Ok, presume you meant between TR202/3 base and emitter, not collector..

What are the voltages between the base/emitter of TR203/4?

First suspect is dried out and leaking electrolytics, but TR203 base to emitter voltage at 0.8+ volts says it's cooked. Doesn't necessarily say why. TR 205 is also off or nearly so, 2.4 V across R211 doesn't look right.

Shouldn't need a matched pair for TR203/204 (edit correction 201/202) to get a stable and usable system, though it may be needed to get the distortion/noise spec. If you want to borrow a distortion meter I've got one.

That 21 volts negative on TR203 collector looks wrong, (80 v or so across R209) confirms that TR203 is not a happy bunny. Looks too far negative, should be nearer mid rail. But TR201 and 202 are nearly completely off, as revealed by + 67V on their collectors. Long tailed pair like this (even if upside-down) should have collectors mid rail or thereabouts. Can't hurt to pull C204 and replace it. Ditto C201 is suspect.

Keep us posted as to progress, do.

Best rgds Simon

edited for correction in italics – though the same is true of TR203/204!

Edited By Simon Williams 3 on 18/08/2019 00:51:42

Hi Neil,

As said above, unplugging the pre-amp does naught WRT C204 – not sure if you checked that cap. The other caps appear non-polarised, so unless old, paper wound types, they should not be any problem.

The key, as Simon said,is that under quiescent conditions, the voltage at TR201/2 collector should be close to mid-rail.

You should be able to check TR201/2 in circuit – the circuit is reasonably balanced in coupled impedances and comparative readings between the two should reveal large deltas. Try the usual ohms readings tween BE, BC, EC, with positive and negative polarities, and compare each transistor. ( discharge PSU caps PSC1/2 first).

If nothing reveals, you may need to remove each and check in your fancy tester..

OR – lift the leg of the mosfet end of R207 . Connect a resistor, value equal to R202, between TR202 base and ground ( midpoint of the PSU caps). On power, the voltage at the collectors of TR201/2 should now be equal and close to mid-rail.

I do the repairs for our local Hi-Fi/Sound system retailer here in Swakopmund and work on equipment of all conceivable types..From shebeens, disco's, restaurants, homes, Churches, Bands….And have followed many a garden path in the search for the fault, so what do I know…

Joe

I think that's a good place to start Neil. I repaired a Hitachi amp – see schematic below – similar to yours , but with stages inverted, and the input long tailed pair had to be well matched – gain delta of 5 gave around hundred millivolts output offset. This amp has pots for offset adjustment, but setting the pot voltages for equal Vbe showed how close the matching had to be. Also,the input LTP in this amp were bonded together, with copper foil wrapped around – for thermal tracking.

Joe

Well, there are not many transistors in that amp so when you swap the last one out, all will work…

I second Adrian's comment on the pot – esp since you said that there was a point where the output FETs got hot – they had to be turned on to some extent for that – if the pot is open frame, clean the track with a little methylated spirits while winding the pot back and forth two or three times. ( note its position and return it to that). The output FET's standing current should vary smoothly while adjusting the pot up or down SLIGHTLY…Beware excess current in the output!

Joe

But you don't have to repair them. They are so cheap that they are disposable. I used to be able to hear 20kHz as a teenager, but I could never hear 200kHz…

Said he, who bought a Pink Triangle turntable because he didn't like the nasty ringing sound of the Lin Sondek LP12.

Well, You are then very lucky in the UK…

Most of the Combo Amps, PA systems and Jam Amps I get to repair here are Peavey, Phonic, and ALTO.

They are considered quite good stuff, and they have shifted most of their classis amps to Class D ( Peavey lagging behind, thank goodness).

A typical 500W+500W amp from Phonic or ALTO cost approx £650.00 . A 'Digital' (class D) version costs around £900.00 . An analogue powered speaker ( Woofer/Sub with a Tweeter) of 400Watts is around £400.00 in 'analogue' and around £600.00 in class D.

When I chase for spares, the agents in South Africa always start there response with 'Ooo, thats a Digital Amp, quite pricey, spares are..', or words to the same effect!

And of course, the next Hate up the list is a class D amp WITH a switchmode powersupply…THAT the agents refer to as a FULLY Digital amp….

And the trend with the agents these days is to not keep component spares – for most of the mid-range power amps they keep a powers supply module and the amplifier module with the latter around 35% the cost of the complete unit..

The only thing to like about that technology is that at my age it's easier to carry up the stairs to the workshop..

And, as a radio ham, I hate them even more..The EMI many of these amps generate on the HF bands is terrible! The filtering is always done down to a cost, with lip service to the regulations..Some of them are dead quiet till you plug in the guitar or a microphone..

Bring back the good Mr Hood anyday!

Rant rant…

Joe