Pye 1005 Achoic - low output one channel

[richieyork]

Hi all

This player is giving me the runaround....

Basically, the r/h channel is low output - about 60% down.

I've done a hum test (touching phono connections - one significantly louder than the other). Verified cartridge is not at fault.

The 'good' side (L/H) power transistors running very hot - the whole assembly gets very warm very quickly, the R/H amp transistors aren't - I think they are simply picking up the transmitted heat from the working side through the chassis.

Having had an issue with one of these machines many moons ago I recalled the capacitors are prone to going way off value so I replaced all the electrolytics in the R/H amp - indeed, they were all pretty shot one way or another. Put the amp back and tested. Same problem...

Thinking the leaky caps may have caused transistor problems I removed and checked all 6 Newmarket transistors on my trusty but cheap Chinese 'MTester' and all showed up correctly and no shorts indicated. So, back in they went.

I also checked a number of resistors and none seemed miles out so left them in place.

I reconnected the amp assembly and powered up again to run some voltage checks (yes, I should've spent time on those earlier...). On powering up both channels I quickly did the hum test on the phono plugs and both channels were now working correctly!!!!!! I was just about to start testing the voltages when there was a crack sound through the speakers and the R/h channel reverted back to it's original low output state. Damn it.......

I've done a few voltage checks on the good channel (L/H) and poorly channel (R/h). Details below. Please note the service sheet suggests 25V supply to the output stage but i'm only getting about 20v - so bear this in mind re below. Factory service sheet figures are shown in brackets
L/H Channel
VT7 (NKT453U)
C 20v (25)
B 8.2 (10.7)
E 7.9 (10.6)
VT8 (NKT453U)
C 7.7 (10.5)
B 0.15 (0.1)
E 0.03 (-)

R/H Channelreadings
VT7
C 20v
B 0.4
E 2.1
VT8
C 0.9v
B 0.4
E 0.04

Any pointers as to the culprit here I'm guessing the output stage on r/h side is only getting partly driven? The transistors seem to be as rare as hens teeth. If you suspect transistors please suggest possible substitutes? It also has some kind of current balancing slider pot which i've checked for continuity and it's ok. I didn't mess with it.

I'm more a valves person so forgive my ignorance in these matters please, I'd just like to get it going especially since it did work albeit for only 30 seconds or so!!

Kind regards
Richard

[Edward Huggins]

These amplifiers are very difficult to Service. After these were introduced in 1964, Pye asked their Dealers to return these to Cambridge, rather than the Dealer should attempt a repair. Those NKT transistors are difficult to source but were still available about 4 years ago. They will be expensive and the circuit will not take too kindly to substitutes. The passive components are very tolerance and value sensitive. And the originals are now 56 years old. Don't tweak any pre-sets until you are more sure as to the respective voltage readings. Pencil mark their current positions beforehand. It does sound like low gain in the AF or Driver stages. Should this be the fault (and if so, the fix is relatively easy) do be aware that the restoration of gain may highlight any underlying PSU smoothing issues. Don't take any short cuts or workarounds. I wish you well.

[G6ONEDave]

With those voltages I would check / replace R28 the 1 ohm between VT8 collector and VT7 emitter. Also check that the loudspeaker crossover has not got shorted capacitors. Hopefully you are not getting dc across the speaker on the afflicted channel. Something else to look at is the driver pair VT5 & VT6, error voltages might be useful for further thoughts.

[Michael Maurice]

I’ve repaired a few of these. On each occasion, carefully replacing the electrolytic capacitors gave a much improved performance.

[richieyork]

Originally Posted by Edward Huggins

These amplifiers are very difficult to Service. After these were introduced in 1964, Pye asked their Dealers to return these to Cambridge, rather than the Dealer should attempt a repair. Those NKT transistors are difficult to source but were still available about 4 years ago. They will be expensive and the circuit will not take too kindly to substitutes. The passive components are very tolerance and value sensitive. And the originals are now 56 years old. Don't tweak any pre-sets until you are more sure as to the respective voltage readings. Pencil mark their current positions beforehand. It does sound like low gain in the AF or Driver stages. Should this be the fault (and if so, the fix is relatively easy) do be aware that the restoration of gain may highlight any underlying PSU smoothing issues. Don't take any short cuts or workarounds. I wish you well.

Oh dear, my anxieties are justified then Edward...However I'm going to plough on. I've not adjusted anything and will steer clear until I've got it working more or less as it should be then I'll balance the current.

[richieyork]

Originally Posted by Michael Maurice

I’ve repaired a few of these. On each occasion, carefully replacing the electrolytic capacitors gave a much improved performance.

Hi Michael - i've replaced all the electrolytics in the faulty channel - many were way adrift of spec but it didn't help.
I have noted a 200uF cap on both amp boards related to the current balance circuit which connects both amps - C18 - i'm wondering if that might be faulty on the good amp and pulling the current away from the poorly amp

I've attached a screen grab of this part of the circuit if it helps you or the other folk on here.

Cheers.

[richieyork]

Originally Posted by G6ONEDave

With those voltages I would check / replace R28 the 1 ohm between VT8 collector and VT7 emitter. Also check that the loudspeaker crossover has not got shorted capacitors. Hopefully you are not getting dc across the speaker on the afflicted channel. Something else to look at is the driver pair VT5 & VT6, error voltages might be useful for further thoughts.

