Another Yale Reverb repair ..

[daxio]

Hi all - found the site through a previous post about repairing a Yale Reverb. Didn't want to make a new post as I just joined the forum so .. but have been advised to go ahead anyway.

So I have a Yale Reverb - it is really nicely put together. Early 80s, discrete components, solid pine. As far as I can make out the gain stage 3 uses half a 4558 and stage 4 is a 4007 for tube sound I guess. Stage 5 is master after the reverb return. I mostly know that by reading around last couple of days and trying to understand the schematic .. attached below.

I have no sound out. Power amp section works. So I made a signal probe and get life after the tone and gain circuits up until to R25 then dead after at the gates and drains of the 4007. I checked the power rails - the 15V rail at R35 is reading 16V but seems to drift up slowly - not sure if significant. The zener only seems to be putting out 3.8V ( I think supposed to be 6.2). The 4007 is reading 1.8V at TP2 source - which looking at the spec sheet seems low - would that stop it functioning completely?

Could the 4007 simply be blown? ( I lent the amp to a friend ..) I ordered some TI CD4007UBE anyway. My other suspect is the 4007 trim pot - which seems to be putting out same voltage no matter where set (about 0.55v).

I have reflowed the pots but not the various cable multi-connector sockets as was the solution to a dead power stage in an older post about this same amp.

Appreciate any advice/pointers. I can solder, hold the schematic the right way up and not electrocute myself, but not too much more.

Cheers

[g1]

Re-soldering all those connectors was always the first step whenever we worked on this series of amps. It's pretty much necessary before doing any troubleshooting as you will otherwise get very inconsistent results.

[daxio]

Re-soldering all those connectors was always the first step whenever we worked on this series of amps. It's pretty much necessary before doing any troubleshooting as you will otherwise get very inconsistent results.

OK I'll get on it - the pots I understand because they get stressed, didn't realise also for the interconnects. I'll report back. Thanks.

[mikeydee77]

Hi Daxio,
Yes a new thread is preferred especially when it is unrelated fault. Hopefully your post will help someone else down the line especially if the thread title descibes the fault well.

I have not seen this amp ever so best go with g1 advice first as sounds like that there is a common fault on these amps
When you check the voltages you want to see your +/- 37v, 24v an 15v supplies being close to spec and certainly symmetrical.

You said that the power amp is working... is that verified by plugging into the power amp in socket?

[daxio]

Hi Daxio,
Yes a new thread is preferred especially when it is unrelated fault. Hopefully your post will help someone else down the line especially if the thread title descibes the fault well.

I have not seen this amp ever so best go with g1 advice first as sounds like that there is a common fault on these amps
When you check the voltages you want to see your +/- 37v, 24v an 15v supplies being close to spec and certainly symmetrical.

You said that the power amp is working... is that verified by plugging into the power amp in socket?

Hi Mike yes I will check those solder joints and as for the power section, yes a) by plugging the pre-amp out of another guitar amp into power amp input or also b) by tapping the reverb tank which pumps some reverb noise into the pre-amp reverb return, after the 4007 (and master vol?) , just before the power amp link up and controllable by the reverb channel pot

[g1]

In the other thread you mentioned only having 3.8V at TP2 (IC4). Check R34, R35, D7, and C100. Feel if IC4 is getting hot.

[daxio]

In the other thread you mentioned only having 3.8V at TP2 (IC4). Check R34, R35, D7, and C100. Feel if IC4 is getting hot.

Reflowed all the cable connectors.
Then turns out my meter was off - it needed a new 9v battery, which doesn't help of course, sorry. OK so all 3 rails check out +ve and -ve symmetrical. The +15V at R35 is reading 14.5V.
I am getting 2.5V at D7 and 1.25V at C21/TP2/pin 14 of IC4.
R34 , R35 read correct resistance (in circuit but they are 56 ohm and 562 ohm only).
Took out the zener and got 6.0V with 12V and a 1k resistor.
Took out cap C21 charges OK to 12V.
IC4 not getting hot BUT R35 is very hot. So 12V drop across 560 ohm gives 144/560 ~ 1/4 Watt. Explains why it is hot. Should be (14.5-6.2)^2/560 = 0.12W - still would be warm - seems a strange way to drop a voltage?

So is there a short? Where - IC4? I put a meter from pin 14 to ground (pin 4) and get 4.5kΩ once C21 is charged, and 1k with test leads reversed? So I am thinking something is breaking down only at 5V to drop that voltage to 1.25V - IC4, C21 or D7. Well I tested D7 and C21 at 12V. That leaves just IC4.

I put a meter across the trim pot and R26/28 - all in circuit so readings meaningless but all high 10s of kΩ so no shorts.

Hope my logic isn't too far off - not my strength as you may have guessed, but I like to learn.

[g1]

Yes, it sounds like you have it all covered and IC4 is likely bad. You can try desoldering the IC4 pins tied to supply (2,11,14) and see if that D7 voltage comes back up to 6V.

[daxio]

Yes, it sounds like you have it all covered and IC4 is likely bad. You can try desoldering the IC4 pins tied to supply (2,11,14) and see if that D7 voltage comes back up to 6V.

OK IC4 source pins desoldered - I now have 6.05V from the zener. R35 now only warm as predicted .. ! So is that it ? Have I found the problem (we sorry) ? If so then I just hope the new 4007 chips are quiet as I have read they are not actually supposed to be audio/analogue ICs at all. Should I solder in a DIP socket so I can swap in different ones? Wow, man I could get the bug for all this - I always loved fixing mechanical/electrical things and helping people out but electronics was always mostly beyond me - a dark art.

[g1]

Yes, that should be it.
The 4007 is just being used for switches, I don't think there is any point socketing it or trying different ones as it shouldn't have any tonal properties.

[daxio]

Yes, that should be it.
The 4007 is just being used for switches, I don't think there is any point socketing it or trying different ones as it shouldn't have any tonal properties.

Oh OK so like a slow start to prevent pops? I saw them being used in distortion pedals so I just thought .. Anyhow we're done, a bit of PCB track repair but all fixed and sounding good! Many thanks g1 and mikeydee77 .

[g1]

Oh OK so like a slow start to prevent pops? I saw them being used in distortion pedals so I just thought ..

Sorry, my mistake, I was thinking of some other circuit. The CD4007 here is being used for clipping, my guess would be to provide a soft clipping just before the power amp hits full power output.