Soundlab CPA 100 Mobile power amp, and surprising output matching.

Now, I needed a mobile amp, 100 watt-ish for a project. Didn't I have something in the loft? Yep, a cheap-and-cheerful Soundlab CPA 100. 75 watts allegedly. That'll do. It had a label on it. "Been reverse poled" ... ugh. Let's have a look.

I wasn't holding out much hope really, the fuse was intact. but I was expecting a rake of burnt out resistors, maybe some PCB tracks blown open...

I was, however presently surprised, just a handful of caps had popped their bases, as a result of being effectively connected backwards!   

Anyway, a few cursory checks, and on with the power. Great. It works. Left it running for a few mins, driving a few watts into a dummy load, when I started thinking... why two transformers?

Now normally an automotive power amp needs a few more volts to drive a larger amount of power into a 4 ohm load... Ohms law applies here again, but we're dealing with a sine wave (for RMS watts)

For example, a "proper" class A only amp will swing 6 volts into our 4 ohm load ( a little less in reality, due to the losses in the transistor), so ohms law :

Power = ((Epeak/1.414) x 2) / R where E-peak is the maximum voltage of the sine wave (it will be less than the supply in reality due to transistor losses)

= (6/1.414) x 2 / 4

=4.2 watts

A push pull amp , will swing our full 12 volts into our load, giving ~18 watts. 

There are other amp topologies, such as Class H "Rail switchers" which will give more, but the limit is usually around 50 watts.

Now normally, our big automotive power amp has a step-up switched mode supply, which generates a nice +/- supply of , say 40 volts, so it can swing 80 volts into our load... mucho power ! As long as the supply is up to it! You don't get something for nothing, so the current consumed at 12 volts from the battery would rise proportionally. If the amp with +/- 40 volt supply actually exists, it will generate about 200 watts in a 4 ohm load, and will need 200 watts (plus a bit for losses in the power supply) from our 12 volt source (That's 16 2/3 amps) In Bridge-tied load, we're looking at more like 750 watts.

So, back to our little Soundlab. Are the transformers a power supply? Nope. So how does it generate more power than it's push pull design allows for?

Those transformers are output transformers. They lower the impedance the output stage sees from the speaker. So , to swing enough current from a 12 volt supply into our load, the load needs to be small.

Let's work it out...

Power = (Epeak/1.414)x2  / Resistance , so 

Resistance = (Epeak/1.414) x 2/Power

= (12/1.414)x2/75 

= 0.226 Ohms.

So our two output transformers change our 4 ohm load to a 0.226 ohm load, which allows more current to be drawn from the output stage, and more voltage to swing into the load.

There's some quite heavy negative feedback from the speaker side to tame an otherwise cheap amplifier into giving reasonable performance, and it doesn't make 75 watts RMS, but more like 75 watts when bridged, so will probably give me sufficient power for my needs. The plus side to having a transformer coupled output is if the output transistors should fail, there will never be DC on the speaker!

Radford STA-15 repairs & renovations. A quick note on biassing.

Telephone rang, a lovely chap called John. 

"I've got a Radford, one of the valves has gone, can you take a look?"

Of course. The trouble is, John lives in Scotland, and posting anything with valves in always worries me, but never fear, UPS did a stunning job and all arrived safely.

It's a rather lovely STA-15, made in 1962.

Ah yes, the disclaimer.....

WARNING. Do NOT try this at home. This amplifier calls for some high HT voltages. The main HT rail is some 390 VDC. It will floor you or kill you if you come into contact with it. It will not warn you, blow a fuse, nor pull your safety trip. It may not give you a second chance. RESPECT IT'S AUTHORITY. 

There are exposed 240VAC mains connections inside the enclosure. 

I will not, under any circumstances, accept any liability if you decide to repair an amplifier for yourself.

Having got that out of the way....

Fairly obvious straight away which valve had failed. Apparently, it had glowed brightly and the fizzed. It's gone down to air. It was one of a pair of Mullard EL34's. Pity, but it's twin read a bit sad on the Avo tester, so was probably due a change anyway. Interestingly enough, the other pair, which read low, but not within the realms of working OK, were Zaerix valves. I never really understand people mixing valves brands between channels, I mean we want the thing to be balanced really. Nothing for it but a new set of valves. A phone call to the lovely people at hotroxuk.com saw a new matched set of four Russian made Tung-Sol on the way. (No connection with Hot Rox, other than a delighted customer)





So, why had the Mullard expired? Switching on with the valves removed, showed the offending valve had a positive voltage on it's grid, with respect to the cathode. Not good. That's equivalent to you holding the throttle flat down of your car for hours... The valve's anode current would have risen, and the anode itself probably glowed bright red trying to dissipate all that heat, followed by the glass envelope failing due to the heat. Sadly, it's an all too common a fault. The capacitor which couples the audio to the grid, after the phase splitter had broken down, and was passing DC. Nothing for it , but a re-cap.