Hi - i've ordered a new 1ohm resistor in case the original has some sort of intermittent fault so that's a start.

I've also noted Langrex sell the two drivers VT5/VT6 so i've ordered a pair, again as a test. Not dirt cheap but if it fixes it, great, if not they will do as a stock item if I ever dare go near one of these again.

I've posted a screenshot of the output stage if it helps.

Cheers!

[G6ONEDave]

Have you checked the capacitors in the crossover circuit C21 & C23? They are located in what might appear to be the power supply, the panel in the middle of the base board. It is actually both power supply and crossover in one assembly.

[richieyork]

I'm going to check those in the morning (Monday) and report back. Either somethings pulling the amp down or it's not being given a nudge to the last two transistors by a fault in the previous pair (VT5/6).....

[Michael Maurice]

I've had trouble with VT5/6 in the past
Good luck.

[richieyork]

Originally Posted by G6ONEDave

Have you checked the capacitors in the crossover circuit C21 & C23? They are located in what might appear to be the power supply, the panel in the middle of the base board. It is actually both power supply and crossover in one assembly.

Checked all the crossover caps - they test ok and no significant leakage...

[richieyork]

OK, this morning's (Monday) update on this awkward machine:

The fault 'seems' to be temperature related....before that thought here's the component changes today.

I replaced ALL the caps in the circuit from VT7 area right through to the output transistors - previously I'd just replaced the electrolytics.

I checked the crossover components - some caps 'straddle' both left and right channels - no issues found.

I replaced any resistors in this area that measured out of tolerance. None were significantly so but given previous experience of this amp they can get fussy over just about anything.

I replaced C15 (200uf)which sits across RV5 - the current balancing pot - on both channels, thinking this might be breaking down.

Having done all of the above I powered it up - both channels working correctly! Bear in mind this happened the other morning from a cold start. As with the other morning, after about a minute or so, 'crack' from the speakers and the panel light dimmed slightly. Fault is back, just like the other morning - low volume r/h channel, l/h channel output stage sounding fine but running very hot - r/h output transistors lukewarm in comparison.

Does this sound like a transistor breaking down Tin whiskers I'd previously pulled them all and checked on a tester and none showed issues.

I've ordered and awaiting delivery of some new old stock replacements - however I cannot find replacement output transistors for love nor money.

I've ordered some freezer spray and then see if I can isolate which part is causing the troubles but given the above update i'm happy to accept any suggestions of things to try!!!


Many thanks
Richard

[Herald1360]

Some voltage checks under fault conditions around VT5 and VT6 on each channel could be informative. A fault in or around either could cause the VT8 hard on symptom seen in the output stage check.

[Edward Huggins]

I can see you're suffering with this. Noting your reference to the crossover components, the wiring for these will look very, er, odd as indeed they are. Don't attempt any alteration to these, but maybe double-check for any highish voltage breakdown within the caps or shorted turns on the coils.

[PJL]

Don't go ordering or replacing anything yet.

Both output transistors are showing a base voltage of 0.4V. This would imply VT5 is turned off so I would look at the connection from its collector to the PSU.

If it's not that then take bce measurements of VT5 and VT4.

[richieyork]

UPDATE - i've managed to fix it!

Again, this morning - powered it up from cold - worked great for about a couple of minutes, then sharp 'crack' from the speakers, the pilot lamp dimmed a little and r/h channel went low output and the l/h channel output transistor pair started to heat up quite dramatically (as before).

I'd thoroughly checked over just about anything and everything on my daily attempts to get this thing working. In despair i'd ordered replacement driver transistors NKT211/NKT713 thinking one or both may be causing the problem. Bear in mind the original driver transistors tested fine on multimeter and my 'all in one' Chinese component tester...

So, this morning I first tried freezing with Servisol Freezer each transistor in the output stage, as soon as the fault kicked in. No joy. No change whatsoever. Fault remained.

I then measured the voltage at collector of VT8 and found it had collapsed to 0.9v. So, I replaced the driver VT6 with one of the new old stock items I received yesterday. After 2 mins it was still working fine. Same after 20 minutes, two hours etc. ALL 4 of the output transistors were barely warm, the current balance control test point was spot on at 0V and both amps giving the correct amount of gain.

So, i've learned a lot. A lot about diagnosing with transistor output stages, a lot about the Pye circuit used here, the fact that transistors can test perfectly but are knackered when deployed in circuit!

A huge thanks to all on here who daily supported my endeavours and gave me invaluable pointers. You kept me going!

Onwards & upwards!

Regards
Richard

[Edward Huggins]

This is great news and well rewarded after all of your persevernace. Also I'm glad you were also able to source those NKT driver transistors. If genuine NOS, they will be well over half a century old!

[richieyork]

Originally Posted by Edward Huggins

This is great news and well rewarded after all of your persevernace. Also I'm glad you were also able to source those NKT driver transistors. If genuine NOS, they will be well over half a century old!

Thanks Edward! I'm thrilled its running again, as is the young owner who loves its style.

Re the Newmarket transistors I found that Langrex had a decent range and they are genuine NOS items

[AC/HL]

Just goes to show that they're just transistorised amplifiers, despite their unorthodox design. Logic rules (as usual).