Bush SRP51. What have I blown?

[DRhodes]

Hi folks.

While changing the stylus with the Bush SRP51 turned on I accidentally touched the wires at the back of the cartridge, creating as massive blast of a buzz.
Since then there's a lack of volume and heavy distortion when the volume is turned up.

Could I have simply blown the speaker or am I looking at a more complicated repair, ie it could be many different things?

All advice welcome.

Thanks, D.

[Chris55000]

Hi!

The SRP51 (and the auxiliary add–on AU51 stereo extension amplifier) is a simple complementary–symmetry audio stage using a BC109 feeding into an AC128/AC176 TO–1 complementary pair of o/p transistors.

As the circuit–diagram is purchasable "up top" I won't post it but it's easy to repair – measure the d.c. voltage at point "13" of the amplifier PCB – there should be about +13.2 to +13.7V here with the player switched on.

If the voltage is very high or very low, you have unfortunately damaged either or both of the AC128/176 output transistors, and it'll be necessary to get replacements – you should always replace both as a pair when germanium devices are involved, as either or both may exhibit "thermal runaway" after feeding them with a lot of 50 c/s "blast" from your finger!

Worse case scenario is that the noise might also have damaged the loudspeaker, which MUST be 15 ohms impedance minimum!

Once you've replaced the transistors, you MUST check the quiescent current as detailed in the service sheet, typically 7–10 mA with no signal, set by adjustment of RV2.

It might also be necessary to readjust RV1 to give +13.7V at point 13 on the PCB after you've fitted new transistors!

If you have a good DVM, after you've got access to the PCB, check RV2 is still reading less than 50 ohms, if it's more than this, you'll need to replace it – the modern value of 47 ohms will do fine!

Chris Williams

PS!

Once you've repaired your amplifier, a far safer test is to tap the pickup arm near to the pickup cartridge with the volume turned up – modern ceramic cartridges are always slightly "microphonic", and you should get a general "liveliness", faint hum or rustling noises when you handle the pickup arm if the cartridge and wiring to it is intact, and you won't damage your amplifier or speaker this way!

PPS!

You can also get good–quality Russian replacement types as well – GT403A (AC128 pnp substitute) and GT404A (AC176 npn substitute) – these were used in cheap Russian radios & record–players and are plentiful, and also the square–block AC128K & AC176K are worth considering as these have a ready–drilled hole to bolt them to heatsinks, chassis–metalwork, etc!

[Edward Huggins]

Hang on a minute - here we go again!
Chris' Post above is an excellent series of suggestions. But I am 75% sure that the OP will not understand a word of this.
The way first time Posters pose their queries will usually give us a clue as to their relative abilities...more sensitive reading of such Posts is always needed.

[John10b]

Chris has given a very detailed answer, but I think Edward is correct.
If it is possible can you get to the speaker(s)?, with the mains DISCONNECTED.
If you can then visually inspect them, damage may be obvious, if they look OK very gently push the cone in and out, just a few millimetres to start. They should move easily with no rubbing, if however, they are rubbing you have probably damaged them.
Hope this helps
John

[mhennessy]

I've literally no idea what record player Chris is talking about here. The SRP51 uses AD161/162 output transistors, and the supply rail is more like 27V.

The cartridges are quite unreliable on these, and it is possible that touching the pins on the back has mechanically disturbed the fragile internal connections.

Obviously, tackling a fault like this requires a logical approach and some test equipment. At this point we know nothing about the OP, so until we learn about their skills, experience and capabilities, the only advice we can give is simple: this is a job for a specialist.

[Chris55000]

Hi!

There's been several references to SRPs with AC127/AC176 transistors, whether that's a mistake with model nos, or mixed–up references from different Trader sheets I'm not sure!

I have to confess I turfed out my red R & TV circuit books years ago because they were taking too much space up, so now when answering Members' questions I just use whatever free snippets I can find, as buying complete sheets is too expensive for most queries!

If I actually have the equipment concerned, then I will buy the bookwork – I bought one up top for the Hacker RP25 my friend owns as the bass/treble controls are seized, involving complete dismantling to repair, so I would want Hacker's own bookwork rather than a Trader sheet!

