Philips EL3536 input stage not working

[Clockwork_TV]

Hi All

I'm trying to fix one channel a Philips EL3536 reel to reel stereo valve recorder. One channel works the other is pretty much dead. The output stage does work when I switch to mono so that narrows it down to the input stage. The signal gets to the ECC83 input (looking with CRO) but doesn't seem to be getting past to the output from this. So I thought it could be the load resistor.

Now here's the thing - when I measure the value of the load resistor (R10 220kohms) with the power off it reads correct. When it's on, it reads about 440kohms - but on the other (working) channel it still reads 220kohms. And both have the correct reading of 68V. Is this the load resistor failing or is it some other part of the valve circuit causing a false reading? Any ideas?

I had also swapped the ECC83 valves round to check it's not the valve.

Cheers

[Station X]

All I can say it that you shouldn't be measuring resistance with power on. Voltages within the circuit will affect the reading.

Having said that, the different reading for the two channels point to a fault somewhere. Are you sure it's the faulty channel which reads 440k when powered up? I'd expect the readings to differ on both channels.

[Clockwork_TV]

I need to check but it's R110 I'm actually trying to look at. It's not exactly obvious which way round things are in the diag (it's like a birds nest, wires going all over the place). Is it ok to switch on with a valve taken out just to see it's the correct channel I'm looking at?

[julie_m]

What Graham said about trying to measure resistors with the machine powered up. An ohmmeter has to force a current through the resistor under test (an analogue meter uses a constant voltage and measures the resulting current, which is why the scale reads backwards -- R = V/I, so the current decreases as the resistance increases; whereas a digital meter uses a constant current and measures the voltage developed across the resistor.)

It would be worth giving your meter a quick sanity check with whatever resistors you have in your parts drawer, just to make sure you haven't damaged it. You won't get very far trying to fault-find with a faulty test meter .....

Originally Posted by Clockwork_TV

Is it ok to switch on with a valve taken out just to see it's the correct channel I'm looking at?

It should be fine.

Tape recorders, being designed to connect to other equipment, necessarily have isolated chassis (not that that stopped some manufacturers .....); so the valve heaters will be in parallel. So pulling out a valve won't cut off the heater current to the rest of the machine. It could increase the heater and possibly the HT voltage, depending on the regulation factor of the power transformer; but a quick, back-of-envelope guesstimate says not by enough to worry about unless it's one of the output valves you're removing. And valve circuits tend to be AC coupled, so you will be unlikely to be upsetting any DC conditions.

You mentioned "R110", so I presume you have a wiring diagram and can therefore make sure before you proceed. As long as the anode(s) of the valve you want to remove are capacitively coupled to the following stage, then it should be safe to lift it -- but do check the schematic.

[Station X]

Here's a circuit extract.

I have to say I'm puzzled because R110 isn't associated with an ECC83. Please could you clarify which valve and resistor you're referring to and which pins you're looking at the signal on.

[yestertech]

R110( R10) is the anode load for the EF86. This handles first stage amplification from the head and also mic. input on record. Check all the voltages around this stage, if necessary by comparison with the working one. As you have a working channel it should be fairly straightforward to compare all the DC voltages between the two to determine what's missing. ( pin-for-pin ) The HT supply is separate for left/right preamp stages so check the HT decouplers ( resistors and associated electrolytics.) If all these check out Ok you may have a broken/dirty switch contact somewhere as there's a lot of record/playback switching shown. Do you get any 'hiss' from the defective channel at all, as this could indicate an open circuit head.( !)

Andy

[Clockwork_TV]

Originally Posted by Station X

Here's a circuit extract.

I have to say I'm puzzled because R110 isn't associated with an ECC83. Please could you clarify which valve and resistor you're referring to and which pins you're looking at the signal on.

Sorry yes you're right it is the EF86, I was writing from memory

[Station X]

OK. As Andy yestertech says, check the voltages on V2 and V102, comparing them with each other and the values shown on the circuit diagram.

[Clockwork_TV]

OK, thanks, will try checking the pins (not easily accessible have to take it apart a bit)