was supposedly the Philips version with a yellow plastic case, it was marked as 5 valve but when I got it open, it was a 4 valve. Lineup: 6AN7, 6N8, 6M5, 6V4. It has an intermittent fault of the volume dropping, and it would pop back up a bit later. The AGC changed in sympathy, so I suppose the 6N8 might have been at fault. However the radio is missing the dial pointer and associated hardware. The cloth or chicken wire on the front is missing, and the back cover has a giant hole. So it will probably get scrapped instead.

Lastly, another 495MA, same colour as the Fleetwood, a fine example of a 4-valve with 6BV7. Changed a few capacitors, and upgraded the 100 ohm bias resistor to 150, just like with the other radio in post #1. It works well with an external antenna. Line-up 6BE6, 6AU6, 6BV7, 6X4.

EDIT: Well, after running the 495MA for a while, it died, with a burning smell. So, back to the bench.


 
 Return to top of page · Post #: 13 · Written at 10:44:02 AM on 22 February 2022.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4724

Replacing some capacitors if they are waxed paper types is a recipe for disaster & all electrolytic caps, not run for ages are liable to present as a short, any electrolytic caps here where there is no storage history, or over 2 year will be put on the reformer before use.

The greatest threat to an output transformer is C22. The plate bypass caps had a high attrition rate and were often over 1KV to handle the audio transients.

The 100 Ohm resistor is the fixed, or "back bias"; Changing that will alter the bias of every valve in the set and by increasing it that will throttle back every valve but not fix the problem that caused the other one to have low volts.

The fixed bias is wholly dependant on the current through R14. Which is the sum total of all cathodes If the voltage is lower than specified then either a valve is faulty eg heater cathode short, or low conduction, or there is another path to CT like the failure of C20. C20 is quite capable of burning out the transformer C21, R15.

I think a little bit of revision is in order. Do not heavy up R14.

As a side note: The second reason for building the second reformer in the face of the1938 VCT having one, was for situations where I suspected a new filter cap had let go; I could power the B rail with tubes out & any dividers disconnected. 25 - 400V. As that is a monitored voltage one can quickly see a problem. On short, or overload the reformers regulator will lock up.

Also; Any set being "burn tested" or just being fired up after overhaul here, will have an analogue voltmeter clipped to B+ (IC Clips).

Marc


 
 Return to top of page · Post #: 14 · Written at 6:07:34 PM on 22 February 2022.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1693

Had a bit of spare time today, so looked at 2 more radios.

Philips 172, valves 6AN7, 6BH5, 6BD7, 6M5, 6V4. Sound low and distorted after a while. The cathode had 0 volts, but it could be restored by wiggling the pin connection. There was a barely noticeable solder splash shorting the cathode to the earth side of the heater. After fixing that, turned the volume to full and it dropped to half by itself. Voltages were unaffected, so tried another speaker, volume restored. The original speaker (Rola C) had dry joints all over. Fixed this, ran it for a while, all good. Put it back on the burn-in bench, failed after an hour. Back on the end of the repair queue.

Hotpoint P64MEX. Yet another 6BE6, 6AU6, 6BV7, 6X4. Would fade out after a while. Radio touted as super-sensitive because of the 6BV7 wonder-valve. Can't see how that works out. This time the problem was a faulty 6BV7 - the grid bias would slowly drop away, so drawing more current all the while and losing volume. I only have 2 spares, and one of them turned out to be faulty too - after 2 minutes the grid would short internally to the cathode and everything got super hot. The last 6BV7 worked - unusually the envelope is taller than usual. But I wasn't going to give up on the original valve just yet. Next thing was to see if a tune-up might help the sensitivity. The aerial padder was way out, as was both sides of the 2ND IF. Now there was lots of volume, for a 4-valve set.

I can see that Marcc doesn't like modifications, but the repairs are for me, so I feel I have the right to try out whatever I want. I had to find a way to stop the 6BV7 from getting hot, so hot that the chassis in the area was hot to the touch. So, once again the bias resistor was changed, from 100 to 200 ohms. This brought the bias up to -5.2 volts. Using the original valve for half an hour and the bias had only dropped to -4.75, and it seemed stable, so I consider the point proven. Also the valve could be touched without fear of getting burnt. The radio is on the burn-in bench, working as I write this.

EDIT: burn-in passed, radio back in storage.


 
 Return to top of page · Post #: 15 · Written at 11:16:06 PM on 22 February 2022.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4724

I do not believe in mods without good reason. If there is a reason as with an STC 59 chassis it will happen. Neither HRSA , Kevin Chant & Radio museum had anything on that chassis & they only used it in one model. It was part of a deceased estate and I initially recognised it from a part I made for it and I had supplied info to try and ID it years before. It actually was a fixer from Dandenong.

It had been hacked to try & get the volume control (original missing) into the grid of the OP valve. There was a mass of WW resistors trying to re-establish bias. This had all failed. On comparison with that series of chasses, I found the Local / DX switch was also conspicuous by its absence. So that led to a reverse engineer, in the process I found the missing pot was 6K and in the cathode bias divider chain.

So having the same bits & valves it was rebuilt to pretty much 58a but with a 5Kpot with adjustment. It decided to fire up no signal so the Oscilloscope & Sig Gen were fired up to find where it got lost. Problem with plate wiring that had been got at, detect first audio.

Also found was a lead dress issue on the mixer in the process that had to be sorted as it was causing a superhet. whistle and then a distortion issue with the Plate Detector.; Also spotted by the Oscilloscope. In conjunction with that the local switch (on /off) was fitted with 68pF series cap which was shorted out on DX. That also resulted in a bias change on the 6D6 via the bias chain to cut it back & stop it over loading the Plate Detector. So I will modify.

Kevin Chant & HRSA now have a copy of that circuit labelled as chassis #59. However, It is noted on it what was done, how & why and its origin.

Modifying to fix a fault not found, can have nasty ramifications. These things run on smoke & if it gets out: That's a serious problem.

Marc


 
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