The fault–finding & repair principles are exactly the same whether small AC128/AC188 etc., or larger TO–66 AD161, etc., transistors are used tho!

Unfortunately the OPs rarely state how experienced they are or what equipment they have, but in the OP's case damage of some sort has obviously been done!

Chris

[Refugee]

Is that not a simple 4-transistor amplifier with one silicon transistor in the first stage?
I would guess that the two core mains lead was wired the wrong way so that there is more capacitave coupling between the mains and the chassis so that the static has spiked the first transistor.

[mhennessy]

Chris, you've had me going for quite a while with this!

I know this particular machine really well. I was lucky enough to inherent one from my Dad when he upgraded to a music centre. I spent most of my childhood trying to convert it into stereo. We ended up cloning the amplifier. No schematics, but only 5 transistors, so how hard could it be? The full line-up is BC114, BC132, BC119, plus the AD161/2 pair.

Having wrecked the player as a kid, I picked up another one about 10 years back. It even came with the AU51 - *and* the special cable with the hard to source DIN plug, which was a real bonus. I could not believe my luck, to be honest - this was before record player prices had shot up, so I only paid a few 10s of pounds (postage was more IIRC). It certainly sounds a lot better than I'd remembered. But I did run into the problem with the bad contacts in the cartridge. I've got several cartridges now - they're quite hard to find, and they're all suffering the same problem to a greater or lesser extent. It is possible to swap parts around, but so much in there has gone hard and brittle with age.

Anyway, because of my long-standing affection for these machines, I am writing an in-depth article about them. I've done a lot of research, but at no point have I run into a version using a TO1 output stage. Which meant that while the work is a long way from being ready, I would have had to held off publishing anything until we could corroborate your recollections.

However, I now believe you were thinking about the SRP52.

Only 1 digit difference, but it makes a huge difference.

The schematic for the 52 can be found here: https://www.vintage-radio.net/forum/...28&postcount=4 - it doesn't match your recollection completely, but it's a closer match than the SRP51, so we might be on to something.

Of course, I might still be wrong. If so, please can you provide a link or similar reference. I've spent about an hour searching this forum but no joy yet.

[Chris55000]

Ah!

It was from a Google Image Search for an SRP51 output stage I found the snippet of circuit my first reply referred to and indeed the SRP52/AU52 was what it should have said – the pdf linked was indeed exactly the same!

The trouble with looking for things of this type is there's that much "noise" from sponsored ads that pay backhanders to Google & (dubious!!) sellers of overpriced pieces of paper covered in toner splatterings masquerading as "service manuals" it's not easy to pick out the exact item correctly, and my apologies for referring to the wrong circuit in error!

I might add the Heathkit STR–1 Stereo Tape Recorder I posted most (excepting the last few operation and advertising pages) of the Assembly Manual for, has almost exactly the same circuit arrangement with a silicon driver and AD161/AD162 output pair as the SRP51, although the layout is totally different of course and the resistor values differ but the general principle is identical!

Chris Williams

[Chris55000]

PS!

I've also seen an "upside–down" version of the SRP51's circuit arranged for positive–earth supply with a pnp silicon driver and the AD162 at the top (collector to –24V) and the AD161 (collector to +ve earth line) at the bottom – presumably because the maker had pnp devices on hand – I think BRC/Thorn and Dynatron have used this version of the circuit on occasions!

[mhennessy]

Mystery solved!

[Techman]

There's been several threads in as many years on these players where the mains transformer has failed and as far as I can remember, not one of these threads has ever reached a conclusion. Either the transformer just fails on its own or something in the amplifier causes it to fail. Tests made on rectifiers seemed to be a bit inconclusive if I remember rightly. An exact replacement transformer has proved to be unavailable and wrong transformers being fitted have caused even more problems, also rectifier replacements have either been faulty from new or incorrectly fitted causing more burn ups! The amplifier needs to be tested on a separate power supply of some sort before sending the original transformer away for rewinding.

The above regarding the transformer is unlikely to be the issue with the OP's player, but if the fault is in the output stage of the amplifier, then a burned out transformer could be the end result, but the actual fault seems to be up in the air at the moment as the OP hasn't returned for an update on this thread as yet and may never do so - a little bit like those other threads on this same player that I mentioned.