The Radford has a lovely symmetrical design.
In this picture I've nearly finished re-capping the left hand channel...

... and there's both sides done. Caps are Vishay and Rifa, they're reliable and are well thought of in Audiophile circles.








I checked the other valves for faults, and emission, and all was found to be in good order. I also did some checks on the values of resistors, especially anode, cathode and screen. Everything was good, and always better than 5% tolerance. Nice!

Now to check the biassing....

There's a lot of BS out there about biassing, some of it comes from manufacturers and importers, some of it folklore, most of it wrong or downright stupid. We need to bias out output stage to get our anode current to less than the maximum plate dissipation of our valve. So we have an EL34. It's maximum plate dissipation is 20 watts. Anything more and we're in trouble, and run the risk of premature failure, the anode glowing red etc etc. Nasty. Don't do it.

So we need to know two things. The current flowing into the anode, and the voltage between the anode and the cathode (not, as some suggest between the anode and ground). That's all the valve "sees" and is all we need to do our calculations.

To make this easy, please welcome the Doz "Bias-o-rama" ... It's an octal socket and an octal plug. All the pins are connected together, except the anode and cathode, which are connected by 1 ohm resistors. There's some flying leads across these resistors. Now measuring the voltage across the resistors, will give us the current flowing in mA. Great.
So, first off, install the bias-o-rama, and valves into the amp. Plug in speakers.

Right, switch on and wait for the valves to warm. It's worth keeping an eye on things here, just in case the bias is miles out, or there's a fault, and our valve starts to red-plate. If it does, switch it off quickly.

Now that everything's ok, measure the voltage between the anode and cathode. In this case a very civilised 390 V. Now measure the voltage across the anode resistor on the "Bias-o-rama"... That gives us the current in mA flowing though the Anode. As these valves are from a matched set, both currents should be within a couple of mA of each other....








Yep , they're good (Thanks HotRox!)


So we have 390v and about 50 mA flowing. Simple ohm's law tells us power = voltage x current, so
390 x 0.050 = 19.5 watts. Less than our maximum allowable 20 watts. (I will point out, this was taken after I'd adjusted the bias, which on the Radford involves changing resistors in the grid feed circuit, there's no pot or anything helpful like that)

Biassing too cool (not enough current flowing) will show up as cross-over distortion.

Some biassing voodoo calls for measurement of the cathode current. I'm not sure why, as this also includes any current flowing into the screen too. Less accurate, but probably near enough. I suppose the cathode is always at a lower voltage, so it's safer.

OK, so now we're happy with the bias, leave the amp running for 15 mins and check again. Still good? (It should be, although I once had a set of clear top 6550's that drifted badly, they went back!)
Right, now go to a darkened room, and play the amp loud for 15 mins. Look carefully at the large grey anode. If there's any signs of it glowing red, or little spots of glowing, it's still biassed too hot. Go and do it again!

So there it is, on test in the living room, much to the wife's delight. Playing the excellent CD "I will be a pilgrim" by Arch Garrison (http://www.archgarrison.co.uk/) , a blend of acoustic guitar and philcordia , some nice folky bits with a psychadelic air to it.

A post script...

My apologies for the poor focus, the infra red from the tubes stops the auto-focus working on the camera (it works by infra-red!), and this was a long exposure. I hope you can see the blue glow around the holes in the anode. This is caused by electrons hitting the glass and causing it to fluoresce. This is normal. If a tube gets "gassy" a similar blue glow can be seen, but very much more pronounced and time for a new valve! 

 

Technics SL-P1200B Compact Disc Player

This belongs to lovely chap in the beautiful country of Wales.

It's a commercial CD player, favored (briefly) by the beeb and independent local radio stations in the late 80's , early 90's. This one dates from 1988.

The -P version was specifically for broadcast use, and instead of the large slider on the right being for pitch control, it's a fader. The pitch control is the small knob on the top right.

Rumour has it the poor things gained a reputation for being a finicky, requiring the CD to be spotless, and was possibly responsible for more "dead-air" back then than anything else. Sad really, because the spec is fantastic, Burr-brown PCM54HP DAC's and Technic's successful "Class AA" bits and bobs.

All this needed was a bit of a service. The laser is fantastic and should give plenty of good service back in the motherland!

Trio (Kenwood) KA-2500 amplifier repair & renovations.

The things that turn up. This arrived from a friend of a friend, for repair.

It looked as if it had been safely stored in a lake for the last 40 years.

It's a fairly straight-forward silicon amp, dating from ~1973. It uses 4 NPN 2SD180 output transistors, capacitively coupled to the speakers.