The other thing worth mentioning for anyone researching these players is that there was a Murphy version of it with a different model number - one of the 'failed' repair threads that I mentioned was for the Murphy version. All this information is from memory, so worth checking out the actual details using the forum search function.

[Chris55000]

Hi!

My estimation of the transformer rating would be 18V at about 300mA max – using oversized transformers on amplifiers with Germanium transistors will produce a rectified voltage that's too much for them!

Whilst you can feed sine–wave drive into this type of amplifier to determine the maximum power supply current, it's not advisable to keep the drive applied for more than two/three seconds in order to prevent possible thermal runaway of the o/p transistors – this is how the designers do it, then quickly note the max. current reading.

The transformer is then designed to provide no more than about half of the max. current recorded (remember this is class B output, so the average supply current at average listening levels is much lower!)

To give the SRP51 example, quiescent supply voltage is 24.5V, (marked on circuit) so the r.m.s. rating is 24.5/1.4 = 17.5V plus two diode drops = 19V r.m.s. max., hence my suggestion of an 18V r.m.s. transformer rated 300mA, e.g., 6VA rating.

Chris Williams

[mhennessy]

The SRP51 should be 27.5V. You're still looking at the SRP52

To help with the transformer question, I decided to take a look at my AU51 as it is easy to bring down from the attic. I reckoned the transformer is probably 20-0-20 off-load to give a rectified DC in the 27-28V region. A standard 18-0-18 unit (or 0-18 and a diode bridge instead of the 2 diodes) would probably do - it will be a little higher off-load, so likely closer to the original. And even if it's a bit lower, we're not talking about an audible change in audio level before clipping.

But mine measured rather low, and had a bit of a hum. Of course, it was faulty!

As has probably been said in other threads, watch out for those pre-set resistors. The 50 ohm resistor to set the quiescent current (RV5) is particularly risky, because as it drifts high, so does the bias.

The quiescent current was around 300mA, and of course it shot up even higher as soon as I touched the preset and it fell apart!

Luckily the PSU output sags readily, so the transistors survived. But of course, this would kill the transformer eventually. I'd recommend replacing this little pre-set resistor as a matter of course.

If replacing one of these transformers, I reckon the original is probably in the 10-20VA range. I wouldn't use a larger one as transformers are easier to find than output transistors.

What makes sourcing the transformer a little tricky is the 6.3V winding for the pilot lamp. I suppose the easiest thing is to replace the lamp with a higher voltage one, and power it from one of the 18V AC halves.

You'll probably run into mechanical issues too, as the transformer is secured to the PSU chassis with 4 bolts via the laminations in each corner. A replacement will probably have a standard mounting clamp. But there's plenty of space in the area, so with care, the only problems that should remain after a replacement are cosmetic.

OK, having written all that, I'm happy the unit has had enough time to settle down with the correct quiescent current of 6-7mA and the midpoint is also fine (that's set with RV4, which is the same type as RV5, but this one hasn't broken yet). The transformer measures 19.5-0-19.5, and HT is 25.7V. Still a little lower than what the manual says, but more than good enough. That number is nominal anyway.

The SRP52 uses a motor overwind which is 18V, hence the lower supply of 24.5V. I imagine that the power rating of the motor overwind is a bit less than the separate transformer of the '51, so there will probably be more sag under load, which helps them to get away with smaller output devices. The '52 will have less power output than the '51, but I'm sure it's perfectly adequate for the intended use. After all, maximum loudness has as much to do with the efficiency of the loudspeaker as it does power output.

Hopefully this is useful information.

Mark

[Chris55000]

Hi!

After all, maximum loudness has as much to do with the efficiency of the loudspeaker as it does power output.

Most definitely true, this is how the Rigonda "Symfonia 003" makes all the noise it does using it's originally supplied speakers, from a stated nominal power output of only 4VA (quoted as such on the English booklet I have for it!) – nowhere near the maximum theoretical output from two U.L. push–pull EL84 equivalents at 250V!

Chris Williams