This unit had obviously seen a couple of repairs in the long-distant past. The first thing I noticed was the mains fuse, which looked like it belonged in a 1960's Morris-Minor rather than the mains input of this amp! 

 8 amps continuous, 15 amp blow.... You can just imagine this being worked a little too hard in the 70's... blasting out some Yes at a party, only to be met with silence ... then someone has a bright idea and pops the bonnet on Dad's Morris!

Rather than the 2 amp requirement!

The mains input filtering cap, C28, was looking in a sorry state, so was snipped out, pending replacement, and mains gently applied via the variac. The power supply was obviously heavily loaded by something, as the voltage across C25 didn't rise as expected, and there was some excessive current draw from the mains. The HT feed wire was disconnected, and the voltage rose to an acceptable level, without the current draw. My immediate thoughts went to the output transistors, our 4 2SD180's , but these tested OK. I started to check the caps in the output stage, and every single one read high ESR, low capacity, leaky or a combination of all three! A blanket re-cap saw no real improvement of current draw when the supply was reconnected. Time to measure some voltages... With the mains set low, to give us an HT of ~5V, bias was obviously miles out, with two of our output transistors (Q2 & Q3) hard to rail. Tracing back showed either Q3 or Q5 were hard on, for no apparent reason... R5 had also risen in value to 1 Megohm or so. Q3 and 5 were removed and tested OK on the transistor tester, and found to read fine for gain and leakage. R5 was replaced. Powering on again showed no improvement. More voltage measurements again hinted that either Q3 or Q5 were the guilty party. Static testing of caps, resistors and transistors proved fruitless. Q3 and Q5 are 2SD317's.... and are so obsolete. Noah himself had discarded his stock (of two, obviously!) Looking at the spec showed an old favorite of mine, the BC547 may well do in that position, although it has a slightly less Vce of 50V as opposed to 70V for the 2SC317. I knew that it would never see more than 50V in this position, so I fitted two, and was rewarded with audio, albeit the output stage was still drawing too much current. Setting up the bias to the output stage (60mA) cooled things somewhat , and I left the amp on test for a while... 

All was well, and the bias stable :) 

All that remained to do was to recap the phono pre-amp board, a quick clean up and send it on it's way. The output capacitors seemed to read OK, and there's very little in the way of DC on the speaker terminals. The main smoothing cap also seems OK... After all that testing I suspect it had reformed itself,  without resorting to the Dreadnaught capacitor reformer.

Freshly re-capped phono-pre. Nice symmetrical design .

 Top down...

 A mixture of construction, some tag-strip , but mostly PCB.

 The finished article...

... and the guilty parties!

Nice selection of in's and out's... Note Tape HD, now that's not High Definition, oh no, that's Tape HeaD! It feeds the phono stage without the RIAA equalisation.







How does it sound? Phono preamp is a bit noisy. Best described as "of it's time" , but a nice piece of earlyish transistor hifi nonetheless.

Mini 1275GT ... more corrosion

 ... and so it continues....

N/S Floor in 

N/S heel board cut out

Check alignment

New N/S heel board cut to size...

... and welded in place.

There needs to be 50.5 inches between these bolts and the ones on the other side.

N/S heel board / boot floor/ rear pocket closer panel welded in

N/S inner sill seat belt mounting point welded in, and door step to a-post panel lined up

... and welded into place

N/S rear quarter repair cut to size and welded.

Nasty near side floor bodge removed

... rear pocket unpleasantness!

Much more to cut out yet!

O/S Inner wing repaired

O/S heel board / rear seat area repaired

... and rotten heel board cut out

more to come ....

Inner wing repair (another view)

Nasty.. most of the floor comes out with the a-post still attached

... nasty

Lower rear pocket is in a bad way due to being sandwiched between two rotting pieces of floor 

.. so a new section is made up and welded in place

Floor taking shape...

Check fit ... and I'm not happy with it... 

... so I turn my attention to the rear heel board again! 

and the closer panel

Outer sill in place... 

... starting to come together... but why wasn't I happy with the fit?

Because the centre cross member was in a bad way .... so I cut it out

.. welded some tabs to a new one to ensure good alignment, and welded in place.

... then the boot floor gets a repair piece...

... or three

Valence checked for fit

Rotten lower section of the rear panel cut out and replacement welded in

Inner wheel arch repair (this is where it all started!)

.. then I hit filler in the top part of the O/S rear quarter...

... so it gets cut out

... and , after two evening's worth of cutting and fettling...

The new panel is fitted and welded in.

Window rail and quarter stiffener panel fitted.

A coat of primer to protect the new metal.

A coat of priner and stone chip for the floor , and refit the freshly painted rear subframe.

Refit the front subframe (again freshly painted), and back on it's wheels.

... still much work to do to the front panel, and of course paint and reassembly, but that's it for now.